George E Gehrels
- Professor, Geosciences
- Distinguished Professor
- Member of the Graduate Faculty
Contact
- (520) 349-4702
- Gould-Simpson, Rm. 529
- Tucson, AZ 85721
- ggehrels@arizona.edu
Degrees
- B.S. Geology
- University of Arizona, Tucson, Arizona, United States
- M.S. Geology
- University of Southern California, Los Angeles, California, United States
- Ph.D. Geology
- California Institute of Technology, Pasadena, California, United States
Awards
- Excellence in Post-Doc Mentoring Award
- University of Arizona, Spring 2021
- AGU Fellow
- AGU, Fall 2015
Interests
No activities entered.
Courses
2024-25 Courses
-
Intro To Oceanography
GEOS 212 (Spring 2025) -
Internship
GEOS 393 (Fall 2024) -
Research
GEOS 900 (Fall 2024)
2023-24 Courses
-
Internship
GEOS 393 (Summer I 2024) -
Independent Study
GEOS 399 (Spring 2024) -
Master's Report
GEOS 909 (Spring 2024) -
Research
GEOS 900 (Spring 2024) -
Independent Study
GEOS 399 (Fall 2023) -
Independent Study
GEOS 599 (Fall 2023) -
Master's Report
GEOS 909 (Fall 2023) -
Physical Geology
GEOS 251 (Fall 2023) -
Research
GEOS 900 (Fall 2023) -
Teaching Geosciences
GEOS 397A (Fall 2023)
2022-23 Courses
-
Internship
GEOS 393 (Summer I 2023) -
Directed Research
GEOS 492 (Spring 2023) -
Historical Geology
GEOS 255 (Spring 2023) -
Independent Study
GEOS 499 (Spring 2023) -
Master's Report
GEOS 909 (Spring 2023) -
Research
GEOS 900 (Spring 2023) -
Teaching Geosciences
GEOS 397A (Spring 2023) -
Directed Research
GEOS 392 (Fall 2022) -
Independent Study
GEOS 399 (Fall 2022) -
Research
GEOS 900 (Fall 2022)
2021-22 Courses
-
Internship
GEOS 393 (Summer I 2022) -
Directed Research
GEOS 392 (Spring 2022) -
Directed Research
GEOS 492 (Spring 2022) -
Independent Study
GEOS 399 (Spring 2022) -
Intro To Oceanography
GEOS 212 (Spring 2022) -
Teaching Geosciences
GEOS 397A (Spring 2022)
2020-21 Courses
-
Honors Thesis
GEOS 498H (Spring 2021) -
Intro To Oceanography
GEOS 212 (Spring 2021) -
Teaching Geosciences
GEOS 397A (Spring 2021) -
Honors Thesis
GEOS 498H (Fall 2020) -
Physical Geology
GEOS 251 (Fall 2020) -
Teaching Geosciences
GEOS 397A (Fall 2020)
2019-20 Courses
-
Intro To Oceanography
GEOS 212 (Spring 2020) -
Teaching Geosciences
GEOS 397A (Spring 2020) -
Physical Geology
GEOS 251 (Fall 2019) -
Teaching Geosciences
GEOS 397A (Fall 2019)
2018-19 Courses
-
Thesis
GEOS 910 (Summer I 2019) -
Dissertation
GEOS 920 (Spring 2019) -
Honors Thesis
GEOS 498H (Spring 2019) -
Intro To Oceanography
GEOS 212 (Spring 2019) -
Mineral-Petrol-Geochem
GEOS 596A (Spring 2019) -
Teaching Geosciences
GEOS 397A (Spring 2019) -
Thesis
GEOS 910 (Spring 2019) -
Dissertation
GEOS 920 (Fall 2018) -
Honors Thesis
GEOS 498H (Fall 2018) -
Independent Study
GEOS 499 (Fall 2018) -
Master's Report
GEOS 909 (Fall 2018) -
Physical Geology
GEOS 251 (Fall 2018) -
Research
GEOS 900 (Fall 2018) -
Teaching Geosciences
GEOS 397A (Fall 2018)
2017-18 Courses
-
Dissertation
GEOS 920 (Spring 2018) -
Intro To Oceanography
GEOS 212 (Spring 2018) -
Master's Report
GEOS 909 (Spring 2018) -
Research
GEOS 900 (Spring 2018) -
Teaching Geosciences
GEOS 397A (Spring 2018) -
Dissertation
GEOS 920 (Fall 2017) -
Independent Study
GEOS 299 (Fall 2017) -
Master's Report
GEOS 909 (Fall 2017) -
Physical Geology
GEOS 251 (Fall 2017) -
Research
GEOS 900 (Fall 2017) -
Teaching Geosciences
GEOS 397A (Fall 2017)
2016-17 Courses
-
Dissertation
GEOS 920 (Spring 2017) -
Master's Report
GEOS 909 (Spring 2017) -
Research
GEOS 900 (Spring 2017) -
Thesis
GEOS 910 (Spring 2017) -
Honors Thesis
GEOS 498H (Fall 2016) -
Independent Study
GEOS 599 (Fall 2016) -
Master's Report
GEOS 909 (Fall 2016) -
Research
GEOS 900 (Fall 2016)
2015-16 Courses
-
Directed Research
GEOS 392 (Spring 2016) -
Directed Research
GEOS 492 (Spring 2016) -
Independent Study
GEOS 599 (Spring 2016) -
Intro To Oceanography
GEOS 212 (Spring 2016) -
Master's Report
GEOS 909 (Spring 2016) -
Physical Geology
GEOS 251 (Spring 2016) -
Research
GEOS 900 (Spring 2016) -
Teaching Geosciences
GEOS 397A (Spring 2016)
Scholarly Contributions
Books
- Davis, G. H., Orent, E. B., Clinkscales, C., Ferroni, F. R., Gehrels, G. E., George, S. W., Guns, K. A., Hanagan, C. E., Hughes, A., Iriondo, A., Jepson, G., Kelty, C., Krantz, R. W., Levenstein, B. M., Lingrey, S. H., Miggins, D. P., Moore, T., Portnoy, S. E., Reeher, L., & Wang, J. W. (2023).
Structural Analysis and Chronologic Constraints on Progressive Deformation within the Rincon Mountains, Arizona: Implications for Development of Metamorphic Core Complexes
. Geological Society of America. doi:10.1130/mwr222 - DeCelles, P. G., Carrapa, B., Ojha, T. P., Gehrels, G. E., & Collins, D. (2020). Structural and thermal evolution of the Himalayan thrust belt in midwestern Nepal. Geological Society of America.
- Harrison, M., Baldwin, S., Caffee, M., Gehrels, G. E., Schoene, B., Schuster, D., & Singer, B. (2015). It’s about time: Opportunities and challenges for U.S. geochronology. Institute for Geophysics and Planetary Physics Publication 6539.
Chapters
- DeCelles, P., Carrapa, B., Horton, B., McNabb, J., Gehrels, G. E., & Boyd, J. (2015). The Miocene Arizaro Basin, central Andean hinterland: Response to partial lithosphere removal?. In Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile(pp 359-385). Memoir 212: Geological Society of America.
- DeCelles, P., Zandt, G., Beck, S., Currie, C., Ducea, M., Kapp, P., Gehrels, G. E., Carrapa, B., Quade, J., & Schoenbohm, L. (2015). Cyclical orogenic processes in the Cenozoic central Andes. In Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile(pp 459-490). Memoir 212: Geological Society of America.
- Einhorn, J., Gehrels, G. E., Vernon, A., & DeCelles, P. (2015). U-Pb zircon geochronology of Neoproterozoic–Paleozoic sandstones and Paleozoic plutonic rocks in the Central Andes (21°S–26°S). In Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile(pp 115-124). Memoir 212: Geological Society of America.
- Quade, J., Dettinger, M., Carrapa, B., DeCelles, P., Murray, K., Huntington, K., Cartwright, A., Canavan, R., Gehrels, G. E., & Clementz, M. (2015). The growth of the central Andes, 22° S–26° S. In Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile(pp 277-308). Memoir 212: Geological Society of America.
- Reiners, P., Thomson, S., Vernon, A., Willett, S., Zattin, M., Einhorn, J., Gehrels, G. E., Quade, J., Pearson, D., Murray, K., & Cavazza, W. (2015). Low-temperature thermochronologic trends across the central Andes, 21ºS-28ºS. In Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile(pp 215-249). Memoir 212: Geological Society of America Memoir.
Journals/Publications
- Kroeger, E. D., McClelland, W. C., Colpron, M., Piercey, S. J., & Gehrels, G. E. (2023).
Detrital zircon U-Pb and Hf isotope signature of Carboniferous and older strata of the Yukon-Tanana terrane in Yukon, Canadian Cordillera: Implications for terrane correlations and the onset of Late Devonian arc magmatism
. Geosphere, 19(4), 1032-1056. doi:10.1130/ges02607.1 - Alberts, D., Gehrels, G. E., Nelson, J., & Roeske, S. (2021). U-Pb and Hf Analyses of detrital zircons from Paleozoic and Cretaceous strata on Vancouver Island, British Columbia: constraints on the Paleozoic tectonic evolution of southern Wrangellia. Lithosphere, 2021(1).
- Cecil, M. R., Gehrels, G. E., Rusmore, M. E., Woodsworth, G. J., Stowell, H. H., Yokelson, I. N., Homan, E., Kitajima, K., & Valley, J. W. (2021). Mantle control on magmatic flare-ups in the southern Coast Mountains batholith, British Columbia. Geosphere, 17(6), 2027--2041.
- George, S., Nelson, J. L., Alberts, D., Greig, C. J., & Gehrels, G. E. (2021). Triassic--Jurassic Accretionary History and Tectonic Origin of Stikinia From U-Pb Geochronology and Lu-Hf Isotope Analysis, British Columbia. Tectonics, 40(4), e2020TC006505.
- Haque, Z., Geissman, J. W., Irmis, R. B., Olsen, P. E., Lepre, C., Buhedma, H., Mundil, R., Parker, W. G., Rasmussen, C., & Gehrels, G. E. (2021). Magnetostratigraphy of the Triassic Moenkopi Formation From the Continuous Cores Recovered in Colorado Plateau Coring Project Phase 1 (CPCP-1), Petrified Forest National Park, Arizona, USA: Correlation of the Early to Middle Triassic Strata and Biota in Colorado Plateau and Its Environs. Journal of Geophysical Research: Solid Earth, 126(9), e2021JB021899.
- Jepson, G., Carrapa, B., George, S. W., Triantafyllou, A., Egan, S. M., Constenius, K. N., Gehrels, G. E., & Ducea, M. N. (2021). Resolving mid-to upper-crustal exhumation through apatite petrochronology and thermochronology. Chemical Geology, 565, 120071.
- McClelland, W. C., Strauss, J. V., Colpron, M., Gilotti, J. A., Faehnrich, K., Malone, S. J., Gehrels, G. E., Macdonald, F. A., & Oldow, J. S. (2021). 'Taters versus sliders: Evidence for a long-lived history of strike-slip displacement along the canadian arctic transform system (CATS).
- Plange, K. M., Gehrels, G. E., Brown, E. H., Schermer, E. R., McClelland, W. C., & Sun, S. (2021). U-Pb Geochronology and Hf Isotope Geochemistry of the Turtleback Complex and East Sound Group, San Juan Islands, Northwestern North American Cordillera. Lithosphere, 2021(1).
- Rasmussen, C., Mundil, R., Irmis, R. B., Geisler, D., Gehrels, G. E., Olsen, P. E., Kent, D. V., Lepre, C., Kinney, S. T., Geissman, J. W., & others, . (2021). U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change. Bulletin, 133(3-4), 539--558.
- Sundell, K. E., Gehrels, G. E., & Pecha, M. E. (2021). Rapid U-Pb geochronology by laser ablation multi-collector ICP-MS. Geostandards and Geoanalytical Research, 45(1), 37--57.
- Thomas, W. A., Gehrels, G. E., Sundell, K. E., & Romero, M. C. (2021). Detrital-zircon analyses, provenance, and late Paleozoic sediment dispersal in the context of tectonic evolution of the Ouachita orogen. Geosphere, 17(4), 1214--1247.
- Balica, C., Ducea, M. N., Gehrels, G. E., Kirk, J., Roban, R. D., Luffi, P., Chapman, J. B., Triantafyllou, A., Guo, J., Stoica, A. M., & others, . (2020). A zircon petrochronologic view on granitoids and continental evolution. Earth and Planetary Science Letters, 531, 116005.
- Dafov, M. N., Carrera, A., Gehrels, G. E., Alberts, D., Pereira, M., Cecil, M. R., Rusmore, M. E., Stowell, H. H., Woodsworth, G. J., & Roeske, S. M. (2020). U-Th-Pb Geochronology and Lu-Hf Isotope Geochemistry of Detrital Zircons in Metasedimentary Rocks of the Southern Coast Mountains Batholith. Lithosphere, 2020(1).
- Gehrels, G., Giesler, D., Olsen, P., Kent, D., Marsh, A., Parker, W., Rasmussen, C., Mundil, R., Irmis, R., Geissman, J., & others, . (2020). LA-ICPMS U--Pb geochronology of detrital zircon grains from the Coconino, Moenkopi, and Chinle formations in the Petrified Forest National Park (Arizona). Geochronology, 2(2), 257--282.
- Rasmussen, C., Mundil, R., Irmis, R. B., Geisler, D., Gehrels, G. E., Olsen, P. E., Kent, D. V., Lepre, C., Kinney, S. T., Geissman, J. W., & others, . (2020). U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change. GSA Bulletin.
- Romero, M. C., Ridgway, K. D., & Gehrels, G. E. (2020). Geology, U-Pb geochronology, and Hf isotope geochemistry across the Mesozoic Alaska Range suture zone (south-central Alaska): Implications for Cordilleran collisional processes and tectonic growth of North America. Tectonics, 39(3), e2019TC005946.
- Sundell, K. E., Gehrels, G. E., & Pecha, M. E. (2020). Rapid U-Pb Geochronology by Laser Ablation Multi-Collector ICP-MS. Geostandards and Geoanalytical Research.
- Thomas, W. A., Gehrels, G. E., Sundell, K. E., Greb, S. F., Finzel, E. S., Clark, R. J., Malone, D. H., Hampton, B. A., & Romero, M. C. (2020). Detrital zircons and sediment dispersal in the eastern Midcontinent of North America. Geosphere, 16(3), 817--843.
- Waite, L., Fan, M., Collins, D., Gehrels, G., & Stern, R. J. (2020). Detrital zircon provenance evidence for an early Permian longitudinal river flowing into the Midland Basin of west Texas. International Geology Review, 62(9), 1224--1244.
- Whitney, D. L., Jacobsen, S., & Gehrels, G. (2020). All Hands on Deck: Implications of the Recent NSF Earth Sciences Decadal Report for the Mineral Sciences. Elements: An International Magazine of Mineralogy, Geochemistry, and Petrology, 16(4), 228--230.
- Craddock, J. P., Ojakangas, R. W., Malone, D. H., Konstantinou, A., Mory, A., Bauer, W., Thomas, R. J., Affinati, S. C., Pauls, K., Zimmerman, U., & others, . (2019). Detrital zircon provenance of Permo-Carboniferous glacial diamictites across Gondwana. Earth-science reviews, 192, 285--316.
- Davis, G. H., Spencer, J. E., & Gehrels, G. E. (2019). Field-trip guide to the Catalina-Rincon metamorphic core complex, Tucson, Arizona. Geologic Excursions in Southwestern North America, 55, 1.
- Eddy, M. P., Iba~nez-Mejia, M., Burgess, S. D., Coble, M. A., Cordani, U. G., DesOrmeau, J., Gehrels, G. E., Li, X., MacLennan, S., Pecha, M., & others, . (2019). GHR 1 Zircon--A New Eocene Natural Reference Material for Microbeam U-Pb Geochronology and Hf Isotopic Analysis of Zircon. Geostandards and Geoanalytical Research, 43(1), 113--132.
- Kent, D. V., Olsen, P. E., Lepre, C., Rasmussen, C., Mundil, R., Gehrels, G. E., Giesler, D., Irmis, R. B., Geissman, J. W., & Parker, W. G. (2019). Magnetochronology of the entire Chinle Formation (Norian age) in a scientific drill core from Petrified Forest National Park (Arizona, USA) and implications for regional and global correlations in the Late Triassic. Geochemistry, Geophysics, Geosystems, 20(11), 4654--4664.
- Pavlis, T. L., Amato, J. M., Trop, J. M., Ridgway, K. D., Roeske, S. M., & Gehrels, G. E. (2019). Subduction polarity in ancient arcs: A call to integrate geology and geophysics to decipher the Mesozoic tectonic history of the Northern Cordillera of North America. GSA Today, 29(11).
- Seedorff, E., Barton, M. D., Gehrels, G. E., Valencia, V. A., Johnson, D. A., Maher, D. J., Stavast, W. J., & Marsh, T. M. (2019). Temporal evolution of the Laramide arc: U-Pb geochronology of plutons associated with porphyry copper mineralization in east-central Arizona. Geologic Excursions in Southwestern North America, 55, 369.
- Spencer, J. E., Richard, S. M., Lingrey, S. H., Johnson, B. J., Johnson, R. A., & Gehrels, G. E. (2019). Reconstruction of Mid-Cenozoic Extension in the Rincon Mountains Area, Southeastern Arizona, USA, and Geodynamic Implications. Tectonics, 38(7), 2338--2357.
- Stickroth, S. F., Carrapa, B., DeCelles, P. G., Gehrels, G. E., & Thomson, S. N. (2019). Tracking the Growth of the Himalayan Fold-and-Thrust Belt From Lower Miocene Foreland Basin Strata: Dumri Formation, Western Nepal. Tectonics, 38(11), 3765--3793.
- Thomas, W. A., Gehrels, G. E., Lawton, T. F., Satterfield, J. I., Romero, M. C., & Sundell, K. E. (2019). Detrital zircons and sediment dispersal from the Coahuila terrane of northern Mexico into the Marathon foreland of the southern Midcontinent. Geosphere, 15(4), 1102--1127.
- Yoshida, M., Gehrels, G. E., Upreti, B. N., & Rai, S. M. (2019). Early Paleozoic zircon ages of the Higher Himalayan Gneisses of the Everest region and their Pan-African/Proto-Himalayan orogenic signature. Journal of Nepal Geological Society, 59, 107--124.
- Cecil, M. R., Rusmore, M. E., Gehrels, G. E., Woodsworth, G. J., Stowell, H. H., Yokelson, I. N., Chisom, C., Trautman, M., & Homan, E. (2018). Along-Strike Variation in the Magmatic Tempo of the Coast Mountains Batholith, British Columbia, and Implications for Processes Controlling Episodicity in Arcs. Geochemistry, Geophysics, Geosystems, 19(11), 4274--4289.
- Chapman, J. B., Dafov, M. N., Gehrels, G., Ducea, M. N., Valley, J. W., & Ishida, A. (2018). Lithospheric architecture and tectonic evolution of the southwestern US Cordillera: Constraints from zircon Hf and O isotopic data. Bulletin, 130(11-12), 2031--2046.
- Chapman, J. B., Robinson, A. C., Carrapa, B., Villarreal, D., Worthington, J., DeCelles, P. G., Kapp, P., Gadoev, M., Oimahmadov, I., & Gehrels, G. (2018). Cretaceous shortening and exhumation history of the South Pamir terrane. Lithosphere, 10(4), 494--511.
- Eddy, M. P., Iba~nez-Mejia, M., Burgess, S. D., Coble, M. A., Cordani, U. G., DesOrmeau, J., Gehrels, G. E., Li, X., MacLennan, S., Pecha, M., & others, . (2018). GHR 1 Zircon--A New Eocene Natural Reference Material for Microbeam U-Pb Geochronology and Hf Isotopic Analysis of Zircon. Geostandards and Geoanalytical Research.
- Holland, M. E., Karlstrom, K. E., Gehrels, G., Shufeldt, O. P., Begg, G., Griffin, W., & Belousova, E. (2018). The Paleoproterozoic Vishnu basin in southwestern Laurentia: Implications for supercontinent reconstructions, crustal growth, and the origin of the Mojave crustal province. Precambrian Research, 308, 1--17.
- Karlstrom, K., Hagadorn, J., Gehrels, G., Matthews, W., Schmitz, M., Madronich, L., Mulder, J., Pecha, M., Giesler, D., & Crossey, L. (2018). Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zircons. Nature Geoscience, 11(6), 438.
- Kent, D. V., Olsen, P. E., Rasmussen, C., Lepre, C., Mundil, R., Irmis, R. B., Gehrels, G. E., Giesler, D., Geissman, J. W., & Parker, W. G. (2018). Empirical evidence for stability of the 405-kiloyear Jupiter--Venus eccentricity cycle over hundreds of millions of years. Proceedings of the National Academy of Sciences, 115(24), 6153--6158.
- Olsen, P. E., Geissman, J. W., Kent, D. V., Gehrels, G. E., Mundil, R., Irmis, R., Lepre, C., Rasmussen, C., Giesler, D., Parker, W. G., & others, . (2018). Colorado Plateau Coring Project, Phase I (CPCP-I): a continuously cored, globally exportable chronology of Triassic continental environmental change from western North America. Scientific Drilling, 24, 15--40.
- Pecha, M. E., Gehrels, G. E., Karlstrom, K. E., Dickinson, W. R., Donahue, M. S., Gonzales, D. A., & Blum, M. D. (2018). Provenance of Cretaceous through Eocene strata of the Four Corners region: Insights from detrital zircons in the San Juan Basin, New Mexico and Colorado. Geosphere, 14(2), 785--811.
- Pullen, A., Ib'a~nez-Mejia, M., Gehrels, G. E., Giesler, D., & Pecha, M. (2018). Optimization of a Laser Ablation-Single Collector-Inductively Coupled Plasma-Mass Spectrometer (Thermo Element 2) for Accurate, Precise, and Efficient Zircon U-Th-Pb Geochronology. Geochemistry, Geophysics, Geosystems, 19(10), 3689--3705.
- Schermer, E. R., Hoffnagle, E. A., Brown, E. H., Gehrels, G. E., & McClelland, W. C. (2018). U-Pb and Hf isotopic evidence for an Arctic origin of terranes in northwestern Washington. Geosphere, 14(2), 835--860.
- Schmidt, J., Hacker, B. R., Ratschbacher, L., Stuebner, K., Stearns, M., Kylander-Clark, A., Cottle, J. M., Webb, A., Gehrels, G., & Minaev, V. (2018). Cenozoic deep crust in the Pamir (vol 312, pg 411, 2011). EARTH AND PLANETARY SCIENCE LETTERS, 493, 172--172.
- Carrapa, B., bin, H., Kapp, P. A., DeCelles, P. G., & Gehrels, G. (2017). Tectonic and erosional history of southern Tibet recorded by detrital chronological signatures along the Yarlung River drainage. GEOLOGICAL SOCIETY OF AMERICA BULLETIN, 129(5-6), 570-581.
- Chapman, J. B., Ducea, M. N., Kapp, P., Gehrels, G. E., & DeCelles, P. G. (2017). Spatial and temporal radiogenic isotopic trends of magmatism in Cordilleran orogens. GONDWANA RESEARCH, 48, 189-204.
- Dehler, C., Gehrels, G., Porter, S., Heizler, M., Karlstrom, K., Cox, G., Crossey, L., & Timmons, M. (2017). Synthesis of the 780-740 Ma Chuar, Uinta Mountain, and Pahrump (ChUMP) groups, western USA: Implications for Laurentia-wide cratonic marine basins. GEOLOGICAL SOCIETY OF AMERICA BULLETIN, 129(5-6), 607-624.
- Gehrels, G. E., McClelland, W. C., & Yokelson, I. (2017). Reply to "Comment on "U-Pb and Hf Isotope Analysis of Detrital Zircons From Mesozoic Strata of the Gravina Belt, Southeast Alaska' by Yokelson Et Al. (2015)". TECTONICS, 36(11), 2741-2743.
- Linde, G. M., Trexler Jr., J. H., Cashman, P. H., Gehrels, G., & Dickinson, W. R. (2017). Three-Dimensional Evolution of the Early Paleozoic Western Laurentian Margin: New Insights From Detrital Zircon U-Pb Geochronology and Hf Isotope Geochemistry of the Harmony Formation of Nevada. TECTONICS, 36(11), 2347-2369.
- Mulder, J. A., Karlstrom, K. E., Fletcher, K., Heizler, M. T., Timmons, J. M., Crossey, L. J., Gehrels, G. E., & Pecha, M. (2017). The syn-orogenic sedimentary record of the Grenville Orogeny in southwest Laurentia. PRECAMBRIAN RESEARCH, 294, 33-52.
- Pearson, D. M., MacLeod, D. R., Ducea, M. N., Gehrels, G. E., & Patchett, P. J. (2017). Sediment underthrusting within a continental magmatic arc: Coast Mountains batholith, British Columbia. TECTONICS, 36(10), 2022-2043.
- Thomas, W. A., Gehrels, G. E., Greb, S. F., Nadon, G. C., Satkoski, A. M., & Romero, M. C. (2017). Detrital zircons and sediment dispersal in the Appalachian foreland. GEOSPHERE, 13(6), 2206-2230.
- Benyon, C., Leier, A., Leckie, D., Hubbard, S., & Gehrels, G. E. (2016). Sandstone provenance and insights into the paleogeography of the McMurray Formation from detrital zircon geochronology, Athabasca Oil Sands, Canada. AAPG Bulletin, 100(2), 269-287.
- Chapman, J., Gehrels, G. E., Ducea, M., Giesler, D., & Pullen, A. (2016). A new method for estimating parent rock trace element concentrations from zircon. Chemical Geology, 439(1), 59-70.
- DeCelles, P., Carrapa, B., Gehrels, G. E., Chakraborty, T., & Ghosh, P. (2016). Along‐Strike Continuity of Structure, Stratigraphy, and Kinematic History in the Himalayan thrust belt: The View from Northeastern India. Tectonics. doi:10.1002/2016TC004298
- Gehrels, G. E. (2016). Algorithms and software for U-Pb geochronology by LA-ICPMS. Geochemistry, Geophysics, and Geosystems. doi:10.1002/2015GC006097
- Giesler, D., Gehrels, G. E., Pecha, M., White, C., Yokelson, I., & McClelland, W. (2016). U-Pb and Hf isotopic analyses of detrital zircons from the Taku terrane, southeast Alaska. Canadian Journal of Earth Sciences, 53, 1-14. doi:10.1139/cjes-2015-0240
- Harrison, M., Baldwin, S., Caffee, M., Gehrels, G. E., Schoene, B., Shuster, D., & Singer, B. (2016). Geochronology: It’s about time. EOS, 96. doi:10.1029/2015EO041901.
- Horstwood, M., Kosler, J., Gehrels, G. E., Jackson, S., McLean, N., Paton, C., Pearson, N., Sircombe, K., Sylvester, P., Vermeesch, P., Bowring, J., Condon, D., & Schoene, B. (2016). Community-Derived Standards for LA-ICP-MS U-Th-Pb Geochronology – Uncertainty Propagation, Age Interpretation and Data Reporting. Geostandards and Geoanalytical Research, 40(3), 311-332. doi:10.1111/j.1751-908X.2016.00379.x
- Leary, R., DeCelles, P., Quade, J., Gehrels, G. E., & Waanders, G. (2016). The Liuqu Conglomerate, southern Tibet: Early Miocene basin development related to deformation within the Great Counter Thrust system. Lithosphere, 8(5), 427-50.
- Linde, G., Trexler, J., Cashman, P., Gehrels, G. E., & Dickinson, W. (2016). Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Roberts Mountains allochthon: New insights into the early Paleozoic tectonics of western North America. Geosphere, 12(3). doi:10.1130/GES01252.1.
- Pecha, M., Gehrels, G. E., McClelland, W., Giesler, D., White, C., & Yokelson, I. (2016). Detrital zircon U‑Pb geochronology and Hf isotope geochemistry of the Yukon-Tanana terrane, Coast Mountains, southeast Alaska. Geosphere, 12(5), 1-19. doi:10 .1130 /GES01303.1
- Pepper, M., Gehrels, G. E., Pullen, A., Ibanez, M., Ward, K., & Kapp, P. (2016). Magmatic History and Crustal Genesis of South America: Constraints from U-Pb ages and Hf isotopes of detrital zircons in modern rivers. Geosphere, 12(5), 1532-1555. doi:10.1130/GES01315.1.
- Pepper, M., Gehrels, G. E., Pullen, A., Ibanez-Mejia, M., Ward, K. M., & Kapp, P. A. (2016). Magmatic history and crustal genesis of western South America: Constraints from U-Pb ages and Hf isotopes of detrital zircons in modern rivers. Geosphere, 12. doi:10.1130/GESO1315.1
- Spencer, J., Pecha, M., Gehrels, G. E., Dickinson, W., Domanik, K., & Quade, J. (2016). Paleoproterozoic orogenesis and quartz-arenite deposition in the Little Chino Valley area, Yavapai tectonic province, central Arizona, USA. Geosphere, 12(6), 1774-1794.
- Thomas, W., Gehrels, G. E., & Romero, M. (2016). Detrital zircons from crystalline rocks along the Southern Oklahoma fault system, Wichita and Arbuckle Mountains, USA. Geosphere, 12(4), 1224-1234. doi:10.1130/GES01316.1
- White, C., Gehrels, G. E., Pecha, M., Giesler, D., Yokelson, I., McClelland, W., & Butler, R. (2016). U-Pb and Hf isotope analysis of detrital zircons from Paleozoic strata of the southern Alexander terrane (southeast Alaska). Lithosphere, 8(1), 83-96. doi:10.1130/L475.1
- Chen, X., Gehrels, G. E., Yin, A., Zhou, Q., & Huang, P. (2015). Geochemical and Nd–Sr–Pb–O isotopic constrains on Permo–Triassic magmatism in eastern Qaidam Basin, northern Qinghai-Tibetan plateau: Implications for the evolution of the Paleo-Tethys. Journal of Asian Earth Sciences, 114, 674-692.
- Cole, D., Myrow, P., Fike, D., Hakim, A., & Gehrels, G. E. (2015). Uppermost Devonian (Famennian) to Lower Mississippian events of the western U.S.. Palaeogeography, Palaeoclimatology, Palaeoecology, 427, 1-19.
- Colpron, M., Crowley, J., Gehrels, G. E., Long, D., Murphy, D., Beranek, L., & Bickerton, L. (2015). Birth of the northern Cordilleran orogen, as recorded by detrital zircons in Jurassic synorogenic strata and regional exhumation in Yukon. Lithosphere, 7(5), 541-562.
- Holland, M., Karlstrom, K., Doe, M., Gehrels, G. E., Pecha, M., Shufeldt, O., Begg, G., Griffin, W., & Belousova, E. (2015). An imbricate midcrustal suture zone: The Mojave-Yavapai Province boundary in Grand Canyon, Arizona. Geological Society of America Bulletin. doi:10.1130/B31232.1
- Ibanez-Mejia, M., Pullen, A., Arenstein, J., Gehrels, G. E., Valley, J., Ducea, M., Moram, A., Pecha, M., & Ruiz, J. (2015). Unraveling crustal growth and reworking processes in complexzircons from orogenic lower-crust: The Proterozoic PutumayoOrogen of Amazonia. Precambrian Research, 267, 285-310.
- Kimbrough, D., Grove, M., Gehrels, G. E., Dorsey, R., Howard, K., Lovera, O., Aslan, A., House, P., & Pearthree, P. (2015). Detrital zircon U-Pb provenance of the Colorado River: A 5 my record of incision into cover strata overlying the Colorado Plateau and adjacent regions. Geosphere, 11(6), 1719-1748.
- Lane, L., Cecile, M., Gehrels, G. E., Kosko, M., & Parrish, R. (2015). Geochronology and structural setting of Latest Devonian – Early Carboniferous magmatic rocks, Cape Kiber, northeast Russia. Canadian Journal of Earth Sciences, 52, 147-160.
- Lane, L., Gehrels, G. E., & Layer, P. (2015). Provenance and paleogeography of the Neruokpuk Formation, northwest Laurentia: An integrated synthesis. Geological Society of America Bulletin. doi:10.1130/B31234.1
- Leary, R., DeCelles, P., Gehrels, G. E., & Morriss, M. (2015). Fluvial deposition during transition from flexural to dynamic subsidence in the Cordilleran foreland basin: Ericson Formation, Western Wyoming, USA. Basin Research, 27(4), 495-516.
- Mako, C., Williams, M., Karlstrom, K., Doe, M., Powicki, D., Holland, M., Gehrels, G. E., & Pecha, M. (2015). Polyphase Proterozoic deformation in the Four Peaks area, central Arizona, and relevance for the Mazatzal orogeny. Geosphere, 11(6), 1975-1995.
- Navarro, E., Astini, R., Belousova, E., Guler, M., & Gehrels, G. E. (2015). Detrital zircon geochronology and provenance of the Chubut Group in the northeast of Patagonia, Argentina. Journal of South American Earth Sciences, 63, 149-161.
- White, C., Gehrels, G. E., Pecha, M., Giesler, D., Yokelson, I., McClelland, W., & Butler, R. (2015). U-Pb and Hf isotope analysis of detrital zircons from Paleozoic strata of the southern Alexander terrane (southeast Alaska). Lithosphere. doi:10.1130/L475.1.
- Yokelson, I., Gehrels, G. E., Pecha, M., Giesler, D., White, C., & McClelland, W. (2015). U-Pb and Hf isotope analysis of detrital zircons from Mesozoic strata of the Gravina belt, southeast Alaska. Tectonics, 34(10), 2052-2066.
- Andreichev, V., Soboleva, A., & Gehrels, G. (2014). U-Pb dating and provenance of detrital zircons from the Upper Precambrian deposits of North Timan. Stratigraphy and Geological Correlation, 22(2), 147--159.
- Benyon, C., Leier, A., Leckie, D. A., Webb, A., Hubbard, S. M., & Gehrels, G. (2014). Provenance of the Cretaceous Athabasca Oil Sands, Canada: Implications for continental-scale sediment transport. Journal of Sedimentary Research, 84(2), 136--143.
- Burton, M. M., Muniz, A. A., Abbott, P. L., Kimbrough, D. L., Haproff, P. J., Gehrels, G. E., & Pecha, M. (2014). Sourcing sandstone cobble grinding tools in southern California using petrography, U--Pb geochronology, and Hf isotope geochemistry. Journal of Archaeological Science, 50, 273--287.
- Carrapa, B., Mustapha, F. S., Cosca, M., Gehrels, G. E., Schoenbohm, L. M., Sobel, E. R., Decelles, P. G., Russell, J. L., & Goodman, P. J. (2014). Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir. Lithosphere, 6, 443-455.
- Carrapa, B., Mustapha, F. S., Cosca, M., Gehrels, G., Schoenbohm, L. M., Sobel, E. R., DeCelles, P. G., Russell, J., & Goodman, P. (2014). Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir. Lithosphere, 6(6), 443--455.
- Chen, X., Gehrels, G., Yin, A., Zhou, Q., & Huang, P. (2014). Geochemical and Nd--Sr--Pb--O isotopic constrains on Permo--Triassic magmatism in eastern Qaidam Basin, northern Qinghai-Tibetan plateau: Implications for the evolution of the Paleo-Tethys. Journal of Asian Earth Sciences.
- DEHLER, C. M., GEHRELS, G., PORTER, S. M., COX, G., HEIZLER, M. T., KARLSTROM, K. E., CROSSEY, L. J., & TIMMONS, J. M. (2014). CHUMP (CHUAR-UINTA MOUNTAIN-PAHRUMP) STRATA OF THE WESTERN US RECORD CRETACEOUS--LIKE OCEAN ANOXIC EVENTS (OAES) BEFORE SNOWBALL EARTH. 2014 GSA Annual Meeting in Vancouver, British Columbia.
- DeCelles, P., Kapp, P., Gehrels, G., & Ding, L. (2014). Paleocene-Eocene foreland basin evolution in the Himalaya of southern Tibet and Nepal: Implications for the age of initial India-Asia collision. Tectonics, 33(5), 824--849.
- DeCelles, P., Zandt, G., Beck, S., Currie, C., Ducea, M., Kapp, P., Gehrels, G., Carrapa, B., Quade, J., & Schoenbohm, L. (2014). Cyclical orogenic processes in the Cenozoic central Andes. Geological Society of America Memoirs, 212, MWR212--22.
- Dickinson, W. R., Karlstrom, K. E., Hanson, A. D., Gehrels, G. E., Pecha, M., Cather, S. M., & Kimbrough, D. L. (2014). Detrital-zircon U-Pb evidence precludes paleo--Colorado River sediment in the exposed Muddy Creek Formation of the Virgin River depression. Geosphere, 10(6), 1123--1138.
- GEHRELS, G. (2014). JURA-CRETACEOUS SINISTRAL SHEAR ZONES ALONG THE WESTERN FLANK OF THE CENTRAL AND SOUTHERN COAST MOUNTAIN. 2014 GSA Annual Meeting in Vancouver, British Columbia.
- Gehrels, G. (2014). Detrital Zircon U-Pb Geochronology Applied to Tectonics. Annual Review of Earth and Planetary Sciences, 42, 127--149.
- Gehrels, G., & Pecha, M. (2014). Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America. Geosphere, 10(1), 49--65.
- Gehrels, G., & Pecha, M. (2014). Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America. Geosphere, 10(1), 49-65.More infoAbstract: U-Pb geochronologic and Hf isotopic analy ses have been conducted on detrital zircons extracted from 36 samples of Neoproterozoic through Triassic passive margin strata from western North America. The data serve as an improved reference for comparison with inboard strata that accumulated on the North American craton and outboard strata belonging to potentially displaced Cordilleran terranes. As expected, this reference documents significant variations in ages and Hf isotope compositions both north-south and also through time. The data also provide insights into the provenance of Cordilleran passive margin strata. During Neoproterozoic, Cambrian, and Early-Middle Devonian time, most grains were shed from relatively local basement rocks and from Meso proterozoic clastic strata containing 1.2-1.0 Ga grains that originated in the Grenville orogen. This pattern was interrupted during Ordovician time, when much of the Cordilleran margin was blanketed by detritus shed from the northern Canadian Shield. Beginning in Late Devonian time, and continuing through late Paleozoic and Triassic time, most regions were dominated by locally derived detritus (largely recycled from underlying strata), but also received 0.7-0.4 Ga grains that were shed from the Franklinian, Caledonian, Appalachian, and Ouachita-Marathon orogens. This pattern is complicated in southern transects as a result of mid-Paleozoic emplacement of off-shelf assemblages onto the continental margin (e.g., Antler orogeny) and construction of Permo-Triassic magmatic arcs along the margin. Our data also provide a robust record of the crustal evolution of western North America, with significant production of juvenile crust during late Archean (3.0- 2.5 Ga) and Paleoproterozoic (1.78-1.6 Ga) time and phases of mainly crustal reworking at 2.0-1.78, 1.5-1.3, 1.2-1.0, and 0.6-0.2 Ga. This history is somewhat different from that of other continents, with western Laurentia comprising a greater overall proportion of juvenile crust, punctuated by greater degrees of crustal reworking between 2.2 and 1.78 Ga and 0.3-0.2 Ga. © 2014 Geological Society of America.
- Ibanez-Mejia, M., Gehrels, G. E., Ruiz, J., Vervoort, J. D., Eddy, M. P., & Li, C. (2014). Small-volume baddeleyite (ZrO 2) U--Pb geochronology and Lu--Hf isotope geochemistry by LA-ICP-MS. Techniques and applications. Chemical Geology, 384, 149--167.
- Lane, L. S., & Gehrels, G. E. (2014). Detrital zircon lineages of late Neoproterozoic and Cambrian strata, NW Laurentia. Geological Society of America Bulletin, 126(3-4), 398--414.
- Lane, L. S., & Gehrels, G. E. (2014). Detrital zircon lineages of late neoproterozoic and Cambrian strata, NW Laurentia. Bulletin of the Geological Society of America, 126(3-4), 398-414.More infoAbstract: The Phanerozoic tectonic evolution of the Arctic is a field of escalating scientific interest. Detrital zircon provenance studies provide vital contributions to clarify the region's tectonic evolution. Northwest Laurentia exposes a broad expanse of Proterozoic and Paleo zoic sedimentary strata for which detrital zircon populations are poorly characterized. Moreover, the significance of sedimentary recycling is becoming better appreciated in light of detrital zircon studies. As more data become available, our understanding of the detrital zircon character of NW Laurentia improves, providing an increasingly reliable baseline at subcontinental resolution against which potentially allochthonous terranes, such as Arctic Alaska, can be assessed. Sandstones of late Neoproterozoic and Cambrian age from NW Canada yield detrital zircon signatures dominated by zircon grains recycled from Proterozoic sedimentary strata. Two Neoproterozoic sandstones from the northern Mackenzie Mountains yield zircon populations sourced from the Mackenzie Mountains Supergroup. Two Lower Cambrian sandstones sourced from the Yukon stable block and deposited in Richardson Trough have zircon populations nearly identical to those of the upper Wernecke Supergroup, locally exposed across the southern Yukon stable block, where they are unconformably overlain by Cambrian strata. Comparisons with similar studies from NW Canada permit generalizations of the patterns of zircon recycling. Four provenance lineages are described that characterize Laurentia-derived successions in NW Canada. © 2014 Geological Society of America.
- Leary, R., DeCelles, P., Gehrels, G., & Morriss, M. (2014). Fluvial deposition during transition from flexural to dynamic subsidence in the Cordilleran foreland basin: Ericson Formation, Western Wyoming, USA. Basin Research.
- Lippert, P. C., Hinsbergen, D. J., & Dupont-Nivet, G. (2014). Early Cretaceous to present latitude of the central proto-Tibetan Plateau: A paleomagnetic synthesis with implications for Cenozoic tectonics, paleogeography, and climate of Asia. Geological Society of America Special Papers, 507, SPE507--01.
- Mahon, R. C., Dehler, C. M., Link, P. K., Karlstrom, K. E., & Gehrels, G. E. (2014). Detrital zircon provenance and paleogeography of the Pahrump Group and overlying strata, Death Valley, California. Precambrian Research, 251, 102--117.
- Mahon, R. C., Dehler, C. M., Link, P. K., Karlstrom, K. E., & Gehrels, G. E. (2014). Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California: A record of the assembly, stability, and breakup of Rodinia. Geological Society of America Bulletin, 126(5-6), 652--664.
- Painter, C. S., Carrapa, B., DeCelles, P. G., Gehrels, G. E., & Thomson, S. N. (2014). Exhumation of the North American Cordillera revealed by multi-dating of Upper Jurassic--Upper Cretaceous foreland basin deposits. Geological Society of America Bulletin, B30999--1.
- Pierce, E., Hemming, S., Williams, T., Flierdt, T., Thomson, S., Reiners, P., Gehrels, G., Brachfeld, S., & Goldstein, S. (2014). A comparison of detrital U--Pb zircon, 40 Ar/39 Ar hornblende, 40 Ar/39 Ar biotite ages in marine sediments off East Antarctica: Implications for the geology of subglacial terrains and provenance studies. Earth-Science Reviews, 138, 156--178.
- Pullen, A., Ib\'a\~nez-Mej\'\ia, M., Gehrels, G. E., Ib\'a\~nez-Mej\'\ia, J. C., & Pecha, M. (2014). What happens when n= 1000? Creating large-n geochronological datasets with LA-ICP-MS for geologic investigations. Journal of Analytical Atomic Spectrometry, 29(6), 971--980.
- Quade, J., Dettinger, M., Carrapa, B., DeCelles, P., Murray, K., Huntington, K., Cartwright, A., Canavan, R., Gehrels, G., & Clementz, M. (2014). The growth of the central Andes, 22° S--26° S. Geological Society of America Memoirs, 212, MWR212--15.
- RUSMORE, M. E., GEHRELS, G. E., CECIL, M. R., STOWELL, H., WOODSWORTH, G. J., PECHA, M., YOKELSON, I., CHISOM, C., TRAUTMAN, M., & BOGUE, R. R. (2014). PLUTONIC AND STRUCTURAL FRAMEWORK, SOUTHERN COAST MOUNTAINS BATHOLITH, BUTE INLET TO MT WADDINGTON: PRELIMINARY RESULTS. 2014 GSA Annual Meeting in Vancouver, British Columbia.
- Siddoway, C. S., & Gehrels, G. E. (2014). Basement-hosted sandstone injectites of Colorado: A vestige of the Neoproterozoic revealed through detrital zircon provenance analysis. Lithosphere, 6(6), 403--408.
- Tochilin, C. J., Gehrels, G. E., Nelson, J., & Mahoney, J. B. (2014). U-Pb and Hf isotope analysis of detrital zircons from the Banks Island assemblage (coastal British Columbia) and southern Alexander terrane (southeast Alaska). Lithosphere, 6(3), 200--215.
- Volkmer, J. E., Kapp, P., Horton, B. K., Gehrels, G. E., Minervini, J. M., & Ding, L. (2014). Northern Lhasa thrust belt of central Tibet: Evidence of Cretaceous--early Cenozoic shortening within a passive roof thrust system?. Geological Society of America Special Papers, 507, 59--70.
- YOKELSON, I., GEHRELS, G., CECIL, M. R., PECHA, M., & RUSMORE, M. E. (2014). U-PB GEOCHRONOLOGY AND FIELD STUDIES IN THE SOUTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA. 2014 GSA Annual Meeting in Vancouver, British Columbia.
- Andreichev, V. L., Soboleva, A. A., & Gehrels, G. E. (2013). U-Pb age of detrital zircons from the Upper Precambrian terrigenous section of North Timan. Doklady Earth Sciences, 450(2), 592-596.
- Ding, L., Yang, D., Cai, F. L., Pullen, A., Kapp, P., Gehrels, G. E., Zhang, L. Y., Zhang, Q. H., Lai, Q. Z., Yue, Y. H., & Shi, R. D. (2013). Provenance analysis of the Mesozoic Hoh-Xil-Songpan-Ganzi turbidites in northern Tibet: Implications for the tectonic evolution of the eastern Paleo-Tethys Ocean. Tectonics, 32(1), 34-48.More infoAbstract: [1] Mesozoic strata of the Hoh-Xil-Songpan-Ganzi complex in northern Tibet are exposed in a vast (> 370,000 km2) triangle-shaped orogenic belt bound by the Longmen Shan thrust belt in the east, the Kunlun terrane and North China block in the north, and the Qiangtang terrane and Yidun arc in the south. These strata consist of Middle-Upper Triassic submarine fan and deep marine facies rocks that were deposited in the Paleo-Tethys Ocean. Late Triassic-Early Jurassic contractional deformation in the eastern Hoh-Xil-Songpan- Ganzi complex marks the demise of the Paleo-Tethys Ocean basin and the accretion of the Gondwana-derived Qiangtang terrane to Eurasia. We conducted geological mapping, regional stratigraphic analyses, and U-Pb geochronology of detrital zircons (n = 4128) on the Mesozoic sequences exposed in the Hoh-Xil-Songpan-Ganzi complex, Kunlun terrane, and Qiangtang terrane. We identify for the first time marine silciclastic sandstone and shale of Jurassic age in the northwestern Hoh-Xil-Songpan-Ganzi complex that unconformably overlie Upper Triassic turbidites. Zircon age data indicate that the Middle-Upper Triassic marine gravity-flow deposits of the Hoh-Xil-Songpan-Ganzi complex were shed from the North and South China blocks, and Middle-Late Triassic ultrahigh-pressure Qinling- Dabie orogenic belt, as well as the Kunlun and Qiangtang terranes. In addition, the detrital zircon results suggest vast sediment source to sink distances (>1500 km) for the Middle- Upper Triassic Hoh-Xil-Songpan-Ganzi strata, which is consistent with tectonic models for the Paleo-Tethys Ocean basin that incorporate significant components of horizontal tectonic transport like opening of large back-arc basins in response to oceanic slab rollback. © 2012. American Geophysical Union. All Rights Reserved.
- Doe, M. F., Jones, J. V., Karlstrom, K. E., Dixon, B., Gehrels, G., & Pecha, M. (2013). Using detrital zircon ages and Hf isotopes to identify 1.48-1.45Ga sedimentary basins and fingerprint sources of exotic 1.6-1.5Ga grains in southwestern Laurentia. Precambrian Research, 231, 409-421.More infoAbstract: Globally rare 1.6-1.5 Ga zircons provide an underutilized correlation tool for Precambrian supercontinent reconstructions. Detrital 1.6-1.5 Ga zircons, long known from the Belt Supergroup, have recently been discovered in multiple metasedimentary successions in southwestern North America. Few igneous or metamorphic sources in this age range are known in Laurentia, implying non-Laurentian provenance. Combined U-Pb and Hf isotopic data offer a robust test of their provenance. New U-Pb detrital zircon ages from quartzite exposed in the Defiance uplift in northeastern Arizona indicate a maximum depositional age of ca. 1476 Ma and a predominance of locally derived ca. 1655 Ma and older detritus. The quartzite contains populations at 1570, 1554, and 1519 Ma that do not have known Laurentian sources and are similar to our new and recently published data from the ca. 1474-1436 Ma Blackjack Formation in south-central Arizona. Based on similarities between age spectra and depositional age constraints, we suggest a correlation between the two stratigraphic sections and postulate that a previously unrecognized, and now largely eroded, 1475-1450 Ma sedimentary basin or set of basins may have extended across much of southwestern Laurentia. We refer to this basin as the Yankee Joe-Defiance basin. Hf isotopic analysis of ca. 1.6-1.5 Ga detrital zircons from both localities yields positive epsilon-Hf (eHf) values of +0.2 to +12 indicating a near-juvenile Hf fingerprint with a mean of ~ +7. This range overlaps with, but is more juvenile than, eHf values of +3 to +7.5 from the Belt basin. In terms of possible non-Laurentian source regions, Hf data from both the North and South Australia cratons overlap with, but are also generally less juvenile than, eHf values of Laurentian zircons. Only the North Australia craton contains the full range of ages and Hf isotope values recognized in the 1.6-1.5 Ga Yankee Joe-Defiance populations. This finding supports plate reconstructions involving Australia as a source for exotic detritus in western Laurentia ca. 1480-1450 Ma. © 2013 Elsevier B.V.
- Fornash, K. F., Patchett, P., Gehrels, G. E., & Spencer, J. E. (2013). Evolution of granitoids in the Catalina metamorphic core complex, southeastern Arizona: U-Pb, Nd, and Hf isotopic constraints. Contributions to Mineralogy and Petrology, 165(6), 1295-1310.More infoAbstract: The Santa Catalina Mountains, SE Arizona, was one of the first metamorphic core complexes to be described. Despite its status as a type example, relatively little is known about precise ages and origins of the intrusive rocks that make up most of the crystalline core. U-Pb and Hf isotopic data by laser ablation-inductively coupled plasma-mass spectrometry from zircons and Nd isotopic results from whole rocks were obtained for 12 granitoids ranging from 1,440 to 26 Ma. Results confirm that the 1.44-Ga Oracle Granite extends through the Catalina Range as variably mylonitic granite and banded gneiss. Laramide intrusions (67-73 Ma) display initial εNd values -5 to -8 and εHf from -7.5 to -9. Magmatic ages for the prominent white granite sills of the Wilderness suite are 46-57 Ma, in agreement with Terrien (2012), and these granites have initial εNd values -8 to -10 and εHf from -7 to -14. Lastly, the undeformed Catalina Granite has an age of 26 Ma, with an initial εNd and εHf of -6 and -8, respectively. Our Nd results agree with limited results from Farmer and DePaolo (89:10141-10160, 1984). Although the Catalina Granite seems to have a significant juvenile component based on Nd and Hf, most of the Laramide and Wilderness intrusions contain Nd and Hf compositions lying close to the evolution of 1.44-Ga Oracle Granites, a fact that is confirmed by the U-Pb data, which show both 1.7- and 1.4-Ga zircon cores in these samples, with 1.4 Ga as the dominant core age. In order to become the dominant source of most of the 72-45-Ma magmas, the Oracle pluton must not only extend across the whole Catalina region, but also have abundant deep-seated equivalents to provide magma sources. © 2013 Springer-Verlag Berlin Heidelberg.
- Gibson, T. M., Myrow, P. M., Macdonald, F. A., Minjin, C., & Gehrels, G. E. (2013). Depositional history, tectonics, and detrital zircon geochronology of Ordovician and Devonian strata in southwestern Mongolia. Bulletin of the Geological Society of America, 125(5-6), 877-893.More infoAbstract: Lower Paleozoic successions of the Gobi- Altai zone of southern Mongolia record an abrupt facies transition from deposition of predominantly fine-grained uppermost Ordovician through lowermost Devonian carbonate and marl facies to deposition of coarse clastic strata of the Lower Devonian Tsakhir Formation. The Tsakhir generally fines upward from alluvial-fan cobble and pebble conglomerate to interbedded coarseand fine-grained marine siliciclastic and carbonate strata, which were deposited within a tectonically active basin. The marine strata, deposited in a storm-influenced proximal to distal fan delta, include unusual event beds that grade from pebble conglomerate to hummocky cross-stratified grainstone and sandstone. These beds represent sediment emplaced by gravity flows during flood events and reworked by large gravity waves associated with storm events. The interpreted link between flood deposition and storm wave reworking supports a hyper pycnal flow interpretation for these deposits. The sudden facies transition at the base of the formation represents the sedimentological and stratigraphic signature of Early Devonian tectonism in the Gobi-Altai zone. The general upward-fining pattern of the Tsakhir is interpreted as a response to the creation of accommodation space at a greater rate than progradation of the fan delta, in large part due to tectonic subsidence, although some component of eustasy may have been involved. The production of steep relief and deposition of associated volcanics suggest a transition from relatively passive deposition to active tectonics in this region during the Lochkovian to Pragian stages of the Early Devonian. We hereinintroduce the term "Tsakhir event" for this important tectonic transition. Range-bounding faults for this event are not preserved, but alluvial-fan deposition, the development of unconformities, renewed subsidence, and magmatism throughout the Gobi-Altai zone all suggest syndepositional tectonism. Detrital zircon spectra from both Ordovician and Devonian strata contain Archean to Paleozoic ages. Minor differences between Ordovician and Devonian samples suggest changes in source regions and/or transport paths prior to, and after, the Tsakhir event. The paleoenvironmental setting of the Tsakhir Formation requires short transport distances, and thus the age spectrum of a sample from this formation represents proximal basement rocks of the Shine Jinst region of the Gobi-Altai zone. Basement rocks are not exposed in the Shine Jinst region, but the wide variety of ages in all of the detrital spectra suggest a nearby continental source. Our detrital age spectra contain peaks that coincide with basement ages and magmatic events on the adjacent Mongolian microcontinent and also have strong similarities with recently published spectra of nearby landmasses in Neoproterozoic to Paleozoic paleogeographic reconstructions, namely, Siberia, North China, eastern Gondwana, and Tarim. These similarities extend to spectra of late Neoproterozoic to middle Paleozoic rocks throughout Gondwanaland and also Siberia, illustrating the somewhat limited utility of detrital spectra for determining the tectonic affinities of crustal blocks at this time in Earth history. © 2013 Geological Society of America.
- Guynn, J., Tropper, P., Kapp, P., & Gehrels, G. E. (2013). Metamorphism of the Amdo metamorphic complex, Tibet: Implications for the Jurassic tectonic evolution of the Bangong suture zone. Journal of Metamorphic Geology, 31(7), 705-727.More infoAbstract: Tibet consists of several terranes that progressively collided with the southern margin of Asia during the Mesozoic following the closure of intervening ocean basins. This Mesozoic amalgamation history, as well as the extent to which it may have contributed to crustal thickening prior to the Cenozoic Indo-Asia collision, remains poorly constrained and strongly debated. Here, we present a metamorphic petrological and U-Pb zircon geochronological study of the Amdo metamorphic complex, one of the few exposures of high-grade metamorphic rocks in central Tibet, located along the Bangong suture between the Qiangtang terrane to the north and the Lhasa terrane to the south. U-Pb ages of metamorphic zircon in gneiss constrain the timing of peak metamorphism at c. 178 Ma, prior to the Early Cretaceous collision between the two terranes. Peak P-T conditions of gneiss within the metamorphic complex are constrained by conventional as well as multi-equilibrium (THERMOCALC v.3.21 and v.3.33) geothermobarometry of two samples of garnet-amphibolite. Whereas THERMOCALC v.3.21 yields similar results as conventional geothermobarometry, THERMOCALC v. 3.33 yields dramatically lower pressures, mostly due to the change in the amphibole activity model used. Using THERMOCALC v.3.21, the two garnet-amphibolite samples yield similar P-T conditions of 0.83 ± 0.06 GPa at 646 ± 33 °C and 0.97 ± 0.06 GPa at 704 ± 35 °C. Plagioclase coronas on the garnet-amphibolite sample with lower peak P-T conditions indicate a period of isothermal decompression. Additional geothermometry on two garnet-free amphibolites yielded similar temperatures of 700-750 °C and suggests similar P-T conditions across most of the complex. However, two exposures of garnet-kyanite schist located along the southern edge of the metamorphic complex yielded slightly lower peak conditions of 0.75-0.85 GPa and 550-610 °C. Petrographic and field relations suggest the difference in metamorphic grade between the schist and gneiss is due to an intervening thrust fault. The existence of the thrust fault indicates that at least part of the exhumation of the complex was due to contractional deformation, possibly during the Lhasa-Qiangtang collision. Our P-T-t results indicate the occurrence of a significant Early Jurassic tectonothermal event along the southern, active margin of the Qiangtang terrane that deeply buried the Amdo rocks. We suggest that the metamorphism is a result of arc-related tectonism that may have been regionally extensive along the southern Qiangtang terrane; geological records of this tectonism may be rarely exposed along strike because of a lack of exhumation or subsequent depositional and structural burial. © 2013 John Wiley & Sons Ltd.
- Košler, J., Sláma, J., Belousova, E., Corfu, F., Gehrels, G. E., Gerdes, A., S., M., Sircombe, K. N., Sylvester, P. J., Tiepolo, M., Whitehouse, M. J., & Woodhead, J. D. (2013). U-Pb detrital zircon analysis - results of an inter-laboratory comparison. Geostandards and Geoanalytical Research, 37(3), 243-259.More infoAbstract: Inter-laboratory comparison of laser ablation ICP-MS and SIMS U-Pb dating of synthetic detrital zircon samples provides an insight into the state-of-the art of sedimentary provenance studies. Here, we report results obtained from ten laboratories that routinely perform this type of work. The achieved level of bias was mostly within ± 2% relative to the ID-TIMS U-Pb ages of zircons in the detrital sample, and the variation is likely to be attributed to variable Pb/U elemental fractionation due to zircon matrix differences between the samples and the reference materials used for standardisation. It has been determined that ~ 5% age difference between adjacent age peaks is currently at the limit of what can be routinely resolved by the in situ dating of detrital zircon samples. Precision of individual zircon age determination mostly reflects the data reduction and procedures of measurement uncertainty propagation, and it is largely independent of the instrumentation, analytical technique and reference samples used for standardisation. All laboratories showed a bias towards selection of larger zircon grains for analysis. The experiment confirms the previously published estimates of the minimum number of grains that have to be analysed in order to detect minor zircon age populations in detrital samples. Une comparaison inter-laboratoires de datation U-Pb d'échantillons de zircons détritiques de synthèse par les méthodes d'ablation laser ICP-MS et SIMS donne un aperçu de l'état de l'art des études de provenance sédimentaire. Nous présentons ici les résultats obtenus par dix laboratoires qui effectuent régulièrement ce type de travail. Le niveau de biais atteint était compris dans l'intervalle ± 2% par rapport aux âges U-Pb ID-TIMS des zircons de l'échantillon détritique et la variation peut certainement être attribué à un fractionnement élémentaire Pb/U variable relié à des différences de matrices entre les zircons des échantillons et les zircons de référence utilisés pour la normalisation. Il a été déterminé qu'environ 5% de différence d'âge entre les pics d'âge adjacents est actuellement à la limite de ce qui peut être résolu en mode routine par la datation in situ d'échantillons de zircons détritiques. La précision de la détermination de l'âge de zircon individuel reflète principalement la réduction des données et les procédures de mesure de la propagation de l'incertitude, et elle est largement indépendante de l'instrumentation, de la technique d'analyse et des échantillons de référence utilisés pour la standardisation. Tous les laboratoires ont montré un biais vers la sélection des plus gros grains de zircon pour l'analyse. L'expérience confirme les estimations précédemment publiées sur le nombre minimum de grains qui doivent être analysés afin de détecter les populations mineures d'âge de zircons dans les échantillons détritiques. © 2013 The Authors. Geostandards and Geoanalytical Research © 2013 International Association of Geoanalysts.
- Kuznetsov, N. B., Soboleva, A. A., Miller, E. L., Udoratina, O. V., Gehrels, G., & Romanyuk, T. V. (2013). First U-Pb datings of detrital zircons from middle and upper paleozoic sandstones of the Polar Urals: Testing the regional tectonic models. Doklady Earth Sciences, 451(1), 692-697.
- May, S. R., Gray, G. G., Summa, L. L., Stewart, N. R., Gehrels, G. E., & Pecha, M. E. (2013). Detrital zircon geochronology from Cenomanian-Coniacian strata in the Bighorn Basin, Wyoming, U.S.A.: Implications for stratigraphic correlation and paleogeography. Rocky Mountain Geology, 48(1), 41-61.More infoAbstract: A high-flux, Late Cretaceous magmatic event in the western United States has been tested as a zircon source for high-resolution chronostratigraphic correlation in coeval sedimentary rocks in northwest Wyoming. Thirteen samples of Cenomanian-Coniacian sandstone in the Bighorn Basin yielded more than 1200 U/Th/Pb detrital zircon ages from the Mowry Shale, the Frontier Formation, and the Cody Shale. In addition, two individual clast ages were obtained from a conglomerate located near the top of the Frontier Formation. These formations are dominated by detrital zircon grains that yield paleontologically constrained depositional or near-depositional ages. Each sample has a minimum of 22 grains comprising the youngest age peak. Individual youngest peak ages range from 99.4 to 87.7 Ma, spanning Cenomanian through Middle Coniacian time (Gradstein et al., 2012). Three of four stratigraphic sections yield samples with minimum age peaks that young upward, are consistent with available paleontological control, and suggest an age resolution of one-two million years despite an estimated analytical error of 2 percent (+/- 2 Ma for 100 Ma samples). An age reversal at the top of the fourth section demonstrates that recycling of older sediments into younger beds can be an important control on the age of zircon populations, even during intervals of sediment accumulation dominated by first-cycle zircons from an active magmatic arc. The presence of nearly depositional age volcanic cobbles at the top of the Frontier Formation implies rapid erosion and transport of coarse material from a volcanic source eastward into the foreland basin. The new detrital zircon data, in conjunction with available paleontological constraints, provide a framework for detailed stratigraphic correlation.
- May, S. R., Gray, G. G., Summa, L. L., Stewart, N. R., Gehrels, G. E., & Pecha, M. E. (2013). Detrital zircon geochronology from the bighorn basin, wyoming, usa: Implications for tectonostratigraphic evolution and paleogeography. Bulletin of the Geological Society of America, 125(9-10), 1403-1422.More infoAbstract: Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, sub sidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, influence paleogeography and sediment-dispersal patterns, they should also yield similar patterns, or facies, of detrital zircon age spectra . Such age-distribution patterns should be documented on the craton in order to make meaningful comparisons to sedimentary rocks from suspect terranes along continental margins. The Rocky Mountains of western North America provide excellent outcrops of sedimentary rocks that record >500 m.y. of tectonostratigraphic evolution. One such Phanerozoic section is exposed along the margins of the Bighorn Basin in northwest Wyoming, from which we report over 4000 U/Th/Pb detrital zircon ages from 48 samples that span a stratigraphic interval from the Middle Cambrian Flathead Sandstone through the Eocene Willwood Formation. These data provide one of the most complete records of detrital zircon age patterns from this part of cratonic North America. The stratigraphic record of the Bighorn Basin is subdivided into four tectonostratigraphic assemblages (TSA1-TSA4). These assemblages record an initial passive margin, followed by a transition to a convergent margin, followed by a marine-dominated retroarc foreland basin, followed by a retroarc foreland segmented by local basement uplifts. This tectonostratigraphic architecture is expressed as four, first-order patterns within the detrital zircon age distributions. TSA1 represents a Paleozoic-Triassic proximal continental margin assemblage dominated by Proterozoic zircons with abundant grains in the 1600-1950 Ma range, a Grenville population at ca. 1100 Ma, and a Phanerozoic population at ca. 420 Ma. TSA2 is a transitional assemblage associated with the Jurassic-Early Cretaceous organization of a west-facing convergent margin and Cordilleran orogen. The TSA2 detrital zircon age distribution is characterized by the appearance of Mesozoic grains, age peaks at ca. 420 and 600 Ma, and a dominant population of Grenville (1.0-1.1 Ga) grains with a suite of Proterozoic grains diminishing in abundance as age increases to 1.9 Ga. TSA3 sedimentary rocks were deposited in the Cretaceous Interior Seaway in a retroarc foreland basin and are dominated by zircons for which ages are close to the depositional age of the strata, reflecting input from the active Idaho Batholith and Sierran segments of the Cordilleran magmatic arc. The older zircon fractions from TSA3 sedimentary rocks are characterized by a dominant detrital zircon age peak at 1.7-1.8 Ga, which probably reflects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 reflects the phase of basin fill associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this assemblage record erosion and redeposition of all previous sedimentary rocks from surrounding basement uplifts. Patterns of detrital zircon ages reflect fundamental changes in paleogeography and sediment dispersal at the 10-100 m.y. time scale and are clearly related to major tectonic events or phases. Detrital zircon ages also provide evidence for linkages between convergent margin processes such as arc magmatism and sedimentation in the retroarc foreland. During these times of strong arc-retroarc linkage, detrital zircon geochronology provides a potentially useful tool for high-resolution chronostratigraphy. © 2013 Geological Society of America.
- McQuarrie, N., Long, S. P., Tobgay, T., Nesbit, J. N., Gehrels, G., & Ducea, M. N. (2013). Documenting basin scale, geometry and provenance through detrital geochemical data: Lessons from the Neoproterozoic to Ordovician Lesser, Greater, and Tethyan Himalayan strata of Bhutan. Gondwana Research, 23(4), 1491-1510.More infoAbstract: Detrital zircon (DZ) ages, augmented with εNd(0) and δ13C isotopic values from 18 new and 22 published samples collected from Lesser Himalayan (LH), Greater Himalayan (GH) and Tethyan Himalayan (TH) rocks in Bhutan, support deposition of >7km of sedimentary rock in late Cambrian-Ordovician time and provide a stratigraphic framework for the pre-collisional Indian margin. Youngest GH DZ grains become younger upsection from 900Ma to 477Ma. Youngest DZ grains in TH samples are ~490-460Ma. Both the LH Jaishidanda Formation (Fm), and the LH Baxa Group overlie Paleoproterozoic LH rocks. The Jaishidanda Fm exhibits distinct populations of youngest DZ peaks, 475-550Ma, and 800-1000Ma. The Baxa Group (Manas, Pangsari, and Phuntsholing formations) contains youngest DZ peaks at both 500-525Ma and 0.9-1.0Ga. However, most samples from the Baxa Group in western Bhutan contain no grains younger than 1.8Ga. Samples from the LH Paro Fm, which sits directly under the MCT in western Bhutan, have youngest DZ peaks at 0.5, 0.8, 1.0, 1.7, 1.8Ga. εNd values generally match DZ spectra, with samples that contain old, youngest grain populations corresponding to more negative εNd signatures. The Paro Fm is an exception where εNd (0) values from quartzite samples are quite negative (-19 to -24) whereas the εNd (0) values from interbedded schist contain younger detritus (-12 to -17). δ13C values from the Jaishidanda, Paro and Manas formations have δ13C values (-1.8 to +6) suggestive of deposition over late Neoproterozoic to Ordovician time. δ13C values from the Pangsari Fm vary from -2.8 to +1.8, compatible with deposition in the early- to middle Neoproterozoic. The young, latest Cambrian-Ordovician grains preserved in TH, GH and LH rocks suggest that the late Cambrian-Ordovician orogeny, documented in GH rocks throughout the orogen, served as a significant sediment source in Bhutan. © 2012 International Association for Gondwana Research.
- Miller, E. L., Soloviev, A. V., Prokopiev, A. V., Toro, J., Harris, D., Kuzmichev, A. B., & Gehrels, G. E. (2013). Triassic river systems and the paleo-Pacific margin of northwestern Pangea. Gondwana Research, 23(4), 1631-1645.More infoAbstract: Detrital zircon U-Pb ages from Triassic strata exposed in the circum-Arctic, analyzed by LA-ICP-MS and SHRIMP-RG, are compared at the regional scale to better understand the paleogeography of northern Pangea and help restore rift opening of the Arctic. Data sets are compared based on their zircon age distributions, cumulative age probability plots, and the K-S test. Three major source regions are characterized. These fed clastic material to transcontinental river systems that transported material from the highlands of northwestern Pangea to its once continuous paleo-Pacific continental margin. The paleo-Lena River System was fed from sources in the Baikalian and Altay-Sayan mountainous regions of Siberia. Zircon populations are characterized by a limited number of Precambrian zircons (~ 1.8-2.0. Ga with fewer ~ 2.5-3.0. Ga), lack of 0.9-1.8. Ga zircons, and a dominant 480-500. Ma and 290-300. Ma age population. The paleo-Taimyr River System was sourced from the Uralian orogenic belt region and deposited along a rifted portion of the Siberia-Baltica margin beginning in the Permo-Triassic. Precambrian zircon populations are similar to those of the paleo-Lena system, and samples closest to Siberia have similar populations in the 480-500. Ma and 290-300. Ma age ranges. Chukotka, Wrangel Island and Lisburne Hills, Alaska, have sparse ages between 900 and 1800. Ma, Ordovician ages are younger (~ 440-450. Ma), and, along with abundant ~ 300. Ma ages, they contain ~ 250-260. Ma and lesser ~ 215-235. Ma zircons, interpreted as derived from silicic volcanic centers associated with Permo-Triassic to Triassic continental flood basalt provinces in Siberia, Taimyr and Kara Sea region. The trans-Laurentian River System was likely fed by rift-related uplift along the proto North Atlantic/Arctic margin and delivered sediment to the Cordilleran margin of Pangea. These samples have no significant upper Paleozoic zircons and have a much broader age range of Precambrian zircons. © 2012 International Association for Gondwana Research.
- Raines, M. K., Hubbard, S. M., Kukulski, R. B., Leier, A. L., & Gehrels, G. E. (2013). Sediment dispersal in an evolving foreland: Detrital zircon geochronology from upper jurassic and lowermost cretaceous strata, alberta basin, Canada. Bulletin of the Geological Society of America, 125(5-6), 741-755.More infoAbstract: The Alberta foreland basin is a classic example of a retro-arc foreland basin, yet the early stages of its development remain poorly understood. Several contrasting hypotheses have been proposed to explain the source areas and dispersal patterns of sediment in western Canada during the Late Jurassic initiation of the foreland basin. Here, we use detrital zircon uranium-lead (U-Pb) geochronology, sandstone petrography, paleocurrent measurements, and regional correlations to reconstruct the early basin evolution, including sediment provenance and depositional history. These data indicate sediment in the early foreland basin was delivered via two principal sedimentary systems: a south-tonorth axial river system, and transverse fluvial systems that emanated from the adjacent Cordillera. Accordingly, sandstones of the Jurassic foreland, associated with the Minnes Group and equivalent Kootenay and Nikanassin formations, are divided into two informal groups, type 1 and type 2. Type 1 sandstones are mature quartz arenites, present along the entire north-south length of the Alberta Basin, and generally at the base of the succession. Type 1 sandstones have zircons with age populations between 980 and 2000 Ma, similar to sediments of Jurassic and Lower Cretaceous strata in the western United States. These deposits are interpreted to have been derived from southern sources and transported axially to the north along the earliest foredeep of the Cordilleran foreland basin. Type 2 sandstones by contrast, are less mature, containing higher quantities of chert and lithic fragments, and are dominated by 1765-2100 Ma zircons with a smaller population at 2500-2800 Ma. The zircon age populations of type 2 sandstones are similar to populations recorded in the Neoproterozoic to Triassic miogeocline strata of the adjacent fold-and-thrust belt. Type 2 sandstones are common in the western, orogenic side of the basin, but they extend eastward across the basin in fluvial sediments in the upper portion of the succession. Changes in provenance and sediment composition are associated with the evolution of paleodrainages and the increasing importance of Cordilleran erosion to the sediment budget. The progressively greater influx of orogen-derived material relative to subsidence displaced the axial fluvial system toward a more cratonward-position. The collected data support the hypothesis that much of the sediment was initially transported northward by an axial drainage network, followed by Cordilleran-sourced sediments fed by transverse river systems. The present study attempts to unravel predictable patterns of sediment dispersal in evolving foreland basins, while testing whether clear changes in sediment composition of the first clastic pulse of sediment are related to hinterland exhumation, changing drainage divides, weathering processes, or varied provenance. © 2013 Geological Society of America.
- Talavera-Mendoza, O., Ruiz, J., Corona-Chavez, P., Gehrels, G. E., Sarmiento-Villagrana, A., García-Díaz, J. L., & Salgado-Souto, S. A. (2013). Origin and provenance of basement metasedimentary rocks from the Xolapa Complex: New constraints on the Chortis-southern Mexico connection. Earth and Planetary Science Letters, 369-370, 188-199.More infoAbstract: The U-Pb (LA-MC-ICPMS) geochronology of Xolapa metasedimentary rocks from Tierra Colorada, Guerrero to Puerto Ángel, Oaxaca in southern Mexico reveals that their protoliths accumulated in two distinctive cycles of sedimentation, one of Early Jurassic age and another of Late Cretaceous age. These ages are younger than thought and demonstrate that Xolapa metasedimentary rocks are not rocks from the Acatlán or Oaxacan complexes or their Paleozoic sedimentary covers as claimed. However, detrital zircon ages indicate that Xolapa sediments received contemporaneous detritus most likely from these assemblages suggesting a probably (para-)autochthonous origin for the Xolapa terrane. Xolapa rocks record two major tectonothermal events of 64-59. Ma and ~34. Ma; the first event produced the high-grade metamorphism and widespread migmatization that characterize Xolapa and the second event is likely related to extended heating produced by coeval arc plutonism.Pre-Jurassic assemblages of the Chortis block of Central America contain zircon populations that significantly coincide with those recorded in both, the Acatlán and Oaxacan complexes and their Paleozoic sedimentary covers as well as with those recorded in Xolapa metasediments, which suggests a spatial connection among these petrotectonic assemblages during much of the Mesozoic.It is proposed that Xolapa was generated in a basin floored by Permian rocks flanked on one side by southern Mexico terranes and on the other side by the Chortis block. Contraction of the basin tied to the approach and accretion of the Guerrero terrane arc assemblages during Late Cretaceous time produced crustal thickening and high-grade metamorphism and migmatization at mid-crustal levels. Diachronic exhumation of Xolapa began during Early Paleogene time very likely promoted by the detachment and migration of the Chortis block. The migration would additionally produce slicing of Xolapa assemblages generating its elongated and juxtaposed structure and the margin truncation that characterizes southern Mexico. © 2013 Elsevier B.V.
- Thomson, S. N., Reiners, P. W., Hemming, S. R., & Gehrels, G. E. (2013). The contribution of glacial erosion to shaping the hidden landscape of East Antarctica. Nature Geoscience, 6(3), 203-207.More infoAbstract: The subglacial topography in East Antarctica has been revealed by airborne radar surveys1-3. However, how this ice-hidden landscape has evolved over time is less well known2-12. Low pre-glacial erosion rates since the Permian period have been reported12, challenging arguments for enhanced erosion during the Cretaceous period4,6. Here we present a record of long-term East Antarctic erosion by applying multiple dating techniques to over 1,400 detrital mineral grains from onshore moraines and offshore sediments of Cretaceous to Quaternary age in the region of Lambert Glacier and Prydz Bay. Ages from pre-glacial sediments support overall low erosion rates before the expansion of the ice sheet, apart from a discrete interval of magmatic heating about 115 Myr ago that is inconsistent with widespread Cretaceous erosion. We find a shift towards younger and broader age distributions since ∼34 Myrago that necessitates spatially localized erosion of over 2 km in the Lambert Glacier catchment over this time. We infer that the trough containing Lambert Glacier was incised almost entirely by selective glacial erosion following initial expansion of the East Antarctic ice sheet. This implies that the early ice sheet was dynamic with ice flow concentrated along fixed ice streams. Copyright © 2013 Macmillan Publishers Limited.
- Alsleben, H., Wetmore, P. H., Gehrels, G. E., & Paterson, S. R. (2012). Detrital zircon ages in Palaeozoic and Mesozoic basement assemblages of the Peninsular Ranges batholith, Baja California, Mexico: Constraints for depositional ages and provenance. International Geology Review, 54(1), 93-110.More infoAbstract: The origin and continuity of Phanerozoic lithostratigraphic terranes in southern and Baja California remain an unsolved issue in Cordilleran tectonics. We present data from eight detrital zircon samples collected across the southern extent of the Peninsular Ranges that help constrain the provenance of detritus and the depositional ages of these basement units. Detrital zircon signatures from units in the eastern Peninsular Ranges correlate with Palaeozoic passive margin assemblages in the southwestern North American Cordillera. Units in the central belt, which consists of Triassic-Jurassic metasedimentary turbidite assemblages that probably deformed in an accretionary prism setting, and Cretaceous metasedimentary and metavolcanic units that represent the remnants of a continental margin arc, were derived from both proximal and more distal sources. The westernmost units, which are locally structurally interleaved with the Triassic through Cretaceous units of the central belt, are Cretaceous deposits that represent a series of collapsed basin complexes located within and flanking the Cretaceous Alisitos volcanic island arc. Cretaceous intra-arc units show little influx of cratonal material until approximately 110Ma, whereas coeval sediments on the northern and eastern flanks of the Alisitos arc contain abundant cratonal detritus. Intra-arc strata younger than approximately 110Ma contain large amounts of Proterozoic and older detrital zircons. These data suggest that basins associated with the Alisitos arc were either too distant or somehow shielded from North American detritus before 110Ma. In the case of the former, increased influx of continental detritus after 110Ma would support a tectonic model in which the arc was separated from North America by an ocean basin and, as the arc approached the continent, associated depositional centres were close enough to receive input from continental sources. © 2012 Copyright Taylor and Francis Group, LLC.
- Bershaw, J., Garzione, C. N., Schoenbohm, L., Gehrels, G., & Tao, L. (2012). Cenozoic evolution of the Pamir plateau based on stratigraphy, zircon provenance, and stable isotopes of foreland basin sediments at Oytag (Wuyitake) in the Tarim Basin (west China). Journal of Asian Earth Sciences, 44, 136-148.More infoAbstract: The Pamir salient is the western expression of mountain growth related to Indo-Eurasian convergence. Though a rough framework has emerged describing the tectonic evolution of the Pamir, detailed knowledge of the spatial and temporal evolution of Cenozoic deformation is necessary to determine how strain progressed through the orogenic belt. Here we present new stratigraphic, zircon provenance, and stable isotope data from Jurassic to Miocene strata along the Pamir's northeastern margin near the town of Oytag (Wuyitake) in the Tarim Basin (west China). Prominent ∼40Ma peaks in Oligocene to early Miocene detrital zircon grains record the erosion of an Eocene belt of shoshonitic rocks in the central to southeastern Pamir. This is roughly coincident with an ∼4‰ shift in the oxygen isotopic composition (δ 18O) of carbonates during the Eocene and/or Oligocene (from an average of -8.7‰ to -12.6‰), suggesting a reorganization of atmospheric circulation during that time. This could have been caused by uplift of Tarim Basin-bounding ranges and/or retreat of the Paratethys Sea. A subsequent change from Eocene to Jurassic aged detrital zircon grains in the early to middle Miocene indicates provenance shifted from source rocks in the central and/or SE Pamir to the hanging wall of the Main Pamir Thrust (MPT), coincident with prograding facies at that time. This suggests deformation progressed outward toward the northeast margin of the Pamir plateau in the early to middle Miocene. Our results corroborate outward advancement of Himalayan deformation, affecting all margins of the Tarim Basin by the middle Miocene. © 2011 Elsevier Ltd.
- Cecil, M. R., Rotberg, G. L., Ducea, M. N., Saleeby, J. B., & Gehrels, G. E. (2012). Magmatic growth and batholithic root development in the northern Sierra Nevada, California. Geosphere, 8(3), 592-606.More infoAbstract: In contrast to the much-studied central and southern Sierra Nevada, relatively little is known about the growth and petro genesis of the batholith in its northern reaches, making it difficult to evaluate range-wide, spatiotemporal trends in batholithic development and the regional extent of eclogite root production and/or loss. New U-Pb ages from northern Sierra plutons reveal a shift between the age of Cretaceous magmatism recorded in the northern Sierra and the timing of an apparent flare-up in the main batholith, indicating that: (1) the northern batholith was more spatially dispersed and emplaced into regions beyond the modern topographic range, and (2) the Cretaceous high-flux event may have occurred over a longer period of time than previously suggested. Relative to the southern Sierra, Nd and Sr isotopic signatures in northern plutons are more primitive, mimicking the predominantly juvenile nature of the terranes into which the plutons are built. Despite differences in isotopic character, however, major and trace element trends are remarkably similar between northern plutons and the rest of the batholith, suggesting that emplacement into juvenile and/or oceanic lithosphere does not inhibit the generation of evolved, arc-type magmatic products. Northern plutons have relatively high La/Yb and Sr/Y and steep rare-earth element patterns, with small to no Eu anomalies. Taken together, these trends are interpreted to indicate deep processing of magmas in equilibrium with a feldspar-poor, amphibolite-rich residue, containing modest amounts of garnet. It is therefore likely that the northern Sierra Nevada batholith was emplaced into relatively thick crust and developed a dense mafic to ultramafic root. Because it is not seismically imaged today, we posit that the root was subsequently lost, perhaps in response to encroachment of proto-Cascade arc volcanism. © 2012 Geological Society of America.
- Chen, X., Gehrels, G., Yin, A., Li, L. i., & Jiang, R. (2012). Paleozoic and Mesozoic basement magmatisms of eastern Qaidam Basin, northern Qinghai-Tibet Plateau: LA-ICP-MS zircon U-Pb geochronology and its geological significance. Acta Geologica Sinica, 86(2), 350-369.More infoAbstract: The eastern margin of the Qaidam Basin lies in the key tectonic location connecting the Qinling, Qilian and East Kunlun orogens. The paper presents an investigation and analysis of the geologic structures of the area and LA-ICP MS zircon U-Pb dating of Paleozoic and Mesozoic magmatisms of granitoids in the basement of the eastern Qaidam Basin on the basis of 16 granitoid samples collected from the South Qilian Mountains, the Qaidam Basin basement and the East Kunlun Mountains. According to the results in this paper, the basement of the basin, from the northern margin of the Qaidam Basin to the East Kunlun Mountains, has experienced at least three periods of intrusive activities of granitoids since the Early Paleozoic, i.e. the magmatisms occurring in the Late Cambrian (493.1±4.9 Ma), the Silurian (422.9±8.0 Ma-420.4±4.6 Ma) and the Late Permian-Middle Triassic (257.8±4.0 Ma-228.8±1.5 Ma), respectively. Among them, the Late Permian - Middle Triassic granitoids form the main components of the basement of the basin. The statistics of dated zircons in this paper shows the intrusive magmatic activities in the basement of the basin have three peak ages of 244 Ma (main), 418 Ma, and 493 Ma respectively. The dating results reveal that the Early Paleozoic magmatism of granitoids mainly occurred on the northern margin of the Qaidam Basin and the southern margin of the Qilian Mountains, with only weak indications in the East Kunlun Mountains. However, the distribution of Permo-Triassic (P-T) granitoids occupied across the whole basement of the eastern Qaidam Basin from the southern margin of the Qilian Mountains to the East Kunlun Mountains. An integrated analysis of the age distribution of P-T granitoids in the Qaidam Basin and its surrounding mountains shows that the earliest P-T magmatism (293.6-270 Ma) occurred in the northwestern part of the basin and expanded eastwards and southwards, resulting in the P-T intrusive magmatism that ran through the whole basin basement. As the Cenozoic basement thrust system developed in the eastern Qaidam Basin, the nearly N-S-trending shortening and deformation in the basement of the basin tended to intensify from west to east, which went contrary to the distribution trend of N-S-trending shortening and deformation in the Cenozoic cover of the basin, reflecting that there was a transformation of shortening and thickening of Cenozoic crust between the eastern and western parts of the Qaidam Basin, i.e., the crustal shortening of eastern Qaidam was dominated by the basement deformation (triggered at the middle and lower crust), whereas that of western Qaidam was mainly by folding and thrusting of the sedimentary cover (the upper crust).
- Dickinson, W. R., Lawton, T. F., Pecha, M., Davis, S. J., Gehrels, G. E., & Young, R. A. (2012). Provenance of the Paleogene Colton Formation (Uinta Basin) And Cretaceous-Paleogene provenance evolution in the Utah foreland: Evidence from U-Pb ages of detrital zircons, paleocurrent trends, and sandstone petrofacies. Geosphere, 8(4), 854-880.More infoAbstract: The fluviodeltaic Colton Formation (Late Paleocene-Early Eocene) forms a lobate depositional system that prograded from the south into the Laramide Uinta Basin of northeastern Utah (United States) with a preserved sediment volume of ~3000 km3 and a maximum thickness of ~1000 m. Joint consideration of detrital zircon ages, paleocurrent trends, and sandstone petrofacies permits an assessment of Colton provenance relations in the context of evolving Cretaceous-Paleogene sedimentation in the Utah foreland. Grains with U-Pb ages younger than 285 Ma derived from the Cordilleran magmatic arc form ~50% of the detrital zircons in arkosic Colton sand, and were transported ~750 km to the Uinta Basin from the Mojave segment of the arc by the California paleoriver. Colton sedimentation was the Paleogene culmination of a persistent pattern of Cretaceous sediment transport northward, subparallel to the Sevier thrust front, to supplement east-directed sediment delivery to the retroarc foreland from the Sevier thrust belt. The ratio of longitudinally to transversely derived sediment was enhanced in foreland strata after Laramide deformation produced intraforeland uplifts that screened the foreland belt from Sevier sources. The relative abundance of arc-derived detrital zircons that were contributed to strata of the Utah foreland increased in late Campanian time and remained high into Eocene time. Detrital zircon populations in Paleogene forearc strata of southern California are compatible with coeval derivation of arc-derived detritus in the forearc sands and the Colton backarc sand from a common paleodrainage divide crossing the Mojave region to connect hinterland Nevadaplano and Mexicoplano uplands to the north and south. © 2012 Geological Society of America.
- Doe, M. F., V., J., Karlstrom, K. E., Thrane, K., Frei, D., Gehrels, G., & Pecha, M. (2012). Basin formation near the end of the 1.60-1.45 Ga tectonic gap in southern Laurentia: Mesoproterozoic Hess Canyon Group of Arizona and implications for ca. 1.5 Ga supercontinent configurations. Lithosphere, 4(1), 77-88.More infoAbstract: Detrital zircon data from the upper parts of the Proterozoic Hess Canyon Group of southern Arizona reveal abundant 1600-1488 Ma detrital zircons, which represent ages essentially unknown from southern Laurentia. This basinal succession concordantly overlies a >2-km-thick-section of 1657 ± 3 Ma rhyolite of the Redmond Formation. The rhyolite is intercalated with and hence contemporaneous with the lower parts of the overlying White Ledges Formation, a 300-m-thick orthoquartzite unit at the base of the Hess Canyon Group. These quartzites contain a unimodal detrital zircon age probability distribution with peak ages of 1778, 1767, and 1726 Ma, supporting regional correlation with other ca. 1.65 Ga quartzite exposures in southwestern Laurentia. However, the ~900-m-thick argillaceous Yankee Joe and minimum 600-m-thick quartzite-rich Blackjack Formations contain younger detrital zircons, with peak ages ranging from 1666 to 1494 Ma and a maximum depositional age of 1488 ± 9 Ma. Prominent age peaks at 1582-1515 Ma and 1499-1488 Ma represent detritus that is exotic and not derived from known southern Laurentian sources. The Blackjack Formation is cut by the 1436 ± 2 Ma Ruin Granite, indicating that deposition, deformation, and intrusion occurred between 1488 and 1436 Ma. This basin likely developed before or in the early stages of the 1.45-1.35 Ga intracontinental tectonism in southwestern Laurentia. Our findings necessitate the presence of an ~170 m.y. disconformity within the Hess Canyon Group and document a previously unrecognized episode of Mesoproterozoic basin sedimentation (>1.5 km of section) between 1488 and 1436 Ma in southern Laurentia. This new record helps to fill the 1.60-1.45 Ga magmatic gap in southern Laurentia and supports hypotheses for a long-lived Proterozoic tectonic margin along southern Laurentia from 1.8 to 1.0 Ga. The 1.6-1.5 Ga detrital zircon ages offer important new constraints for ca. 1.5 Ga Nuna reconstructions and for the paleogeography of contemporaneous basins such as the Belt Basin in western Laurentia. © 2012 Geological Society of America.
- Gehrels, G. (2012). Detrital Zircon U-Pb Geochronology: Current Methods and New Opportunities. Tectonics of Sedimentary Basins: Recent Advances, 45-62.More infoAbstract: Detrital zircon geochronology is rapidly evolving into a very powerful tool for determining the provenance and maximum depositional age of clastic strata. This rapid evolution is being driven by the increased availability of ion probes and laser ablation inductively coupled plasma (ICP) mass spectrometers, which are able to generate ages efficiently and with sufficient accuracy for most applications. Although large numbers of detrital zircon ages are generated each year, significant uncertainties remain in how data are acquired, which ages are used, how data are plotted, and how age distributions are compared. Improvements in current methods may come from enhanced precision/ accuracy of age determinations, better tools for extracting critical information from age spectra, abilities to determine other types of information (e.g., Hf, O, Li isotope signatures, rare earth element (REE) patterns, cooling ages, structural information) from the dated grains, and availability of a database that provides access to detrital zircon age determinations from sedimentary sequences around the world. © 2012 Blackwell Publishing Ltd.
- Girardi, J. D., Patchett, P. J., Ducea, M. N., Gehrels, G. E., Cecil, M. R., Rusmore, M. E., Woodsworth, G. J., Pearson, D. M., Manthei, C., & Wetmore, P. (2012). Elemental and isotopic evidence for granitoid genesis from deep-seated sources in the Coast Mountains Batholith, British Columbia. Journal of Petrology, 53(7), 1505-1536.More infoAbstract: Major element, trace element and Nd-Sr isotopic data are presented for 82 plutonic rocks from the southern Coast Mountains Batholith (CMB) in British Columbia, Canada, ranging in emplacement age from 210 to 50 Ma. The rocks are part of a large composite magmatic arc batholith, which the major element data show to be of calc-alkaline affinity. The majority of CMB samples lack the depletion in Eu that would be consistent with equilibration of magmas and plagioclase-bearing crystalline residues or fractionates, suggesting that equilibration took place deeper than the pressure limit of plagioclase stability at 35-40 km depth. The CMB samples show a wide variation in the slope of normalized rare earth element (REE) patterns, with chondrite-normalized La/Yb ratios above 10 being mostly confined to periods of high magmatic flux in the arc at 160-140, 120-80, and 60-50 Ma. The clearest relationships between major and trace elements are negative correlations between SiO. 2 and each of Sc, Y, and the heavier REE Gd to Lu. Nd and Sr isotopes mostly document juvenile origins for the granitoids, but show variations to higher . 87Sr/. 86Sr and lower ε. Nd during high-flux periods. The results are interpreted to indicate a deep origin for most CMB magmas, below ∼40 km where mafic to intermediate rock assemblages previously added to the arc crust by mantle melting were transformed to an (amphibole-bearing)-eclogite facies cumulate or restite, such that melting residues consisted mainly of two pyroxenes, garnet and variable proportions of amphibole. Thickened orogenic crust, for which there is clear geological evidence during the period 100-80 Ma, promoted this process. During high-flux periods, larger amounts of older rocks, mostly mafic rocks and some metasediments added to the base of the arc during orogenic shortening, became involved in magma genesis. © The Author 2012. Published by Oxford University Press. All rights reserved.
- Guynn, J., Kapp, P., Gehrels, G. E., & Ding, L. (2012). U-Pb geochronology of basement rocks in central Tibet and paleogeographic implications. Journal of Asian Earth Sciences, 43(1), 23-50.More infoAbstract: The ages and paleogeographic affinities of basement rocks of Tibetan terranes are poorly known. New U-Pb zircon geochronologic data from orthogneisses of the Amdo basement better resolve Neoproterozoic and Cambro-Ordovician magmatism in central Tibet. The Amdo basement is exposed within the Bangong suture zone between the Lhasa and Qiangtang terranes and is composed of granitic orthogneisses with subordinate paragneisses and metasedimentary rocks. The intermediate-felsic orthogneisses show a bimodal distribution of Neoproterozoic (920-820. Ma) and Cambro-Ordovician (540-460. Ma) crystallization ages. These and other sparse basement ages from Tibetan terranes suggest the plateau is underlain by juvenile crust that is Neoproterozoic or younger; its young age and weaker rheology relative to cratonic blocks bounding the plateau margins likely facilitated the propagation of Indo-Asian deformation far into Asia. The Neoproterozoic ages post-date Rodinia assembly and magmatism of similar ages is documented in the Qaidaim-Kunlun terrane, South China block, the Aravalli-Delhi craton in NW India, the Eastern Ghats of India, and the Prince Charles mountains in Antarctica. The Amdo Neoproterozoic plutons cannot be unambiguously related to one of these regions, but we propose that the Yangtze block of the South China block is the most likely association, with the Amdo basement representing a terrane that possibly rifted from the active Yangtze margin in the middle Neoproterozoic. Cambro-Ordovician granitoids are ubiquitous throughout Gondwana as a product of active margin tectonics following Gondwana assembly and indicate that the Lhasa-Qiangtang terranes were involved in these tectono-magmatic events. U-Pb detrital zircon analysis of two quartzites from the Amdo basement suggest that the protoliths were Carboniferous-Permian continental margin strata widely deposited across the Lhasa and Qiangtang terranes. The detrital zircon age spectra of the upper Paleozoic Tibetan sandstones and other rocks deposited in East Gondwana during the late Neoproterozoic and Paleozoic are all quite similar, making it difficult to use the age spectra for paleogeographic determinations. There is a suggestion in the data that the Qiangtang terrane may have been located further west along Gondwana's northern boundary than the Lhasa terrane, but more refined spatial and temporal data are needed to verify this configuration. © 2011 Elsevier Ltd.
- Lackey, J. S., Cecil, M. R., Windham, C. J., Frazer, R. E., Bindeman, I. N., & Gehrels, G. E. (2012). The fine gold intrusive suite: The roles of basement terranes and magma source development in the early Cretaceous Sierra Nevada batholith. Geosphere, 8(2), 292-313.More infoAbstract: The Fine Gold Intrusive Suite is one of the largest (>2000 km2) and oldest intrusive complexes in the Sierra Nevada batholith (California, USA), and therefore contains a wealth of information about nascent magmatic processes in a convergent margin arc. Because the suite intrudes both accreted oceanic and/or island-arc terranes and continental crust, it provides perspective on how convergent margin magmatism recycles existing crust versus reworking of fringing island arcs into continental crust. Such insight informs our understanding of how continental crust formation may have operated in the Phanerozoic as compared to earlier in Earth history. New zircon U-Pb geochronology shows that the largely tonalitic suite was emplaced over ~19 m.y. (124-105 Ma), in three pulses that young from west to east. The most recent domain is nested within the previous ones, such that lobes of magma protruding from the main bodies of the Bass Lake Tonalite (the primary member of the Fine Gold Intrusive Suite) are older than interior areas. Zircon δ18O (6.1‰-8.0‰) and εHf (-4.7 + 6.4) show temporal trends indicating that early magmas were source mixtures of mantle with as much as 45% Paleozoic to Mesozoic oceanic and/or arc rocks, whereas later magmas contain greater inputs (to 50%) of Proterozoic North American crust. Older domains in the suite were likely generated from isolated sources, including initial high Sr/Y (to ~90), high Na2O magmas consistent with garnetbearing sources inferred to be relatively deep. Higher 87Sr/86Sr, lower εHf, and higher Rb/Sr values in younger plutons show a source that tapped greater proportions of North American crust and was presumably more organized and larger, given its more homogeneous isotopic and trace element traits. Our findings also show that expression of the 87Sr/86Sr = 0.706 isopleth in arc magmas may be delayed until magma sources are sufficiently vigorous to melt and incorporate aged continental crust. Therefore, Sri values of older stitching plutons may better record the position of discrete terrane boundaries, whereas younger plutons will record the magmatically average position of terrane boundaries. Although the Fine Gold Intrusive Suite is comparable to the Late Cretaceous voluminous intrusive suite of eastern Sierran suites in terms of duration and age zoning of magmatism, the influence of preexisting basement compositions and differing degrees of organization of the magma sources with age is more pronounced. In addition, the findings show that recycling of fringing arc terranes into continental crust is relatively rapid and that estimates of the growth of Phanerozoic continental crust from such reprocessing should be revised upward. © 2012 Geological Society of America.
- Lukens, C. E., Carrapa, B., Singer, B. S., & Gehrels, G. (2012). Miocene exhumation of the Pamir revealed by detrital geothermochronology of Tajik rivers. Tectonics, 31(2).More infoAbstract: The Pamir mountains are the western continuation of the Tibetan-Himalayan system, the largest and highest orogenic system on Earth. Detrital geothermochronology applied to modern river sands from the western Pamir of Tajikistan records the history of sediment source crystallization, cooling, and exhumation. This provides important information on the timing of tectonic processes, relief formation, and erosion during orogenesis. U-Pb geochronology of detrital zircons and 40Ar/ 39Ar thermochronology of white micas from five rivers draining distinct tectonic terranes in the western Pamir document Paleozoic through Cenozoic crystallization ages and a Miocene (13-21 Ma) cooling signal. Detrital zircon U-Pb ages show Proterozoic through Cenozoic ages and affinity with Asian rocks in Tibet. The detrital 40Ar/ 39Ar data set documents deep and regional exhumation of the Pamir mountains >30 Myr after Indo-Asia collision, which is best explained with widespread erosion of metamorphic domes. This exhumation signal coincides with deposition of over 6 km of conglomerates in the adjacent foreland, documenting high subsidence, sedimentation, and regional exhumation in the region. Our data are consistent with a high relief landscape and orogen-wide exhumation at ∼13-21 Ma and correlate with the timing of exhumation of the Pamir gneiss domes. This exhumation is younger in the Pamir than that observed in neighboring Tibet and is consistent with higher magnitude Cenozoic deformation and shortening in this part of the orogenic system. Copyright 2012 by the American Geophysical Union.
- Pearson, D. M., Kapp, P., Reiners, P. W., Gehrels, G. E., Ducea, M. N., Pullen, A., Otamendi, J. E., & Alonso, R. N. (2012). Major Miocene exhumation by fault-propagation folding within a metamorphosed, early Paleozoic thrust belt: Northwestern Argentina. Tectonics, 31(4).More infoAbstract: The central Andean retroarc thrust belt is characterized by a southward transition at ∼22S in structural style (thin-skinned in Bolivia, thick-skinned in Argentina) and apparent magnitude of Cenozoic shortening (>100 km more in the north). With the aim of evaluating the abruptness and cause of this transition, we conducted a geological and geo-thermochronological study of the Cachi Range (∼24-25S), which is a prominent topographic feature at this latitude. Our U-Pb detrital zircon results from the oldest exposed rocks (Puncoviscana Formation) constrain deposition to mainly Cambrian time, followed by major, Cambro-Ordovician shortening and ∼484 Ma magmatism. Later, Cretaceous rift faults were locally inverted during Cenozoic shortening. Coupled with previous work, our new (U-Th)/He zircon results require 8-10 km of Miocene exhumation that was likely associated with fault-propagation folding within the Cachi Range. After Miocene shortening, displacement on sinistral strike-slip faults demonstrates a change in stress state to a non-vertically oriented 3. This change in stress state may result from an increase in gravitational potential energy in response to significant crustal thickening and/or lithospheric root removal. Our finding of localized Cenozoic shortening in the Cachi Range increases the estimate of the local magnitude of shortening, but still suggests that significantly less shortening was accommodated south of the thin-skinned Bolivian fold-thrust belt. Our results also underscore the importance of the pre-existing stratigraphic and structural architecture in orogens in influencing the style of subsequent deformation. © 2012. American Geophysical Union. All Rights Reserved.
- Rainbird, R., Cawood, P., & Gehrels, G. (2012). The Great Grenvillian Sedimentation Episode: Record of Supercontinent Rodinia's assembly. Tectonics of Sedimentary Basins: Recent Advances, 583-601.More infoAbstract: One of Earth's greatest mountain-building episodes, the Grenvillian orogeny, occurred with the assembly of the supercontinent Rodinia at the end of the Mesoproterozoic era, about 1.2-1.0 billion years ago. Weathering and erosion of the Grenvillian mountain chain, the roots of which can be traced today for nearly 12,000 km, produced huge volumes of sedimentary detritus that were dispersed by an enormous system of braided rivers. Erosion, denudation, and sediment throughput were enhanced by a lack of vegetation and vigorous continental weathering under a climate that favored strong chemical alteration. The enormity of the erosional episode and broad extent of river system that drained the Grenvillian Mountains was first recognized with the advent of detrital zircon geochronology as a tool of provenance analysis. Initially, zircon grains of Grenvillian age were recovered from early Neoproterozoic sedimentary basins located in northwestern Canada, more than 3000km away from the nearest probable sources in the Grenville Province of eastern Laurentia. Paleocurrents derived from cross-bedding in thick fluvial deposits preserved in these basins showed regionally consistent west-northwesterly transport, lending support to the paleogeographic model. Correlative strata, located thousands of kilometers to the south, in the Canadian and US Cordillera, exhibit similar detrital zircon age distributions providing further support for the large-scale river system. These data also indicate that the fluvial system was laterally extensive and likely originated from multiple sources along the great length of the Grenvillian mountain front. Deposits representing the proximal parts of the system have now been recognized in the subsurface of the central US, where they comprise several stratigraphic sequences that can be tied to the various stages of tectonic evolution of the Grenvillian orogeny. The sequences correlate well with outcrop exposures preserved in the Midcontinent Rift system and Great Lakes region to the north. Among these are syn-collisional rift deposits and post-collisional foreland basin deposits displaying features such as axial flow patterns that indicate deposition by trunk rivers flowing parallel to the mountain front. Similar stratigraphic successions are preserved around the North Atlantic in Scotland, Shetland, East Greenland, Svalbard, and Norway. Detrital zircon grains from these successions are dominated by late Paleoproterozoic and late Mesoproterozoic ages inferred to have been derived from source terranes of the Grenville Province in eastern Laurentia. Detrital zircon geochronology indicates that Grenville-age detritus was reworked into numerous Phanerozoic successions around the globe. Some of this detritus was derived directly from uplift and erosion of Grenville Province rocks or recycling of detritus from Grenvillian foreland basin deposits during Appalachian-Hercynian orogenesis and assembly of the Pangea supercontinent. Late Mesoproterozoic detritus has been continually recycled into younger stratigraphic successions and remains a significant component of modern river sediments. © 2012 Blackwell Publishing Ltd.
- Soboleva, A. A., Kuznetsov, N. B., Miller, E. L., Udoratina, O. V., Gehrels, G., & Romanyuk, T. V. (2012). First results of U - Pb dating of detrital zircons from basal horizons of Uralides (Polar Urals). Doklady Earth Sciences, 445(2), 962-968.
- Thomson, S. N., Gehrels, G. E., Ruiz, J., & Buchwaldt, R. (2012). Routine low-damage apatite U-Pb dating using laser ablation-multicollector- ICPMS. Geochemistry, Geophysics, Geosystems, 13(1).More infoAbstract: Apatite is a common U-bearing accessory mineral with a U-Pb closure temperature of ∼500C, making U-Pb dating of apatite a potentially valuable thermochronometer. However, its low U concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods. We overcome previous limitations by using a Nu Plasma multicollector ICPMS with an attached short-pulse excimer laser, and by identifying two new matrix-matched reference apatites to correct for elemental fractionation: gem-quality 485 Myr old apatite from Madagascar which we independently characterized by ID-TIMS analysis, and 523.5 Ma apatite from the McClure Mountain syenite (source of the 40Ar/ 39Ar reference MMhb). Common Pb is corrected using measured 204Pb isobarically corrected for Hg interference and a five-step iterative process using Stacey and Kramers' common Pb model. We accurately reproduce ages of numerous independently characterized apatites, regularly achieving precision of
- Tochilin, C. J., Reiners, P. W., Thomson, S. N., Gehrels, G. E., Hemming, S. R., & Pierce, E. L. (2012). Erosional history of the Prydz Bay sector of East Antarctica from detrital apatite and zircon geo-and thermochronology multidating. Geochemistry, Geophysics, Geosystems, 13(11).More infoAbstract: Approximately 98% of East Antarctica is covered by the East Antarctic Ice Sheet (EAIS), which has covered parts of the continent since the early Oligocene (34Ma) and obscures evidence about the region's tectonic and erosional history. To better constrain the subglacial record, we analyzed geo-and thermochronologic dates of Oligocene-Quaternary sediments from Prydz Bay, which drains ∼16% of the EAIS. We used multidating techniques, measuring U-Pb, fission track, and (U-Th)/He dates on apatite and zircon grains and 40Ar/39Ar dates on hornblende grains to determine crystallization and cooling ages. Apatite and zircon U-Pb dates and hornblende 40Ar/39Ar dates are dominantly ∼500 Ma, recording Pan-African metamorphism and magmatism. Zircon fission track dates record cooling at ∼250-300Ma and ∼120 Ma from Permian-Triassic (300-201Ma) rifting and Cretaceous (120Ma) magmatic resetting. Mean apatite fission track dates decrease from ∼280-210Ma in early Oligocene samples, with lag times decreasing from ∼250-180 My, indicating increasing erosion rates. Miocene-Quaternary (10.7-0Ma) samples show a smaller range from ∼180 to ∼150 Ma. Youngest measured apatite He ages also decrease from ∼100 Ma to ∼25 Ma in Oligocene-Miocene samples. These results indicate increasing erosion rates (0.2km/My) in catchments draining to Prydz Bay in the early Oligocene, with slower erosion since the late Miocene. This erosion was likely achieved by glacial incision into pre-existing valleys, reaching depths of ∼2.8-3.0km by the late Miocene. This is consistent with EAIS models showing a transition to less erosive, cold-based conditions following the mid-Miocene climatic optimum. © 2012. American Geophysical Union. All Rights Reserved.
- Tochilin, C., Dickinson, W. R., Felgate, M. W., Pecha, M., Sheppard, P., Damon, F. H., Bickler, S., & Gehrels, G. E. (2012). Sourcing temper sands in ancient ceramics with U-Pb ages of detrital zircons: A southwest Pacific test case. Journal of Archaeological Science, 39(7), 2583-2591.More infoAbstract: Through use of methodology common in sedimentary geology, we apply U-Pb ages of detrital zircons to source nonlocal temper sand in an ancient ceramic assemblage recovered from Roviana Lagoon of the New Georgia Group in the Solomon Islands. Most potsherds from the Roviana Lagoon contain local volcanic sand as temper, but a small number of sherds contain anomalous granitic temper sand that does not appear to be local. To determine the origin of the anomalous temper, ages of zircons from the anomalous Roviana sherds are compared with ages of zircons in materials from Lizard Island off the Queensland coast and in sand from Muyuw Island in the Solomon Sea where generically similar granitic sands occur. U-Pb analyses of grains from the Roviana sherds yield Middle Miocene ages, while analyses of grains from Lizard Island granitic bedrock, sand, and local potsherds yield much older Permian-Triassic ages, disproving any possibility that the Roviana sherds were derived from Lizard Island, but suggesting local production of the Lizard Island sherds. Ages of grains in a sand sample from Muyuw Island are nearly identical to the ages of grains in the Roviana sherds. All grains in the Muyuw sand are Middle Miocene in age, overlapping closely with the Roviana age population. This strong similarity in detrital zircon signals indicates that the Roviana temper was likely derived from Muyuw Island sands. Our test case for the use of U-Pb ages of detrital zircons in sourcing temper sands is of only regional significance, and not of intrinsic global interest. The methodology, however, should find wide applicability for sourcing temper sands in many parts of the world, for it provides more specific data for the origins of tempers than either petrographic or chemical analysis. © 2012 Elsevier Ltd.
- Alexander, A., Yin, A., Harrison, T. M., Célérier, J., Gehrels, G. E., Manning, C. E., & Grove, M. (2011). Cenozoic tectonic history of the Himachal Himalaya (northwestern India) and its constraints on the formation mechanism of the Himalayan orogen. Geosphere, 7(4), 1013-1061.More infoAbstract: A central debate for the evolution of the Himalayan orogen is how the Greater Himalayan Crystalline complex in its core was emplaced during the Cenozoic Indo-Asian collision. Addressing this problem requires knowledge of the structural relationship between the South Tibet detachment fault (STD) and the Main Central thrust (MCT) that bound these rocks from above and below. The fault relationship is exposed in the Himachal Himalaya of northwestern India, where they merge in their updip direction and form a frontal branch line that has been warped by subsequent top-to-the-southwest shear deformation. To elucidate how the two major crustal-scale faults evolved in the western Himalaya, we conducted integrated geologic research employing field mapping, pressure-temperature (P-T) analyses, U-Pb zircon geochronology, trace and rare earth element (REE) geochemistry, and thermochronology. Our field study reveals complex geometric relationships among major thrusts with large-magnitude shortening within each thrust sheet. Three successive stages of top-to-the-southwest thrust development are recognized: (1) imbricate stack development, (2) translation of large thrust sheets along low-angle detachments and backthrusting along the STD, and (3) development of duplex systems via underplating. This kinematic process can be quantified by our new analytical data: (1) P-T determinations show 7-9 kbar and 450-630 °C conditions across the STD. The lack of a metamorphic discontinuity across the fault is consistent with a backthrust interpretation. (2) U-Pb zircon geochronology yields ca. 830 Ma and ca. 500 Ma ages of granitoids in the MCT hanging wall, ca. 1.85 Ga ages of granitic gneisses in both the MCT hanging wall and footwall, and 8-6 Ma ages of granitic pegmatites in the MCT footwall. These ages help define regional chronostratigraphy, and the youngest ages reveal a previously unknown intrusion phase. (3) Trace element and REE geochemistry of 1.85 Ga, 830 Ma, and 500 Ma granitoids are characteristic of remelted continental crust, constraining the protolith tectonic setting. (4) U-Pb geochronology of detrital zircon reveals that siliciclastic sedimentary sequences above the STD, below the MCT, and between these two faults have similar age spectra with Neoproterozoic youngest age peaks. This result implies that the STD and MCT each duplicated the same stratigraphic section. (5) Th-Pb geochronology of monazite included in MCT hanging-wall garnet yields Paleozoic and early Tertiary ages, indicating Paleozoic and early Tertiary metamorphism in these rocks. (6) The 40Ar/39Ar thermochronology of the K-feldspar from southern MCT hangingwall rocks evinces cooling below 220-230 °C ca. 13-19 Ma or later, constraining the thrust development history. We use these results to derive a tectonic model of crustal shortening across the Himachal Himalaya involving early thickening, tectonic wedging emplacement of the Greater Himalayan Crystalline complex between the MCT and STD, and continued growth of the Himalayan thrust wedge by accretion of thrust horses from the Indian footwall. © 2011 Geological Society of America.
- Cardona, A., Valencia, V. A., Bayona, G., Duque, J., Ducea, M., Gehrels, G., Jaramillo, C., Montes, C., Ojeda, G., & Ruiz, J. (2011). Early-subduction-related orogeny in the northern Andes: Turonian to Eocene magmatic and provenance record in the Santa Marta Massif and Rancheria Basin, northern Colombia. Terra Nova, 23(1), 26-34.More infoAbstract: The timing of orogeny in the northern Andes and the mechanism driving it are still debated. We have studied the age, composition and provenance of granitoids and sandstones of the Santa Marta Massif and Rancheria Basin, northern Colombia, to relate deep-seated and surface tectonic processes attending the Late Cretaceous-Palaeogene history of the northern Andes. Our results indicate the development of five tectonic episodes: (1) collision of northwestern South America with a 92-80 Ma Caribbean arc (70 Ma); (2) late-collisional to early-subduction metamorphism and magmatism (65 Ma); (3) distal accumulation of a thick siliciclastic sequence (60-58 Ma); (4) renewed arc magmatism (58-50 Ma); and (5) magmatic quiescence and block uplift (post-50 Ma). The first episodes are related to the onset of subduction, and the last episode is related to shallow subduction and oblique convergence. Similar events in Colombia and Ecuador reveal that the Late Cretaceous-Eocene orogeny of the northern Andes was influenced by the collision and subduction of the Caribbean oceanic plate. © 2010 Blackwell Publishing Ltd.
- Cecil, M. R., Gehrels, G., Ducea, M. N., & Patchett, P. J. (2011). U-Pb-Hf characterization of the central Coast Mountains batholith: Implications for petrogenesis and crustal architecture. Lithosphere, 3(4), 247-260.More infoAbstract: We present U-Pb geochronologic and Hf isotopic data from 29 plutonic samples within the Coast Mountain batholith, north-coastal British Columbia and southeast Alaska. Hf isotopic values do not correlate with age or variation in magmatic fl ux, but rather they increase systematically from west (εHf[t] = +2 to +5) to east (εHf[t] = +10 to +13) in response to changing country rock assemblages. By comparing our pluton Hf data with previously reported Nd-Sr and detrital zircon characteristics of associated country rocks, we identify three crustal domains in an area where crustal affi nity is largely obscured by metamorphism and voluminous pluton intrusion: (1) a western domain, emplaced into continental-margin strata of the Banks Island assemblage; (2) a central domain, emplaced into the Alexander terrane; and (3) an eastern domain, underlain by the Stikine terrane and its inferred metamorphic equivalents. Between the interpreted Alexander and Stikine terranes, there is a zone of variable εHf(t) (+2 to +13) that coincides with the suture zone separating inboard (Stikine and Yukon-Tanana) from outboard (Alexander and associated) terranes. This variation in εHf(t) values apparently results from the structural imbrication of juvenile (Alexander and Stikine) and evolved (Yukon-Tanana) terranes along mid-Cretaceous thrust faults and the latest Cretaceous-early Tertiary Coast shear zone. Shifts in the Hf values of plutons across inferred terranes imply that they are separated at lower- to midcrustal levels by steep boundaries. Correlation between these Hf values and the isotopic character of exposed country rocks further implies the presence of those or similar rocks at magma-generation depths. © 2011 Geological Society of America.
- Centeno-García, E., Busby, C., Busby, M., & Gehrels, G. (2011). Evolution of the Guerrero composite terrane along the Mexican margin, from extensional fringing arc to contractional continental arc. Bulletin of the Geological Society of America, 123(9-10), 1776-1797.More infoAbstract: The western margin of Mexico is ideally suited for testing two opposing models for the growth of continents along convergent margins: accretion of exotic island arcs by the consumption of entire ocean basins versus accretion of fringing terranes produced by protracted extensional processes in the upper plate of a single subduction zone. We present geologic and detrital zircon evidence that the Zihuatanejo terrane of the Guerrero composite terrane originated from the latter mechanism. The evolution of the Zihuatanejo terrane can be explained by extensional and compressional processes operating entirely within the upper plate of a long-lived subduction zone that dipped east under the Mexican margin. This process controlled crustal growth by continental margin rifting and addition of new igneous and volcaniclastic material during extension, followed by accretion and thickening of the crust during contraction. Prior to this study, all Mesozoic rocks in the western part of the Guerrero composite terrane were considered to be part of a single arc. However, we divide it into four distinctive tectonostratigraphic assemblages: (1) a Triassic-Early Jurassic accretionary complex (Arteaga complex); (2) a Jurassic to earliest Cretaceous extensional volcanic arc assemblage; (3) an Early Cretaceous extensional arc assemblage; and (4) a Santonian-Maastrichtian compressional arc assemblage. (1) The Arteaga subduction complex forms the basement to the Zihuatanejo terrane and includes Grenville, Pan-African, and Permiandetrital zircon suites that match the Potosi fan of the Mexican mainland. (2) The Jurassic to earliest Cretaceous extensional volcanic arc assemblage shows a Callovian-Tithonian (ca. 163-145 Ma) peak in magmatism; extensional unroofing began in this time frame and continued into through the next. (3) The Early Cretaceous extensional arc assemblage has two magmatic peaks: one in the Barremian- Aptian (ca. 129-123 Ma), and the other in the Albian (ca. 109 Ma). In some localities, rapid subsidence produced thick, mainly shallowmarine volcano-sedimentary sections, while at other localities, extensional unroofing of all older assemblages resulted in recycling of zircon from all older units (1, 2, 3). (4) For the Santonian-Maastrichtian compressional arc assemblage, our new detrital zircon dates show for the first time that arc volcanic rocks of this age are present in the coastal Zihuatanejo terrane. The contractional arc developed atop assemblages 1-3, which were shortened between Turonian and Santonian time (ca. 93 and 84 Ma). Taken together, the western Zihuatanejo terrane records a more protracted history of arc magmatism than has yet been dated in other terranes of western Mexico, but it closely matches the history of Baja California to the northwest. © 2011 Geological Society of America.
- Collo, G., Dávila, F. M., Nóbile, J., Astini, R. A., & Gehrels, G. (2011). Clay mineralogy and thermal history of the Neogene Vinchina Basin, central Andes of Argentina: Analysis of factors controlling the heating conditions. Tectonics, 30(4).More infoAbstract: The Vinchina Foreland Basin, western Argentina, contains a ∼7 km thick nonmarine stratigraphy, chronologically constrained within the Mio-Pliocene (circa 19-3.4 Ma), and where distribution of Illite/Smectite interstratified phases has shown a progressive smectite-illitization progress (R0 → R1 → R3), is consistent with an incipient burial history. R0 represents randomly mixed-layered illite/smectite normally found at shallow depths, as this ordering is not stable at ∼120C. In the Vinchina Basin, however, the R0 is still persistent at ∼7 km depth, and its appearance even in the deepest levels is consistent with previous interpretations of low burial temperatures based on thermochronologic studies of detrital apatites. The maximum paleotemperature estimation and basin depth imply geothermal gradient as low as ∼15C/km, which allowed an estimate of heat flow values between 33 and 42 mW/m2, that would rise to between ∼40 and 51 mW/m2 when the sedimentation rate (thermal blanketing) is taken into account. These values were only reported for cold basins and represent a paleothermal state of a refrigerated lithosphere. We suggest the central Andes were dominated since the Miocene by heat transfer derived mostly from crustal contributions with a minimum input from the asthenosphere. This refrigerated lithosphere is typical of segments affected by flat subduction. Preliminary thermal models based on previous geodynamic approaches support our conclusions. Copyright © 2011 by the American Geophysical Union.
- Druschke, P., Hanson, A. D., Wells, M. L., Gehrels, G. E., & Stockli, D. (2011). Paleogeographic isolation of the cretaceous to eocene sevier hinterland, east-central nevada: Insights from U-Pb and (U-Th)/he detrital zircon ages of hinterland strata. Bulletin of the Geological Society of America, 123(5), 1141-1160.More infoAbstract: The Late Cretaceous to Paleogene Sevier hinterland of east-central Nevada is widely regarded as an orogenic plateau that has since undergone topographic collapse. New U-Pb detrital zircon age data consisting of 1296 analyses from the Lower Cretaceous Newark Canyon Formation and the Upper Cretaceous to Eocene Sheep Pass Formation indicate that Precambrian detrital zircon populations recycled from local Paleozoic strata are dominant. Subordinate Mesozoic zircon populations are derived mainly from local backarc volcanic centers of Late Jurassic and Early Cretaceous age, while ca. 38-36 Ma detrital zircon age peaks record the local onset of Eocene volcanism. Sevier hinter land deposits of east-central Nevada lack significant Triassic, Early Jurassic, and Late Cretaceous populations common in terranes of western Nevada and the Sierra Nevada magmatic arc. These data suggest that long-term evolution of the Sevier Plateau involved geographic isolation through a combination of high relief and rugged topography related to Early Cretaceous shortening, and continued isolation through development of latest Cretaceous to Eocene internally drained, extensional basins. The (U-Th)/He zircon ages obtained from the Sheep Pass Formation record late Paleozoic, Early Cretaceous, and Late Cretaceous cooling through 180 °C. Preservation of late Paleozoic (ca. 265 Ma) cooling ages indicates that much of the Upper Paleozoic section within east-central Nevada that contributed detritus to the Sheep Pass basin was un affected by deep thrust burial, or by burial beneath thick Mesozoic sedimentary cover. Early Cretaceous (ca. 135 Ma) cooling ages are potentially coeval with shortening along the central Nevada fold-and-thrust belt, although ca. 80 Ma cooling ages within the Sheep Pass Formation are coeval with hinter land midcrustal extension. Together, these new data provide support for previous interpretations that the Sevier hinterland represents an ancient high-elevation orogenic plateau, and that the latest Cretaceous locally marks a transition from contraction to extension. © 2011 Geological Society of America.
- Gehrels, G. E., Blakey, R., Karlstrom, K. E., Timmons, J. M., Dickinson, B., & Pecha, M. (2011). Detrital zircon U-Pb geochronology of Paleozoic strata in the Grand Canyon, Arizona. Lithosphere, 3(3), 183-200.More infoAbstract: We determined U-Pb ages for detrital zircons from 26 samples of Paleozoic sandstone from the Grand Canyon. Cambrian strata yield mainly ca. 1.44 and 1.7-1.8 Ga ages that indicate derivation from nearby basement rocks of the Yavapai Province. Devonian strata contain zircons of 1.6-1.8 Ga, 1.34-1.40 Ga, and ca. 520 Ma, suggesting derivation from the Mazatzal and Yavapai Provinces, midcontinent region, and the Amarillo-Wichita uplift, respectively. Mississippian strata record a major change in provenance, with predominantly 415-475 Ma and 1030-1190 Ma grains interpreted to have been shed from the central Appalachian orogen. Pennsylvanian strata contain subequal proportions of 1.4-1.8 Ga grains derived from basement rocks exposed in the Ancestral Rocky Mountains and 409-464 and ca. 1070 Ma grains derived from the Appalachians. Permian strata contain abundant Appalachian zircons, including 270-380 Ma grains, and a lesser proportion of grains derived from the Ancestral Rocky Mountains. Transcontinental transport during Mississippian through Permian time is interpreted to have occurred in large river systems, facilitated by northeasterly trade winds during low sea level and by coastal currents. A compilation of young ages from all Upper Paleozoic strata yields age peaks of 270-365 Ma, 395-475 Ma, and 515-640 Ma, an excellent match for Alleghanian, Acadian, Taconic, and Neoproterozoic (peri-Gondwanan) episodes of magmatism along the Appalachian margin. Lag times of the youngest grains in these Upper Paleozoic strata average ~25 m.y., suggesting relatively rapid exhumation and erosion of Appalachian source regions. © 2011 Geological Society of America. © 2011 Geological Society of America.
- Ibanez-Mejia, M., Ruiz, J., Valencia, V. A., Cardona, A., Gehrels, G. E., & Mora, A. R. (2011). The Putumayo Orogen of Amazonia and its implications for Rodinia reconstructions: New U-Pb geochronological insights into the Proterozoic tectonic evolution of northwestern South America. Precambrian Research, 191(1-2), 58-77.More infoAbstract: Outcrops of late Meso- to early Neoproterozoic crust in northwestern South America are restricted to isolated exposures of basement inliers within the northern Andes of Colombia, Peru and Venezuela. However, evidence for the existence of an autochthonous Stenian-Tonian belt in northern Amazonia that is undisturbed by Andean orogenesis has not been recognized so far. Here we report ~1200 new single-zircon U-Pb geochronological analyses from 19 Proterozoic rock samples of northwestern South America collected from the Garzón and Las Minas Andean cordilleran inliers, drill-core samples from the foreland basin basement, and outcrops of cratonic Amazonia in eastern Colombia (western Guyana shield). Our new geochronological results document the existence of a previously unrecognized Meso- to Neoproterozoic orogenic belt buried under the north Andean foreland basins, herein termed the Putumayo Orogen, which has implications for Proterozoic tectonic reconstructions of Amazonia during the assembly of the supercontinent Rodinia. Based on the interpretations of new and pre-existing data, we propose a three-stage tectonometamorphic evolution for this orogenic segment characterized by: (1) development of a pericratonic fringing-arc system outboard of Amazonia's leading margin during Mesoproterozoic time from ~1.3 to 1.1. Ga, where the protoliths for metaigneous and metavolcanosedimentary units of the Colombian and Mexican inliers would have originated in a commonly evolving Colombian-Oaxaquian fringing-arc system; (2) amphibolite-grade metamorphism and migmatization between ca. 1.05 and 1.01. Ga by inferred amalgamation of these parautochtonous arc terranes onto the continental margin, and (3) granulite-grade metamorphism at ~0.99. Ga during continent-continent collision related to Rodinia final assembly. Along with additional paleogeographic constraints, this new geochronological framework suggests that the final metamorphic phase of the Putumayo Orogen was likely the result of collisional interactions with the Sveconorwegian province of Baltica, in contrast to previously proposed models that place this margin of Amazonia as the conjugate of the Grenville province of Laurentia. © 2011 Elsevier B.V.
- Jacobson, C. E., Grove, M., Pedrick, J. N., Barth, A. P., Marsaglia, K. M., Gehrels, G. E., & Nourse, J. A. (2011). Late Cretaceous-early Cenozoic tectonic evolution of the southern California margin inferred from provenance of trench and forearc sediments. Bulletin of the Geological Society of America, 123(3-4), 485-506.More infoAbstract: During the Late Cretaceous to early Cenozoic, southern California was impacted by two anomalous tectonic events: (1) under-plating of the oceanic Pelona-Orocopia-Rand schists beneath North American arc crust and craton; and (2) removal of the western margin of the arc and inner part of the forearc basin along the Nacimiento fault. The Pelona-Orocopia-Rand schists crop out along a belt extending from the southern Sierra Nevada to southwestern Arizona. Protolith and emplacement ages decrease from >90 Ma in the northwest to
- Kuznetsov, N. B., Orlov, S., Miller, E. L., Shazillo, A. V., Dronov, A. V., Soboleva, A. A., Udoratina, O. V., & Gehrels, G. (2011). First results of U/Pb dating of detrital zircons from Early Paleozoic and Devonian sandstones of the Baltic-Ladoga region (south Ladoga Area). Doklady Earth Sciences, 438(2), 759-765.More infoAbstract: The first results of U/Pb isotopic dating (LA ICP MS) of detrital zircons from sands from the Middle Cambrian Sablinka Formation, Upper Cambrian Ladoga Formation, Low Ordovician Tosna Formation, and calcareous sands from Syas' Formation (Sargaevskii horizon of the Upper Frasnian) from Baltica-Ladoga Glint (BLG) of the Southern Ladoga area are presented. The obtained ages of detrital zircons span the intervals 492.7 ± 5.1-3196.4 ± 5.1 Ma (Sablino Formation); 577.9 ± 7-2972.6 ± 13.4 Ma (Ladoga Formation); 509.4 ± 8.5-3247.6 ± 10.1 Ma (Tosna Formation); 451.1 ± 14.7-2442.2 ± 6.9 Ma (Syas' Formation). A comparison of the obtained isotopic ages of detrital zircons to ages of crystalline complexes composing the Kola-Karelian, Svecofennian, and Sveconorwegian domains of Baltic Shield and Pre-Uralian-Timanian structures of Subpolar and Polar Urals and basement of Pechora Basin was carried out. It is proposed that the Middle Paleozoic sedimentary basin accumulated Upper Frasnian rocks of Syas' Formation. The basin ranged northward from the present-day BLG and occupied the eastern part of the Baltic Shield. © 2011 Pleiades Publishing, Ltd.
- Leier, A. L., & Gehrels, G. E. (2011). Continental-scale detrital zircon provenance signatures in Lower Cretaceous strata, western North America. Geology, 39(4), 399-402.More infoAbstract: Lower Cretaceous strata and the underlying sub-Cretaceous unconformity in western North America record a profound, but poorly understood change in sedimentation patterns and basin dynamics in the Cordilleran foreland basin. To better understand the regional sedimentary systems and provenance during Early Cretaceous time, we sampled 10 Lower Cretaceous sandstone and conglomerate units that overlie the sub-Cretaceous unconformity in Canada and the United States for detrital zircon uranium-lead (U-Pb) geochronology. These Lower Cretaceous strata contain two distinct detrital zircon U-Pb age signatures. A "northern" signature, present in strata in Alberta and British Columbia, contains zircons with ages of ca. 120 Ma and 1850 Ma, and is composed of zircons from the Cordilleran arc and grains recycled from strata of the Canadian miogeocline. A "southern" signature, present in strata from southwestern United States to central Montana, contains zircons with ages of ca. 160 Ma, ca. 250-650 Ma, and ca. 1040 Ma, and consists of zircons from the Cordilleran arc and grains recycled from late Paleozoic strata and Mesozoic eolianite units in the western United States. We propose that the differences in detrital zircon U-Pb age populations between northern and southern areas of western North America are due to differences in zircon populations in the sediment source strata exposed in the contemporaneous thrust belt, and possibly a subtle paleohydraulic divide in Montana. These distinct provenance signatures along the Cordillera suggest that the mechanisms responsible for Early Cretaceous changes in foreland basin dynamics occurred along the length of the Cordillera in both the U.S. and Canada. © 2011 Geological Society of America.
- Long, S., McQuarrie, N., Tobgay, T., Rose, C., Gehrels, G., & Grujic, D. (2011). Tectonostratigraphy of the lesser Himalaya of Bhutan: Implications for the along-strike stratigraphic continuity of the northern Indian margin. Bulletin of the Geological Society of America, 123(7-8), 1406-1426.More infoAbstract: New mapping in eastern Bhutan, in conjunction with U-Pb detrital zircon and delta;13C data, defines Lesser Himalayan tectonostratigraphy. The Daling-Shumar Group, 2-6 km of quartzite (Shumar Formation) overlain by 3 km of schist (Daling Formation), contains ~1.8-1.9 Ga intrusive orthogneiss bodies and youngest detrital zircon peaks, indicating a Paleoproterozoic deposition age. The Jaishidanda Formation, 0.5-1.7 km of garnet-biotite schist and quartzite, stratigraphically overlies the Daling Formation beneath the Main Central thrust, and yields youngest detrital zircon peaks ranging from ~0.8-1.0 Ga to ca. 475 Ma, indicating a Neoproterozoic-Ordovician(?) deposition age range. The Baxa Group, 2-3 km of quartzite, phyllite, and dolomite, overlies the Daling-Shumar Group in the foreland, and yields ca. 0.9 Ga to ca. 520 Ma youngest detrital zircon peaks, indicating a Neoproterozoic-Cambrian(?) deposition age range. Baxa dolomite overlying quartzite containing ca. 525 Ma detrital zircons yielded delta;13C values between +3‰ and +6‰, suggesting deposition during an Early Cambrian positive δ13C excursion. Above the Baxa Group, the 2-3 km thick Diuri Formation diamictite yielded a ca. 390 Ma youngest detrital zircon peak, suggesting correlation with the late Paleozoic Gondwana supercontinent glaciation. Finally, the Permian Gondwana succession consists of sandstone, siltstone, shale, and coal. Our deposition age data from Bhutan: (1) reinforce suggestions that Paleoproterozoic (~1.8-1.9 Ga) Lesser Himalayan deposition was continuous along the entire northern Indian margin; (2) show a likely east ward continuation of a Permian over Cambrian unconformity in the Lesser Himalayan section identified in Nepal and northwest India; and (3) indicate temporal overlap between Neoproterozoic-Paleozoic Lesser Himalayan (proximal) and Greater Himalayan-Tethyan Himalayan (distal) deposition. © 2011 Geological Society of America.
- Martin, A. J., Burgy, K. D., Kaufman, A. J., & Gehrels, G. E. (2011). Stratigraphic and tectonic implications of field and isotopic constraints on depositional ages of Proterozoic Lesser Himalayan rocks in central Nepal. Precambrian Research, 185(1-2), 1-17.More infoAbstract: In the Himalaya of central Nepal, uncertainty in the absolute depositional ages and the relative stratigraphic positions of several formations in the Proterozoic part of the Lesser Himalayan series has hindered structural mapping, east-west correlation of units and structures, and interpretations of basin architecture during both the Proterozoic and Paleozoic. At the stratigraphic base of the Lesser Himalayan series in central Nepal, 441 new U/Pb isotopic ages of detrital zircons from the previously defined type Kuncha, Kushma, and Fagfog formations constrain the maximum possible depositional ages for these clastic units to be c. 1900, 1770, and 1810. Ma, respectively. 182 detrital zircon U/Pb isotopic ages confirm the identity of another sample of the Fagfog Formation, and field relations indicate that this unit rests depositionally on the Kushma Formation with no obvious unconformity or lithologic difference. This interpretation, combined with the recognition that the term "Kushma Formation" historically has been applied incongruously to apparently different units in different parts of Nepal, leads us to recommend abandoning the term "Kushma Formation" and replacing it with "lower Fagfog Formation." The overlying part of the Fagfog then becomes the "upper Fagfog Formation." Because the 1770. Ma constraint on the maximum depositional age comes from the lower part of the unit, the entire Fagfog Formation must have been deposited after 1770. Ma. The depositional ages of both the lower and upper parts of the Fagfog must be younger than the Kuncha Formation because it was intruded by a granite at 1878 ± 22. Ma (2-sigma). Thus an unconformity lasting at least 86. M.y. probably separates the Kuncha Formation (deposited between 1900 and 1856. Ma, considering uncertainties) and the Fagfog Formation (deposited after 1770. Ma). The lower Fagfog Formation is present in the Kali Gandaki region but apparently pinches out to the east because it is not present structurally beneath the Kathmandu nappe.Suggestions about the depositional age of the carbonate-dominated Malekhu Formation at the top of the Proterozoic part of the Lesser Himalayan series have ranged from Mesoproterozoic to late Paleozoic. These age estimates largely stem from correlation with parts of the Krol succession some 500km to the west in northwest India, and changing interpretations of the depositional age of the Krol rocks. 104 new measurements of carbon isotope abundances in carbonate from a 300m measured section through type exposures of the Malekhu Formation reveal a narrow range of δ13C values between -1.7 and +0.2‰ (VPDB) and a mean of -0.9±0.4‰ (1 standard deviation). These values are inconsistent with correlation with the Krol succession, which preserves large amplitude positive and negative excursions in δ13C values. Comparison with known marine carbonate δ13C values through time suggests deposition of the Malekhu Formation prior to c. 1300Ma. Upper and lower bounds for deposition of the Proterozoic part of the Lesser Himalayan series thus are c. 1300 and c. 1900Ma, though actual deposition likely occurred during only a portion of this interval. Further, a profound unconformity representing at least 900M.y. separates the Malekhu Formation from the depositionally overlying Carboniferous-Permian Sisne Formation. No upper Mesoproterozoic, Neoproterozoic, or lower Paleozoic rocks have been found in the Lesser Himalayan series in central Nepal. The carbon isotope stratigraphy of the Malekhu Formation in central Nepal matches that of the Buxa Formation exposed in the Ranjit window of Sikkim to the east, but does not match the chemostratigraphy of rocks correlated with the Buxa Formation in eastern Bhutan and Arunachal Pradesh, northeast India. © 2010 Elsevier B.V.
- Miller, E. L., Kuznetsov, N., Soboleva, A., Udoratina, O., Grove, M. J., & Gehrels, G. (2011). Baltica in the Cordillera?. Geology, 39(8), 791-794.More infoAbstract: U-Pb ages of detrital zircon suites from Paleozoic strata in the Arctic Alaska-Chukotka terrane (AAC), Alexander terrane, northern Sierra terrane, and eastern Klamath terrane of the North American Cordillera suggest an exotic Gondwana or Baltic origin. We evaluate these hypotheses with U-Pb ages of detrital zircon suites from Cambrian-Devonian strata of northern Baltica. Precambrian zircon populations (ca. 0.8-3.0 Ga) from Baltica compare well with similar age detritus in the AAC and Cordilleran terranes, but the amount and age of younger Neoproterozoic and Ordovician-Silurian components are variable. The AAC shares its stratigraphy with Baltica and has the most similar detrital zircon suites. Closing the Arctic places the AAC against the Lomonosov Ridge and the edge of Baltica in pre-Cretaceous time. After the Caledonian orogeny and before the Ural Mountains formed, the Baltica, AAC, and Cordilleran margins shared a Devonian-Carboniferous rift history and became along-strike portions of a Carboniferous-Permian continental margin. This rifting event might have been responsible for the initial separation of Baltica and Caledonian affinity terranes from this margin. © 2011 Geological Society of America.
- Pullen, A., Kapp, P., Gehrels, G. E., Ding, L., & Zhang, Q. (2011). Metamorphic rocks in central Tibet: Lateral variations and implications for crustal structure. Bulletin of the Geological Society of America, 123(3-4), 585-600.More infoAbstract: Insights about lateral variations in the age, composition, and structure of the central Tibetan crust are provided by geologic investigations of metamorphic rocks in the Qiangtang terrane. Previous studies have shown that a tectonic mélange of Triassic age with blueschist- and eclogite-bearing blocks within a greenschist-facies matrix is exposed over an E-W distance of ~600 km in the central Qiangtang terrane. New mapping shows that the mélange extends over a N-S distance of ~150 km, nearly to the trace of the early Mesozoic Jinsha suture in the north. The mélange , exposed structurally beneath Upper Paleozoic to Mesozoic strata in the footwalls of early Mesozoic normal faults, is composed mostly of Paleozoic metasedimentary and crystalline rocks. These findings support the hypothesis that a large part of the central and northern Qiangtang terrane crust is composed of supracrustal rocks. The Duguer Range,
- Pullen, A., Kapp, P., McCallister, A. T., Chang, H., Gehrels, G. E., Garzione, C. N., Heermance, R. V., & Ding, L. (2011). Qaidam Basin and northern Tibetan Plateau as dust sources for the Chinese Loess Plateau and paleoclimatic implications. Geology, 39(11), 1031-1034.More infoAbstract: The Chinese Loess Plateau of central Asia is composed of interbedded loess and paleosol layers, deposited during glacial and interglacial cycles, respectively, during the past ~2.5 m.y. Understanding the provenance of loess is fundamental to reconstructing wind patterns during Quaternary glacial periods. We determined and compared U-Pb ages on zircon crystals from Loess Plateau strata and potential source areas. The results indicate that the loess was largely derived from the Qaidam Basin and the northern Tibetan Plateau to the west, both of which exhibit spatially extensive geomorphic landforms indicative of past (interpreted as pre-Holocene) wind erosion and/or defl ation by westerly winds. This challenges the current paradigm that the loess of the Chinese Loess Plateau was largely sourced from deserts located to the northwest, as observed in the modern interglacial climate. We propose that during glacial periods, the mean annual positions of the polar jet streams were shifted equatorward, resulting in more southerly tracks for dust-generating storms and suppression of the East Asian monsoon by inhibiting the subtropical jet from shifting northward across the Tibetan Plateau. © 2011 Geological Society of America.
- Schmidt, J., Hacker, B. R., Ratschbacher, L., Stübner, K., Stearns, M., Kylander-Clark, A., Cottle, J. M., Alexander, A., Webb, G., Gehrels, G., & Minaev, V. (2011). Cenozoic deep crust in the Pamir. Earth and Planetary Science Letters, 312(3-4), 411-421.More infoAbstract: Multiple high-grade crystalline domes across the Pamir contain Barrovian facies-series metapelites with peak metamorphic assemblages of garnet + kyanite ± staurolite + biotite + oligoclase ± K-white mica. Thermobarometry yields pressures of 6.5-8.2 kbar and temperatures of 600-650 °C for the Kurgovat dome in the northwestern Pamir, 9.4 kbar and 588 °C for the west-central Yazgulom dome, 9.1-11.7 kbar and 700-800 °C for the east-central Muskol dome, and 6.5-14.6 kbar and 700-800 °C for the giant Shakhdara dome in the southwestern Pamir. These new data indicate exhumation of the Pamir crystalline domes from crustal depths of ~. 30-40. km. New titanite, monazite and zircon geochronology, in conjunction with published ages, illustrate that this metamorphism is Oligocene-Miocene in all but the Kurgovat dome (where it is Triassic). If the Pamir had a pre-collisional crustal thickness less than 30. km and if the India-Asia convergence within the Pamir is less than 600. km, the current 70. km-thick crust could have been created by plane strain with no net gain or loss of material. Alternatively, if the pre-collisional crustal thickness was greater than 30. km or India-Asia convergence within the Pamir is more than 600. km, significant loss of continental crust must have occurred by subhorizontal extrusion, erosion, or recycling into the mantle. Crustal recycling is the most likely, based on deep seismicity and Miocene deep crustal xenoliths. © 2011 Elsevier B.V.
- Spencer, J. E., Richard, S. M., Gehrels, G. E., Gleason, J. D., & Dickinson, W. R. (2011). Age and tectonic setting of the Mesozoic McCoy Mountains Formation in western Arizona, USA. Bulletin of the Geological Society of America, 123(7-8), 1258-1274.More infoAbstract: The McCoy Mountains Formation consists of Upper Jurassic to Upper Cretaceous siltstone, sandstone, and conglomerate exposed in an east-west-trending belt in southwestern Arizona and southeastern California. At least three different tectonic settings have been proposed for McCoy deposition, and multiple tectonic settings are likely over the ~80 m.y. age range of deposition. U-Pb isotopic analysis of 396 zircon sand grains from at or near the top of McCoy sections in the southern Little Harquahala, Granite Wash, New Water, and southern Plomosa Mountains, all in western Arizona, identifi ed only Jurassic or older zircons. A basaltic lava fl ow near the top of the section in the New Water Mountains yielded a U-Pb zircon date of 154.4 ± 2.1 Ma. Geochemically similar lava fl ows and sills in the Granite Wash and southern Plomosa Mountains are inferred to be approximately the same age. We interpret these new analyses to indicate that Mesozoic clastic strata in these areas are Upper Jurassic and are broadly correlative with the lowermost McCoy Mountains Formation in the Dome Rock, McCoy, and Palen Mountains farther west. Six samples of numerous Upper Jurassic basaltic sills and lava fl ows in the McCoy Mountains Formation in the Granite Wash, New Water, and southern Plomosa Mountains yielded initial εNd values (at t = 150 Ma) of between +4 and +6. The geochemistry and geochronology of this igneous suite, and detrital-zircon geochronology of the sandstones, support the interpretation that the lower McCoy Mountains Formation was deposited during rifting within the western extension of the Sabinas-Chihuahua-Bisbee rift belt. Abundant 190-240 Ma zircon sand grains were derived from nearby, unidentifi ed Triassic magmatic-arc rocks in areas that were unaffected by younger Jurassic magmatism. A sandstone from the upper McCoy Mountains Formation in the Dome Rock Mountains (Arizona) yielded numerous 80-108 Ma zircon grains and almost no 190-240 Ma grains, revealing a major reorganization in sediment-dispersal pathways and/or modifi cation of source rocks that had occurred by ca. 80 Ma. © 2011 Geological Society of America.
- Brown, E. H., Gehrels, G. E., & Valencia, V. A. (2010). Chilliwack composite terrane in northwest washington: Neoproterozoic-silurian passive margin basement, ordovician-silurian arc inception. Canadian Journal of Earth Sciences, 47(10), 1347-1366.More infoAbstract: The Chilliwack composite terrane in northwest Washington is part of an assemblage of mid-Paleozoic arc terranes extending from California to Alaska. Some terranes bear evidence of exotic origin, whereas others apparently formed proximal to western Laurentia, posing a complex problem in unraveling the Paleozoic accretionary history of the Cordillera. In our proposed broader definition, the Chilliwack composite terrane includes the volcanic and sedimentary East Sound and Chilliwack groups, and the plutonic and metamorphic Turtleback and Yellow Aster complexes. New zircon ages indicate that the plutonic and volcanic rocks are mutually related as parts of the same arc complex and that its inception was as old as Late Ordovician to Silurian, older than most other parts of the mid-Paleozoic terrane assemblage. Base-ment to the arc complex is a passive margin assemblage of metamorphosed quartzose sandstone and calc-silicate rock of the Yellow Aster Complex, bracketed in age by ca. 1000 Ma detrital zircons and 418 Ma intrusive rocks. This association of paragneiss basement and overlying and (or) intruding arc resembles that of older parts of the extensive Yukon-Tanana terrane in the northern Cordillera. Detrital zircon ages support a western Laurentian pericratonic origin for the paragneiss basement and the overlying arc. However, an early to mid-Paleozoic connection of this assemblage to the exotic outboard Alexander terrane is also indicated, based on (1) Mesoproterozoic and early Paleozoic detrital zircons in Devonian sedi-mentary rocks of the arc, and also in certain other pericratonic Devonian terranes and strata of the miogeocline; (2) Late Ordovician - Silurian igneous ages; and (3) an earliest Devonian or older metamorphic age of the basement paragneiss.
- Calkins, J. A., Zandt, G., Girardi, J., Dueker, K., Gehrels, G. E., & Ducea, M. N. (2010). Characterization of the crust of the Coast Mountains Batholith, British Columbia, from P to S converted seismic waves and petrologic modeling. Earth and Planetary Science Letters, 289(1-2), 145-155.More infoAbstract: The late Triassic to early Tertiary Coast Mountains Batholith (CMB) of British Columbia provides an ideal locale to study the processes whereby accreted terranes and subduction-related melts interact to form stable continental crust of intermediate to felsic composition and complementary ultramafic residuals. Seismic measurements, combined with calculated elastic properties of various CMB rock compositions, provide a window into the deep-crustal lithologies that are key to understanding the processes of continental growth and evolution. We use a combination of seismic observations and petrologic modeling to construct hypothetical crustal sections at representative locations across the CMB, then test the viability of these sections via forward modeling with synthetic seismic data. The compositions that make up our petrologic forward models are based on calculations using the free energy minimization program Perple_X to predict mineral assemblages at depth for the bulk compositions of exposed plutonic rocks collected in the study area. Seismic data were collected along two transects in west-central British Columbia: a southern line that crossed the CMB near the town of Bella Coola (near 52° N), and a northern line centered on the towns of Terrace and Kitimat (near 54° N). Along both transects, seismic receiver functions reveal high Vp/Vs ratios near the Insular/Intermontane terrane boundary and crustal thickness increasing from 26 ± 3 km to 34 ± 3 km (at the 1 sigma certainty level) from west to east across the Coast Shear Zone (CSZ). On the southern line, we observe an anomalous region of complex receiver functions and diminished Moho signals beneath the central portion of the CMB. Our petrologic and seismic profiles show that observed seismic data from much of the CMB can be well-matched in terms of crustal thickness and structure, average Vp/Vs, and amplitude of the Moho converted phase, without including ultramafic residual material in the lower crust. © 2009 Elsevier B.V. All rights reserved.
- Cecil, M. R., Ducea, M. N., Reiners, P., Gehrels, G., Mulch, A., Allen, C., & Campbell, I. (2010). Provenance of Eocene river sediments from the central northern Sierra Nevada and implications for paleotopography. Tectonics, 29(6).More infoAbstract: Geochronology of fluvial deposits can be used to characterize provenance, the paleotopography of sediment source regions, and the development of regional drainage systems. We present U-Pb and (U-Th)/He ages of detrital zircon grains from Eocene gravels preserved in several paleoriver systems along the western flank of the central and northern Sierra Nevada. These ages allow us to trace the sourcing of detritus in paleorivers and to constrain the evolution of the Sierra Nevada range front. U-Pb zircon age distributions are bimodal, with a dominant peak between 110 and 95 Ma and smaller but significant peaks in the Middle to Late Jurassic, matching the predominant ages of the Sierra Nevada batholith. A small fraction (
- Davis, S. J., Dickinson, W. R., Gehrels, G. E., Spencer, J. E., Lawton, T. F., & Carroll, A. R. (2010). The Paleogene California River: Evidence of Mojave-Uinta paleodrainage from U-Pb ages of detrital zircons. Geology, 38(10), 931-934.More infoAbstract: U-Pb age spectra of detrital zircons in samples from the Paleogene Colton Formation in the Uinta Basin of northeastern Utah and the Late Cretaceous McCoy Mountains Formation of southwestern Arizona (United States) are statistically indistinguishable. This finding refutes previous inferences that arkosic detritus of the Colton was derived from cratonic basement exposed by Laramide tectonism, and instead establishes the Cordilleran magmatic arc (which also provided sediment to the McCoy Mountains Formation) as the primary source. Given the existence of a north-south-trending drainage divide in eastern Nevada and the north-northeast direction of Laramide paleoflow throughout Arizona and southern Utah, we infer that a large river system headed in the arc of the Mojave region flowed northeast ~700 km to the Uinta Basin. Named after its source area, this Paleogene California River would have been equal in scale but opposite in direction to the modern Green River-Colorado River system, and the timing and causes of the subsequent drainage reversal are important constraints on the tectonic evolution of the Cordillera and the Colorado Plateau. © 2010 Geological Society of America.
- Dickinson, W. R., & Gehrels, G. E. (2010). Insights into North American Paleogeography and Paleotectonics from U-Pb ages of detrital zircons in Mesozoic strata of the Colorado Plateau, USA. International Journal of Earth Sciences, 99(6), 1247-1265.More infoAbstract: Individual U-Pb ages for 5,655 detrital zircons (DZ) in 61 sandstone samples from Mesozoic strata of the Colorado Plateau and nearby areas provide insights into paleogeographic relations across the interior of North America and the paleotectonic evolution of North American continental margins. Pre-Mesozoic DZ grains derived either directly, or ultimately through sediment recycling, from distant sources in eastern North America are more abundant than DZ grains derived from the nearby Cordilleran magmatic arc of western North America. Sediment dispersal patterns included Triassic fluvial transport of detritus westward from the Ouachita orogen uplifted along the northern flank of rift highlands precursor to the oceanic Gulf of Mexico, Jurassic eolian transport southward into widespread ergs from deflation of floodplains of transcontinental paleorivers with headwaters in pre-Atlantic Appalachian highlands, and Jurassic-Cretaceous recycling of eolianite DZ from retroarc Sevier thrust sheets and from sedimentary cover of the Mogollon paleohighlands flanking the Border rift system. © 2009 Springer-Verlag.
- Dickinson, W. R., Gehrels, G. E., & Marzolf, J. E. (2010). Detrital zircons from fluvial Jurassic strata of the Michigan basin: Implications for the transcontinental Jurassic paleoriver hypothesis. Geology, 38(6), 499-502.More infoAbstract: The U-Pb age spectrum of detrital zircons in sandstone of the fluvial Middle Jurassic Ionia Formation in the Michigan basin, United States, resembles the age spectra in Jurassic eolianites of the Colorado Plateau, except that a Neoproterozoic (725-510 Ma) subpopulation present in the eolianites is absent from the Michigan basin strata. The detrital zircon data are compatible with the hypothesis that the Ionia Formation was deposited by northern tributaries of a transcontinental Jurassic paleoriver system that transported detritus from headwaters in Atlantic rift highlands of the Appalachian region to floodplains and deltas in the present northern Rocky Mountains region. Previous detrital zircon studies have indicated that sedimented lowlands in that paleogeographic position were deflated by paleowinds to feed eolian sand southward (in present coordinates) into the Colorado Plateau ergs (sand seas). © 2010 Geological Society of America.
- Dickinson, W. R., Gehrels, G. E., & Marzolf, J. E. (2010). Detrital zircons from fluvial Jurassic strata of the Michigan basin: Implications for the transcontinental Jurassic paleoriver hypothesis: Reply. Geology, 38(12), e229-e230.
- Dickinson, W. R., Gehrels, G. E., & Stern, R. J. (2010). Late Triassic Texas uplift preceding Jurassic opening of the Gulf of Mexico: Evidence from U-Pb ages of detrital zircons. Geosphere, 6(5), 641-662.More infoAbstract: We use U-Pb ages for 2655 individual detrital zircon grains in 30 samples of Upper Triassic (Carnian-Norian) sandstones of the southwestern USA (our data) and northern Mexico (other data) to infer regional Late Triassic provenance relations and tectonic features, which included a prerift uplift in Texas precursory to Jurassic opening of the Gulf of Mexico. Detrital zircons in sandstones from central paleorivers of the Chinle-Dockum fluvial system on the High Plains (USA) and Colorado Plateau, and from the marine terminus of the fluvial system along the eastern flank of the Auld Lang Syne backarc basin in the Great Basin, reflect regional dispersal of sediment from the Ouachita orogen and adjacent Mesoproterozoic basement exposed on the northern flank of the rift uplift. The composite grain population of Ouachita-derived sands displays dominant U-Pb age peaks at 1100-1050 Ma (Grenvillian) and subordinate Neoproterozoic (630-550 Ma) and Paleozoic (475-400 Ma) age peaks inferred to document recycling of sand grains from the Ouachita system. The Ouachita detritus was supplemented by contributions from Mesoproterozoic basement rocks of southwest Laurentia (compound 1805-1655 Ma and unitary 1440 Ma age peaks) and from Permian-Triassic arc assemblages of northeastern Mexico (composite 285-215 Ma age peak). Master Chinle-Dockum paleodrainages extended east-southeast-west-northwest for 2000 km from Texas to Nevada, flowing along the axis of a backarc trough formed by dynamic subsidence behind the Cordilleran magmatic arc, from headwaters in the Texas prerift uplift to the sediment trap of the backarc marine basin where the Auld Lang Syne Group accumulated. Sandstones from peripheries of the linked fluvial and marine depositional systems contain contrasting populations of detrital zircons derived either from the relict Amarillo-Wichita uplift, which fed abundant Cambrian zircons derived from an uplifted aulacogen-floor igneous assemblage, to selected basal units of the fluvial system, or from the Cordilleran arc assemblage to the south and west. The youngest Dockum sample from the Ouachita foreland on the High Plains contains abundant Devonian zircons probably derived from the Ouachita metamorphic core zone now present only in the Texas subsurface. Sandstones along the southern flank of the Chinle backarc basin on the southern Colorado Plateau contain populations of detrital zircons similar to those in sandstones of the Antimonio-Barranca forearc basin (Sonora) that was farther south beyond the arc axis. Sediment transported longitudinally from the Texas uplift to the El Alamar paleoriver and Potosí subsea fan of northeastern Mexico contains populations of detrital zircons similar but not identical to those in Chinle-Dockum fluvial strata of the High Plains shed transversely from the rift uplift. © 2010 Geological Society of America.
- Dickinson, W. R., Stair, K. N., Gehrels, G. E., Peters, L., Kowallis, B. J., Blakey, R. C., Amar, J. R., & Greenhalgh, B. W. (2010). Geological note: U-Pb and 40ar/39ar ages for a tephra lens in the middle jurassic page sandstone: First direct isotopic dating of a mesozoic eolianite on the colorado plateau. Journal of Geology, 118(2), 215-221.More infoAbstract: Joint U-Pb and 40Ar/39Ar geochronology for pyroclastic zircon and biotite crystals from tephra lenses intercalated within eolianite of the Middle Jurassic Page Sandstone on the Colorado Plateau provides the first direct isotopic age for a Jurassic eolianite on the Colorado Plateau and indicates a chronostratigraphic age of 171.5-169.5 Ma for the Page Sandstone. With stated uncertainties in the age boundaries of global stratigraphic stages taken into account, the isotopic age of lower Bajocian is coordinate with the inferred biostratigaphic age of upper Bajocian from lateral correlation of Page Sandstone with equivalent marine strata. Comparative isotopic ages imply that the Page Sandstone was deposited during the same time frame as the Temple Cap Formation, inferred conventionally to be older, and that the J1 and J2 surfaces of the Colorado Plateau are therefore not reliable chronostratigraphic marker horizons. © 2010 by The University of Chicago.
- Ducea, M. N., Otamendi, J. E., Bergantz, G., Stair, K. M., Valencia, V. A., & Gehrels, G. E. (2010). Timing constraints on building an intermediate plutonic arc crustal section: U- Pb zircon geochronology of the Sierra Valle Frtil-La Huerta, Famatinian arc, Argentina. Tectonics, 29(4).More infoAbstract: The Sierra Valle Frtil Range in northwestern Argentina exposes a tilted crustal section through the Ordovician Famatinian arc, from >25 km to shallow crustal paleodepths. Fourteen new U-Pb zircon crystallization ages of magmatic rocks from Sierra Valle Frtil area show that this section of the arc was built over a short time interval during the Ordovician, between 485 and 465 Ma. Zircon rim ages demonstrate that high-grade metamorphism and migmatization were synchronous with magmatic emplacement. Inherited ages in some of the plutonic rocks as well as detrital zircons in the metasedimentary framework suggest that the Famatinian arc was emplaced into a thick miogeoclinal cover to the thinned margin of the proto-South American continent in the Ordovician, which represents a part of Gondwana. Docking of the Precordilleran terrane outboard of proto-South America led to the cessation of arc magmatism in the Valle Frtil area and preservation of the arc in its early stages after
- Hampton, B. A., Ridgway, K. D., & Gehrels, G. E. (2010). A detrital record of Mesozoic island arc accretion and exhumation in the North American Cordillera: U-Pb geochronology of the Kahiltna basin, southern Alaska. Tectonics, 29(4).More infoAbstract: The stratigraphic record of Mesozoic arc accretion in the North American Cordillera is preserved in a discontinuous belt of clastic strata that are exposed inboard (cratonward) of the allochthonous Wrangellia composite terrane in southern Alaska, western Canada, and Washington State. LA-ICPMS analyses of eight samples (n = 714 detrital zircon grains) collected at different stratigraphic intervals from the Jurassic-Cretaceous Kahiltna assemblage in southern Alaska reveals a bulk U-Pb age distribution of Precambrian-Mesozoic age grains (Mz 74%, Pz 11%, Pc 15%). A comparison of U-Pb ages from older to younger stratigraphic intervals within the Kahiltna assemblage reveals three stages of exhumation and basin development during arc accretion. Stages include (1) an initial Late Jurassic-Early Cretaceous pre/early collisional phase during which detritus was derived almost solely from Middle-Late Jurassic and Early Cretaceous magmatic sources of the outboard Wrangellia composite terrane (Mz 100%, Pz 0%, Pc, 0%), (2) a second Early Cretaceous syncollisional phase that reflects the introduction of Paleozoic and Precambrian detritus from the inboard Intermontane belt (Mz 84%, Pz 11%, Pc 5%) and an upsection increase in older detrital zircon grains compared to Mesozoic age grains (Mz 65%, Pz 11%, Pc 24%), and (3) a final Early-Late Cretaceous late/postcollisional phase that represents continued detrital contributions from Precambrian-Mesozoic source areas (Mz 19%, Pz 22%, Pc 59%) located inboard and outboard of the Kahiltna basin. Similar bulk trends in detrital zircon age populations have been reported from along-strike, age-equivalent strata of the Gravina belt (Mz 74%, Pz 20%, Pc 6%) in southeastern Alaska suggesting that similar provenance trends may exist in basins along this >2000 km -long collisional zone. © 2010 by the American Geophysical Union.
- Horstwood, M., Gehrels, G., & Bowring, J. (2010). Improving consistency in laser ablation geochronology. Eos, 91(28), 247-.More infoAbstract: Workshop on Data Handling in LA-ICP-MS U-Th-Pb Geochronology; San Francisco, California, 12-13 December 2009; The use of uranium-thorium-lead (U-Th-Pb) laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) geochronology involves rapid analysis of U-and Th-rich accessory minerals. It routinely achieves 1-2% precision for U-Th-Pb dates constituting detrital mineral age spectra and for dating igneous and metamorphic events. The speed and low setup and analysis cost of LA-ICP-MS U-Th-Pb geochronology has led to a proliferation of active laboratories. Tens of thousands of analyses are produced per month, but there is little agreement on how to transform these data into accurate U-Th-Pb dates. Recent interlaboratory blind comparisons of zircon samples indicate that resolvable biases exist among laboratories and the sources of bias are not fully understood. Common protocols of data reduction and reporting are essential for scientists to be able to compare and interpret these data accurately.
- Johnston, S. M., Hartz, E. H., Brueckner, H. K., & Gehrels, G. E. (2010). U-Pb zircon geochronology and tectonostratigraphy of southern Liverpool Land, East Greenland: Implications for deformation in the overriding plates of continental collisions. Earth and Planetary Science Letters, 297(3-4), 512-524.More infoAbstract: The East Greenland Caledonides formed in the overriding plate as Baltica was subducted westward beneath Laurentia from 460 to 360. Ma, and offer a unique opportunity to investigate lower crustal deformation in the overriding plates of continental collisions. Field work and new zircon geochronology from gneisses in southern Liverpool Land, exposed in the hinterland ~. 100. km east of the nearest Caledonian gneisses, define three tectonostratigraphic units that are, from the bottom up, the eclogite. +. peridotite-bearing Tværdal complex and the granulite-facies Jættedal complex in the footwall of the top-N Gubbedalen shear zone, and the Hurry Inlet granite and associated paragneiss screens in its immediate hangingwall. Zircons from Tværdal complex gneisses yield metamorphic rims that cluster in age from 409 to 401. Ma and overgrow magmatic cores of 1674 and 1665. Ma in two samples, and range from ~. 1800-1000. Ma in a third sample. In contrast, zircons from three samples in the Jættedal complex and two samples in the paragneiss screens of the Hurry Inlet granite yield metamorphic rims that cluster in age from 438 to 417. Ma with Archean-Early Neoproterozoic detrital cores. A cross-cutting granitic dike in the Jættedal complex yields an age of 394. Ma. Archean-Early Neoproterozoic detrital zircons associated with ~. 440-420. Ma metamorphism in the Liverpool Land paragneisses suggests correlation with the Krummedal sequence and the Hagar Bjerg thrust sheet of Laurentian affinity. 1670. Ma cores in the Tværdal complex, and ~. 400. Ma eclogite-facies metamorphism, allow correlation of the Tværdal complex with the Western Gneiss Region in Norway, and it may therefore be of Baltican affinity. Furthermore, the contact between the older Jættedal complex with the younger Tværdal complex requires the existence of a structure, named the Ittoqqortoormiit shear zone herein, which juxtaposed these rocks prior to the initiation of normal-sense slip along the Gubbedalen shear zone. This work provides geochronologic evidence for continental underplating of the overriding plate by the subducting plate during orogenesis, and supports models for high-pressure exhumation in continental collisional settings that identify separate structures associated with initial emplacement in the lower-middle crust and subsequent upper-crustal exhumation. © 2010 Elsevier B.V.
- Lawton, T. F., Hunt, G. J., & Gehrels, G. E. (2010). Detrital zircon record of thrust belt unroofing in Lower Cretaceous synorogenic conglomerates, central Utah. Geology, 38(5), 463-466.More infoAbstract: U-Pb detrital zircon (DZ) ages (n = 807) from Lower Cretaceous and lowermost Upper Cretaceous synorogenic conglomerate and interbedded sandstone deposited in and near the foredeep of the Cordilleran foreland basin in central Utah indicate stratigraphic compositional variation among the deposits. Eight DZ age populations, ranging from Archean through Mesozoic, are present in the synorogenic foredeep deposits in varying proportions and permit definition of three compositional suites, termed here chronofacies. Chronofacies A, present in uppermost Neocomian-lowermost Aptian foredeep deposits, contains Archean through early Paleozoic DZ grains derived from Jurassic-Pennsylvanian strata of the thrust belt. Chronofacies B, in Aptian-Albian foredeep deposits, contains Archean and Paleoproterozoic grains with an age peak near 1850 Ma, a population distribution similar to that of lower Paleozoic quartzites of the thrust belt. Chronofacies C, in Albian-lower Cenomanian foredeep deposits, contains a trimodal population distribution of Paleoproterozoic and Mesoproterozoic grains similar to that of Cambrian-Neoproterozoic quartzite strata of the thrust belt. The chronofacies of the foredeep deposits thus record systematic erosional unroofing of the thrust belt during Early Cretaceous time. © 2010 Geological Society of America.
- Leier, A. L., Mcquarrie, N., Horton, B. K., & Gehrels, G. E. (2010). Upper oligocene conglomerates of the Altiplano, Central Andes: The record of deposition and deformation along the margin of a Hinterland basin. Journal of Sedimentary Research, 80(8), 750-762.More infoAbstract: The Altiplano Plateau is a high-elevation, internally drained basin located in the hinterland of the Central Andean fold-thrust belt of Bolivia. Cenozoic strata exposed along the margins of the basin provide a unique record of deposition and deformation in this region and can also be applied to understanding synorogenic sedimentation in other fold-thrust belts. We examined Oligocene conglomerate units deposited along the margin of the nascent Altiplano in an effort to better understand how this large hinterland basin has evolved, and examine the relationship between upper crustal deformation and sedimentation in the interior of the Central Andean fold-thrust belt. Facies associations indicate initial deposition occurred in alluvial-fan and braidplain settings, and growth strata in these units record syndeformational deposition. Facies associations in overlying units contain a greater proportion of fine-grained material and were deposited in isolated fluvial channels surrounded by well drained-floodplain deposits. These units are less deformed relative to the alluvial-fan and braidplain deposits, suggesting that deformation waned as deposition continued. Clast counts and paleocurrents indicate that much of the Oligocene sediment was locally derived from Paleozoic strata and deposited in semi-isolated basins located between major thrusts and folds. Uranium-lead ages of detrital zircons from the conglomerate beds correspond to those from surrounding Paleozoic strata, supporting the hypothesis that most of sediment in the Oligocene conglomerate beds were derived directly from these older units. The youngest population of detrital zircon ages in the conglomerate beds are identical to the inferred ages of the deposits themselves. Collectively, the data indicate that the eastern margin of the Altiplano during Oligocene time was dominated by active upper crustal deformation, with alluvial-fan and braidplain deposition occurring in topographic lows. As deformation waned, deposition shifted from alluvial fans to isolated fluvial channels surrounded by extensive floodplains. In the regional setting, Upper Oligocene sediments exposed along the eastern margin of the Altiplano represent the remnants of a feeder zone to large fluvial distributary systems that occupied and infilled the center of the Altiplano. Copyright © 2010, SEPM (Society for Sedimentary Geology).
- Memeti, V., Gehrels, G. E., Paterson, S. R., Thompson, J. M., Mueller, R. M., & Pignotta, G. S. (2010). Evaluating the Mojave-Snow Lake fault hypothesis and origins of central Sierran metasedimentary pendant strata using detrital zircon provenance analyses. Lithosphere, 2(5), 341-360.More infoAbstract: Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) detrital zircon age data and detailed mapping of metasedimentary and metavolcanic pendants in the central Sierra Nevada are used to examine the age and origin of the metasediments, and to search for evidence of the location and history of the Cretaceous Mojave-Snow Lake fault. Quartzites from the Snow Lake, Benson Lake, May Lake, and Quartzite Peak pendants yield age spectra that best match Neoproterozoic to Ordovician passive-margin strata, thus supporting the presence of displaced passive-margin strata now preserved in Sierran pendants. Sediments at Cinko Lake, Strawberry Mine, and NE of Snow Lake are interpreted to be Early Jurassic to Early Cretaceous and marine, and probably not equivalents of the Fairview Valley Formation at Black Mountain as previously interpreted. Without this correlation, the suggested 400 km of displacement along the Mojave-Snow Lake fault is unconstrained, the exact location of origin for these passive-margin and Jurassic marine metasediments is uncertain, and the nature of the contact between these two sediment packages, which has been suggested to be an angular unconformity, is speculative and deserving of a more detailed evaluation. The timing of displacement along the inferred dextral Snow Lake fault is constrained to be between ca. 145 Ma, the maximum depositional age of Jurassic strata at Cinko Lake, and 102 Ma and 103-108 Ma, the age of the oldest intruding pluton and the youngest volcanic rocks juxtaposed along the stratigraphic break with adjacent eugeoclinal rocks. © 2010 Geological Society of America.
- Park, H., Barbeau Jr., D. L., Rickenbaker, A., Bachmann-Krug, D., & Gehrels, G. (2010). Application of foreland basin detrital-zircon geochronology to the reconstruction of the southern and central Appalachian orogen. Journal of Geology, 118(1), 23-44.More infoAbstract: We report the U-Pb age distribution of detrital zircons collected from central and southern Appalachian foreland basin strata, which record changes of sediment provenance in response to the different phases of the Appalachian orogeny. Taconic clastic wedges have predominantly ca. 1080-1180 and ca. 1300-1500 Ma zircons, whereas Acadian clastic wedges contain abundant Paleozoic zircons and minor populations of 550-700 and 1900-2200 Ma zircons consistent with a Gondwanan affinity. Alleghanian clastic wedges contain large populations of ca. 980-1080 Ma and ca. 2700 Ma and older Archean zircons and fewer Paleozoic zircons than occur in the Acadian clastic wedges. The abundance of Paleozoic detrital zircons in Acadian clastic wedges indicates that the Acadian hinterland consisted of recycled material and Taconic-aged plutons, which provided significant detritus to the Acadian foreland basin. The appearance of Pan-African/Brasiliano- and Eburnean/Trans-Amazonian-aged zircons in Acadian clastic wedges suggests a Devonian accretion of the Carolina terrane. In contrast, the relative decrease in abundance of Paleozoic detrital zircons coupled with an increase of Archean and Grenville zircons in Alleghanian clastic wedges indicates the development of an orogenic hinterland consisting of deformed passive margin strata and Grenville basement. The younging-upward age progression in Grenville province sources revealed in Taconic through Alleghanian successions suggest a reverse unroofing sequence that indicates at least two cycles of Grenville zircon recycling.
- Rioux, M., Mattinson, J., Hacker, B., Kelemen, P., Blusztajn, J., Hanghøj, K., & Gehrels, G. (2010). Intermediate to felsic middle crust in the accreted Talkeetna arc, the Alaska Peninsula and Kodiak Island, Alaska: An analogue for low-velocity middle crust in modern arcs. Tectonics, 29(3).More infoAbstract: Seismic profiles of several modern arcs have identified thick, low-velocity midcrustal layers (Vp = 6.0-6.5 km/s) that are interpreted to represent intermediate to felsic plutonic crust. The presence of this silicic crust is surprising given the mafic composition of most primitive mantle melts and could have important implications for the chemical evolution and bulk composition of arcs. However, direct studies of the middle crust are limited by the restricted plutonic exposures in modern arcs. The accreted Talkeetna arc, south central Alaska, exposes a faulted crustal section from residual subarc mantle to subaerial volcanic rocks of a Jurassic intraoceanic arc and is an ideal place to study the intrusive middle crust. Previous research on the arc, which has provided insight into a range of arc processes, has principally focused on western exposures of the arc in the Chugach Mountains. We present new U-Pb zircon dates, radiogenic isotope data, and whole-rock geochemical analyses that provide the first high-precision data on large intermediate to felsic plutonic exposures on Kodiak Island and the Alaska Peninsula. A single chemical abrasion-thermal ionization mass spectrometry analysis from the Afognak pluton yielded an age of 212.87 ± 0.19 Ma, indicating that the plutonic exposures on Kodiak Island represent the earliest preserved record of Talkeetna arc magmatism. Nine new dates from the extensive Jurassic batholith on the Alaska Peninsula range from 183.5 to 164.1 Ma and require a northward shift in the Talkeetna arc magmatic axis following initial emplacement of the Kodiak plutons, paralleling the development of arc magmatism in the Chugach and Talkeetna mountains. Radiogenic isotope data from the Alaska Peninsula and the Kodiak archipelago range from εNd(t) = 5.2 to 9.0 and 87Sr/86Srint = 0.703515 to 0.703947 and are similar to age-corrected data from modern intraoceanic arcs, suggesting that the evolved Alaska Peninsula plutons formed by extensive differentiation of arc basalts with little or no involvement of preexisting crustal material. The whole-rock geochemical data and calculated seismic velocities suggest that the Alaska Peninsula represents an analogue for the low-velocity middle crust observed in modern arcs. The continuous temporal record and extensive exposure of intermediate to felsic plutonic rocks in the Talkeetna arc indicate that evolved magmas are generated by repetitive or steady state processes and play a fundamental role in the growth and evolution of intraoceanic arcs. © 2010 by the American Geophysical Union.
- Shufeldt, O. P., Karlstrom, K. E., Gehrels, G. E., & Howard, K. E. (2010). Archean detrital zircons in the Proterozoic Vishnu Schist of the Grand Canyon, Arizona: Implications for crustal architecture and Nuna supercontinent reconstructions. Geology, 38(12), 1099-1102.More infoAbstract: U-Pb dating of 1035 detrital zircons from 12 spatially distributed samples of the Paleoproterozoic Vishnu Schist (Arizona, United States) reveals a bimodal 207Pb/206Pb age probability diagram with peaks at 1.8 Ga and 2.5 Ga. Surprisingly, only 13% of detrital zircon ages overlap with the published depositional age range of 1750-1741 Ma. The similarity of the age distributions in all samples constrains possible suturing of crustal blocks to pre-Vishnu Schist deposition rather than during the peak 1710-1680 Ma deformation. Of all grains analyzed, 15% overlap at 2σ with the 1.84 ± 1 Ga Elves Chasm orthogneiss of the western Grand Canyon. This supports field evidence that the Vishnu Schist was deposited on 1.84 Ga arc basement rather than in a juvenile 1.75 Ga arc setting. Archean grains of 3.8-2.5 Ga compose 30% of all grains. A comparison of the ages older than 2.2 Ga from the Vishnu Schist (495 grains) with compilations of zircon ages from other cratons does not support provenances in the Wyoming, South China, or Siberian cratons; instead sources may be located in Gawler craton of Australia, North China craton, or Antarctica. If the detrital zircons were far-traveled, this is a new constraint for viable reconstructions of the Nuna supercontinent. However, given the high percentage of pre-1.8 Ga zircons, unexposed proximal basement sources are more likely, resulting in a model by which Vishnu sediments were derived from Mojave province crust that consists of Archean and 1.9-1.8 Ga crust, now in the subsurface, that was unroofed during Vishnu deposition. © 2010 Geological Society of America.
- Tobgay, T., Long, S., McQuarrie, N., Ducea, M. N., & Gehrels, G. (2010). Using isotopic and chronologic data to fingerprint strata: Challenges and benefits of variable sources to tectonic interpretations, the Paro Formation, Bhutan Himalaya. Tectonics, 29(6).More infoAbstract: We combine detrital zircons (DZ) and epsilon neodymium (ε Nd) signatures with field mapping in the Paro Formation in western Bhutan. DZ age spectra are strongly variable and display signatures that have been used to uniquely identify both Greater Himalayan (GH) and Lesser Himalayan (LH) strata. DZ age peaks from six quartzite samples require sources for ∼0.5, 0.8, 1.2, 1.4, 1.7, 1.8, and 2.5 Ga zircons in the Paro Formation. The youngest (∼0.5 Ga) zircons argue for a Cambrian maximum deposition age. Two samples have a youngest 1.8 Ga peak typically attributed to Paleoproterozoic LH rocks. A ∼450 Ma crystallization age from two granite samples constrains the minimum deposition age as Ordovician. New εNd signatures from six detrital samples from the Paro Formation show significant variation with lithology. Schists have εNd(0) values between -12.0 and -16.9, while quartzite values vary between -18.8 and -24.5. These data imply that the Paro Formation was derived from both young and old sources, with DZ and εNd values obtained from the same quartzite samples requiring old detritus while the εNd values obtained from interbedded schist require younger detritus. Using published isotopic and chronologic definitions of Himalayan strata, schist-rich layers would be considered GH, while the interbedded quartzite would be LH. Thus, the Paro Formation refutes the generally accepted notion that different Himalayan tectonostratigraphic zones have unique DZ and εNd signatures. Our data recommend caution in the use of DZ and εNd signatures for tectonic interpretation, especially when making correlations with studies that extend 1000s of km along strike. Copyright 2010 by the American Geophysical Union.
- Valencia, V. A., Ducea, M., Talavera-Mendoza, O., Gehrels, G., Ruiz, J., & Shoemaker, S. (2010). Erratum to U-Pb geochronology of granitoids in the north-western boundary of the Xolapa Terrane [Rev. Mex. Cienc. Geol., 26 (2009), 189-200]. Revista Mexicana de Ciencias Geologicas, 27(2), 387-.
- Weislogel, A. L., Graham, S. A., Chang, E. Z., Wooden, J. L., & Gehrels, G. E. (2010). Detrital zircon provenance from three turbidite depocenters of the Middle-Upper Triassic Songpan-Ganzi complex, central China: Record of collisional tectonics, erosional exhumation, and sediment production. Bulletin of the Geological Society of America, 122(11-12), 2041-2062.More infoAbstract: To test the idea that the voluminous upper Middle to Upper Triassic turbidite strata in the Songpan-Ganzi complex of central China archive a detrital record of Dabie ultrahigh-pressure (UHP) terrane unroofing, we report 2080 single detrital U-Pb zircon ages by sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis from 29 eastern Songpan-Ganzi complex sandstone samples. Low (
- Yin, A., Dubey, C. S., Webb, A. A., Kelty, T. K., Grove, M., Gehrels, G. E., & Burgess, W. P. (2010). Geologic correlation of the Himalayan orogen and Indian craton: Part 1. Structural geology, U-Pb zircon geochronology, and tectonic evolution of the Shillong Plateau and its neighboring regions in NE India. Bulletin of the Geological Society of America, 122(3-4), 336-359.More infoAbstract: The Himalayan orogen has experienced intense Cenozoic deformation and widespread metamorphism, making it diffi cult to track its initial architecture and the subsequent deformation path during the Cenozoic India-Asia collision. To address this issue, we conducted structural mapping and U-Pb zircon geochronology across the Shillong Plateau, Mikir Hills, and Brahmaputra River Valley of northeastern India, located 30-100 km south of the eastern Himalaya. Our work reveals three episodes of igneous activity at ca. 1600 Ma, ca. 1100 Ma, and ca. 500 Ma, and three ductile-deformation events at ca. 1100 Ma, 520-500 Ma, and during the Cretaceous. The first two events were contractional, possibly induced by assembly of Rodinia and Eastern Gondwana, while the last event was extensional, possibly related to breakup of Gondwana. Because of its prox imity to the Himalaya, the occurrence of 500 Ma contractional deformation in northeastern India implies that any attempt to determine the magnitude of Cenozoic deformation across the Himalayan orogen using Proterozoic strata as marker beds must first remove the effect of early Paleozoic deformation. The lithostratigraphy of the Shillong Plateau established by this study and its correlation to the Himalayan units imply that the Greater Himalayan Crystalline Complex may be a tectonic mixture of Indian crystalline basement, its Proterozoic-Cambrian cover sequence and an early Paleozoic arc. Although the Shillong Plateau may be regarded as a rigid block in the Cenozoic, our work demonstrates that distributed active left-slip faulting dominates its interior, consistent with earthquake focal mechanisms and global positioning system velocity fields across the region. © 2010 Geological Society of America.
- Amato, J. M., Toro, J., Miller, E. L., Gehrels, G. E., Farmer, G. L., Gottlieb, E. S., & Till, A. B. (2009). Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions. Bulletin of the Geological Society of America, 121(9-10), 1219-1235.More infoAbstract: The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska-Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska-Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700-540 Ma, with prominent peaks at 720-660 Ma, 620-590 Ma, 560-510 Ma, 485 Ma, and 440-400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic-Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska-Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0-1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska-Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska-Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paleogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin. © 2009 Geological Society of America.
- Barbeau Jr., D. L., Davis, J. T., Murray, K. E., Valencia, V., Gehrels, G. E., Zahid, K. M., & Gombosi, D. J. (2009). Detrital-zircon geochronology of the metasedimentary rocks of north-western Graham Land. Antarctic Science, 22(1), 65-78.More infoAbstract: Metasedimentary rocks constitute an important but comparatively poorly understood part of the Antarctic Peninsula. Herein we report single-grain U-Pb detrital-zircon ages from samples of the Trinity Peninsula and Botany Bay Groups of north-western Graham Land. All studied samples are dominated by a large and narrowly defined population of late Palaeozoic zircons. Significant earlymiddle Palaeozoic and minor Neoproterozoic and Mesoproterozoic sub-populations constitute the majority of pre-Carboniferous grains. These detrital-zircon age populations are consistent with sediment derivation entirely from western Gondwana sources. Despite the clear Gondwana signatures, our data suggest that the Trinity Peninsula Group province was either a parautochthonous peri-Gondwanan terrane later accreted to the Antarctic Peninsula, or a significant topographic barrier precluded voluminous sediment contributions from the interior of Gondwana. Statistical comparisons with similar metasedimentary complexes of southern South America, the South Shetland Islands and eastern New Zealand indicate a diversity of sediment provenance not previously recognized, but may provide a means to better determine the pre-break-up configuration of western Gondwana. Although insufficient to definitively restore Antarctic Peninsula components adjacent to South American complexes, some Trinity Peninsula Group samples exhibit robust affinities to the Miers Bluff Formation in the South Shetland Islands and the Duque de York and Main Range Metamorphic Complexes of the Patagonian Andes. © 2009 Antarctic Science Ltd.
- Barbeau Jr., D. L., Gombosi, D. J., Zahid, K. M., Bizimis, M., Swanson-Hysell, N., Valencia, V., & Gehrels, G. E. (2009). U-Pb zircon constraints on the age and provenance of the rocas verdes basin fill, tierra del fuego, argentina. Geochemistry, Geophysics, Geosystems, 10(12).More infoAbstract: The Late Jurassic to Early Cretaceous Rocas Verdes basin constitutes one of the most poorly understood components of the southernmost Andes. As a result, accurate reconstructions and interpretations of deformation associated with the Andean orogeny and the kinematics of Scotia arc development also remain poorly constrained. In this data brief, we report U-Pb zircon ages from sandstones of the Rocas Verdes basin fill and from a crosscutting pluton in the southernmost Andes of Argentine Tierra del Fuego. Derrital samples contain predominant Early to early Middle Cretaceous (circa 130-105 Ma) U-Pb zircon age populations, with very small or single-grain middle Mesozoic and Proterozoic subpopulations. A very small subpopulation of Late Cretaceous ages in one sample raises the unlikely possibility that parts of the Rocas Verdes basin are younger than perceived. A sample from a crosscutting syenitic pegmatite yields a crystallization age of 74.7 +2.2/-2.0 Ma. The data presented herein encourage further geochronologic evaluation of the Rocas Verdes basin in order to better constrain the depositional ages and provenance of its contents. Copyright 2009 by the American Geophysical Union.
- Barbeau Jr., D. L., Olivero, E. B., Swanson-Hysell, N. L., Zahid, K. M., Murray, K. E., & Gehrels, G. E. (2009). Detrital-zircon geochronology of the eastern Magallanes foreland basin: Implications for Eocene kinematics of the northern Scotia Arc and Drake Passage. Earth and Planetary Science Letters, 284(3-4), 489-503.More infoAbstract: U/Pb detrital-zircon geochronology of eleven sandstones collected from Cretaceous through Oligocene strata of the eastern Magallanes foreland basin of southernmost Argentina records a dramatic provenance shift near the end of the middle Eocene at ca. 39 Ma. From the Late Cretaceous through most of the middle Eocene, detrital zircons reaching the foreland basin were dominantly contributed from ≤ 140 Ma sources, most likely derived from the Patagonian-Fuegian magmatic arc. In contrast, detrital-zircon populations of sampled upper Eocene and Oligocene strata are dominated by 150-190 Ma and pre-Mesozoic grains presumably derived from ignimbrites and granitoids associated with the break-up of Gondwana and from metasedimentary rocks of the Cordillera Darwin metamorphic complex, respectively. Exposures of these units occur in the hinterland of the Fuegian Andes thrust belt and inboard of the Patagonian-Fuegian batholith, suggesting that middle to late Eocene shortening in the rear of the Fuegian orogenic wedge structurally dammed batholith-derived sediment from reaching the foreland basin while contributing Jurassic and pre-Mesozoic detritus from hinterland thrust sheets. The timing of this interpreted deformation is in agreement with (1) independent structural, stratigraphic, and thermochronometric evidence of middle Paleogene deformation in the Fuegian Andes, and (2) marine Nd isotope ratio data that reveal initial penetration of Pacific water through the Drake Passage, thereby suggesting a possible link between the kinematics of the Fuegian Andes, the opening of Drake Passage, and if related, the Oi-1 glaciation of Antarctica. © 2009 Elsevier B.V. All rights reserved.
- Bovet, P. M., Ritts, B. D., Gehrels, G., Abbink, A. O., Darby, B., & Hourigan, J. (2009). Evidence of miocene crustal shortening in the North Qilian Shan from cenozoic stratigraphy of the Western Hexi Corridor, Gansu Province, China. American Journal of Science, 309(4), 290-329.More infoAbstract: New sedimentologic, stratigraphie, and compositional data from the Paleogene-Neogene stratigraphie succession exposed in the northwest Hexi Corridor and within the North Qilian Shan, provide evidence to suggest that crustal shortening in the North Qilian Shan fold-thrust belt initiated during die Miocene. The section is composed of four lithostratigraphic units: Oligocene-Miocene fine- to coarse-grained Unit 1, Miocene conglomeratic Unit 2, and Pliocene-Pleistocene conglomeratic Units 3 and 4. Unit 3 lies in angular unconformity over both Units 1 and 2, and Unit 4 contains a progressive unconformity. The onset of conglomerate deposition at the base of Unit 2 suggests an increase in depositional energy, which we interpret as me result of proximal orogenesis in the North Qilian Shan fold and tíirust belt. Supporting evidence includes the appearance of strongly northeast-trending paleocurrents, indicat-ing paleoflow away from the Qilian Shan, clast lidiologies that match sources in the North Qilian Shan, and sandstone with detrital framework modes that indicate a recycled orogen source. In contrast, Unit 1 contains paleocurrent indicators that are variable but generally trend northward and sandstone and clast compositions which are more diagnostic of a continental block source. Detrital zircon age determinations from Unit 1 are also not consistent with a source in the North Qilian Shan; rather, they suggest a provenance in hinterland regions within the South Qilian Shan and North Qaidam terranes. In sum, these results are all consistent with initiation of proximal uplift of the North Qilian Shan during deposition of the gradational transition from Unit 1 to Unit 2, demonstrating shortening in the Qilian Shan before die late Miocene. This comprehensive study tightens our understanding of when far-field stress related to the India-Eurasia continent-continent collision reached the northeastern edge of the Tibetan Plateau.
- Cina, S. E., Yin, A., Grove, M., Dubey, C. S., Shukla, D. P., Lovera, O. M., Kelty, T. K., Gehrels, G. E., & Foster, D. A. (2009). Gangdese arc detritus within the eastern Himalayan Neogene foreland basin: Implications for the Neogene evolution of the Yalu-Brahmaputra River system. Earth and Planetary Science Letters, 285(1-2), 150-162.More infoAbstract: In order to assess the spatial and temporal extent of sediment transport from the Gangdese batholith of Tibet to the eastern Himalayan Neogene foreland basin, we performed U-Pb and Lu-Hf analyses on eleven sandstone samples from three locations within the Arunachal and Sikkim Himalaya. We also analyzed detrital zircons from eight modern river sand samples of the Yalu-Brahmaputra River system and its major tributaries in the eastern Himalaya. Results from the river sands are used to contrast the provenance characteristics of the Gangdese arc in southern Tibet with nominally equivalent arc rocks east of the Himalaya in the northernmost Indo-Burma Ranges. Our results indicate that the deposition of Gangdese batholith-derived sediment within the eastern Himalayan foreland: (1) occurred throughout Late Miocene and Pliocene time (~ 10-3 Ma), (2) was limited to the Arunachal Himalaya, and (3) was sourced north of the Himalaya. This detritus may have been deposited by a transverse Himalayan river, such as the Subansiri River, as suggested by high percentages of the Gangdese-derived zircons within the Neogene samples (15-31%) and S- to SW-oriented paleocurrent directions from two of the Neogene sample localities. At this time, our preferred model to explain the data invokes capture of an originally westward-flowing Yalu River by the Subansiri River at ~ 10 Ma, followed by capture of the Yalu River by the Siang River at ~ 3-4 Ma. © 2009 Elsevier B.V. All rights reserved.
- Crawford, M. L., Klepeis, K. A., Gehrels, G. E., & Lindline, J. (2009). Mid-Cretaceous-Recent crustal evolution in the central Coast orogen, British Columbia and southeastern Alaska. Special Paper of the Geological Society of America, 456, 97-124.More infoAbstract: The Coast orogen of western coastal British Columbia and southeastern Alaska is one of the largest batholithic belts in the world. This paper addresses the structure and composition of the crust in the central part of this orogen, as well as the history of its development since the mid-Cretaceous. The core of the orogen consists of two belts of metamorphic and plutonic rocks: the western metamorphic and thick-skinned thrust belt comprising 105-90-Ma plutons and their metamorphic country rocks, and the Coast Plutonic Complex on the east, with large volumes of mainly Paleogene magmatic rocks and their high-temperature gneissic host rocks. These two belts are separated by the Coast shear zone, which forms the western boundary of a Paleogene magmatic arc. This shear zone is subvertical, up to 5 km wide, and has been seismically imaged to extend to and offset the Moho. Lithologic units west of the Coast shear zone record contractional deformation and crustal thickening by thrusting and magma emplacement in the mid-Cretaceous. To the east, the Coast Plutonic Complex records regional contraction that evolves to regional extension and coeval uplift and exhumation after ca. 65 Ma. Igneous activity in the Complex formed a Paleogene batholith and gave rise to high crustal temperatures, abundant migmatite and, as a result, considerable strain localization during deformation. In both belts, during each stage of the orogeny, crustal-scale deformation enabled and assisted magma transport and emplacement. In turn, the presence of magma, as well as its thermal effects in the crust, facilitated the deformation. After 50 Ma, the style of crustal evolution changed to one dominated by periods of extension oriented approximately perpendicular to the orogen. The extension resulted in tilting of large and small crustal blocks as well as intra-plate type magmatic activity across the orogen. Seismic-refl ection and refraction studies show that the crust of this orogen is unusually thin, probably due to the periods of orogen-perpendicular stretching. Magmatic activity west of the Coast shear zone in the Late Oligocene and Miocene was related to one period of orogen-parallel transtension along the margin. Small-scale, mafi c, mantle-derived volcanic activity continues in the region today. The change from convergence to translation and extension is related to a major plate reorganization in the Pacifi c that led to a change from subduction of an oceanic plate to northwestward translation of the Pacifi c plate along the northwest coast of North America. Although it has been proposed that this orogen is the site of major (up to 4000 km) pre-Eocene northward terrane translation, there is little evidence for such large-scale displacement or for the kind of discontinuity in the geological record that such displacement would entail. © 2009 The Geological Society of America.
- Dickinson, W. R., & Gehrels, G. E. (2009). U-Pb ages of detrital zircons in Jurassic eolian and associated sandstones of the Colorado plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment. Bulletin of the Geological Society of America, 121(3-4), 408-433.More infoAbstract: U-Pb ages for 1655 individual detrital zircon grains in 18 samples of eolian and associated marine and fluvial sandstones of the Glen Canyon and San Rafael Groups from the Colorado Plateau and contiguous areas shed light on patterns of Jurassic sediment dispersal within Laurentia. Most detrital zircon grains in Jurassic eolianites were derived ultimately from basement provinces older than 285 Ma in eastern and central Laurentia, rather than from rock assemblages of the nearby Cordilleran margin. The most prominent peaks of constituent age populations at 420 Ma, 615 Ma, 1055 Ma, and 1160 Ma reflect derivation from Paleozoic, Neoproterozoic, and Grenvillian sources within the Appalachian orogen or its sedimentary cover. Sediment was transported to a position upwind to the north of the Colorado Plateau by a transcontinental paleoriver system with headwaters in the central to southern Appalachian region, but subordinate non-Appalachian detritus was contributed by both northern and southern tributaries during sediment transit across the continent. Subordinate detrital zircons younger than 285 Ma in selected Middle to Upper Jurassic eolianites were derived from the Permian-Triassic East Mexico and the Mesozoic Cordilleran magmatic arcs. Lower Jurassic fluvial sandstones typically contain a mixture of detrital zircons redistributed from eolian sand and derived from the East Mexico arc, which lay up-current to the southeast. Zircons in marine Curtis sandstone were largely reworked from underlying Entrada eolianite, with minor contributions from the Jurassic backarc igneous assemblage of the Great Basin. Once mature quartzose detritus was dispersed widely across southwest Laurentia by a transcontinental paleoriver system and paleowinds, which deposited extensive Jurassic ergs, durable zircon grains were recycled by multiple intraregional depositional systems. Lower Jurassic fluvial sand is locally composed, however, of detritus derived from the nearby Cordilleran magmatic arc assemblage and its Precambrian basement. © 2008 Geological Society of America.
- Dickinson, W. R., & Gehrels, G. E. (2009). Use of U-Pb ages of detrital zircons to infer maximum depositional ages of strata: A test against a Colorado Plateau Mesozoic database. Earth and Planetary Science Letters, 288(1-2), 115-125.More infoAbstract: We test the research strategy of using youngest U-Pb ages of detrital zircons to constrain the maximum depositional ages of strata containing the zircon grains by comparing U-Pb ages of detrital zircons in 58 samples of Mesozoic sandstone from the Colorado Plateau and adjacent areas with depositional ages known independently from biostratigraphy. Our analysis confirms the validity of the research strategy but indicates that results vary somewhat depending upon how youngest grain age is specified. We use four alternate measures of youngest age which vary from least to most statistically robust as follows: (a) youngest single grain age, (b) youngest graphical age peak controlled by more than one grain age; (c) mean age of the youngest two or more grains that overlap in age at 1σ, (d) mean age of the youngest three or more grains that overlap in age at 2σ. We also calculated the "youngest detrital zircon age" generated by Isoplot 2008 but do not recommend that model age as a youngest-age measure. In general, the youngest-age measures based on multiple grain ages are more consistently compatible with depositional ages, but the youngest single grain ages are compatible with depositional age for > 90% of samples, and lie within 5 Ma of depositional age for ~60% of samples. Selected minor discrepancies between youngest grain age and depositional age may reflect stratigraphic miscorrelations rather than errors in U-Pb geochronology. © 2009 Elsevier B.V. All rights reserved.
- Dickinson, W. R., Lawton, T. F., & Gehrels, G. E. (2009). Recycling detrital zircons: A case study from the Cretaceous Bisbee Group of southern Arizona. Geology, 37(6), 503-506.More infoAbstract: Statistically indistinguishable U-Pb age spectra for detrital zircon populations from the quartzose petrofacies of the fl uvial Lower Cretaceous Bisbee Group (southeastern Arizona) and from quartzose eolianites of the Middle to Upper Jurassic San Rafael Group (eastern Colorado Plateau) support the inference that quartzose detritus was reworked into the Bisbee basin of the border rift belt from older eolianites exposed along the uplifted rift shoulder to the north. The comparative data indicate that age spectra of detrital zircons can be reproduced faithfully during sediment recycling, and the possibility of zircon recycling should be considered for many provenance interpretations based on U-Pb ages of detrital zircons. Analogous relations between U-Pb age spectra of detrital zircons in the Lower Cretaceous basal McCoy Mountains Formation (California) and in Lower to Middle Jurassic eolianites of the western Colorado Plateau to the north suggest similar reworking of detritus from sedimentary cover of the rift shoulder into the keel of the rift belt farther west. © 2009 Geological Society of America.
- Druschke, P., Hanson, A. D., Wells, M. L., Rasbury, T., Stockli, D. F., & Gehrels, G. (2009). Synconvergent surface-breaking normal faults of Late Cretaceous age within the Sevier hinterland, east-central Nevada. Geology, 37(5), 447-450.More infoAbstract: The hinterland of the Sevier orogenic belt of western North America is widely interpreted as a Cretaceous to Paleogene orogenic plateau. Although evidence for mid-crustal extension of Late Cretaceous age within the Sevier hinterland is widespread, coeval surface-breaking normal fault systems have not been documented. New 1:12,000-scale mapping within the type section of the latest Cretaceous to Eocene Sheep Pass Formation of east-central Nevada suggests that deposition occurred in response to normal fault movement recording up to 4 km of Late Cretaceous and Paleogene stratigraphic throw. Intrabasinal normal faulting caused lateral thickness variations within the basal Sheep Pass Formation, although upper members are largely unaffected. An extensional basin setting best explains the fanning of bedding dips, the deposition of megabreccia, and the presence of syndepositional normal faults within the Sheep Pass Formation. Deposition of the basal member of the Sheep Pass Formation is bracketed between ca. 81.3 ± 3.7 Ma and 66.1 ± 5.4 Ma, based on the (U-Th)/He cooling ages of detrital zircons, and on a U-Pb carbonate age derived from the overlying lacustrine limestone member. These new data provide the strongest evidence to date for the existence of Late Cretaceous, surface-breaking normal faults in the Sevier hinterland. Normal faulting was coeval with mid-crustal hinterland extension and with continued contraction within the Sevier foreland to the east. © 2009 Geological Society of America.
- Gehrels, G. E., Rusmore, M., Woodsworth, G., Crawford, M., Andronicos, C., Hollister, L., Patchett, J., Ducea, M., Butler, R., Klepeis, K., Davidson, C., Friedman, R., Haggarf, J., Mahoney, B., Crawford, W., Pearson, D., & Girardi, J. (2009). U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution. Bulletin of the Geological Society of America, 121(9-10), 1341-1361.More infoAbstract: Previously published and new U-Pb geochronologic analyses provide 313 zircon and 59 titanite ages that constrain the igneous and cooling history of the Coast Mountains batholith in north-coastal British Columbia. First-order findings are as follows: (1) This segment of the batholith consists of three portions: a western magmatic belt (emplaced into the outboard Alexander and Wrangellia terranes) that was active 177-162 Ma, 157-142 Ma, and 118-100 Ma; an eastern belt (emplaced into the inboard Stikine and Yukon-Tanana terranes) that was active ca. 180-110 Ma; and a 100-50 Ma belt that was emplaced across much of the orogen during and following mid-Cretaceous juxtapo sition of outboard and inboard terranes. (2) Magmatism migrated eastward from 120 to 80 (or 60) Ma at a rate of 2.0-2.7 km/Ma, a rate similar to that recorded by the Sierra Nevada batholith. (3) Magmatic flux was quite variable through time, with high (>35-50 km3/Ma per km strike length) flux at 160-140 Ma, 120-78 Ma, and 55-48 Ma, and magmatic lulls at 140-120 Ma and 78-55 Ma. (4) High U/Th values record widespread growth (and/or recrystallization) of metamorphic zircon at 88-76 Ma and 62-52 Ma. (5) U-Pb ages of titanite record rapid cooling of axial portions of the batholith at ca. 55-48 Ma in response to east-side-down motion on regional extensional structures. (6) The magmatic history of this portion of the Coast Mountains batholith is consistent with a tectonic model involving formation of a Late Jurassic-earliest Cretaceous magmatic arc along the northern Cordilleran margin; duplication of this arc system in Early Cretaceous time by >800 km (perhaps 1000-1200 km) of sinistral motion (bringing the northern portion outboard of the southern portion); high-flux magmatism prior to and during orthogonal mid-Cretaceous terrane accretion; low-flux magmatism during Late Cretaceous-Paleocene dextral transpressional motion; and high-flux Eocene magmatism during rapid exhumation in a regime of regional crustal extension. © 2009 Geological Society of America.
- González-León, C. M., Valencia, V. A., Lawton, T. F., Amato, J. M., Gehrels, G. E., Leggett, W. J., Montijo-Contreras, O., & Fernández, M. A. (2009). The lower Mesozoic record of detrital zircon U-Pb geochronology of Sonora, México, and its paleogeographic implications. Revista Mexicana de Ciencias Geologicas, 26(2), 301-314.More infoAbstract: Detrital zircon U-Pb geochronology from each of the formations of the Triassic-Lower Jurassic Barranca and El Antimonio groups of central and northwestern Sonora and from the Lower Jurassic Basomari and Middle Jurassic Lily formations of northern Sonora indicate they contain distinctive zircon populations. A Proterozoic population has peak ages near 1.8, 1.7, 1.6, 1.4, and 1.1 Ga. A population of Permo-Triassic grains with important peak ages near 269, 254, 245, 234 and 227 Ma. A third population of Early Jurassic age (~190 Ma) is only present in the middle member of the Lower Jurassic Sierra de Santa Rosa Formation and in the Basomari Formation. The fourth population of Middle Jurassic zircons with age peaks near 168 and 162 Ma is only present in the Lily Formation. A fifth population of Neoproterozoic and Paleozoic zircons, present only in the Basomari and Lily Formations, has Silurian and Devonian grains (~430 - 380 Ma) and Neoproterozoic grains (590 - 547 Ma). Possible source areas for these populations are Proterozoic igneous and metamorphic basement and/or Neoproterozoic and Paleozoic sandstones of southwestern USA and Sonora, the mostly Triassic magmatic arc of the Mojave Desert in California and the Permo-Triassic arc of northern Mexico, the Jurassic continental magmatic arc of southwestern North America, and the Jurassic eolian sand seas in Arizona. Regional lithofacies, fossils, and paleocurrents indicate that the Barranca Group records a large fluvio-deltaic system on the margin of the El Antimonio marine basin, and Proterozoic and Permo-Triassic zircon populations common to the Barranca and El Antimonio Groups indicate that detritus was derived from the same source areas to the north. Lithofacies, age, and detrital zircon populations of the Basomari and Lily Formations indicate that they were deposited within the Jurassic magmatic arc of North America. The Basomari and Lilly Formations contain abundant Early Jurassic zircon grains, as does the middle member of the Sierra de Santa Rosa Formation; however, the Sierra de Santa Rosa Formation lacks a Neoproterozoic and Paleozoic grain population present in the Basomari and Lily Formations. The Basomari Formation, which is located north of the proposed trace of the Mojave-Sonora megashear, contains Proterozoic granitic clasts derived from Caborcan basement, which suggests that the Caborca block must have been located close to the Basomari basin by Early Jurassic time, a relation that contradicts the existence of the Mojave-Sonora megashear. The new data also indicate a maximum Early Triassic depositional age for the previously undated Arrayanes Formation and correlation with the Antimonio Formation on the basis of a shared young detrital zircon peak age at ~254 Ma. A Coyotes Formation sample fails to confirm its supposed Early Jurassic age as it only yielded Proterozoic grains. Young zircon grain ages in the Lily Formation indicate a maximum Middle Jurassic age for that formation.
- Johnston, S., Gehrels, G., Valencia, V., & Ruiz, J. (2009). Small-volume U-Pb zircon geochronology by laser ablation-multicollector-ICP-MS. Chemical Geology, 259(3-4), 218-229.More infoAbstract: U-Pb zircon geochronology is hampered by problems acquiring meaningful geologic ages on zoned grains that retain isotope signatures from multiple growth or thermal events. We present a new method using laser ablation-multicollector-inductively coupled plasma-mass spectrometry to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12-14 μm in diameter by 4-5 μm in depth (< 3 ng of zircon). Using Channeltron multipliers to monitor Pb intensities in conjunction with a total ion counting method and errors calculated as function of the number of counts, the small-volume technique reproduced published ages on eight Mesoproterozoic-Cretaceous secondary zircon standards precise and accurate within 2%, and an age ∼ 1 Ma too young on a Oligocene-aged grain. Two initial applications of the small-volume technique - the detrital zircon provenance of fine-grained mudstones and shales and the creation of zircon U-Pb age maps to investigate the detrital and metamorphic history of a granulite-facies paragneiss - demonstrate the utility of this technique to a variety of geologic problems and confirm the viability of laser ablation-multicollector-inductively coupled plasma-mass spectrometry as a tool for high spatial resolution U-Pb geochronology.
- Lawton, T. F., Bradford, I. A., Vega, F. J., Gehrels, G. E., & Amato, J. M. (2009). Provenance of Upper Cretaceous-Paleogene sandstones in the foreland basin system of the Sierra Madre Oriental, northeastern Mexico, and its bearing on fluvial dispersal systems of the Mexican Laramide Province. Bulletin of the Geological Society of America, 121(5-6), 820-836.More infoAbstract: Sandstone petrography and detrital zircon U-Pb analysis of Upper Cretaceous - Paleogene sandstones in the foreland basin of the Sierra Madre Oriental in northeastern Mexico indicate long-distance sediment transport from arc, basement, and thrust-belt sources lying to the west, northwest, and south. The basin fill, termed the Difunta Group, consists of sublitharenites, litharenites, feldspathic litharenites, and lithic arkoses derived from mixed sources that included sedimentary rocks, magmatic arc rocks, and subordinate basement rocks. Six age populations comprise the detrital zircon content of the sandstones: Proterozoic (1900-900 Ma), early Paleozoic (500-400 Ma), late Paleozoic-Early Triassic (288-235 Ma), Jurassic (180-151 Ma), Early Cretaceous (150-111 Ma), and Late Cretaceous-Paleogene (110-54 Ma). These grains were derived from several arc terranes, ranging in age from Permian to Paleogene, in western Mexico and the southwestern United States, from sedimentary rocks and possibly interbedded tuffs of the Sierra Madre Oriental orogen and from basement sources or their derivative sandstones of the southwestern United States. The petrographic and geochronologic provenance data corroborate existing models for derivation of much foreland detritus from arc sources to the west, identify the Sierra Madre orogen itself as an important source for sediment, and these data modify the Late Cretaceous - Paleogene paleogeography of Mexico to include a long, orogen-parallel fluvial system with headwaters in the southwestern United States. The difference in average ages of the youngest grains in the sandstones and their inferred depositional ages is 10.5 m.y., indicating that the initial coarse fill of the foreland basin was derived from early Laramide uplift and eastward arc migration in northwestern Mexico and the southwestern United States. © 2009 Geological Society of America.
- Valencia, V. A., Ducea, M., Talavera-Mendoza, O., Gehrels, G., Ruiz, J., & Shoemaker, S. (2009). U-Pb geochronology of granitoids in the north-western boundary of the Xolapa Terrane. Revista Mexicana de Ciencias Geologicas, 26(1), 189-200.More infoAbstract: The Sierra Madre del Sur, a Mesozoic-Cenozoic magmatic arc in southern Mexico, was studied using U-Pb zircon geochronology. Undeformed to slightly deformed plutons from two transects were sampled at the limit between the Guerrero and Xolapa terranes, in order to constrain the magmatic history, nature of the basement and terrane boundaries. Four samples from the Zihuatanejo, Guerrero, transect within the Guerrero terrane, yielded crystallization ages of 41.8 ± 1.4, 43.4 ± 1.6, 40.8 ± 1.4 and, 41.8 ± 4.6 Ma. No inherited zircons were detected in these plutons indicating that pre-existing zircons from continental basement or sediments are not a significant component in these rocks. Five samples from the Atoyac, Guerrero transect within the Xolapa terrane, yielded crystallization ages of 53.5 ± 1.9, 52.7 ± 1.9, 57.3 ± 2.2, 54.4 ± 1.7, and 57.0 ± 2.1 Ma, analogous to the ages reported for the Acapulco intrusive. One sample of this transect yielded an age of 40.2 Ma with an inherited component of 58-64 Ma, similar to the ages determined for the first five samples. Several clusters of Mesozoic inherited zircons with ages of 72-74 Ma, 83-87 Ma, 90-92 Ma, 105-111 Ma and, 143-153 Ma, indicate that the magmatism in the Xolapa terrane was active since the Jurassic, and that multiple episodes of magmatism occurred during the Cretaceous. Inherited zircons also indicate that processes of assimilation and recycling of previous intrusive bodies have played an important role in the evolution of the Xolapa Complex. Older Paleozoic (~320 Ma; ~360 Ma) and Grenvillian (~960-1085 Ma) inherited zircons ages suggest an affinity of the Xolapa Complex with the Acatlán and Oaxaca Complexes, even though the metasedimentary basement of the Xolapa complex (of unknown age) may be the source of these Paleozoic and Grenvillian zircons. The presence of inherited zircons in the Atoyac transect suggests that the limit between the Xolapa and Guerrero terranes is located between these two transects.
- Vega-Granillo, R., Talavera-Mendoza, O., Meza-Figueroa, D., Ruiz, J., López-Martínez, M., & Gehrels, G. E. (2009). Pressure-temperature-time evolution of high-pressure rocks of the Acatlán Complex (southern Mexico): Implications for the evolution of the Iapetus and Rheic Oceans: Reply. Bulletin of the Geological Society of America, 121(9-10), 1460-1464.
- Amato, J. M., Boullion, A. O., Serna, A. M., Sanders, A. E., Farmer, G. L., Gehrels, G. E., & Wooden, J. L. (2008). Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico. Bulletin of the Geological Society of America, 120(3-4), 328-346.More infoAbstract: New U-Pb zircon ages, geochemistry, and Nd isotopic data are presented from three localities in the Paleoproterozoic Mazatzal province of southern New Mexico, United States. These data help in understanding the source regions and tectonic setting of magmatism from 1680 to 1620 Ma, the timing of the Mazatzal orogeny, the nature of postorogenic maginatism, Proterozoic plate tectonics, and provide a link between Mazatzal subblocks in Arizona and northern New Mexico. The data indicate a period from 1680 to 1650 Ma in which juvenile felsic granitoids were formed, and a later event between 1646 and 1633 Ma, when these rocks were deformed together with sedimentary rocks. No evidence of pre-1680 Ma rocks or inherited zircons was observed. The igneous rocks have ENd(t) from -1.2 to +4.3 with most between +2 and +4, suggesting a mantle source or derivation from similar-aged crust. Nd isotope and trace element concentrations are consistent with models for typical are magmatism. Detrital zircon ages from metasedimentary rocks indicate that sedimentation occurred until at least 1646 Ma. Both local and Yavapai province sources contributed to the detritus. All of the samples older than ca. 1650 Ma are deformed, whereas undeformed porphyroblasts were found in the contact aureole of a previously dated 1633 Ma gabbro. Regionally, the Mlazatzal orogeny occurred mainly between 1654 and 1643 Ma, during final accretion of a series of island arcs and intervening basins that may have amalgamated offshore. Rhyolite magmatism in the southern Mazatzal province was coeval with gabbro intrusions at 1633 Ma and this bimodal magmatism may have been related to extensional processes following arc accretion. © 2007 Geological Society of America.
- Dickinson, W. R., & Gehrels, G. E. (2008). Sediment delivery to the Cordilleran foreland basin: Insights from U-Pb ages of detrital zircons in Upper Jurassic and Cretaceous strata of the Colorado Plateau. American Journal of Science, 308(10), 1041-1082.More infoAbstract: In late Mesozoic time, the southern Cordilleran foreland basin was bounded on the west by the Sevier thrust belt and on the south by the Mogollon highlands. Paleocurrent indicators in fluvial and fluviodeltaic strata imply sediment delivery into the basin from both tectonic features. Ages of detrital zircons in sandstones of the basin provide insights into the nature of the sediment sources. Upper Jurassic and Lower Cretaceous fluvial strata were deposited as sediment blankets across the width of the basin but Upper Cretaceous marginal-marine facies were restricted to the basin margin, with marine facies in the basin interior. Most Upper Jurassic and Lower Cretaceous fluvial sandstones contain heterogeneous age populations of Precambrian and Paleozoic detrital zircons largely recycled from Jurassic eolianites uplifted within the Sevier thrust belt or antecedent highlands, and exposed as sedimentary cover over the Mogollon highlands, with only minor contributions of Mesozoic zircon grains from the Cordilleran magmatic arc along the continental margin. Sources in Yavapai-Mazatzal Proterozoic basement intruded by anorogenic Mesoproterozoic plutons along the Mogollon highlands were significant for the Westwater Canyon Member of the Upper Jurassic Morrison Formation and for early Upper Cretaceous (Turonian) fluviodeltaic depositional systems, in which arc-deirived Cordilleran zircon grams are more abundant than in older and younger units composed dominantly of recycled detritus. Detrital zircons confirm that the Salt Wash and Westwater Canyon Members of the Morrison Formation formed separate foreland megafans of different provenance. Late Upper Cretaceous (Campanian) fluvial sand- stones include units containing mostly recycled sand lacking arc-derived grains in the Sevier foredeep adjacent to the Sevier thrust front, and units derived from both Yavapai-Mazatzal basement and the Cordilleran arc farther east, with some mingling of sand from both sources at selected horizons within the Sevier foredeep. Evidence for longitudinal as well as transverse delivery of sediment to the foreland basin shows that paleogeographic and isostatic analyses of thrust-belt erosion, sediment loads, and basin subsidence in foreland systems need to allow for derivation of foreland sediment in significant volumes from sources lying outside adjacent thrust belts.
- Dickinson, W. R., & Gehrels, G. E. (2008). U-PB ages of detrital zircons in relation to paleogeography: Triassic paleodrainage networks and sediment dispersal across southwest Laurentia. Journal of Sedimentary Research, 78(12), 745-764.More infoAbstract: Integration of detrital zircon studies with data on biostratigraphy, isopachs, facies, and paleocurrents can improve reconstructions of paleogeography for ancient fluvial systems. U-Pb ages for 1808 individual detrital zircon grains in 20 samples of Triassic fluvial sandstone on the Colorado Plateau and High Plains, U.S.A., help constrain paleodrainage paths and patterns of sediment dispersal across southwest Laurentia. The dominant paleoflow was from southeast to northwest from sources as distant as the Ouachita orogen, and segments of Mesoamerica beyond it, to the Cordilleran margin in the Great Basin. Detrital zircons in Middle Triassic Moenkopi strata deposited along the distal eastern flank of the Sonoma foreland basin were derived from varied sources to the south and southeast, as were similar populations of detrital zircons in Upper Triassic Chinle strata exposed along the southern margin of the Colorado Plateau. Basal Chinle (-Dockum) strata of the High Plains to the east contain detrital zircon populations dominated by Cambrian grains derived from the nearby Amarillo-Wichita uplift, as do Gartra strata of northeast Utah to which the Cambrian grains were transported across eroded roots of Ancestral Rocky Mountains uplifts along upstream reaches of the Eagle paleoriver of northwest Colorado. The central locus of the Chinle-Dockum fluvial system lay along the tectonic furrow of an elongate backarc basin formed by dynamic subsidence inland from the Cordilleran magmatic arc. A lower Chinle-Dockum trunk paleoriver and the upper Chinle-Dockum Cottonwood paleovalley document longitudinal paleoflow parallel to the axis of the basin. Detrital zircon populations show that headwaters of both paleodrainages tapped the Ouachita orogen, the Permian-Triassic East Mexico arc, and associated rock assemblages of southwest Laurentia and adjacent Mesoamerica. Sediment sources in the Mesozoic Cordilleran arc became increasingly more prominent during Late Triassic time. Copyright © 2008, SEPM (Society for Sedimentary Geology).
- Gehrels, G. E., Valencia, V. A., & Ruiz, J. (2008). Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation-multicollector-inductively coupled plasma-mass spectrometry. Geochemistry, Geophysics, Geosystems, 9(3).More infoAbstract: U-Th-Pb geochronology by laser ablation-multicollector-inductively coupled plasma-mass spectrometry initiated during the mid to late 1990s as a reconnaissance tool, capable of generating ages of only moderate precision from relatively large volumes of zircon. New developments in instrumentation and experimental methodology, as described herein and by other researchers, now make it possible it to correct for common Pb accurately (using measured 204Pb), to acquire geochronologic information rapidly (30-40 unknowns/h), to generate U-Pb ages with an accuracy of better than 1% for most zircon standards, and to conduct analyses on much smaller (e.g., 10 μm by 6 μm) volumes of material. These capabilities are driving important advances in many aspects of Earth science research. Copyright 2008 by the American Geophysical Union.
- Grove, M., Bebout, G. E., Jacobson, C. E., Barth, A. P., Kimbrough, D. L., King, R. L., Zou, H., Lovera, O. M., Mahoney, B. J., & Gehrels, G. E. (2008). The Catalina Schist: Evidence for middle Cretaceous subduction erosion of southwestern North America. Special Paper of the Geological Society of America, 436, 335-361.More infoAbstract: The Catalina Schist underlies the inner southern California borderland of southwestern North America. On Santa Catalina Island, amphibolite facies rocks that recrystallized and partially melted at ca. 115 Ma and at 40 km depth occur atop an inverted metamorphic stack that juxtaposes progressively lower grade, high-pressure/temperature (PT) rocks across low-angle faults. This inverted metamorphic sequence has been regarded as having formed within a newly initiated subduction zone. However, subduction initiation at ca. 115 Ma has been diffi cult to reconcile with regional geologic relationships, because the Catalina Schist formed well after emplacement of the adjacent Peninsular Ranges batholith had begun in earnest. New detrital zircon U-Pb age results indicate that the Catalina Schist accreted over a ̃20 m.y. interval. The amphibolite unit metasediments formed from latest Neocomian to early Aptian (122-115 Ma) craton-enriched detritus derived mainly from the pre-Cretaceous wall rocks and Early Cretaceous volcanic cover of the Peninsular Ranges batholith. In contrast, lawsonite-blueschist and lower grade rocks derived from Cenomanian sediments dominated by this batholith's plutonic and volcanic detritus were accreted between 97 and 95 Ma. Seismic data and geologic relationships indicate that the Catalina Schist structurally underlies the western margin of the northern Peninsular Ranges batholith. We propose that construction of the Catalina Schist complex involved underthrusting of the Early Cretaceous forearc rocks to a subcrustal position beneath the western Peninsular Ranges batholith. The heat for amphibolite facies metamorphism and anatexis observed within the Catalina Schist was supplied by the western part of the batholith while subduction was continuous along the margin. Progressive subduction erosion ultimately juxtaposed the high-grade Catalina Schist with lower grade blueschists accreted above the subduction zone by 95 Ma. This coincided with an eastern relocation of arc magmatism and emplacement of the ca. 95 Ma La Posta tonalite-trondjhemite- granodiorite suite of the eastern Peninsular Ranges batholith. Final assembly of the Catalina Schist marked the initial stage of the Late Cretaceous-early Tertiary craton-ward shift of arc magmatism and deformation of southwestern North America that culminated in the Laramide orogeny. © 2008 The Geological Society of America.
- Grove, M., Gehrels, G. E., Cotkin, S. J., Wright, J. E., & Zou, H. (2008). Non-Laurentian cratonal provenance of Late Ordovician eastern Klamath blueschists and a link to the Alexander terrane. Special Paper of the Geological Society of America, 438, 223-250.More infoAbstract: The tectonic significance of early Paleozoic convergent-margin rocks of the Alexander and Sierran-Klamath terranes is poorly understood. New phengite 40Ar/39Ar and Rb-Sr results from the schist of Skookum Gulch of the Yreka subterrane in the Klamath Mountains (454 ± 10 Ma) confirm that blueschists are the oldest known subduction-zone rocks of the western North American Cordillera. The blueschists are juxtaposed with kilometer-scale tectonic blocks of ca. 565 Ma tonalite. Detrital zircons from the blueschists require close proximity to a diverse source of cratonal or derivative supracrustal rocks and preclude formation within an isolated intra-oceanic setting. This strong cratonal provenance (mostly 1.0-2.0 Ga, with resolved concentrations of 1.49-1.61 Ga zircon) is also exhibited by adjacent Early Devonian lower greenschist units of the Yreka subterrane (Duzel phyllite and Moffett Creek Formation). Additional results from temporally equivalent arc-derived sedimentary units (Sissel Gulch graywacke and Gazelle Formation) yield strongly unimodal zircon age distributions of early Paleozoic zircon. The results indicate that the Yreka sub-terrane formed at an Ordovician-Silurian- Early Devonian convergent margin near a Mesoproterozoic-Paleoproterozoic craton and Ediacaran crust. Appreciable 1.491.61 Ga zircon within the Yreka subterrane is compatible with a recent biogeographic analysis that indicates a non-Laurentian origin for the eastern Klamath terrane. Additional new data reveal that key early Paleozoic convergent-margin rocks within the northern Sierran-Klamath and Alexander terranes share similar arc and cratonal provenance, including 1.49-1.61 Ga zircon. We hypothesize that the rocks from all three areas are dispersed tectonic fragments that were derived from the same convergent margin and were independently transported to western North America. Of the oro-genic source regions indicated by previous paleomagnetic and biogeographic analysis, the detrital zircon provenance favors western Baltica over eastern Australia. © 2008 Geological Society of America.
- Kelty, T. K., Yin, A., Dash, B., Gehrels, G. E., & Ribeiro, A. E. (2008). Detrital-zircon geochronology of Paleozoic sedimentary rocks in the Hangay-Hentey basin, north-central Mongolia: Implications for the tectonic evolution of the Mongol-Okhotsk Ocean in central Asia. Tectonophysics, 451(1-4), 290-311.More infoAbstract: Understanding the development of the Central Asian Orogenic System (CAOS), which is the largest Phanerozoic accretionary orogen in the world, is critical to the determination of continental growth mechanisms and geological history of central Asia. A key to unraveling its geological history is to ascertain the origin and tectonic setting of the large flysch complexes that dominate the CAOS. These complexes have been variably interpreted as deep-marine deposits that were accreted onto a long-evolving arc against large continents to form a mega-accretionary complex or sediments trapped in back-arc to fore-arc basins within oceanic island-arc systems far from continents. To differentiate the above models we conducted U-Pb geochronological analyses of detrital-zircon grains from turbidites in the composite Hangay-Hentey basin of central Mongolia. This basin was divided by a Cenozoic fault system into the western and eastern sub-basins: the Hangay Basin in the west and Hentey basin in the east. This study focuses on the Hentey basin and indicates two groups of samples within this basin: (1) a southern group that were deposited after the earliest Carboniferous (∼ 339 Ma to 354 Ma) and a northern group that were deposited after the Cambrian to Neoproterozoic (∼ 504 Ma to 605 Ma). The samples from the northern part of the basin consistently contain Paleoproterozoic and Archean zircon grains that may have been derived from the Tuva-Mongol massif and/or the Siberian craton. In contrast, samples from the southern part of the basin contain only a minor component of early Paleozoic to Neoproterozoic zircon grains, which were derived from the crystalline basement bounding the Hangay-Hentey basin. Integrating all the age results from this study, we suggest that the Hangay-Hentey basin was developed between an island-arc system with a Neoproterozoic basement in the south and an Andean continental-margin arc in the north. The initiation of the southern arc occurred at or after the early Carboniferous, allowing accumulation of a flysch complex in a long-evolving accretionary complex. © 2007 Elsevier B.V. All rights reserved.
- King, D. S., Klepeis, K. A., Goldstein, A. G., Gehrels, G. E., & Clarke, G. L. (2008). The initiation and evolution of the transpressional Straight River shear zone, central Fiordland, New Zealand. Journal of Structural Geology, 30(4), 410-430.More infoAbstract: Structural data and U/Pb geochronology on zircon from central Fiordland, New Zealand show the role of pre-existing structural heterogeneities in the kinematic evolution of a newly discovered zone of transpression. The Straight River shear zone consists of steep zones of high strain that are superimposed onto older fabrics across a 10 × 80 km region. The older foliation formed during two periods of tectonism: contraction and magmatism of mostly Carboniferous (∼312-306 Ma) age and Early Cretaceous batholith emplacement ending by 113.4 ± 1.7 Ma followed by extension that ceased by 88.4 ± 1.2 Ma. The primary mechanism for the formation of steep shear zone foliations was the folding of these older fabrics. Conjugate crenulation cleavages associated with the folding record shortening at high angles to the shear zone boundaries. Fold axial surfaces and axial planar cleavages strike parallel to the shear zone with increasing strain as they progressively steepened to subvertical. In most areas, shear sense flips from oblique-sinistral (east-side-down component) to oblique-dextral (west-side-down) across zones of intermediate and high strain. High strain zones display subvertical mineral lineations, steep strike-slip faults and shear sense indicators that record strike-slip motion across the steep lineations. These patterns reflect triclinic transpression characterized by narrow zones of mostly strike-slip deformation and wide zones of mostly contraction. Zones of high strain align with offshore traces of late Tertiary strike-slip faults, suggesting that a previously undocumented component of late Tertiary shortening and strike-slip motion is accommodated within Fiordland. © 2007 Elsevier Ltd. All rights reserved.
- McQuarrie, N., Robinson, D., Long, S., Tobgay, T., Grujic, D., Gehrels, G., & Ducea, M. (2008). Preliminary stratigraphic and structural architecture of Bhutan: Implications for the along strike architecture of the Himalayan system. Earth and Planetary Science Letters, 272(1-2), 105-117.More infoAbstract: Preliminary mapping and stratigraphic correlation of Lesser Himalayan rock in eastern Bhutan using field characteristics, U-Pb detrital zircon dating, and e{open}Nd geochemistry define the first-order stratigraphic architecture of the Indian passive margin sequence in the eastern Himalaya. We use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern Himalayan fold-thrust belt in Bhutan. We propose that Lesser Himalayan rock in Bhutan can be divided into lower Lesser Himalayan rocks with a Paleoproterozoic detrital zircon signal, and upper Lesser Himalayan rocks with detrital zircon signals of ∼ 1000-500 Ma. The ∼ 500 Ma detrital grains are from rocks in the frontal portions of the fold-thrust belt north of the Main Boundary thrust as well as directly in the footwall of the Main Central thrust in the hinterland. Our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross-section through the Kuru Chu valley in eastern Bhutan which provides the first image of the geometry and amount of shortening through Bhutan. The Main Frontal thrust tilts a 6 km of thick section of Neogene foreland basin deposits. These deposits are separated from folded and faulted upper Lesser Himalayan rocks by the Main Boundary thrust. North of the Main Boundary thrust, we propose two duplex systems. The southernmost duplex contains 9 repeated sections of upper Lesser Himalayan units, including the Permian Gondwana Sequence and the Cambrian (or younger) Baxa Group. The northernmost duplex is located in the footwall of the Main Central thrust, and is comprised of two repeated sections of the Proterozoic Shumar and Daling Formations. The southern boundary of this northern duplex is the Shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the Ramgarh thrust in Nepal and India. We propose that the development of the duplex systems passively folded overlying Tethyan and Greater Himalayan rocks. Minimum shortening for this part of the Himalayan fold-thrust belt is 359 km, all of which has occurred from 22 Ma to present defining a long-term shortening rate of 16 mm/yr. © 2008 Elsevier B.V. All rights reserved.
- Miller, E. L., Soloviev, A., Kuzmichev, A., Gehrels, G., Toro, J., & Tuchkova, M. (2008). Jurassic and cretaceous foreland basin deposits of the Russian arctic: Separated by birth of the makarov basin?. Norsk Geologisk Tidsskrift, 88(4), 201-226.More infoAbstract: The age and mode of formation of the various sub-basins of the Amerasian Basin of the Arctic Ocean remain unknown. Jurassic-Cretaceous synorogenic foreland basin deposits are the youngest stratigraphic units deposited in the Russian Arctic prior to rifting and formation of the Amerasian Basin. U-Pb dating of detrital zircon suites (6 samples, ∼100 zircons each) by LA-ICP-MS reveal that sandstones in the New Siberian Islands have nearly identical sources to those in Chukotka and the South Anyui Zone (SAZ) despite evidence for proximal derivation and little transport. These include abundant Precambrian (∼2.1-1.7 Ga), Late Paleozoic (∼330-250 Ma) and lesser Mesozoic (∼175 to 145 Ma) ages; youngest zircons are likely derived from Main Belt granites in the N. Verkoyansk. The foreland basin and its counterpart the orogenic highlands either extended continuously for ∼1400 km along strike, or the localities studied were once much closer together. We hypothesize that rifting/extension associated with formation of the Makarov Basin and development of the SAZ as a transform fault might be one way of explaining the present separation of the study sites.
- Poole, F. G., Gehrels, G. E., & Stewart, J. H. (2008). Significance of detrital zircons in upper Devonian ocean-basin strata of the Sonora allochthon and Lower Permian synorogenic strata of the Mina Mexico foredeep, central Sonora, Mexico. Special Paper of the Geological Society of America, 442, 121-131.More infoAbstract: U-Pb isotopic dating of detrital zircons from a conglomeratic barite sandstone in the Sonora allochthon and a calciclastic sandstone in the Mina Mexico foredeep of the Minas de Barita area reveals two main age groups in the Upper Devonian part of the Los Pozos Formation, 1.73-1.65 Ga and 1.44-1.42 Ga; and three main age groups in the Lower Permian part of the Mina Mexico Formation, 1.93-1.91 Ga, 1.45-1.42 Ga, and 1.1-1.0 Ga. Small numbers of zircons with ages of 2.72-2.65 Ga, 1.30-1.24 Ga, ca. 2.46 Ga, ca. 1.83 Ga, and ca. 0.53 Ga are also present in the Los Pozos sandstone. Detrital zircons ranging in age from 1.73 to 1.65 Ga are considered to have been derived from the Yavapai, Mojave, and Mazatzal Provinces and their transition zones of the southwestern United States and northwestern Mexico. The 1.45-1.30 Ga detrital zircons were probably derived from scattered granite bodies within the Mojave and Mazatzal basement rocks in the southwestern United States and northwestern Mexico, and possibly from the Southern and Eastern Granite-Rhyolite Provinces of the southern United States. The 1.24-1.0 Ga detrital zircons are believed to have been derived from the Grenville (Llano) Province to the east and northeast or from Grenvilleage intrusions or anatectites to the north. Several detrital zircon ages ranging from 2.72 to 1.91 Ga were probably derived originally from the Archean Wyoming Province and Early Paleoproterozoic rocks of the Lake Superior region. These older detrital zircons most likely have been recycled one or more times into the Paleozoic sandstones of central Sonora. The 0.53 Ga zircon is believed to have been derived from a Lower Cambrian granitoid or meta-morphic rock northeast of central Sonora, possibly in New Mexico and Colorado, or Oklahoma. Detrital zircon geochronology suggests that most of the detritus in both samples was derived from Laurentia to the north, whereas some detritus in the Permian synorogenic foredeep sequence was derived from the evolving accretionary wedge to the south. Compositional and sedimentological differences between the continental-rise Los Pozos conglomeratic barite sandstone and the foredeep Mina Mexico calciclastic sandstone imply different depositional and tectonic settings. © 2008 The Geological Society of America.
- Prokopiev, A. V., Toro, J., Miller, E. L., & Gehrels, G. E. (2008). The paleo-Lena River - 200 m.y. of transcontinental zircon transport in Siberia. Geology, 36(9), 699-702.More infoAbstract: An immense wedge of Carboniferous to Jurassic siliciclastic strata accumulated on the Verkhoyansk passive margin of the Siberian craton. U-Pb ages of detrital zircons from Pennsylvanian to Middle Jurassic sandstones are remarkably consistent and show a systematic change in the proportion of age populations. Most zircons originated from the southern margin of Siberia, which was tectonically active in the Paleozoic and early Mesozoic, and were transported to the Verkhoyansk margin by a major transcontinental river system that existed for ∼200 m.y., the paleo-Lena River. Specific sources are the Angara-Vitim batholith of Transbaikalia (315 and 291-288 Ma age peaks), plutons of the Altay-Sayan region of the Central Asia fold belt (494-482 Ma), Proterozoic granitoids of northern Transbaikalia and the East Sayan Range (1888-1832 Ma), and minor contributions from the Siberian Platform and Aldan Shield (2900-2300 Ma). The paleo-Lena River met its demise when the Verkhoyansk margin was deformed in the Late Jurassic and Cretaceous, and sediment was diverted north to the Arctic Ocean. Thus, the life span of major transcontinental drainage systems can be comparable to that of the plate boundaries that surround them. © 2008 The Geological Society of America.
- Pullen, A., Kapp, P., Gehrels, G. E., Vervoort, J. D., & Ding, L. (2008). Triassic continental subduction in central tibet and Mediterranean-style closure of the Paleo-Tethys Ocean. Geology, 36(5), 351-354.More infoAbstract: The Qiangtang metamorphic belt (QMB) in central Tibet is one of the largest and most recently documented high-pressure (HP) to near-ultrahigh-pressure (near-UHP) belts on Earth. Lu-Hf ages of eclogite- and blueschist-facies rocks within the QNB an 244-223 Ma, indistinguishable from the age of UHP metamorphism in the Qinling-Dabie orogen. Results of a U-Pb detrital zircon study suggest that protoliths of the QMB include upper Paleozoic Qiangtang continental margin strata and sandstones that were derived from a Paleozoic arc terrane that developed within the Paleo-Tethys Ocean to the north. We attribute QMB HP metamorphism to continental collision between the Qiangtang terrane and a Paleo-Tethys arc terrane. This collision, and the coeval South China-North China collision, may have slowed convergence between Laurasia and Gondwana-derived terranes and initiated Mediterranean-style rollback and backarc basin development within much of the remnant Paleo-Tethys Ocean realm. © 2008 The Geological Society of America.
- Valencia, V. A., Eastoe, C., Ruiz, J., Ochoa-Landin, L., Gehrels, G., González-Leon, C., Barra, F., & Espinoza, E. (2008). Hydrothermal evolution of the porphyry copper deposit at La Caridad, Sonora, Mexico, and the relationship with a neighboring high-sulfidation epithermal deposit. Economic Geology, 103(3), 473-491.More infoAbstract: Four main stages of alteration and mineralization are present at La Caridad, Sonora, Mexico. In stage I, quartz veins are associated with orthoclase-anhydrite-biotite hydrothermal alteration within the La Caridad intrusive complex andA4th pervasive biotitization in andesites and diorites. A zone of propylitic alteration surrounds this biotitic zone. Weak molybdenite ± chalcopyrite ± magnetite ± pyrite ± sphalerite mineralization formed in stage I. stage II, the main hydrothermal mineralization event, is represented by quartz veins with pyrite-sericite + chlorite and is associated with chalcopyrite + pyrite ± molybdenite. In stage III, lead-zinc-silver mineralization formed in veins that were emplaced peripherally to the main system, locally overprinting the earlier events. In stage IV, intermediate-sulfidation mineralization, represented by quartz-tennantite-chalcopyrite-pyrite-sericite veinlets, formed in the core of the porphyry deposit system. The La Caridad Antigua mine, located -3 km east of the La Caridad orebody, has a high-sulfidation epithermal assemblage that includes pyrophillite, kaolinite, alunite, quartz, and harite as gangue, and a variety of sulfides including, chalcopyrite, pyrite, enargite, tetrahedrite-tennantite, and chalcocite. A new 206Pb/238U age of zircons from an associated porphyry stock at La Caridad Antigua is 55.0 ± 1.7 Ma, identical to previous U/Pb ages determined in the porphyry Cu deposit and consistent with a spatial and temporal link between La Caridad and La Caridad Antigua deposits. Fluid inclusion data indicate that stages I and II at La Caridad were deposited by saline hydrothermal fluid at 360° to 460° and 330° to 410°C, respectively. stages III and IV were deposited by low-salinity hydrothermal fluid at 33° to 370° and 260° to 320°C, respectively. oxygen and hydrogen isotopes indicate deposition of stages I, II, and IV either from magmatic vapor or from mixtures of magmatic and highly evaporated (lacustrine?) formation water and mixing with meteoric water in stages II and III. Sulfur and oxygen isotopes in stage 1 anhydrite indicate external derivation of sulfate and mixing of igneous and external water sources. © 2008 Society of Economic Geologists, Inc.
- Yin, A., Dang, Y., Wang, L., Jiang, W., Zhou, S., Chen, X., Gehrels, G. E., & McRivette, M. W. (2008). Cenozoic tectonic evolution of Qaidam basin and its surrounding regions (Part 1): The southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin. Bulletin of the Geological Society of America, 120(7-8), 813-846.More infoAbstract: Cenozoic Qaidam basin, the largest active intermountain basin inside Tibet, figures importantly in the debates on the history and mechanism of Tibetan plateau formation during the Cenozoic Indo-Asian collision. To determine when and how the basin was developed, we conducted detailed geologic mapping and analyses of a dense network of seismic reflection proffies from the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin. Our geologic observations provide new constraints on the timing and magnitude of Cenozoic crustal thickening in northern Tibet. Specifically, our work shows that the southernmost part of the Qilian Shan-Nan Shan thrust belt and contractional structures along the northern margin of Qaidam basin were initiated in the Paleocene-early Eocene (65-50 Ma), during or immediately after the onset of the Indo-Asian collision. This finding implies that stress was transferred rapidly through Tibetan lithosphere to northern Tibet from the Indo-Asian convergent front located >1000 km to the south. The development of the thrust system in northern Qaidam basin was driven by motion on the Altyn Tagh fault, as indicated by its eastward propagation away from the Altyn Tagh fault. The eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of Qaidam basin, suggesting thrust loading was the main control on the basin formation and evolution. The dominant structure in northern Qaidam basin is a southwest-tapering triangle zone, which started to develop since the Paleocene and early Eocene (65-50 Ma) and was associated with deposition of an overlying southwest-thickening, growth-strata sequence. Recognition of the triangle zone and its longevity in northern Qaidam basin explains a long puzzling observation that Cenozoic depocenters have been located consistently along the central aids of the basin. This basin configuration is opposite to the prediction of classic foreland-basin models that require the thickest part of foreland sediments deposited along basin edges against basin-bounding thrusts. Restoration of balanced cross sections across the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin suggests that Cenozoic shortening strain is highly inhomogeneous, varying from ∼20% to >60%, both vertically in a single section and from section to section across the thrust belt. The spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of Cenozoic rotations in different parts of Qaidam basin. The observed crustal shortening strain also implies that no lower-crustal injection or thermal events in the mantle are needed to explain the current elevation (∼3000-3500 m) and crustal thickness (45-50 km) of northern Qaidam basin and the southern Qilian Shan-Nan Shan thrust belt. Instead, thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern Qaidam basin and the southern Qilian Shan-Nan Shan region. © 2008 Geological Society of America.
- van, T., Hemming, S. R., Goldstein, S. L., Gehrels, G. E., & Cox, S. E. (2008). Evidence against a young volcanic origin of the Gamburtsev Subglacial Mountains, Antarctica. Geophysical Research Letters, 35(21).More infoAbstract: The Gamburtsev Subglacial Mountains of East Antarctica are among Earth's most enigmatic mountain ranges. They are situated in the middle of Antarctica, buried under hundreds of meters of ice, and reach an elevation of more than 3000 m. These characteristics have given rise to speculation about their origin and composition, in particular whether they are a hotspot or remnants of ancient orogenic events. We studied fluvio-deltaic sands from ODP Site 1166 in Prydz Bay which were deposited prior to the onset of East Antarctic glaciation to address the question of their origin. Results for U-Pb dating of detrital zircons reveal an important age population of ∼530 Ma, which is accompanied by a dominant 40Ar/39Ar age population of ∼519 Ma for detrital hornblendes. The data show no sign of young volcanic contributions, suggesting an old continental origin of the mountains, leaving as enigmatic the cause of their great height and size. Copyright 2008 by the American Geophysical Union.
- Amato, J. M., Bogar, M. J., Gehrels, G. E., Farmer, G. L., & McIntosh, W. C. (2007). The tlikakila complex in southern alaska: A suprasubduction-zone ophiolite between the wrangellia composite terrane and north america. Special Paper of the Geological Society of America, 431, 227-252.More infoAbstract: The Tlikakila complex is a northeast-striking ∼5-km-wide and ∼75-km-long belt of lower greenschist-facies sedimentary and igneous rocks in the Lake Clark region of south-central Alaska. It forms the only exposures of pre-Cretaceous rocks between the Peninsular terrane and the Farewell terrane. Protoliths include basalt, gabbro, ultramafic rocks, limestone, chert, mudstone, chert-pebble conglomerate, and minor quartz sandstone. Geochemical analyses of igneous rocks indicate primitive island arc compositions. Rare earth element (REE) patterns of the volcanic rocks and gabbro are flat with most elements between 5 and 15 times chondrite values. Initial 87Sr/86Sr isotope ratios range from 0.7042 to 0.7065. εNd ranges from +9.3 to +2.6. The complex is older than 192 Ma based on a 40Ar/ 39Ar date of white mica from a pegmatite vein in a metasedimentary rock and younger than ca. 293 Ma based on the youngest zircon in a chert-pebble conglomerate. Detrital zircon ages suggest source rocks included the Yukon-Tanana and Wrangellia-Alexander terranes. Previously reported Norian conodonts in nearby correlative limestone indicate at least some of the complex is Late Triassic. Metamorphism reached peak temperatures of 350-450 °C based on mineral assemblages in metapelite and metabasite. Three 40Ar/ 39Ar dates of biotite from metapelite indicate metamorphism at 177 ± 1 Ma. We interpret the Tlikakila complex as a dismembered suprasubductionzone ophiolite that originated near a trench above a north-dipping subduction zone in Late Triassic time. This subduction eventually created the Talkeetna arc. Asecond northdipping subduction zone closed the intervening basin between the Talkeetna arc and the southern Alaska continental margin. Metamorphism and deformation of the Tlikakila complex was coeval with either collision of the arc or shallowing of this slab at ca. 177 Ma. Copyright © 2007 The Geological Society of America.
- Amato, J. M., Rioux, M. E., Kelemen, P. B., Gehrels, G. E., Clift, P. D., Pavlis, T. L., & Draut, A. E. (2007). U-Pb geochronology of volcanic rocks from the jurassic talkeetna formation and detrital zircons from prearc and postarc sequences: Implications for the age of magmatism and inheritance in the talkeetna arc. Special Paper of the Geological Society of America, 431, 253-271.More infoAbstract: Six samples collected from pre-, syn-, and post-Talkeetna arc units in southcentral Alaska were dated using single-grain zircon LA MC-ICP-MS geochronology to assess the age of arc volcanism and the presence and age of any inherited components in the arc. The oldest dated sample comes from a volcanic breccia at the base of the Talkeetna Formation on the Alaska Peninsula and indicates that initial arc volcanism began by 207 ± 5 Ma. A sedimentary rock overlying the volcanic section in the Talkeetna Mountains has a maximum depositional age of
- Brown, E. H., & Gehrels, G. E. (2007). Detrital zircon constraints on terrane ages and affinities and timing of orogenic events in the San Juan Islands and North Cascades, Washington. Canadian Journal of Earth Sciences, 44(10), 1375-1396.More infoAbstract: Detrital zircon geochronology of this report pertains to Cretaceous orogeny in northwest Washington, an event that involved blueschist metamorphism and emplacement of nappes in the San Juan Islands - northwest Cascades thrust system and continental arc magmatism and associated Barrovian metamorphism in the neighboring Coast Plutonic Complex. Structurally low in the thrust system, quartzose gneiss of the Yellow Aster Complex yields an entirely Precambrian suite of detrital zircons, with an age pattern that is similar to that of Ordovician miogeoclinal rocks and the outboard Yukon-Tanana, Yreka, and Shoo Fly terranes elsewhere in the Cordillera. Midway in the nappe pile of the northwest Cascades, sandstone in the Bell Pass Mélange has a zircon age population of 110 Ma, an age that together with the spectrum of exotic materials associated with the sandstone suggests the mélange was a major zone of dislocation during mid-Cretaceous nappe emplacement. At a high level in the thrust system are nappes of the Fidalgo Complex, Lummi Formation, Constitution Formation, and Easton Metamorphic Suite, all with a prominent age peak in the range of 148-155 Ma. These units appear to be mutually related, represent inter-arc marginal basins, and are correlative with terranes in the western Klamath Mountains. The Nooksack Formation, foorwall to nappes in the Cascades, has a zircon population at 114 Ma, providing a maximum age of nappe emplacement. The Tonga Formation of the Coast Plutonic Complex bears zircons that indicate a depositional age of
- Dávila, F. M., Astini, R. A., Jordan, T. E., Gehrels, G., & Ezpeleta, M. (2007). Miocene forebulge development previous to broken foreland partitioning in the southern Central Andes, west-central Argentina. Tectonics, 26(5).More infoAbstract: In the distal region of the modern flat-slab segment in the southern Central Andes, an unusual stack of middle Miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west. These features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system. Sedimentary thickness farther east and far from the Cordilleran tectonic loads suggests accommodation and preservation driven by "nonisostatic" dynamic subsidence. Regional overlapping relationships and basin modeling suggest that the Modern broken foreland (present Sierras Pampeanas) can be interpreted as a reactivation of a formerly partitioned broad forebulge. Copyright 2007 by the American Geophysical Union.
- Fletcher, J. M., Grove, M., Kimbrough, D., Lovera, O., & Gehrels, G. E. (2007). Ridge-trench interactions and the Neogene tectonic evolution of the Magdalena shelf and southern Gulf of California: Insights from detrital zircon U-Pb ages from the Magdalena fan and adjacent areas. Bulletin of the Geological Society of America, 119(11-12), 1313-1336.More infoAbstract: The Magdalena fan is an apparently beheaded submarine depocenter that has figured prominently in reconstructions of middle to late Miocene Pacific-North American plate interactions. The deposit accumulated rapidly at the base of the continental slope on top of newly formed oceanic crust of the Magdalena microplate from 14.5 to 13 Ma. Subduction of this crust ceased as the Pacific-Magdalena spreading center encountered the trench. The widely accepted two-phase kinematic model for the formation of the Gulf of California holds that ∼300 km of dextral shear between the Pacific and North American plates occurred along faults west of Baja California prior to the onset of dextral-transtensional shearing in the gulf ca. 6 Ma. We measured 1796 detrital zircon U-Pb ages from 65 samples in an effort to characterize the provenance of the fan, determine its source region, and define the cumulative dextral slip along faults offshore of southwestern Baja California. Zircons from the fan are dominantly 120-65 Ma with subordinate 15-35 Ma grains. Excellent matches to the fan can be obtained by mixing Magdalena shelf strata and/or adding detritus from the west-draining portion of the Los Cabos block. The same cannot be accomplished with zircons from the east-draining portion of the Los Cabos block and mainland Mexico. Our results favor a western Baja source region for the fan and suggest that cumulative dextral slip along faults west of Baja was
- Gleason, J. D., Finney, S. C., Peralta, S. H., Gehrels, G. E., & Marsaglia, K. M. (2007). Zircon and whole-rock Nd-Pb isotopic provenance of Middle and Upper Ordovician siliciclastic rocks, Argentine Precordillera. Sedimentology, 54(1), 107-136.More infoAbstract: Graptolite-bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold-thrust belt record the disintegration of a long-lived Cambro-Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U-Pb detrital zircon, and Nd-Pb whole-rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo-lithic sandstones collected in the eastern and central Precordillera yield complex U-Pb zircon age spectra dominated by 1.05-1.10 Ga zircons, secondary populations of 1.22, 1.30, and 1.46 Ga, rare 2.2 and 1.8 Ga zircons, and a minor population (
- Gleason, J. D., Gehrels, G. E., Dickinson, W. R., Patchett, P. J., & Kring, D. A. (2007). Laurentian sources for detrital zircon grains in turbidite and deltaic sandstones of the Pennsylvanian Haymond Formation, Marathon Assemblage, West Texas, U.S.A.. Journal of Sedimentary Research, 77(11-12), 888-900.More infoAbstract: U-Pb ages of individual detrital zircon grains from deltaic and turbidite sandstones of the lower to middle Pennsylvanian Haymond Formation (Marathon basin, west Texas) range from 320 Ma to 3 Ga, indicating a mixed provenance broadly similar to that of Ouachita Carboniferous turbidites and related fluvio-deltaic facies of Arkansas-Oklahoma. Differences with the Ouachita assemblage are attributed to the unique depositional setting of the Haymond Formation, which records final closure of a Hercynian-age remnant ocean basin at the western termination of the Ouachita-Marathon suture zone. Tectonic and sedimentary recycling along subduction zones separating Gondwanan (upper plate) and Laurentian (lower plate) blocks likely contributed to the Laurentia-dominated distribution of age provinces observed in the zircon population. Detrital-zircon age spectra are nearly identical for both facies of the Haymond Formation, reinforcing the hypothesis that subaerial fan or braid delta sandstones, and foredelta submarine-ramp turbidites, were part of an integrated depositional system fed by the same sources. Copyright © 2007, SEPM (Society for Sedimentary Geology).
- Hampton, B. A., Ridgway, K. D., O'Neill, J. M., Gehrels, G. E., Schmidt, J., & Blodgett, R. B. (2007). Pre-, syn-, and postcollisional stratigraphic framework and provenance of upper triassic-upper cretaceous strata in the northwestern talkeetna mountains, alaska. Special Paper of the Geological Society of America, 431, 401-438.More infoAbstract: Mesozoic strata of the northwestern Talkeetna Mountains are located in a regional suture zone between the allochthonous Wrangellia composite terrane and the former Mesozoic continental margin of North America (i.e., the Yukon-Tanana terrane). New geologic mapping, measured stratigraphic sections, and provenance data define a distinct three-part stratigraphy for these strata. The lowermost unit is greater than 290 m thick and consists of Upper Triassic-Lower Jurassic mafic lavas, fossiliferous limestone, and a volcaniclastic unit that collectively we informally refer to as the Honolulu Pass formation. The uppermost 75 m of the Honolulu Pass formation represent a condensed stratigraphic interval that records limited sedimentation over a period of up to ca. 25 m.y. during Early Jurassic time. The contact between the Honolulu Pass formation and the overlying Upper Jurassic-Lower Cretaceous clastic marine strata of the Kahiltna assemblage represents a ca. 20 m.y. depositional hiatus that spans the Middle Jurassic and part of Late Jurassic time. The Kahiltna assemblage may to be up to 3000 m thick and contains detrital zircons that have a robust U-Pb peak probability age of 119.2 Ma (i.e., minimum crystallization age/maximum depositional age). These data suggest that the upper age of the Kahiltna assemblage may be a minimum of 10-15 m.y. younger than the previously reported upper age of Valanginian. Sandstone composition (Q-43% F-30% L-27%-Lv-71% Lm-18% Ls-11%) and U-Pb detrital zircon ages suggest that the Kahiltna assemblage received igneous detritus mainly from the active Chisana arc, remnant Chitina and Talkeetna arcs, and Permian-Triassic plutons (Alexander terrane) of the Wrangellia composite terrane. Other sources of detritus for the Kahiltna assemblage were Upper Triassic-Lower Jurassic plutons of the Taylor Mountains batholith and Devonian-Mississippian plutons; both of these source areas are part of the Yukon-Tanana terrane. The Kahiltna assemblage is overlain by previously unrecognized nonmarine strata informally referred to here as the Caribou Pass formation. This unit is at least 250 m thick and has been tentatively assigned an Albian-Cenomanian-to-younger age based on limited palynomorphs and fossil leaves. Sandstone composition (Q-65% F-9% L-26%-Lv-28% Lm-52% Ls-20%) from this unit suggests a quartz-rich metamorphic source terrane that we interpret as having been the Yukon-Tanana terrane. Collectively, provenance data indicate that there was a fundamental shift from mainly arc-related sediment derivation from sources located south of the study area during Jurassic-Early Cretaceous (Aptian) time (Kahiltna assemblage) to mainly continental margin-derived sediment from sources located north and east of the study area by Albian-Cenomanian time (Caribou Pass formation). We interpret the threepart stratigraphy defined for the northwestern Talkeetna Mountains to represent pre- (the Honolulu Pass formation), syn- (the Kahiltna assemblage), and post- (the Caribou Pass formation) collision of the Wrangellia composite terrane with the Mesozoic continental margin. A similar Mesozoic stratigraphy appears to exist in other parts of south-central and southwestern Alaska along the suture zone based on previous regional mapping studies. New geologic mapping utilizing the three-part stratigraphy interprets the northwestern Talkeetna Mountains as consisting of two northwest-verging thrust sheets. Our structural interpretation is that of more localized thrust-fault imbrication of the three-part stratigraphy in contrast to previous interpretations of nappe emplacement or terrane translation that require large-scale displacements. Copyright © 2007 The Geological Society of America.
- Kalbas, J. L., Ridgway, K. D., & Gehrels, G. E. (2007). Stratigraphy, depositional systems, and provenance of the lower cretaceous kahiltna assemblage, western alaska range: Basin development in response to oblique collision. Special Paper of the Geological Society of America, 431, 307-343.More infoAbstract: The Kahiltna assemblage of southern Alaska crops out in an 800-km-long belt that forms the core of much of the rugged Alaska Range. New sedimentologic, provenance, and geologic mapping data suggest that the Kahiltna assemblage exposed in the western Alaska Range represents a late Early Cretaceous to Late Cretaceous marine basin that formed in response to oblique collision between a composite island-arc terrane and the Mesozoic continental margin of North America. The Kahiltna assemblage in the study area crops out in two belts located north and south of the Denali fault system. Measured stratigraphic sections show that the Kahiltna assemblage in the southern outcrop belt has a minimum thickness of 5560 m and consists of eight siliciclastic lithofacies that represent tabular and weakly channelized mixed sand-mud submarine-fan systems that developed in a base-of-slope environment of deposition. Our analysis of the Kahiltna assemblage located north of the Denali fault indicates the presence of similar lithofacies along with additional strata that we interpret to represent outer-shelf and/or upperslope (slope apron) depositional environments. Geologic mapping for this study identified the depositional basement of both outcrop belts as UpperTriassic to Lower Jurassic marine-volcanic and volcaniclastic strata that form the upper part of the Mystic subterrane. Detrital zircon data constrain the depositional age of most of the Kahiltna assemblage in the study area to Early Cretaceous time (Aptian or Albian) or later and suggest a significantly younger timing of basin development than previously recognized. Compositional data indicate that sandstone and conglomerate of the Kahiltna assemblage were derived from both Mesozoic continental margin and composite island-arc terrane sources. Modal sandstone compositions (n = 41) are consistent with a mixed arc and recycled orogen provenance (Q23F 9L68; Qm11F9Lt80). Detrital zircons from sandstone collected in the lower part of the Kahiltna assemblage yield Precambrian (32%), Paleozoic (12%), and Mesozoic (56%) U-Pb ages. Concordant ages are consistent with the age distributions of Proterozoic, Devonian, Mississippian, and Triassic-Jurassic plutonic rocks of the former continental margin that formed the northern boundary of the Kahiltna basin. Plutons of the Talkeetna and Chisana arcs, part of the composite island-arc terrane located south of the basin, also probably contributed to the abundance of detrital zircons with ages between 200 and 163 Ma and between 124 and 106 Ma, respectively. Our new findings indicate that by Early Cretaceous time, the North American continental margin and composite island-arc terrane were in close enough proximity for both to contribute sediment to the Kahiltna basin. Stratigraphic, structural, and geochronologic relationships presented here, combined with previous regional studies of Mesozoic strata in the suture zone, suggest that the Kahiltna assemblage is the product of oblique island-arc terrane collision. Oblique collision resulted in the juxtaposition of continental margin and oceanic strata within thrust sheets along the closing suture zone. Dominantly west- and southwest-directed submarine-fan systems transported detritus axially away from the closing suture zone and into the along-strike marine basin represented by most of the Kahiltna assemblage exposed in the western Alaska Range. Comparisons with along-strike uplifted Mesozoic marine basins suggest westward time-transgressive closure of a suture zone that extends from British Columbia to southwestern Alaska. Copyright © 2007 The Geological Society of America.
- Klepeis, K. A., King, D., Paoli, M. D., Clarke, G. L., & Gehrels, G. (2007). Interaction of strong lower and weak middle crust during lithospheric extension in western New Zealand. Tectonics, 26(4).More infoAbstract: Exhumed sections of the middle and lower crust in western New Zealand reveal how deformation was partitioned within a thermally and rheologically evolving crustal column during Cretaceous continental extension. Structural data, P-T determinations, and U-Pb geochronology from central Fiordland and the Paparoa Range in Westland show that extension initiated in the lower crust by ∼ 114 Ma as a period of arc-related magmatism waned. Initially, deformation was localized into areas that were weakened by heat and magma. However, these hot, weak zones were ephemeral. During the perio 114-111 Ma, lower crustal fabrics record a rapid progression from magmatic flow to high-temperature deformation at the garnet-granulite facies (T > 700°C, P = 12 kbar) to cooler deformation at the upper amphibolite facies (T = 550-650°C, P = 7-9 kbar). Lower crustal cooling and compositional contrusts between mafic granulites and hydrous metasedimentary material resulted in a middle crust that was weak relative to the lower crust. Between circa 111 and circa 90 Ma, focused subhorizontal flow and vertical thinning in a weak middle crust led to the collapse of the upper crust and the unroofing of midcrustal material. During this period, arrays of conjugate-style shear zones transferred displacements vertically and horizontally through the crust, resulting in a structural style that resembles crustal-scale boudinage. The New Zealand example of continental extension shows that a weak middle crust and a relatively cool, highly viscous lower crust can result in a localized style of extension, including the formation of metamorphic core complexes that exhume the middle crust but not the lower crust. Copyright 2007 by the American Geophysical Union.
- Lease, R. O., Burbank, D. W., Gehrels, G. E., Wang, Z., & Yuan, D. (2007). Signatures of mountain building: Detrital zircon U/Pb ages from northeastern Tibet. Geology, 35(3), 239-242.More infoAbstract: Although detrital zircon has proven to be a powerful tool for determining provenance, past work has focused primarily on delimiting regional source terranes. Here we explore the limits of spatial resolution and stratigraphic sensitivity of detrital zircon in ascertaining provenance, and we demonstrate its ability to detect source changes for terranes separated by only a few tens of kilometers. For such an analysis to succeed for a given mountain, discrete intrarange source terranes must have unique U/Pb zircon age signatures and sediments eroded from the range must have well-defined depositional ages. Here we use ∼1400 single-grain U/Pb zircon ages from northeastern Tibet to identify and analyze an area that satisfies these conditions. This analysis shows that the edges of intermontane basins are stratigraphically sensitive to discrete, punctuated changes in local source terranes. By tracking eroding rock units chronologically through the stratigraphic record, this sensitivity permits the detection of the differential rock uplift and progressive erosion that began ca. 8 Ma in the Laji Shan, a 10-25-km-wide range in northeastern Tibet with a unique U/Pb age signature. © 2007 Geological Society of America.
- Manuszak, J. D., Ridgway, K. D., Trop, J. M., & Gehrels, G. E. (2007). Sedimentary record of the tectonic growth of a collisional continental margin: Upper jurassic-lower cretaceous nutzotin mountains sequence, eastern alaska range, alaska. Special Paper of the Geological Society of America, 431, 345-377.More infoAbstract: Upper Jurassic-Lower Cretaceous sedimentary strata of the Nutzotin basin, the Nutzotin Mountains sequence, crop out in the Nutzotin and Mentasta Mountains of the eastern Alaska Range. These strata represent one of the best-exposed and leastmetamorphosed examples of a basin that is interpreted to have formed during collision of an allochthonous volcanic arc (i.e., the Wrangellia terrane) with a continental margin. New stratigraphic, geologic mapping, and provenance data indicate that the Nutzotin basin formed as a retroarc foreland basin along the northern margin (present coordinates) of the Wrangellia terrane. Coeval with basin development along the northern margin, sedimentary basins and plutons located along the southern margin of the Wrangellia terrane were being incorporated into a regional fold-and-thrust belt. This fold-and-thrust belt, located south of the Nutzotin basin, exposed multiple structural levels of the Wrangellia terrane that were eroded and provided sediment that was transported northward and deposited in the Nutzotin basin. New sedimentologic and stratigraphic data from the ∼3 km thick (minimum thickness) Nutzotin Mountains sequence define a three-part stratigraphy. The lower part consists of Upper Jurassic (Oxfordian to Tithonian) conglomerate with outsized limestone clasts (>10 m in diameter) and interbedded sandstone and shale that grade basinward into mainly black shale with minor micritic limestone and isolated lenses of conglomerate. The middle part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) normal-graded sandstone and shale interbedded with massive tabular sandstone and lenticular conglomerate. The upper part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) mudstone with distinctive fossil-rich horizons and minor interbedded sandstone. The overall stratigraphy of the Nutzotin Mountains sequence represents a general upward-shallowing and upward-coarsening package that represents a general transition from distal mud-rich submarine-fan strata to more proximal sand-rich submarine-fan strata that are in turn overlain by marine shelf strata. Feldspathic sandstone compositions (Q6F67L27), eastward and northeastward directed paleocurrent indicators, diagnostic clasts in conglomerate, and detrital zircon U-Pb ages of 151- 147 Ma (n = 8) and 159-156 Ma (n = 2) indicate that sediment in the Nutzotin basin was derived primarily from the Wrangellia terrane and the Chitina and Chisana arcs that intrude the Wrangellia terrane. The stages of deformation documented in the Nutzotin Mountains sequence provide insight into the growth of collisional continental margins by the tectonic incorporation of basinal strata. Our data show that strata of the Nutzotin basin have been deformed into an accretionary wedge by north-dipping thrust faults and related overturned folds above a north-dipping décollement. Displacement on this décollement was the product of northward underthrusting of basinal strata beneath the former continental margin and resulted in southward tectonic transport of distal basinal strata of the Nutzotin Mountains sequence strata over both more proximal basinal strata and the Wrangellia terrane. Previously published K-Ar ages from plutons that crosscut both the décollement and folded Nutzotin Mountains sequence strata indicate that contractional deformation ended between 117 and 105 Ma. Regionally, the Nutzotin Mountains sequence represents part of a series of Mesozoic sedimentary basins located along the inboard margin of the Wrangellia composite terrane that have similar depositional styles and were all subsequently incorporated into accretionary wedges that dip toward the former continental margin. These deformed strata define a continentalscale suture zone that extends along the northwestern Cordillera for over 2000 km. Copyright © 2007 The Geological Society of America.
- Nelson, J., & Gehrels, G. (2007). Detrital zircon geochronology and provenance of the southeastern Yukon-Tanana terrane. Canadian Journal of Earth Sciences, 44(3), 297-316.More infoAbstract: Two samples of late Paleozoic grit and Late Mississippian quartzite-chert conglomerate collected from southeastern Yukon-Tanana terrane (YTT) - a composite thrust sheet resting structurally above North American parautochthonous strata and intervening imbricate sheets of the late Paleozoic oceanic Slide Mountain terrane - yielded, respectively, 89 and 74 concordant or nearly concordant (
- Rioux, M., Hacker, B., Mattinson, J., Kelemen, P., Blusztajn, J., & Gehrels, G. (2007). Magmatic development of an intra-oceanic arc: High-precision U-Pb zircon and whole-rock isotopic analyses from the accreted Talkeetna arc, south-central Alaska. Bulletin of the Geological Society of America, 119(9-10), 1168-1184.More infoAbstract: The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intra-oceanic arc crustal section. Arc-related units include all levels of a lithospheric column, from residual mantle harzburgites to sub-aerial volcanic rocks, and provide a rare opportunity to study intrusive arc processes directly. We present the first high-precision U-Pb zircon ages and an extensive new data set of 143Nd/144Nd and 87Sr/86Sr isotopic analyses from Talkeetna arc plutonic rocks. These data provide new insight into the timing and extent of Taikeetna arc magmatism, the tectonic development of the arc, and the role of preexisting crustal material in the generation of arc magmas. New analyses from the exposed arc crustal section in the Chugach Mountains indicate that the Talkeetna arc began to develop as a juvenile [εNd(t) = 6.0-7.8 and 87Sr/86Srint = 0.703379-0.703951] intra-oceanic arc between 202.1 and 181.4 Ma. This initial arc plutonism was followed ca. 180 Ma by a northward shift in the arc magmatic axis and generation of a large plutonic suite in the Talkeetna Mountains. Plutons from the eastern Talkeetna Mountains yield U-Pb zircon ages of 177.5-168.9 Ma and are isotopically similar to the Chugach Mountains intrusions [εNd(t) = 5.6-7.2 and 87Sr/86Srint = 0.703383-0.703624]. However, plutons from the western Taikeetna Mountains batholith have more evolved initial isotopic ratios [εNd(t) = 4.0-5.5 and 87Sr/86Srint = 0.703656-0.706252] and contain inherited xenocrystic Carboniferous-Triassic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into arc magmas and require amalgamation of the Talkeetna arc with the Wrangellia terrane by ca. 153 Ma. As a whole, the combined U-Pb zircon and isotopic data from the Chugach and Talkeetna Mountains indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of preexisting crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust. © 2007 Geological Society of America.
- Talavera-Mendoza, O., Ruiz, J., Gehrels, G. E., Valencia, V. A., & Centeno-García, E. (2007). Detrital zircon U/Pb geochronology of southern Guerrero and western Mixteca arc successions (southern Mexico): New insights for the tectonic evolution of southwestern North America during the late Mesozoic. Bulletin of the Geological Society of America, 119(9-10), 1052-1065.More infoAbstract: Late Jurassic-Cretaceous arc-related volcaniclastic rocks from the southern Guerrero and western Mixteca terranes of Mexico were analyzed by U-Pb detrital zircon geochronology (laser ablation-multicollector-inductively coupled plasma-mass spectroscopy) to place constraints on the depositional history and provenance of the rocks. Pre-Middle Jurassic basement rocks and sandstone from the Upper Cretaceous Mexcala Formation were also analyzed to define the origin and provenance of the prevolcanic substratum, and the time of accretion of Guerrero composite terrane sequences. Data from the Taxco-Taxco Viejo, Teloloapan, and Arcelia assemblages indicate that the youngest (129-141 Ma) zircon fraction in each sequence was derived from local volcanic sources, whereas older populations (ca. 247-317, 365-459, 530-617, 712-878, 947-964, 1112-1188, 1350-1420, 1842-1929, 2126-2439, and 2709-3438 Ma) show sediment influx from varied sources, most likely through grain recycling. The major zircon clusters in these sequences match the populations recorded in the nearby Acatán Complex. In contrast, the Huetamo sample is dominated by Lower Cretaceous (ca. 126 Ma) zircons of local volcanic provenance, and the Zihuatanejo sample contains zircon clusters (ca. 259, ca. 579, and ca. 947-1162Ma) comparable to major populations recorded in the underlying Arteaga Complex. A sample from the Middle Triassic-Middle Jurassic Arteaga Complex at Tzitzio contains zircon clusters (ca. 202-247, ca. 424, ca. 600, ca. 971, and ca. 2877 Ma) consistent with an ultimate derivation from both North American and South American sources. The sample from the Las Ollas suite contains comparable zircon populations (ca. 376-475, ca. 575, ca. 988-1141, and ca. 2642-2724 Ma), and it is interpreted to be part of the prevolcanic basement. In contrast, the youngest zircon cluster (ca. 105 Ma) in the Mexcala Formation coincides with the major volcanic events in the Taxco-Taxco Viejo, Teloloapan, and Arcelia assemblages, whereas the older clusters (ca. 600, ca. 953, ca. 1215, ca. 1913, and ca. 2656-2859 Ma) broadly match the major populations recorded in rocks from the Acatlán Complex. These new data combined with available geochemical and isotopic data indicate that the Taxco-Taxco Viejo arc assemblage developed on continental crust. The Acatlán Complex is the most plausible candidate. The Teloloapan and Arcelia arc assemblages were developed on oceanic crust as offshore arcs facing the Acatlán Complex. The Zihuatanejo terrane assemblages were developed on the Arteaga Complex, and evidence no influence from the Acatlán Complex. This suggests that these assemblages were formed farther away or in a restricted basin. The Guerrero composite and Mixteca arc successions are coeval with the Alisitos arc of northern Mexico and in part with the Nevada and Klamath ranges of the southwestern United States, and with the arc series from the Greater and Lesser Antilles and northern South America. Data indicate that during late Mesozoic time, southwestern North America was a site of intensive volcanism in a complex arc-trench system similar to that of the east Pacific. Our data are consistent with a diachronic accretion of the Guerrero composite terrane sequences, beginning during late Cenomanian time with the amalgamation of the Teloloapan and probably the Arcelia assemblages, and finishing at the end of Cretaceous time with the accretion of the Zihuatanejo terrane assemblages. © 2007 Geological Society of America.
- Vega-Granillo, R., Talavera-Mendoza, O., Meza-Figueroa, D., Ruiz, J., Gehrels, G. E., López-Martínez, M., & C., J. (2007). Pressure-temperature-time evolution of Paleozoic high-pressure rocks of the Acatlán Complex (southern Mexico): Implications for the evolution of the Iapetus and Rheic Oceans. Bulletin of the Geological Society of America, 119(9-10), 1249-1264.More infoAbstract: New thermobarometric and U/Pb and 40Ar/39Ar geochronologic data coupled with ages obtained from the Acatlán Complex, the basement of the Mixteco terrane of southern Mexico, reveal the existence of three distinctive high-pressure metamorphic events of early to middle Paleozoic age, each recorded in a separate lithologicai suite. Xayacatlán suite eclogites with oceanic affinity underwent peak metamorphism at 609-491 °C and 13-12 kb during the Early Ordovician (ca. 490-477 Ma, U-Pb zircon), followed by a partial overprint at 600 °C and ∼9.6 kb and then at 500 °C and ∼6.7 kb. An overprinting event at 525-500 °C and ∼9.5 kb is ascribed to the Devonian. The pressure-temperature (P-T) path of the Xayacatlán suite indicates a subduction-exhumation process followed by tectonically related reburial. Ixcamilpa suite blueschists with oceanic affinity underwent epidote-blueschist metamorphism (T, 200-390 °C; P, 6-9 kb) and then epidoteamphibolite (T, 390-580 °C; P, 9-6 kb) events ascribed to the Late Ordovician-Early Silurian. Esperanza suite eclogites with continental affinity underwent peak metamorphism at 830-730 °C and 17-15 kb. Amphibole from eclogite yields a 430 ± 5 Ma 49Ar/39Ar age, dating the high-pressure (HP) event. P-T paths of high-temperature (HT) eclogites like those of the Esperanza suite have been related to the collision of continental blocks. Partial overprinting occurred at 690-640 °C and 14-10 kb prior to 374 ± 2 Ma (40Ar/39Ar, phengite). The three HP suites were tectonically juxtaposed at different times before the Mississippian Period, resulting in the closure of the Iapetus Ocean. Phengite 40Ar/39Ar geochronology reveals the existence of a widespread tectonothermal event between 345 and 323 Ma, which may be related to the juxtaposition of the HP-composed block and the Gondwanan-affinity Cosoltepec suite, causing the closure of the Rheic Ocean. The tectonothermal events in the Acatlán Complex coincide in time, physical conditions, and tectonic setting with events in the Appalachian-Caledonian orogen, suggesting their relation. On that basis the geology of the Acatlán Complex can lead to a more comprehensive understanding of the tectonic evolution of the Appalachian orogen and of the Gondwana-Laurentia interactions preceding the Pangean assembly. © 2007 Geological Society of America.
- Walsh, E. O., Hacker, B. R., Gans, P. B., Grove, M., & Gehrels, G. (2007). Protolith ages and exhumation histories of (ultra)high-pressure rocks across the Western Gneiss Region, Norway. Bulletin of the Geological Society of America, 119(3-4), 289-301.More infoAbstract: The timing of protolith formation, ultrahigh-pressure (UHP) subduction, and subsequent exhumation for the ultrahigh-pressure to high-pressure units across the eastern part of the Western Gneiss Region, Norway, were assessed using U/Pb zircon, Th/Pb monazite, and 40Ar/39Ar white mica ages. U/Pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the Baltica basement before the entire mass was subducted to (ultra)high pressure. Eclogites within the allochthons across the entire Western Gneiss Region are Caledonian and show a degree of zircon (re)crystallization that increases with peak pressure, permitting the interpretation that the entire region underwent synchronous subduction. 40Ar/39Ar white mica ages of 399 Ma indicate that the eastern part of the Western Gneiss Region had been exhumed to shallow crustal levels while UHP metamorphism was ongoing farther west, indicating a westward dip to the slab. The 40Ar/39Ar white mica ages also show a clear east-to-west gradient across the entire Western Gneiss Region, indicating that the Western Gneiss Region rose diachronously to crustal levels from east to west between 399 and 390 Ma. © 2007 Geological Society of America.
- Yin, A., Manning, C. E., Lovera, O., Menold, C. A., Chen, X., & Gehrels, G. E. (2007). Early paleozoic tectonic and thermomechanical evolution of ultrahigh-pressure (UHP) metamorphic rocks in the Northern Tibetan Plateau, Northwest China. International Geology Review, 49(8), 681-716.More infoAbstract: Coesite- and diamond-bearing ultrahigh-pressure (UHP) metamorphic rocks represent continental materials that were once subducted to depths of >90 km. Identifying how these rocks were subsequently returned to Earth's surface has been a major challenge. Opinions on this matter vary widely, ranging from vertical extrusion of a coherent continental slab to channel flow of tectonically mixed mélange. To address this problem, we conducted integrated research across the North Qaidam UHP metamorphic belt using structural mapping, petrologic studies, and geochronologic and thermochronologic analyses. Our regional synthesis indicates that the early Paleozoic Qilian orogen, within which the North Qaidam UHP metamorphic belt was developed, was created by protracted southward oceanic subduction. The process produced a wide mélange belt and the Qilian magmatic arc. Arc magmatism was active between 520 and 400 Ma, coeval with North Qaidam UHP metamorphism. The North Qaidam UHP metamorphic belt also spatially overlaps the early Paleozoic Qilian magmatic arc. Petrologic, geochronologic, and geochemical studies indicate that the protolith of the UHP metamorphic rocks was a mixture of continental and mafic/ultramafic materials, derived either from oceanic mélanges or pieces of a rifted continental margin tectonically incorporated into an oceanic subduction channel. These observations require that the North Qaidam UHP metamorphic rocks originated at least in part from continental crust that was subducted to mantle depths and then transported across a mantle wedge into a coeval arc during oceanic subduction. Upward transport of the UHP rocks may have been accommodated by rising diapirs launched from a mélange channel on top of an oceanic subducting slab. To test this hypothesis, we developed a quantitative model that incorporates existing knowledge on thermal structures of subduction zones into the mechanics of diapir transport. Using this model, we are able to track P-T and T-t paths of individual diapirs and compare them with the observed P-T and T-t paths from North Qaidam. The main physical insight gained from our modeling is that the large variation of the observed North Qaidam P-T paths can be explained by a combination of temporal and spatial variation of thermal structure and mechanical strength of the mantle wedge above the early Paleozoic Qilian subduction slab. Hotter P-T trajectories can be explained by a high initial temperature (∼800°C) of a diapir that travels across a relatively strong mantle wedge (i.e., activation energy E = 350 kJ/mol for dry olivine), while cooler P-T paths may be explained by a diapir with initially low temperature (∼700°C) that traveled through a weaker mantle wedge, with its strength at least two orders of magnitude lower than that of dry olivine. This latter condition could have been achieved by hydraulic weakening of olivine aggregates in the mantle wedge via fluid percolation through the mantle wedge during oceanic subduction. Copyright © 2007 by V.H. Winston & Son, Inc. All rights reserved.
- Becker, T. P., Thomas, W. A., & Gehrels, G. E. (2006). Linking late Paleozoic sedimentary provenance in the Appalachian basin to the history of Alleghanian deformation. American Journal of Science, 306(10), 777-798.More infoAbstract: The Pennsylvanian and Permian strata in the Appalachian basin are generally interpreted to be the erosional record of the Alleghanian orogeny. Advances in our understanding of the kinematic and tectonic history of the Alleghanian orogen from study of the orogenic interior have not been integrated into interpretations of the late Paleozoic stratigraphic record. This comparative study of detrital-zircon U-Pb ages from the youngest preserved synorogenic deposits in the Alleghanian foreland (Permian Dunkard Group) to previously published ages from Early and Middle Pennsylvanian-age deposits reveals a subtle, but perhaps significant, shift in the detrital-zircon-age population. The Permian detrital-zircon ages lack 1700 to 1900 Ma and Archean-age grains characteristically present in the Pennsylvanian-age deposits, which are interpreted to indicate recycling of the lower Paleozoic passive margin. Instead, the Permian detrital-zircon-age populations are representative of the ages of rocks within the Appalachian orogenic interior. This change in the detrital-zircon-age population correlates temporally to a shift from transpressionally-inspired oblique deformation during the Pennsylvanian to foreland-vergent contraction during the Early Permian. The study demonstrates the utility of detrital-zircon dating in sedimentary basins associated with complex and protracted tectonism beyond what can be ascertained by petrographic techniques alone.
- Bloch, J. D., Timmons, J. M., Crossey, L. J., Gehrels, G. E., & Karlstrom, K. E. (2006). Mudstone petrology of the Mesoproterozoic Unkar Group, Grand Canyon, U.S.A.: Provenance, weathering, and sediment transport on intracratonic Rodinia. Journal of Sedimentary Research, 76(9-10), 1106-1119.More infoAbstract: The synthesis of mudstone petrology and interbedded sandstone detrital zircon geochronology provides insights into the provenance, weathering regime, hydrodynamic effects, and diagenesis of the Mesoproterozoic Unkar Group. Unkar mudstones were derived largely from the Grenville Orogen (GO) of southwest Texas and the adjacent Yavapai-Mazatzal (YM) and Southern Granite Rhyolite (SGR) terranes. Detrital zircon data indicate distinct pulses of GO-derived sediment during Hakatai and Dox Formation deposition whereas Shinumo sediment contains a larger component of YM and SGR material. An increase in plagioclase and biotite abundances in the Dox further suggest an orogenic pulse. Bulk chemistry, including elevated Cr and Ni abundances and REE systematics, implicate a heterogeneous provenance for Unkar sediment best approximated by a mix of granite or granodiorite with some basalt. Weathering of Unkar sediment, as determined from mudstone and crystalline-source bulk-chemical trends, is characterized as moderate (CIA values between 55 and 70) indicative of a temperate climate in an orogenic setting. Small (less than 10 wt %) amounts of detrital kaolinite and chlorite are consistent with a moderate (temperate) weathering regime. Illite, the dominant clay mineral in the Unkar Group, is largely the 2M1 polytype and detrital in origin. However, it is estimated that ∼ 7% of the K in the Escalante Creek Member results from K-metasomatism and may form up to ∼ 15 wt% authigenic 1M or 1Md illite. The clay-size fraction and zircon are the chief contributors to the REE budget in Unkar mudstones. Lithostratigraphic variations in the HREE distributions can be attributed partially to variable zircon abundance in the silt-size fraction, a likely function of hydrodynamic sorting. Age constraints on the duration of Unkar Group sedimentation range from 30 to 100 My for the ∼ 1100 m conformable Shinumo-Dox succession and result in compacted sedimentation rates of between 1.0 and 3.6 cm/103 yr. These rates are comparable to Mesozoic foreland-basin settings. Copyright © 2006, SEPM (Society for Sedimentary Geology).
- Darby, B. J., & Gehrels, G. (2006). Detrital zircon reference for the North China block. Journal of Asian Earth Sciences, 26(6), 637-648.More infoAbstract: U-Pb analyses of 250 single detrital zircons from Upper Proterozoic to Ordovician strata collected from the Zhuozi Shan in north-central China provide a detrital zircon reference for the North China block, a major crustal entity in the Asian tectonic collage. The results, which range in age from 1.72 to 2.97 Ga, shed new light on the age of the crystalline basement in North China, much of which is covered by younger sedimentary units. In addition, this detrital zircon reference can be used to help determine the provenance of clastic sedimentary units and for assessing validity of paleogeographic and regional tectonic models that include the complex history of Asian continental amalgamation, terrane accretion, and subsequent translation that is ongoing today. © 2006 Elsevier Ltd. All rights reserved.
- Ducea, M. N., Gehrels, G. E., Shoemaker, S., Ruiz, J., & Valencia, V. A. (2006). Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology: Reply. Bulletin of the Geological Society of America, 118(5-6), 765-767.
- Guynn, J. H., Kapp, P., Pullen, A., Heizler, M., Gehrels, G., & Ding, L. (2006). Tibetan basement rocks near Amdo reveal "missing" mesozoic tectonism along the Bangong suture, central Tibet. Geology, 34(6), 505-508.More infoAbstract: The U-Pb and 40Ar/39Ar studies of a unique exposure of crystalline basement along the Jurassic-Early Cretaceous Bangong suture of central Tibet reveal previously unrecognized records of Mesozoic metamorphism, magmatism, and exhumation. The basement includes Cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185-170 Ma. The basement cooled to ∼300 °C by 165 Ma and was exhumed to upper crustal levels in the hanging wall of a south-directed thrust system during Early Cretaceous time. We attribute Jurassic metamorphism and magmatism to the development of a continental arc during Bangong Ocean subduction, and Early Cretaceous exhumation to northward continental underthrusting of the Lhasa terrane beneath the Qiangtang terrane. We speculate that a Jurassic arc extended regionally along the length of the Bangong suture, but in all other places in Tibet has been buried, either depositionally or structurally, beneath supracrustal assemblages. © 2006 Geological Society of America.
- Hacker, B. R., Wallis, S. R., Ratschbacher, L., Grove, M., & Gehrels, G. (2006). High-temperature geochronology constraints on the tectonic history and architecture of the ultrahigh-pressure Dabie-Sulu Orogen. Tectonics, 25(5).More infoAbstract: New U/Pb zircon and Th/Pb monazite ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane. Combined with Sm/Nd ages, Rb/Sr ages, inclusion relationships, and geologic relationships, they help define the timing of peak recrystallization, the timing of subsequent amphibolite-facies metamorphism, and the architecture of the Dabie-Sulu suture zone between the collided Sino-Korean and Yangtze cratons. The data indicate a ∼15 Myr record of UHP recrystallization, the first clearly documented for a giant UHP terrane; this requires that continental subduction in the Dabie-Sulu orogen involved multiple UHP tectonic or recrystallization events. A 244-236 Ma "precursor" UHP event, seen only in the Dabie Shan, was followed by a second, ∼230-220 Ma "main" UHP event, which was itself terminated by a 220-205 Ma amphibolite-facies overprint. Older eclogite-facies events seen in the Qinling segment of this orogenic belt raise the possibility that these rocks have undergone (U)HP metamorphism three or four times, but at present, there is no geochronological evidence in the Dabie-Sulu area to support this. The subduction of the lower, Yangtze plate did not proceed in a simple fashion: The ages of inherited zircon cores demonstrate that a ribbon continent of Yangtze affinity escaped subduction and became wedged against the Sino-Korean plate hanging wall. Copyright 2006 by the American Geophysical Union.
- Miller, E. L., Toro, J., Gehrels, G., Amato, J. M., Prokopiev, A., Tuchkova, M. I., Akinin, V. V., Dumitru, T. A., Moore, T. E., & Cecile, M. P. (2006). New insights into Arctic paleogeography and tectonics from U-Pb detrital zircon geochronology. Tectonics, 25(3).More infoAbstract: To test existing models for the formation of the Amerasian Basin, detrital zircon suites from 12 samples of Triassic sandstone from the circum-Arctic region were dated by laser ablation-inductively coupled plasma-mass spectrometry (ICP-MS). The northern Verkhoyansk (NE Russia) has Permo-Carboniferous (265-320 Ma) and Cambro-Silurian (410-505 Ma) zircon populations derived via river systems from the active Baikal Mountain region along the southern Siberian craton. Chukotka, Wrangel Island (Russia), and the Lisburne Hills (western Alaska) also have Permo-Carboniferous (280-330 Ma) and late Precambrian-Silurian (420-580 Ma) zircons in addition to Permo-Triassic (235-265 Ma), Devonian (340-390 Ma), and late Precambrian (1000-1300 Ma) zircons. These ages suggest at least partial derivation from the Taimyr, Siberian Trap, and/ or east Urals regions of Arctic Russia. The northerly derived Ivishak Formation (Sadlerochit Mountains, Alaska) and Pat Bay Formation (Sverdrup Basin, Canada) are dominated by Cambrian-latest Precambrian (500-600 Ma) and 445-490 Ma zircons. Permo-Carboniferous and Permo-Triassic zircons are absent. The Bjorne Formation (Sverdrup Basin), derived from the south, differs from other samples studied with mostly 1130-1240 Ma and older Precambrian zircons in addition to 430-470 Ma zircons. The most popular tectonic model for the origin of the Amerasian Basin involves counterclockwise rotation of the Arctic Alaska-Chukotka microplate away from the Canadian Arctic margin. The detrital zircon data suggest that the Chukotka part of the microplate originated closer to the Taimyr and Verkhoyansk, east of the Polar Urals of Russia, and not from the Canadian Arctic. Copyright 2006 by the American Geophysical Union.
- Solari, L. A., Schaaf, P., Ducea, M. N., Gehrels, G. E., Shoemaker, S., Ruiz, J., & Valencia, V. A. (2006). Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-PB zircon geochronology: Discussion. Bulletin of the Geological Society of America, 118(5-6), 764-767.
- Talavera-Mendoza, O., Ruiz, J., Gehrels, G. E., Meza-Figueroa, D. M., Vega-Granillo, R., & Valencia, V. A. (2006). Reply to comment on "U-Pb geochronology of the Acatlán Complex and implications for the Paleozoic paleogeography and tectonic evolution of southern Mexico" by Talavera et al.. Earth and Planetary Science Letters, 245(1-2), 476-480.More infoAbstract: Nance et al. raise three issues regarding the geochronology and tectonic interpretations of the Acatlán Complex of southern Mexico presented in our paper [1]. They specifically inquire about: (1) the Mesoproterozoic crystallization age reported for megacrystic granites and tonalitic gneisses of the Tecolapa suite (formerly part of the Esperanza Granitoids); (2) the Taconian-age of the high-pressure (HP) metamorphism of Xayacatlán Formation; and (3) the provenance of Xayacatlán rocks. They propose an alternative model for the evolution of the Acatlán Complex. We welcome the comment and the opportunity to reiterate the main points of our work. We take also the opportunity to give the precise locations of the samples dated in our work (Table 1). The geochronological and geological data presented in our paper underscore the conclusions presented by several authors (including some of Nance's co-authors) indicating that the stratigraphy of the complex as originally defined is oversimplified and that it should not be further used to avoid confusion and misinterpretations [2,3]. Unfortunately, much of the discussion by Nance et al. is based on the old stratigraphic scheme. Furthermore, the comment does not provide geographic, petrographic or geological information of dated samples (or they refer to in press papers) making it hard to determine if we are discussing the same rocks or units. Below, we address the three main points in the Nance et al. comment. Age of the Esperanza Granitoids and the Tecolapa suite: Nance et al. use U/Pb ages of some granitic bodies (La Noria, Los Hornos, Palo Liso), which they include in the Esperanza Granitoids unit to propose a Middle Ordovician (∼ 460 ± 10 Ma) crystallization age for the whole unit. However, geochronological data and critical geological evidence presented in our paper and in other studies [4,5] demonstrate that the Esperanza Granitoids as defined by them, include granitic bodies formed by different magmatic events of a different age. The evidence for this assertion follows. Firstly, megacrystic granites from La Noria-Los Hornos were dated by Yañez et al. [6] (by TIMS) at 371 ± 34 Ma. We have recently confirmed this age with new TIMS U/Pb geochronology yielding an identical age of 371 ± 17 Ma, thus confirming the existence of a Devonian magmatic event. Furthermore, the classic Esperanza Granitoids are affected by eclogitic metamorphism [4]. La Noria-Los Hornos, on the other hand, are locally deformed but do not show any evidence of eclogite metamorphism. Ortega-Gutierrez et al. [4] demonstrate that La Noria-Los Hornos granites and the Esperanza Granitoids are unrelated plutons, a statement that is supported by our data. Secondly, megacrystic granites belonging to the classic Esperanza Granitoids yield an Early Silurian TIMS age (440 ± 18 Ma) [4], which was confirmed by LA-ICPMS ages (442 ± 5 Ma) in our paper [1]. An older, 471 Ma SHRIMP age was recently reported for granites from the same area based on a single concordant zircon [3]. This latter age is identical within analytical uncertainty to the age (470 ± 16 Ma) reported in our work for petrographically identical granites from the Piaxtla area. The combined data therefore indicate that the classic Esperanza Granitoid magmatism ranged from 471 to 440 Ma. Thirdly, there is a suite of K-feldspar megacrystic granitoids and tonalitic to grabbroic gneisses outcropping in the Olinalá region, which yielded Mesoproterozoic (1163-1043 Ma) crystallization ages, which we named the Tecolapa suite. Rocks of this suite have been confused with the Esperanza Granitoids as defined by Ortega-Gutiérrez et al. [4] and were long considered as part of this unit. The ages reported in our paper were based on the analysis of twenty-five to fifty bipyramidal zircons from two distinctive samples (CU-920 and ACA-505) using a spot size of 50 μm in one sample and 35 μm in the other. We analyzed tips as close as possible from borders to determine the crystallization age. Our reaported magmatic ages are defined by zircon clusters composed of 17 grains in sample CU-920 and 19 in sample ACA-505 and not on a single or a few zircons even if concordant. Because Pb-loss commonly produces analyses that are younger than the crystallization age but are still analytically concordant, we rely on clustering as an indicative that an age is robust. CL imaging revealed, however, that some zircons do contain thin rims (generally less than 15 μm wide), which were originally not possible to analyze accurately with the size of our laser beam. In order to determine the age and nature of these rims, we performed new analyses using a smaller spot size of 10 μm. The new analyses show that the rims are ∼ 465 My old as claimed by Nance et al., and as we interpreted in our paper as the time of Pb-loss. However, the nature of this age is uncertain because the U/Th ratios of the rims span from magmatic (U/Th < 7) to clearly metamorphic (U/Th > 10) values. Geological evidence, however, suggest that overgrowths are more probably of metamorphic origin because rocks from the Tecolapa suite are unequivocally intruded by post-tectonic granites dated at 478-472 Ma (Early Ordovician) preventing a younger crystallization age for the Tecolapa rocks. Thus, available geochronological data and geological evidence support a Mesoproterozoic crystallization age for these rocks and justify their separation from the Ordovician-Early Silurian granites of the classic Esperanza Granitoids. The presence of Mesoproterozoic rocks in the Acatlán complex is a new finding that indicates that their origin dates back to the configuration of Rodinia. Nance et al. suggest that this Grenvillian element could be related to the evolution of the Oaxaca Complex, a Mesoproterozoic terrane of South American origin. The age of Tecolapa granites (1163-1043 Ma) match better with the major magmatic pulse (∼ 1100 Ma) in the Grenville province of SW North America than with the main magmatism in Oaxaca and in the Grenville province of South America (∼ 980-990 Ma) [5]. Furthermore, the close relationship of Tecoloapa rocks with those of the Xayacatlán and Ixcamilpa rocks showing Laurentian affinities also point out to a North American origin. Age of the high-pressure metamorphism: Nance et al. propose a Mississippian (∼ 345 Ma) age for the HP metamorphism recorded in some units of the Acatlán Complex based on two U/Pb ages from an eclogite and a migmatite from the Piaxtla group. Following the original definition of the Piaxtla group [4], these authors assume that rocks of the Xayacatlán and Esperanza Granitoids units share a single HP metamorphic event. As outlined in our paper, there is convincing geological and geochronological evidence to propose the existence of three major events of HP metamorphism affecting different units of the Acatlán Complex.(1)In the Olinalá region, in the western Acatlán Complex, eclogites and garnet-amphibolites of the Xayacatlán Formation are overthrusted by metabasites and schists of El Rodeo Formation. The latter are affected by a greenschist, low-pressure metamorphism and, to our knowledge; there is no evidence indicating that this unit was affected by a HP event. Given the contrasting metamorphic conditions of both assemblages, eclogitic metamorphism in the Xayacatlán Formation and the greenschist metamorphism in the El Rodeo Formation must have occurred before their juxtaposition. The age of juxtaposition is constrained by El Progreso leucogranite dated at 476 ± 8 Ma, which intrudes the contact between the Xayacatlán and El Rodeo formations providing a minimum Early Ordovician age for the HP, eclogitic metamorphism in the Xayacatlán Formation.(2)Detrital zircon geochronology in rocks of the Ixcamilpa suite indicates a maximum Middle Ordovician depositional age. Rocks of this suite underwent blueschist metamorphism of poorly resolved age but it is evident that it took place after Middle Ordovician preventing any linkage with the eclogitic metamorphism of Xayacatlán Formation.(3)Reported ages for the classic Esperanza Granitoids range from 471 to 440 Ma. There are eclogites and migmatites intimately associated with augen gneisses. The age of this eclogitic metamorphism is also poorly constrained but it certainly occurred after 440 Ma, the youngest confirmed age for Esperanza rocks. This metamorphism therefore cannot be related to eclogitic metamorphism of Xayacatlán Formation, thereby confirming the existence of two distinctive phases of eclogitic metamorphism in the Acatlán Complex. Unfortunately, Nance et al. do not provide geographic nor petrographic information of their dated samples making it impossible to be sure if their ages were obtained from rocks of Xayacatlán or Esperanza Granitoids formations, both of which contain eclogites and migmatites. From our own data, we infer that Nance et al. ages are from rocks belonging to the Esperanza Granitoids unit for which we obtained Devonian to Mississippian Ar/Ar cooling ages. If so, their ages do not disagree with the Early Ordovician age we inferred for the Xayacatlán eclogites since metamorphisms in the Esperanza Granitoids and Xayacatlán units are not genetically related. Provenance of Xayacatlán rocks: In our paper, we proposed a Laurentian provenance for the Ixcamilpa blueschist suite based mainly on the presence of a significant population of detrital zircons at ∼ 477 Ma and a smaller but distinctive population at ∼ 1821 Ma, both of exclusive Laurentian affinity.
- Valencia, V. A., Barra, F., Weber, B., Ruiz, J., Gehrels, G., Chesley, J., & Lopez-Martinez, M. (2006). Re-Os and U-Pb geochronology of the El Arco porphyry copper deposit, Baja California Mexico: Implications for the Jurassic tectonic setting. Journal of South American Earth Sciences, 22(1-2), 39-51.More infoAbstract: New geochronologic data from the El Arco porphyry copper deposit, Baja California, Mexico, yield a Middle Jurassic crystallization U-Pb age of 164.7 ± 6.5 Ma for zircons from a granodiorite porphyry that hosts the Cu-Au-(Mo) mineralization, and four Re-Os molybdenite ages yield a weighted average age of 164.1 ± 0.4 Ma. These ages disagree with previous K-Ar age determinations that suggest a correlation of the intrusive rocks of El Arco-Calmalli area with the eastern Peninsular Ranges batholith of Cretaceous age. Instead, the volcanic-plutonic rocks of the El Arco-Calmalli area are more properly associated with the San Andres-Cedros volcanic-plutonic complex (∼166 Ma). El Arco-Calmalli block likely was part of the intraoceanic arc system described in the Vizcaino-Cedros region and accreted to the continent during the Cretaceous (98-110 Ma). The presence of a continental Jurassic magmatic arc in the mainland and the occurrence of El Arco-Calmalli (San Andres-Cedros) volcanic-plutonic complex indicate two Jurassic magmatic arcs at this latitude (continental and oceanic). © 2006 Elsevier Ltd. All rights reserved.
- Valencia, V. A., Noguez-Alcántara, B., Barra, F., Ruiz, J., Gehrels, G., Quintanar, F., & Valencia-Moreno, M. (2006). Re-Os molybdenite and LA-ICPMS-MC U-Pb zircon geochronology for the Milpillas porphyry copper deposit: Insights for the timing of mineralization in the Cananea District, Sonora, Mexico. Revista Mexicana de Ciencias Geologicas, 23(1), 39-53.More infoAbstract: New geochronological data presented here improves the understanding of the temporal evolution of the Cananea Mining District, and particularly of the Milpillas porphyry copper deposit (northeastern Sonora, Mexico). Uranium-lead zircon analyses, using laser ablation ICP-MS multi-collector, from the quartz monzonite porphyry unit that host the mineralization at the Milpillas deposit, yielded a crystallization age of 63.9 ± 1.3 Ma (2-sigma). Re-Os molybdenite ages from two drill core samples from more than 500 m depth yielded an identical age of 63.1 ± 0.4 Ma (2-sigma), suggesting a restricted period of mineralization. These ages indicate that the Milpillas deposit is the oldest Laramide porphyry copper deposit recognized so far in the Cananea District. Our new Re-Os data in addition to previous Re-Os data, suggest that mineralization within the district, occurred within a ∼4 m.y. period in three discrete pulses at ∼59 Ma, ∼61 Ma and ∼63 Ma. This is in contrast to the previous model in which mineralization at the Cananea District was the result of a continuous hydrothermal system that started at ∼62 Ma and ended at ∼52 Ma.
- Weislogel, A. L., Graham, S. A., Chang, E. Z., Wooden, J. L., Gehrels, G. E., & Yang, H. (2006). Detrital zircon provenance of the Late Triassic Songpan-Ganzi complex: Sedimentary record of collision of the North and South China blocks. Geology, 34(2), 97-100.More infoAbstract: Using detrital zircon geochronology, turbidite deposystems fed from distinct sediment sources can be distinguished within the Songpan-Ganzi complex, a collapsed Middle to Late Triassic turbidite basin of central China. A southern Songpan-Ganzi deposystem initially was sourced solely by erosion of the Qinling-Dabie orogen during early Late Triassic time, then by Qinling-Dabie orogen, North China block, and South China block sources during middle to late Late Triassic time. A northern Songpan-Ganzi system was sourced by erosion of the Qinling-Dabie orogen and the North China block throughout its deposition. These separate deposystems were later tectonically amalgamated to form one complex and then uplifted as the eastern Tibet Plateau. © 2006 Geological Society of America.
- Weislogel, A. L., Graham, S. A., Chang, E. Z., Wooden, J. L., Gehrels, G. E., & Yang, H. (2006). Detrital zircon provenance of the Late Triassic Songpan-Ganzi complex: Sedimentary record of collision of the North and South China blocks: Reply. Geology, 34(1), e107-e108.
- Yin, A., Dubey, C. S., Kelty, T. K., Gehrels, G. E., Chou, C. Y., Grove, M., & Lovera, O. (2006). Structural evolution of the Arunachal Himalaya and implications for asymmetric development of the Himalayan orogen. Current Science, 90(2), 195-206.More infoAbstract: Geologic mapping and stratigraphiv correlation of low-grade Precambrian Lesser Himalayan units using U-Pb detrital zircon dating reveal the existence of a Main Central Thrust (MCT) window and a prominent ductile thrust zone within the Greater Himalayan Crystallines in the Arunachal Himalaya of NE India. The newly discovered MCT window is cut and offset by several active north-trending normal faults extending from southeast Tibet, indicating the fault is no longer active. Ion-microprobe dating of monazite inclusions in garnets from the MCT zone indicates that the fault was active at 10.1 ± 1.4 Ma. Our structural data together with a synthesis of existing geologic maps suggest that the eastern Himalaya is composed of a large thrust duplex with the folded MCT as the roof fault. The total amount of crustal shortening accommodated by the duplex and the MCT south of the South Tibetan Detachment may exceed 500 km, which is probably greater than the amount of crustal shortening across the central Himalaya in Nepal and definitely greater than the amount of shortening across the western Himalaya in Pakistan. The observed systematic variation of crustal shortening suggests that Himalayan crustal thickening and uplift are uneven along strike, which may be in response to the westward decrease in convergence rate between India and Asia during the Cenozoic.
- Amidon, W. H., Burbank, D. W., & Gehrels, G. E. (2005). Construction of detrital mineral populations: Insights from mixing of U-Pb zircon ages in Himalayan rivers. Basin Research, 17(4), 463-485.More infoAbstract: Fission-track, U-Pb and Pb-Pb analyses of detrital heavy mineral populations in depositional basins and modern river sediments are widely used to infer the exhumational history of mountain belts. However, relatively few studies address the underlying assumption that detrital mineral populations provide an accurate representation of their entire source region. Implicit in this assumption is the idea that all units have equal potential to contribute heavy minerals in proportion to their exposure area in the source region. In reality, the detrital mineral population may be biased by variable concentrations of minerals in bedrock and differential erosion rates within the source region. This study evaluates the relative importance of these two variables by using mixing of U-Pb zircon ages to trace zircon populations from source units, through the fluvial system, and into the foreland. The first part of the study focuses on the Marsyandi drainage in central Nepal, using tributaries that drain single formations to define the U-Pb age distributions of individual units and using trunk river samples to evaluate the relative contributions from each lithology. Observed mixing proportions are compared with proportions predicted by a simple model incorporating lithologic exposure area and zircon concentration. The relative erosion rates that account for the discrepancy between the observed and predicted mixing proportions are then modelled and compared with independent erosional proxies. The study also compares U-Pb age distributions from four adjacent drainages spanning ∼ 250 km along the Himalayan front using the Kolmogorov-Smirnov statistic and statistical estimates of the proportion of zircon derived from each upstream lithology. Results show that, along this broad swath of rugged mountains, the U-Pb age distributions are remarkably similar, thereby allowing data from more localized sources to be extrapolated along strike. © 2005 Blackwell Publishing Ltd.
- Amidon, W. H., Burbank, D. W., & Gehrels, G. E. (2005). U-Pb zircon ages as a sediment mixing tracer in the Nepal Himalaya. Earth and Planetary Science Letters, 235(1-2), 244-260.More infoAbstract: This paper presents a new approach to quantify sediment mixing based on the mixing of U - Pb zircon age distributions within sediment. Two statistical techniques are presented to determine the proportion in which two known age distributions combine to create a known mixed age distribution. These techniques are then used to determine relative erosion rates between adjacent drainage basins above and below the Main Central Thrust (MCT) in the central Nepal Himalaya. The MCT region is coincident with an abrupt north-south change in geomorphic character and mineral cooling ages that are thought to represent an erosional response to higher rock uplift rates north of the MCT zone. However, it is unclear whether the ongoing deformation responsible for the differential uplift rates is: (1) focused on the MCT; (2) at depth along a crustal scale ramp; or (3) along newly mapped thrust faults south of the MCT. Our study explores this issue by comparing modern erosion rates with longer-term erosion rates determined from mineral cooling ages. Zircons were separated from modern river sand and dated by LA-MC-ICPMS before the measured isotopic ratios and ages were used in 1-d and 2-d mixing calculations. The 1-d technique creates probability density functions of zircon ages for each sample and then uses both an iterative and inverse approach to estimate mixing between samples. In contrast, the 2-d technique estimates mixing between probability "fields" defined by the measured 238U/206Pb and 207Pb/206Pb ratios. Given a finite mixture with perfect sample representation, both techniques produce perfect mixing estimates across a range of mixing proportions. Modeling results demonstrate that given imperfect subsample representation of the complex parent age distribution, differing degrees of subsample smoothing may be required to achieve an accurate mixing estimate. Using mixing of zircon ages as a quantitative proxy for sediment mixing requires a correction for the concentration of zircon in the river sediment. Two new methods for establishing zircon concentration in river sediment are presented demonstrating the existence of 2- to 5-fold differences in zircon concentration between adjacent drainages. Relative erosion rates are estimated by determining the zircon mixing ratio between adjacent drainages which are then normalized by the ratio of zircon concentrations and the ratio of drainage areas. Results show ∼3 times higher modern erosion rates south of the MCT in the northernmost Lesser Himalaya. Future applications of this new technique may include reach-scale sediment transport dynamics, improved sedimentary basin analysis, and better interpretation of foreland mineral cooling ages. © 2005 Elsevier B.V. All rights reserved.
- Barbeau Jr., D. L., Ducea, M. N., Gehrels, G. E., Kidder, S., Wetmore, P. H., & Saleeby, J. B. (2005). U-Pb detrital-zircon geochronology of northern Salinian basement and cover rocks. Bulletin of the Geological Society of America, 117(3-4), 466-481.More infoAbstract: Salinia is an out-of-place granitic terrane in central coastal California whose debated origin is critical to understanding the tectonic history of southwestern North America. Salinian metasedimentary and sedimentary rocks that respectively host and cover its predominant arc rocks should contribute important data about its origin and kinematic history, but pervasive intrusion, high-grade metamorphism and Cenozoic erosion of the Salinian block have inhibited their widespread characterization and correlation. To further address these problems, we report 605 U-Pb detrital-zircon geochronologic ages collected by laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) from seven Salinian metasedimentary framework (Sur Series) and sedimentary cover samples. Samples collected from the Sur Series contain Late Archean (2.5-2.9 Ga), late Paleoproterozoic (1.6-1.9 Ga), Mesoproterozoic (0.9-1.5 Ga), Neoproterozoic (0.65-0.8 Ga), Paleozoic (250-450 Ma), and possibly Mesozoic U-Pb detrital-zircon ages. Samples collected from Upper Cretaceous cover units have various age-peak distributions, which collectively include late Paleoproterozoic (1.6-1.8 Ga), early Mesoproterozoic (1.35-1.55 Ga), Permo-Triassic (220-290 Ma), and Jurassic-Cretaceous (80-190 Ma) peaks. From these data, several interpretations are made. (1) Maximum depositional ages of the Sur Series and cover intervals are 280-360 Ma and 78-90 Ma, respectively. (2) The presence of Late Archean, early Paleoproterozoic, and Neoproterozoic zircons in Salinian metasedimentary rocks suggest that uplift and erosion of adjacent basins recycled sediment onto Salinia. (3) The abundant pre-Mesoproterozoic detrital-zircon ages in Sur Series and cover units preclude the possibility that Salinia originated in southern Mexico, as has been previously suggested. (4) Five of six key detrital-zircon age peaks identified in Salinian basement and cover units are nowhere more closely arranged than in the Mojave Desert-Peninsular Ranges region of Baja and southern Alta California. (5) Paleozoic and early Mesozoic detrital zircons in Sur Series and cover units match the ages of several plutonic events that occurred along the western margin of North America-however, Permian ages favor a Mojave Desert origin over other candidates. Collectively, these and other data suggest that Salinia resided in the Mojave Desert-Peninsular Ranges region from the late Paleozoic until the Late Cretaceous, after which it was rapidly exhumed, deposited upon, and then translated outboard and northward to its current position. © 2005 Geological Society of America.
- Becker, T. P., Thomas, W. A., Samson, S. D., & Gehrels, G. E. (2005). Detrital zircon evidence of Laurentian crustal dominance in the lower Pennsylvanian deposits of the Alleghanian clastic wedge in eastern North America. Sedimentary Geology, 182(1-4), 59-86.More infoAbstract: The late Paleozoic Alleghanian orogeny culminated with the continent-continent collision between Laurentia and Gondwana in the construction of the supercontinent Pangea. Within the Appalachian basin, the sedimentological record of the early stages of the continental collision is preserved as late Mississippian-age siltstones and mudstones that mark the end of carbonate production. An upward transition to coarse, quartz-rich, cratonward-prograding, fluvial deposits of Pennsylvanian age is cited to mark the initiation of continental collision and cratonward advancement of the tectonic load. Seven samples of early Pennsylvanian (early-middle Morrowan) sandstones and conglomerates from the Appalachian basin were collected along the orogen from eastern Pennsylvania to central Alabama for U-Pb dating of detrital zircons. All of the samples reveal a dominance of zircons with ages that can be attributed to crust-forming events in Laurentia. A small percentage of the detrital zircons have ages (530-680 Ma and 2000-2200 Ma) that correspond to the Pan-African/Brasiliano orogenic belts and Trans-Amazonian/Eburnian cratons typical of Gondwanan crust. Although recycling of pre-orogenic strata may disproportionately bias the detrital-zircon population toward ages of Laurentian affinity, the dominance of the Laurentian "signature" in the Appalachian basin along strike suggests that the early components of the Alleghanian orogen were composed mainly of recycled Laurentian crust. Detrital-zircon populations are not indicative of substantial incorporation of Gondwanan crust in the orogenic highlands. On the basis of the timing of dextral displacement along shear zones in the Appalachian hinterland, the early Pennsylvanian clastic deposits preserved in the Appalachian basin likely correspond to transpressional exhumation of the Laurentian margin, consistent with hypotheses of oblique collision between Gondwana and Laurentia in the late Paleozoic. © 2005 Elsevier B.V. All rights reserved.
- Butler, R. F., Gehrels, G. E., & Davidson, C. (2005). Reply to comment by M.E. Beck and B.A. Housen on "Paleomagnetism and geochronology of the Ecstall pluton in the coast mountains of British Columbia: Evidence for local deformation rather than large-scale transport". Journal of Geophysical Research B: Solid Earth, 110(1), 1-3.
- Colpron, M., Gladwin, K., Johnston, S. T., Mortensen, J. K., & Gehrels, G. E. (2005). Geology and juxtaposition history of the Yukon-Tanana, Slide Mountain, and Cassiar terranes in the Glenlyon area of central Yukon. Canadian Journal of Earth Sciences, 42(8), 1431-1448.More infoAbstract: In central Yukon, the pericratonic Yukon-Tanana terrane (YT) is juxtaposed with the Cassiar terrane (CT, parautochthonous North America) along the Tummel fault zone (TFZ), a 3-4 km wide, northwest-trending belt comprising imbricate fault slices of Slide Mountain terrane (SM, greenstone, chert, serpentinite) and synorogenic clastic rocks. North-east of the TFZ, the CT comprises Paleozoic metapelitic rocks, marble, and amphibolite of continental margin affinity. To the southwest, the YT consists of a pre-Late Devonian metasedimentary complex overlain and intruded by Mississippian clastic, volcanic, and plutonic successions of continental are affinity. In the TFZ, Middle to Late Permian ocean-floor basalt of the SM shows evidence of crustal contamination, suggesting deposition at the edge of a margical ocean basin. Deformation features in the TFZ include early ductile fabrics overprinted by younger brittle structures. Triassic synorogenic clastic rocks in the TFZ, and at the base of a klippen above the YT, suggest that terrane imbrication began shortly after the Early Triassic. 40Ar/39Ar mica ages from the region suggest cooling of the YT, SM, and part of CT below 300 °C by Early Jurassic time. Pervasive brittle structures in the Ragged Lake klippe, which roots into the TFZ, indicate brittle thrusting of the SM over the CT in post-Triassic time. Early Cretaceous plutons intrude the CT (Glenlyon Batholith) and the TFZ (leucogabbro) and impose a contact aureole that extends westward into the YT. Steep brittle structures that deformed the TFZ also affect, in part, the Glenlyon Batholith but do not significantly offset its contact aureole. Consequently, little displacement can have occurred along the TFZ after Early Cretaceous time. © 2005 NRC Canada.
- Finney, S., Peralta, S., Gehrels, G., & Marsaglia, K. (2005). The Early Paleozoic history of the Cuyania (greater Precordillera) terrane of western Argentina: Evidence from geochronology of detrital zircons from Middle Cambrian sandstones. Geologica Acta, 3(4), 339-354.More infoAbstract: U-Pb geochronology of large detrital zircons populations is a powerful tool for interpreting sandstone provenance. Here, it is applied to three Middle Cambrian sandstones from the Precordillera of Argentina with the purpose of using the provenance interpretations to test paleogeographic and paleotectonic models proposed for the Cuyania or Precordillera terrane. Two samples from the La Laja Formation have distinctive detrital zircon age distributions. All zircon grains fall within unimodal populations of 1688-1200 Ma in one sample and 1559-1316 Ma in the other. Of these grains, 23% and 65%, respectively, are within the age range of the North American magmatic gap (1610-1490 Ma), indicating a non-Laurentian provenance. A very different sample was taken from a sandstone interval in a large olistolith within the Estancia San Isidro Formation. Its zircon population is dominated by a single, prominent 615-511 Ma age cluster, which is indicative of a provenance in a Brasiliano orogenic belt. The absence of zircons with Grenvillian ages (1200 to 950 Ma) is difficult to reconcile with paleogeographic and geotectonic models in which Cuyania rifted from Laurentia in Cambrian or Ordovician time. The data are most consistent with models in which Cuyania rifted from the southern margin of West Gondwana. Given a Cambrian association with Gondwana and a post-Ordovician arrival at its present position in Gondwana, the Cuyania terrane must have migrated along the southern and western margins of Gondwana during the Ordovician Period. © UB-ICTJA.
- Gillis, R. J., Gehrels, G. E., Ruiz, J., & Antonio, L. (2005). Detrital zircon provenance of Cambrian-Ordovician and Carboniferous strata of the Oaxaca terrane, southern Mexico. Sedimentary Geology, 182(1-4), 87-100.More infoAbstract: U-Pb geochronologic analyses have been conducted on 135 detrital zircon grains from Paleozoic strata of the Oaxaca terrane of southern Mexico. The grains are mainly mid-Proterozoic in age (age probability peak of 993 Ma), with subordinate clusters of ∼358 and ∼472 Ma. The mid-Proterozoic ages can be used to evaluate three possible paleopositions for the Oaxaca terrane during mid-Proterozoic time. Formation in proximity to northwest South America appears most likely, as southwestern portions of the Amazon craton (and perhaps basement massifs in the northern Andes) contain igneous rocks of the appropriate age. A more specific test of ties to these regions is not yet possible due to the limited number of U-Pb determinations from the region. Formation in proximity to Grenville-age rocks in northeast North America is less likely, as detrital zircons shed from these rocks are mostly older than 993 Ma. Previously proposed links with Grenville-age rocks in southwestern North America are not supported by our data, as there is little overlap of ages from the Oaxaca terrane and from Grenville-derived detrital zircons in southwestern US and northwestern Mexico. The presence of Paleozoic grains in our samples does not help constrain the Late Proterozoic-early Paleozoic displacement of the Oaxaca terrane, as magmatism of this age was apparently widespread within and adjacent to the Iapetus and Rheic ocean basins. © 2005 Elsevier B.V. All rights reserved.
- González-León, C. M., Stanley, G. D., Gehrels, G. E., & Centeno-García, E. (2005). New data on the lithostratigraphy, detrital zircon and Nd isotope provenance, and paleogeographic setting of the El Antimonio Group, Sonora, Mexico. Special Paper of the Geological Society of America, 393, 259-282.More infoAbstract: The El Antimonio Group is herein proposed as a new lithostratigraphic unit that encompasses the Antimonio, Río Asunción, and Sierra de Santa Rosa Formations in a revised nomenclature from Lucas and Estep (1999b). The type section for the Antimonio, Río Asunción, and the lower part of the Sierra de Santa Rosa Formations is located in the Sierra del Álamo, whereas the representative upper part of the Sierra de Santa Rosa Formation is located in the mountains of same name in northwestern Sonora. The ∼4.5-km-thick sedimentary succession of this group is abundantly fos- siliferous, and its biostratigraphic age is constrained between the Late Permian and Early Jurassic. The 3.4-km-thick section that crops out in the Sierra del Alamo is divided into 14 unconformity-bounded sequences that are tens to hundreds of meters thick and grade from the base upward from a fluvial to shallow marine conglomerate to open marine shale. The El Antimonio succession is correlated with several other Triassic and Jurassic sections that are known in Sonora, all of which are located south of the proposed trace of the Mojave-Sonora megashear. The closest Triassic and Lower Jurassic sections that are located north of the Mojave-Sonora megashear that we correlate with the El Antimonio are known in southern Nevada and southeastern California and include the Moenkopi, Virgin Limestone, Union Wash, Silverlake, and Fairview Valley Formations and the Kings sequence. On the basis of these proposed correlations, we suggest that the El Antimonio Group was deposited in an evolving shallow shelf (Upper Permian-Triassic) to fore-arc basin (Lower Jurassic) that was originally positioned adjacent to southern California and later translated to its present position, along with the Caborca block, by left-lateral Jurassic displacement of the Mojave-Sonora megashear. In this proposed paleogeography, a lower Mesozoic magmatic arc that accumulated volcanic, volcaniclastic, and shallow marine sedimentation in the Mojave Desert and along the California-Nevada border separated the El Antimonio basin from a shallow shelf that developed to the north. New U-Pb geochronology on detrital zircon and Sm/Nd isotope and petrographic data from terrigenous samples of the El Antimonio Group may help to elucidate its provenance and to support this paleogeography. Zircon grains from samples of the lower, middle, and upper parts of the El Antimonio Group yielded ages that cluster around 1.8, 1.6-1.7, 1.4, and 1.00-1.18 Ga and 340, 270-240, and 190 Ma. The Pro- terozoic zircons are interpreted to indicate provenance from the basement provinces of the southwestern United States, although a reworked source for these grains is also possible as they are present in the Cordilleran miogeocline and off-shelf assemblages of Nevada and California and in Proterozoic and Paleozoic strata in Sonora. The closest known sources for the Permian and Lower Triassic zircons are plutons and volcanic rocks that formed a lower Mesozoic magmatic arc extending from southeastern to northern California and western Nevada. Probable sources for the single zircon grain dated at 340 Ma are the Sierra-Klamath terranes, according to interpretation by other authors of grains of similar age in rocks of Nevada. Grains dated around 190 Ma in the youngest sample most probably reveal provenance from the Lower Jurassic magmatic arc of southeastern California or southern Arizona. The Sm/Nd isotopic data from three samples of the lower, middle, and upper parts of the El Antimonio Group indicate a progressive decrease in model ages, from the base upward (TDM = 1.9-1.8 to TDM = 1.13 Ga) of this succession, indicating a most probable derivation from the Yavapai and Grenville provinces in the southwestern United States. Sandstone and conglomerate clast composition in the El Antimonio Group indicate mixed sources of provenance from sedimentary and vulcanoplutonic origin. These most probably correspond to the Proterozoic and Paleozoic sedimentary successions of southwestern North America and to the Triassic-Jurassic magmatic arc of this same region, respectively. © 2005 Geological Society of America.
- Marcotte, S. B., Klepeis, K. A., Clarke, G. L., Gehrels, G., & Hollis, J. A. (2005). Intra-arc transpression in the lower crust and its relationship to magmatism in a Mesozoic magmatic arc. Tectonophysics, 407(3-4), 135-163.More infoAbstract: Structural observations and U-Pb geochronology from Fiordland, New Zealand support a model of partitioned transpression within the lower crust of an early Mesozoic magmatic arc called the Median Batholith. We use this lower crustal section to test whether transpression was an efficient mechanism for transporting magma through the deep lithosphere. A continentward migration of magmatic activity occurred within the margin of Gondwana after ∼140 Ma followed by a period of concentrated magmatism in a vertical, 12-15 km wide lower crustal shear zone after ∼119 Ma. The shear zone, named the Indecision Creek Shear Zone, contains variably oriented dioritic intrusions and displays systematic variations in the three-dimensional orientation of ductile structures. From the margins to the center of the shear zone the pitch of stretching lineations on foliation surfaces changes from 10-35° to 55-82° with increasing finite strain. This increase in pitch is accompanied by a steepening and counter-clockwise rotation of foliation planes. These and other structural patterns indicate that arc-parallel sinistral oblique-slip and strike-slip displacements occurred at the shear zone margins and that deformation in its center was dominated by horizontal arc-normal shortening and near vertical extrusion. This style of partitioned transpression reflects the effects of rheological contrasts created by a heterogeneous pattern of magmatism within the arc. Field relationships and U-Pb dates on zircon suggest that the shear zone formed along the boundary between outboard (older) and inboard (younger) parts of the batholith and facilitated the transfer of small volumes of magma vertically through the lower crust until at least ∼111 Ma, when convergence and arc magmatism waned. © 2005 Elsevier B.V. All rights reserved.
- Rusmore, M. E., Woodsworth, G. J., & Gehrels, G. E. (2005). Two-stage exhumation of midcrustal arc rocks, Coast Mountains, British Columbia. Tectonics, 24(5), 1-25.More infoAbstract: New geologic data from the Central Gneiss Complex along Douglas Channel help delineate the burial and exhumation of the Late Cretaceous to Eocene Coast Mountains magmatic arc. Arc plutonism was on going between ∼90 and 60 Ma, replaced by formation of dikes to 52 Ma. Supracrustal rocks were buried to midcrustal levels by 90 Ma, and garnet and kyanite grew from ∼90 through 70 Ma. Subsequent exhumation was nearly isothermal and took place in two stages. The first occured from ∼70 to 59 Ma as the arc contracted obliquely and sillimanite replaced kyanite. Exhumation was slow (∼0.5 mm/yr), probably accomplished by erosion aided by coaxial crustal thinning. Exhumation rates about doubled in the second stage of exhumation, after 59 Ma and before ∼52 Ma. Penetrative deformation ended prior to intrusion of the Quattoon pluton at 59 Ma and ∼6.5 kbar. Production of cordierite rims on garnet at 4.5 kbar and ∼700°C signaled the end of near-isothermal decompression prior to 52 Ma. Rapid cooling (100°C/106 years) of the Central Gneiss Complex followed cordierite growth; temperatures dropped to ∼250° by ∼48 Ma. The increase in exhumation rate and the subsequent rapid cooling are attributed to excision of >6 km of crust on a detachment system on the northeastern side of the Central Gneiss Complex. Comparison to other parts of the Coast Mountains arc, the Sierra Nevada, and Fiordland, New Zealand, shows that the amount and tempo of exhumation vary greatly within and between arcs, suggesting that the processes accommodating exhumation vary significantly. Copyright 2005 by the American Geophysical Union.
- Talavera-Mendoza, O., Ruiz, J., Gehrels, G. E., Meza-Figueroa, D. M., Vega-Granillo, R., & Campa-Uranga, M. F. (2005). U-Pb geochronology of the Acatlán Complex and implications for the Paleozoic paleogeography and tectonic evolution of southern Mexico. Earth and Planetary Science Letters, 235(3-4), 682-699.More infoAbstract: Even though the Acatlán Complex in southern Mexico contains the largest exposure of Paleozoic rocks in Mexico, it is commonly ignored in reconstructions of Pangea because of poor geochronologic data. Presently, this complex is understood to be composed of metasedimentary units (Cosoltepec, Magdalena, Chazumba and Tecomate Formations), a major magmatic suite (Esperanza Granitoids), and a suite with eclogites and blueschists (Xayacatlán Fm). Sedimentary cover includes unmetamorphosed upper Paleozoic units. Here we provide single-crystal laser ablation U-Pb geochronology of the metasedimentary and magmatic suites of the Acatlán Complex and its upper Paleozoic sedimentary cover. The data reveal a complex geological evolution recording tectonic events from the assembly of Rodinia to the break-up of Pangea. Data for the Esperanza Granitoids record three major tectonothermal events: (1) a Grenvillian (1165 ± 30 to 1043 ± 50 Ma), (2) a Taconian (478 ± 5 to 471 ± 5 Ma), and (3) a Salinian (Acatecan; 461 ± 9 to 440 ± 14 Ma). Eclogitic rocks from the Xayacatlán Formation of Neoproterozoic-Early Ordovician age contain detrital zircons derived most probably from the southwestern North America Grenville province. Data for the blueschists are consistent with a Middle Ordovician depositional age and derivation from Laurentian sources. The Tecomate Formation is composed of two unrelated units of contrasting age and lithology: a Neoproterozoic-Early Ordovician, arc- and rift-related volcanosedimentary unit containing detrital zircons derived from the southwestern North America Grenville province; and an essentially sedimentary unit containing Early Permian fauna. The Cosoltepec Formation has a maximum Devonian depositional age and contains detrital zircons consistent with derivation from South American sources. The age of the Magdalena and Chazumba Formations is established to be Late Pennsylvanian-Early Permian. These units contain detrital zircons indicating ultimate derivation from both North and South America crustal sources. The Late Paleozoic sedimentary cover contains detrital zircons shed mainly from Grenvillian sources with a significant contribution of Pennsylvanian magmatic rocks. The new U-Pb geochronologic data indicate that the traditional stratigraphic scheme used for the Acatlán Complex needs complete revision. Data further indicate that the earliest stages of the tectonic evolution of the Acatlán Complex are tied to the evolution of Rodinia and that the actual configuration of the Acatlán Complex was ultimately achieved by amalgamation of the Magdalena-Chazumba suite during the final stages of Pangea assembly. The Early Jurassic tectonothermal event affecting only the Chazumba and Cosoltepec units to produce the Magdalena Migmatite is related to the break-up of Pangea and the opening of the Gulf of Mexico. The Acatlán Complex contains a section of the suture between Laurentia and Gondwana with some sediments arriving from Laurentia and others from Gondwana and mirrors the structure and evolution of the Appalachian-Caledonian chains of North America. © 2005 Elsevier B.V. All rights reserved.
- Timmons, J. M., Karlstrom, K. E., Heizler, M. T., Bowring, S. A., Gehrels, G. E., & Crossey, L. J. (2005). Tectonic inferences from the ca. 1255-1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis. Bulletin of the Geological Society of America, 117(11-12), 1573-1595.More infoAbstract: The Unkar Group of the Grand Canyon Supergroup is one of the best-preserved remnants of Mesoproterozoic sedimentary rocks in the southwestern United States. It provides an exceptional record of intracratonic basin formation and associated tectonics kinematically compatible with protracted "Grenville-age" NW-directed shortening. New U/Pb age determinations from an airfall tephra at the base of the Unkar Group dates the onset of deposition at ca. 1255 Ma, and 40Ar/39Ar K-feldspar thermochronology in the Grand Canyon indicates that basement rocks cooled through 150 °C between ca. 1300 and 1250 Ma, refining exhumation rates of basement rocks just prior to Unkar deposition. Abrupt thickness and facies changes in conglomerate and dolomite of the Bass Formation (lower Unkar Group) associated with NE-striking monoclinal flexures indicate NW-directed synsedimentary contraction at ca. 1250 Ma. A large disconformity (∼ 75 m.y. duration) is inferred between the lower and upper Unkar Group and is located below the upper Hakatai Shale, as documented by detrital zircons. A second style of Unkar Group deformation involved the development of half grabens and full grabens that record NE-SW extension on NW-striking, high-angle normal faults. Several observations indicate that NW-striking normal faulting was concurrent with upper Unkar deposition, mafic magmatism, and early Nankoweap deposition: (1) intraformational faulting in the Bass Formation, (2) intraformational faulting in the 1070 Ma (old Rb/Sr date) Cardenas Basalt and lower Nankoweap Formation, (3) syntectonic relationships between Dox deposition and 1104 Ma (new Ar/Ar date) diabase intrusion, and (4) an angular unconformity between Unkar Group and Nankoweap strata. The two tectonic phases affecting the Unkar Group (ca. 1250 Ma and ca. 1100 Ma) provide new insight into tectonics of southern Laurentia: (1) Laramide-style (monoclines) deformation in the continental interior at ca. 1250 Ma records Grenville-age shortening; and (2) ca. 1100 Ma detrital muscovite (Ar/Ar) and zircon (U/Pb) indicate an Unkar Group source in the Grenville-age highlands of, southwestern Laurentia during development of NW-striking extensional basins. We conclude that far-field stresses related to Grenville-age orogenesis (NW shortening and orthogonal NE-SW extension) dominated the sedimentary and tectonic regime of southwestern Laurentia from 1250 to 1100 Ma. © 2005 Geological Society of America.
- Butzer, C., Butler, R. F., Gehrels, G. E., Davidson, C., O'Connell, K., & Crawford, M. L. (2004). Neogene tilting of crustal panels near Wrangell, Alaska. Geology, 32(12), 1061-1064.More infoAbstract: A late Oligocene-Miocene igneous complex south and west of Wrangell, Alaska, contains mafic dikes that yield a discordant paleomagnetic direction (inclination, I = 70.4°; declination, D = 39.3°; α95 = 4.8°; N = 72 sites). Combined with local and regional geobarometric, metamorphic, and structural observations, the discordant paleomagnetic direction indicates east-side-up tilt by 16° about a tilt axis with azimuth = 8°. Neogene tilt of crustal blocks in the Insular superterrane accounts for much of the paleomagnetic discordance in Cretaceous plutons without the coastwise translation of >1000 km, as suggested by the Baja British Columbia hypothesis. © 2004 Geological Society of America.
- Ducea, M. N., Gehrels, G. E., Shoemaker, S., Ruiz, J., & Valencia, V. A. (2004). Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology. Bulletin of the Geological Society of America, 116(7-8), 1016-1025.More infoAbstract: The Xolapa Complex of southern Mexico is composed of mid-crustal arc-related gneisses of poorly resolved ages, intruded by undeformed Cenozoic calc-alkaline plutons. Twelve undeformed and deformed tonalitic/granodioritic samples from three transects across the Sierra Madre del Sur (Acapulco, Puerto Escondido, and Puerto Angel) were chosen for U-Pb zircon analysis. The measurements were performed on single crystals of zircons, using a multiple-collector laser-ablation inductively coupled plasma-mass spectrometer (MC-LA-ICP-MS). About 20-30 crystals were measured from each sample. Three gneisses and migmatites from the eastern transect (Puerto Angel), located 30-42 km from the coast, yielded Grenville-aged zircons (970-1280 Ma), suggesting that the samples represent Oaxacan basement, not deformed Xolapa Complex. The central transect (Puerto Escondido) yielded Oligocene ages (25-32 Ma) on undeformed plutons as well as mid-Mesozoic and Permian ages on gneisses. Most samples along the Puerto Escondido transect contain inherited ca. 1.1 Ga xenocrystals of zircons. The western transect (Acapulco) yielded Late Jurassic-Early Cretaceous ages (160-136 Ma) on gneisses, and Paleocene (55 Ma) and Oligocene (34 Ma) ages on undeformed plutons, with no inherited Grenville ages. The older ages and xenocrystic zircons in arc-related Xolapa Complex mirror the crustal ages found in neighboring terranes (Mixteca and Oaxaca) to the north of the Xolapa Complex, suggesting an autochthonous origin of Xolapa with respect to its neighboring north-bounding terranes. The new data and previously published ages for Xolapa suggest that metamorphism and migmatization of the deformed arc rocks took place prior to the Cenozoic. Eocene and Oligocene plutons representing renewed arc-related magmatism in the area are common throughout Xolapa, and probably represent the more deeply exposed continuation of the Sierra Madre Occidental arc to the northwest. The available U-Pb data argue against the previously proposed eastward migration of magmatism between Acapulco and Puerto Angel during the Oligocene. © 2004 Geological Society of America.
- Klepeis, K. A., Clarke, G. L., Gehrels, G., & Vervoort, J. (2004). Processes controlling vertical coupling and decoupling between the upper and lower crust of orogens: Results from Fiordland, New Zealand. Journal of Structural Geology, 26(4), 765-791.More infoAbstract: The pre-Cenozoic configuration of western New Zealand allows determination of the effects of magmatism and a changing lower crustal rheology on the evolution of a Cretaceous orogen from upper to lower crustal levels (10-50 km). Beginning at ∼126 Ma, a composite batholith dominated by diorite was emplaced into the lower crust. During emplacement, deformation was partitioned into zones weakened by magma and heat, leading to the development of two layer-parallel shear zones at the upper and lower contacts of the batholith. Transient vertical decoupling of the crust above and below the batholith occurred from ∼126 Ma until ∼120 Ma as magma was emplaced into and moved through a weak, thick lower crust. By ∼116 Ma, however, much of the batholith had crystallized and the lowermost crust had cooled from 750 °C< T
- Thomas, W. A., Astini, R. A., Mueller, P. A., Gehrels, G. E., & Wooden, J. L. (2004). Transfer of the Argentine Precordillera terrane from Laurentia: Constraints from detrital-zircon geochronology. Geology, 32(11), 965-968.More infoAbstract: Ages from U-Pb and 207Pb/206Pb analyses of detrital zircons from synrift sandstone in the Lower Cambrian Cerro Totora Formation of the Argentine Precordillera have strong similarities to those from late synrift sandstones in the Lower Cambrian Rome Formation of southern Laurentia (Alabama). Ages of detrital zircons from the Cerro Totora sample cluster at 1160-970 Ma (60% of analyzed zircons , 1490-1300 Ma (24%), and 1890-1640 Ma (16%). Ages from two Rome samples cluster at 1240-970 Ma (32% of analyzed zircons), 1540-1270 Ma (31%), 1840-1610 Ma (14%), 1970-1890 Ma (5%), and 2930-2310 Ma (18%). The ages of detrital zircons from the Rome and Cerro Totora sandstones are consistent with sediment supply from the Grenville and older Proterozoic Laurentian provinces, and the older cluster in the Rome sandstones corresponds in age to the Laurentian Archean Superior province. Neither the Rome nor Cerro Totora samples include components younger than Grenville, and the lack of zircons from distinctly Gondwanan provinces is consistent with a Laurentian provenance. The detrital-zircon ages support previous interpretations that the Precordillera was rifted from the Ouachita embayment of Laurentia during Early Cambrian time and subsequently was transferred to Gondwana. © 2004 Geological Society of America.
- Yue, Y., Ritts, B. D., Graham, S. A., Wooden, J. L., Gehrels, G. E., & Zhang, Z. (2004). Slowing extrusion tectonics: Lowered estimate of post-Early Miocene slip rate for the Altyn Tagh fault. Earth and Planetary Science Letters, 217(1-2), 111-122.More infoAbstract: Determination of long-term slip rate for the Altyn Tagh fault is essential for testing whether Asian tectonics is dominated by lateral extrusion or distributed crustal shortening. Previous slip-history studies focused on either Quaternary slip-rate measurements or pre-Early Miocene total-offset estimates and do not allow a clear distinction between rates based on the two. The magmatic and metamorphic history revealed by SHRIMP zircon dating of clasts from Miocene conglomerate in the Xorkol basin north of the Altyn Tagh fault strikingly matches that of basement in the southern Qilian Shan and northern Qaidam regions south of the fault. This match requires that the post-Early Miocene long-term slip rate along the Altyn Tagh fault cannot exceed 10 mm/year, supporting the hypothesis of distributed crustal thickening for post-Early Miocene times. This low long-term slip rate and recently documented large pre-Early Miocene cumulative offset across the fault support a two-stage evolution, wherein Asian tectonics was dominated by lateral extrusion before the end of Early Miocene, and since then has been dominated by distributed crustal thickening and rapid plateau uplift. © 2003 Elsevier B.V. All rights reserved.
- Andronicos, C. L., Chardon, D. H., Hollister, L. S., Gehrels, G. E., & Woodsworth, G. J. (2003). Strain partitioning in an obliquely convergent orogen, plutonism, and synorogenic collapse: Coast Mountains Batholith, Biritish Columbia, Canada. Tectonics, 22(2), 7-1 - 7-24.More infoAbstract: We describe the crustal structure of the Coast Mountains batholith between 54° and 55°N, within the Canadian Cordillera, with emphasis on emplacement of the 7 km thick Kasiks sill complex (KSC). Kinematic patterns that developed during emplacement of the KSC are the result of interactions between magma transport, magma accumulation and regional deformation. The sills were emplaced during NW directed normal shearing and flattening of country rocks that host the KSC. A ∼2 km thick shallowly NE dipping mylonite zone cuts the eastern side of the KSC. Kinematic indicators within the mylonite zone record top to the east normal displacements. Structural analysis shows that mylonite formation occurred during subvertical shortening and east-northeast, subhorizontal extension. U'Pb zircon age dates show that ENE directed normal shearing along the eastern side of the KSC and WNW directed normal shearing within the KSC occurred contemporaneously between ∼54 and 51 Ma, indicating strong strain partitioning between the mylonite and the KSC. This pattern of strain partitioning is interpreted to have been driven by return flow of melt-laden crust in response to tectonic denudation of the upper crust. Seismic profiling shows that many of these structures extend to mid and lower crustal depths. Comparison of our results with other regions within the Canadian Cordillera indicates that orogen-scale right-lateral strike-slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes. Thus the crustal structure of the Coast Mountains batholith was the result of early Tertiary batholith construction during dextral oblique convergence and synorogenic collapse.
- Chen, X., George, G., Wang, X., Yang, F., & Chen, Z. (2003). Granite from North Altyn Tagh, NW China: U-Pb geochronology and tectonic setting. Bulletin of Mineralogy Petrology and Geochemistry, 22(4), 294-298.More infoAbstract: ID TIMS single zircon U-Pb geochronology is applied to determine the age of the big Kuoshibulake porphyritic granite in North Altyn Tagh, NW China, which gives a crystallization age of 443 ± 5 Ma. The age of the granite is of basis for understanding the main tectonic characteristics in this area and also provides a good reference for the division of the strata there.
- Chen, X., Yin, A., Gehrels, G. E., Cowgill, E. S., Grove, M., Harrison, T. M., & Wang, X. (2003). Two phases of Mesozoic north-south extension in the eastern Altyn Tagh range, northern Tibetan Plateau. Tectonics, 22(5), 8-1 - 8-22.More infoAbstract: The >300-km long, east striking Lapeiquan fault lies in the eastern Altyn Tagh range along the northern margin of the Tibetan Plateau and was interpreted as a north dipping thrust in early studies. However, our mapping shows that the fault is a south dipping normal fault juxtaposing Archean-Proterozoic gneisses beneath an early Paleozoic volcanic and sedimentary sequence. Its dip angle varies from
- Dickinson, W. R., & Gehrels, G. E. (2003). U-Pb ages of detrital zircons from Permian and Jurassic eolian sandstones of the Colorado Plateau, USA: Paleogeographic implications. Sedimentary Geology, 163(1-2), 29-66.More infoAbstract: Detrital zircon grains (n=468) from eolian sandstones of Permian and Jurassic sand seas on the Colorado Plateau of southwest Laurentia fall into six separable age populations defined by discrete peaks on age-probability plots. The eolian sands include significant contributions from all Precambrian age belts of the Laurentian craton and all key plutonic assemblages of the Appalachian orogen marking the Laurentia-Gondwana suture within Pangaea. Nearly half the detrital zircon grains were derived ultimately from Grenvillian (1315-1000 Ma), Pan-African (750-500 Ma), and Paleozoic (500-310 Ma) bedrock sources lying within or along the flank of the Appalachian orogen. Recycled origins for Appalachian-derived grains, except for temporary residence of synorogenic detritus in the Appalachian foreland basin or in deformed Ouachita flysch and molasses along tectonic strike, are precluded by regional geology and known geochronology from other Laurentian sedimentary assemblages. We infer that transcontinental Permian and Jurassic river systems transported detritus of Appalachian provenance westward across the subdued surface of the Laurentian craton, for deposition as proximate sources for eolian systems feeding the ergs, on unconsolidated fluvial plains, deltas, and strandlines that lay up-paleowind along or near the Cordilleran paleoshoreline north and northeast of the Colorado Plateau. The postulated river systems headed in the remnant Appalachian orogen (Permian) or the incipient Atlantic rift belt (Jurassic), and additional transport of the Appalachian-derived detritus toward the Colorado Plateau was achieved by longshore drift of sediment southward along the Cordilleran paleoshoreline under the influence of prevailing trade winds in the Permian-Jurassic tropics. Only a quarter of the eolianite detrital zircons were derived or recycled from Mesoproterozoic (1470-1335 Ma) and younger Paleoproterozoic (1800-1615 Ma) basement of the Ancestral Rocky Mountains province adjacent to the Colorado Plateau. The final quarter of eolianite detrital zircons were derived from older Paleoproterozoic (2200-1800 Ma) and Archaean (3015-2580 Ma) basement of the Laurentian shield, or recycled from its sedimentary cover. Both Laurentian shield and Ancestral Rockies detritus may have entered the same transcontinental river systems (through tributary streams), or the same Cordilleran strandline system (by longshore drift), responsible for the delivery of Appalachian-derived sediment to positions near the Colorado Plateau ergs. As Colorado Plateau ergs received contributions from all the potential bedrock sources contiguous with Permian-Jurassic Laurentia and its orogenic-taphrogenic margins, detrital zircon studies of analogous ancient erg deposits elsewhere may help test reconstructions of Rodinia and other ancient paleocontinents by providing proxy records of the full age ranges of bedrock sources distributed across the surfaces of entire landmasses. © 2003 Elsevier B.V. All rights reserved.
- Ducea, M. N., Lutkov, V., Minaev, V. T., Hacker, B., Ratschbacher, L., Luffi, P., Schwab, M., Gehrels, G. E., McWilliams, M., Vervoort, J., & Metcalf, J. (2003). Building the Pamirs: The view from the underside. Geology, 31(10), 849-852.More infoAbstract: The Pamir mountains are an outstanding example of extreme crustal shortening during continental collision that may have been accommodated by formation of a thick crust - much thicker than is currently thought - and/or by continental subduction. We present new petrologic data and radiometric ages from xenoliths in Miocene volcanic rocks in the southeastern Pamir mountains that suggest that Gondwanan igneous and sedimentary assemblages were underthrust northward, buried to >50-80 km during the early stage of the India-Asia collision, and then heated and partly melted during subsequent thermal relaxation before finally being blasted to the surface. These xenoliths, the deepest crustal samples recovered from under any active collisional belt, provide direct evidence for early Cenozoic thickening of the Pamirs and lower-crustal melting during collision; the xenoliths also suggest that the present mountain range was a steady-state elevated plateau for most of the Cenozoic.
- Finney, S., Gleason, J., Gehrels, G., Peralta, S., & Acenolaza, G. (2003). Early Gondwanan connection for the Argentine Precordillera terrane. Earth and Planetary Science Letters, 205(3-4), 349-359.More infoAbstract: The Precordillera of Argentina is widely accepted as an exotic terrane of Laurentian (North American) affinity. Newly acquired U/Pb ages on individual detrital zircons from Lower Cambrian and Upper Ordovician quartz sandstone beds in the Argentine Precordillera indicate a Gondwanan provenance not associated with any known part of Laurentia. Accordingly, the Precordillera terrane is likely underlain by basement rock of Gondwanan affinity. In addition, detrital zircons from the Upper Ordovician sandstone bed provide no evidence for a Mid Ordovician position against the inboard Famatina arc. These results demand critical re-evaluation of widely held assumptions regarding the paleogeography of the Argentine Precordillera. © 2002 Elsevier Science B.V. All rights reserved.
- Gehrels, G. E., Yin, A., & Wang, X. (2003). Detrital-zircon geochronology of the northeastern Tibetan plateau. Bulletin of the Geological Society of America, 115(7), 881-896.More infoAbstract: U-Pb geochronologic analyses have been conducted on 413 detrital-zircon grains collected from 16 samples in the Altun Shan, Nan Shan, and Qilian Shan. The samples come primarily from quartz arenites and metaturbidites of Middle to Late Proterozoic age and from feldspathic and volcanic clast-rich sandstones of early Paleozoic age. Zircon grains in Proterozoic strata resting on Tarim basement yielded mainly 2.0-1.9 Ga ages, whereas Proterozoic strata of the Qaidam and Qilian terranes yielded mainly ca. 930-820 Ma and ca. 1.9-1.1 Ga ages. The younger grains were apparently shed from local igneous rocks, whereas the grains older than 1.1 Ga were shed from an undetermined continental source. Grains in the lower Paleozoic strata are mainly ca. 500-430 Ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting. Our detrital-zircon ages are consistent with a model (first proposed by E.R. Sobel and N. Arnaud) in which early Paleozoic magmatism occurred within a single northeast-facing magmatic arc that was constructed across an assemblage of Middle to Late Proterozoic accretionary complexes, remnants of magmatic arcs, and shallow-marine strata. This arc system was accreted to the Tarim and Sino-Korean cratons during Silurian-Devonian time. The resulting suture has been reactivated as Tertiary thrust faults that currently define the structural and topographic margin of the Tibetan plateau. Our data also provide two new estimates for the offset along the eastern Altyn Tagh fault. A belt of Middle Proterozoic shallow-marine strata is offset by ∼400 km, whereas a belt of 490-480 Ma magmatic arc rocks is offset by ∼370 km. These values are generally similar to the 350-400 km offset reported in most previous studies.
- Gehrels, G. E., Yin, A., & Wang, X. (2003). Magmatic history of the northeastern Tibetan Plateau. Journal of Geophysical Research B: Solid Earth, 108(9), ETG 5-1 - 5-14.More infoAbstract: The northeastern margin of the Tibetan Plateau is underlain by the Qaidam and Qilian terranes, which consist primarily of mid-Proterozoic through lower Paleozoic oceanic and arc-type assemblages that have been accreted to the southern margin of the Tarim/Sino-Korean craton. Most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the Tarim/Sino-Korean craton during early Paleozoic time. The main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins, or as a broad expanse of accretionary complexes. Our geochronologic data support a model, suggested by Sobel and Arnaud [1999], in which the Qaidam and Qilian terranes are separated from the Tarim/Sino-Korean craton by a mid-Paleozoic suture that closed along a southwest dipping subduction zone. The basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of ∼920-930 Ma granitoids. Early Paleozoic arc-type magmatism occurred between ∼480 and ∼425 Ma, apparently sweeping southwestward across much of the Qilian and Qaidam terranes. Accretion-related magmatism along the inboard margin of the Qilian terrane occurred between ∼423 Ma and ∼406 Ma. Following Silurian-Devonian accretion, the region has experienced late Paleozoic and Mesozoic uplift and erosion and has been severely overprinted by Tertiary thrusting, uplift, and strike-slip motion along the Altyn Tagh fault. Correlation of geologic features and magmatic histories between the Altun Shan and the Nan Shan suggests that the eastern Altyn Tagh fault has a total left-lateral offset of ∼375 km.
- Kidder, S., Ducea, M., Gehrels, G., Patchett, P. J., & Vervoort, J. (2003). Tectonic and magmatic development of the Salinian Coast Ridge Belt, California. Tectonics, 22(5), 12-1 - 12-20.More infoAbstract: We present new field, structural, petrographic, and geochronologic data on a rare midcrustal (∼25 km) exposure of a Cordilleran arc, the Coast Ridge Belt, located in the Santa Lucia Mountains of central California. The study area is composed primarily of a deformed suite of upper amphibolite to granulite facies rocks (the "Sur Series"), which is dominated by metaigneous tonalites, diorites, and gabbros with subordinate metasedimentary quartzite and marble. Inherited zircons in magmatic rocks suggest that the provenance of framework rocks is drawn heavily from miogeoclinal formations and that sedimentation occurred in the late Paleozoic or later. Minor magmatism in the Coast Ridge Belt began in the Early or Middle Cretaceous, but magmatic activity was most intense during a short period time from 93 to 81 Ma, based on U-Pb zircon ages of a felsic gneiss and two less-deformed diorites. The time period 93-81 Ma also brackets a period of extensive thickening and high-temperature ductile deformation. While a thrusting cause for ductile deformation cannot be ruled out, we favor the hypothesis that the exposed rocks correspond to a zone of return flow of supracrustal rocks locally displaced by granitoid plutons in the shallower crust. Magmatism ended throughout Salinia between 81 and 76 Ma, coincident with the attainment of peak pressure and temperature conditions of 0.75 GPa and 800°C. Exhumation followed immediately, bringing the Coast Ridge Belt to the surface within 8 My at a rate of at least 2-3 mm/yr. Exhumation was coincident with an episode of extensional collapse that has been documented elsewhere in the southern California arc during the early Laramide orogeny and that may be related to underthrusting of the forearc at that time. Copyright 2003 by the American Geophysical Union.
- Riggs, N. R., Ash, S. R., Barth, A. P., Gehrels, G. E., & Wooden, J. L. (2003). Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone. Bulletin of the Geological Society of America, 115(11), 1315-1323.More infoAbstract: Zircons from the Black Forest Bed, Petrified Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 ± 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded U-PB ages of 2706 ± 6 Ma to 206 ± 6 Ma. Eleven of these ages lie between 211 and 216 ± 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 ± 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide 206Pb*/238U ages from 214 ± 2 Ma to 200 ± 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of the age of the bed. We further propose that, even if some degree of reworking has occurred, the very strong concentration of ages at ca. 213 Ma provides a maximum age for the Black Forest Bed of 213 ± 1.7 Ma. The actual age of the bed may be closer to 209 Ma. Dating continental successions is very difficult when distinct ash beds are not clearly identified, as is the case in the Chinle Formation. Detrital zircons in the Black Forest Bed, however, are dominated by an acicular morphology with preserved delicate terminations. The shape of these crystals and their inferred environment of deposition in slow-water settings suggest that the crystals were not far removed from their site of deposition in space and likely not far in time. Plinian ash clouds derived from explosive eruptions along the early Mesozoic Cordilleran margin provided the crystals to the Chinle basin, where local conditions insured their preservation. In the case of the Black Forest Bed, the products of one major eruption may dominate the volcanic contribution to the unit. Volcanic detritus in the Chinle Formation was derived from multiple, distinct sources. Coarse pebble- to cobble-size material may have originated in eastern California and/or western Arizona, where Triassic plutons are exposed. Fine-grained detritus, in contrast, was carried in ash clouds that derived from caldera eruptions in east-central California or western Nevada.
- Robinson, D. M., Dupont-Nivet, G., Gehrels, G. E., & Zhang, Y. (2003). The Tula uplift, northwestern China: Evidence for regional tectonism of the northern Tibetan Plateau during late Mesozoic-early Cenozoic time. Bulletin of the Geological Society of America, 115(1), 35-47.More infoAbstract: Geologic mapping combined with petrographic and geochronologic studies in the Tula uplift of western China provides insights into the tectonic evolution of the northern edge of the Tibetan Plateau. The Mesozoic and early Cenozoic history of the area is preserved in the Tula uplift, which includes basin strata now exposed in a large syncline, pre-Mesozoic metamorphic basement, and Cretaceous plutons. Petrographic analyses of Upper Jurassic through Paleogene syntectonic sandstones show that rocks in the area were derived from lithologically diverse source terranes consisting of sedimentary, metasedimentary, and igneous rocks. These relationships imply that uplift in the Tula area began in Late Jurassic time and that uplift of the Tibetan Plateau's northern edge may have been initiated long before the early Tertiary India- Asia collision. Continued orogenic activity in the Tula area is recorded by intrusion of ca. 74 Ma granitoid bodies, latest Cretaceous to Paleogene shortening, uplift of Precambrian basement rock, the syntectonic nature of Cretaceous and Paleogene sandstones, and folding of all the basin strata into a regional north-vergent syncline. The northern range-bounding thrust of the Tula uplift has been recently active, suggesting that uplift and thickening continue in the northern Tibetan Plateau.
- Butler, R. F., Gehrels, G. E., Baldwin, S. L., & Davidson, C. (2002). Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large-scale transport. Journal of Geophysical Research B: Solid Earth, 107(1), 3-1 - 3-13.More infoAbstract: Samples for geochronologic, geobarometric, and paleomagnetic analyses were collected across the northern portion of the Ecstall pluton southeast of Prince Rupert, British Columbia. Al-in-hornblende geobarometry indicates pressures from 740 ± 10 to 840 ± 30 MPa corresponding to crystallization depths of ~25 to ~30 km. U/Pb analyses of zircons from western, central, and eastern localities within the pluton yield crystallization ages of 91.5 ± 1.0 Ma, 90.8 ± 1.0 Ma, and 90.5 ± 1.0 Ma, respectively. Rock magnetic experiments, reflected light microscopy, and thermal demagnetization behavior suggest that natural remanent magnetism is carried by low-Ti titanohematite. Unblocking temperatures of the characteristic remanent magnetization (ChRM) are dominantly in the 560°C to 630°C range, with age of magnetization approximated by the 40 Ar/39 Ar hornblende ages of 84.2 ± 0.10 Ma on the western margin and 76.4 ± 0.6 Ma in the center of the pluton. Site-mean ChRM directions were isolated for paleomagnetic samples from 23 sites and are distributed along a small circle with subhorizontal axis at ~340° azimuth. ChRM directions from the central portion of the pluton are concordant with the expected Cretaceous magnetic field direction, while ChRM directions from the western margin are discordant by >70°. Folding of the Ecstall pluton, either during Late Cretaceous west directed thrust transport above the convex upward Prince Rupert Shear Zone or during younger deformation of the pluton and underlying shear zone, can account for the paleomagnetic data and is consistent with the geochronologic, geobarometric, and structural geologic observations.
- Gehrels, G. E. (2002). Detrital zircon geochronology of the Taku terrane, southeast Alaska. Canadian Journal of Earth Sciences, 39(6), 921-931.More infoAbstract: U-Pb geochronologic studies have been conducted on 60 detrital zircon grains from Permian(?) and Triassic metasandstones of the Taku terrane in central southeast Alaska. The resulting ages are mainly in the range 349-387 Ma, with five additional grains that yield probable ages ranging from ∼906 to ∼2643 Ma. These ages are similar to the ages of detrital zircons in Carboniferous and older rocks of the Yukon-Tanana terrane, which lies directly east of the Taku terrane. In contrast, these ages are different from the ages of detrital zircon grains in the Alexander terrane to the west. The data are accordingly consistent with models in which the Taku terrane is a western component of the Stikine and Yukon-Tanana terranes, and that this crustal fragment is separated by a fundamental tectonic boundary from rocks of the Alexander and Wrangellia terranes to the west.
- Gehrels, G. E., Stewart, J. H., & Ketner, K. B. (2002). Cordilleran-margin quartzites in Baja California - Implications for tectonic transport. Earth and Planetary Science Letters, 199(1-2), 201-210.More infoAbstract: One of the current controversies in Cordilleran tectonics concerns the position of Baja California prior to ∼ 300 km of opening of the Gulf of California. Geologic arguments, together with paleomagnetic results from Lower Cretaceous volcanic rocks, suggest that the rocks of the Baja Peninsula formed and evolved along the coast of northwestern Mexico prior to opening of the Gulf. In contrast, paleomagnetic data from Cretaceous-early Tertiary plutonic rocks and clastic strata have been interpreted by some workers to suggest that Baja was located near southern Mexico at approximately 80 Ma. The presence of similar detrital zircon ages in lower Paleozoic quartzites of northeast Baja and in lower Paleozoic strata east of the Gulf, in northwestern Mexico and southwestern US, provides strong support for the northern paleoposition, suggesting that Baja has been transported northward by only ∼ 300 km. © 2002 Elsevier Science B.V. All rights reserved.
- Gleason, J. D., Finney, S. C., & Gehrels, G. E. (2002). Paleotectonic implications of a Mid- to Late-Ordovician provenance shift, as recorded in sedimentary strata of the Ouachita and southern Appalachian Mountains. Journal of Geology, 110(3), 291-304.More infoAbstract: North American Ordovician strata record a large shift in their neodymium isotopic composition (ΔεNd = ~ 7) at around 450 Ma. As part of a continuing effort to understand this phenomenon, we studied the provenance of Middle and Upper Ordovician clastic sedimentary rocks along a transect through the southern Appalachian and Ouachita Mountains using a combination of high-resolution graptolite-neodymium isotopic analysis and U-Pb dating of detrital zircon. Taconian (Blountian)-age clastic sediment (ca. 465 Ma) in the southern Appalachians (εNd = -8) carries a neodymium isotopic signature distinct from strata of the same age in the Ouachita region (εNd = -15). However, the Blountian signature (εNd = -8) becomes firmly established in the eastern Ouachita region by 455 Ma (Arkansas), and by 450 Ma in the westernmost part of the Ouachita fold belt (Oklahoma). In the Ouachitas, craton-dominated sources (εNd = -15) apparently mixed with orogenic sources (εNd = -8) in variable proportions over a several-million-year period before orogenic sources became dominant. We conclude from this that, superimposed on a general westward regional shift in sediment sources with time, there were also complex local effects involving multiple (unmixed) sediment sources that persisted long after the initial pulse of orogenic material arrived. The combined "simultaneous" nature of the isotopic shift, an Ordovician sea-level high stand, and the emergence of the Appalachian-Taconian-Caledonian orogenic belt as a primary sediment source, leads us to conclude that by 450 Ma, seafloor south of North America was being supplied by well-mixed, isotopically homogeneous sediment delivered from uplifted fold-thrust belts and foreland basins of the Appalachian Taconian highlands. U-Pb detrital zircon ages from bracketing sandstone units reinforce the Nd evidence for a complete changeover in provenance between 465 Ma (abundant Archean-age zircons) and 440 Ma (no Archean-age zircons) in the Ouachita region.
- Butler, R. F., Gehrels, G. E., & Kodama, K. P. (2001). Baja British Columbia hypothesis. GSA Today, 11(6), 4-10.More infoAbstract: The Baja British Columbia (Baja B.C.) hypothesis derives primarily from observed discordant paleomagnetic directions of Insular and Intermontane terranes interpreted to indicate post-mid-Cretaceous northward transport by up to 4000 km with respect to the continental interior. Recent paleomagnetic results from near Prince Rupert, British Columbia, document that discordant directions in plutonic rocks of this area are primarily due to local deformation rather than transport from a Cretaceous location along the southwest margin of North America. Expanded paleomagnetic studies of Cretaceous rocks at Duke Island and MacColl Ridge, Alaska, have led to much smaller estimates of latitudinal motion for Insular terranes than previously concluded. Recognition of likely chemical remagnetization of Cretaceous sequences in the Tyaughton and Methow basins, along with effects of compaction shallowing on marine sediments of the Nanaimo Group, indicate that paleomagnetic data from these rocks may not require the magnitude of northward transport suggested by the Baja B.C. paleogeography. Instead, the paleomagnetic observations are consistent with a Cretaceous paleogeography that limits post-mid-Cretaceous northward motion to ∼ 1000 km. The resulting paleogeography has the appealing feature of an Andean-like, continuous subduction-related magmatic arc along the Cordilleran margin during Cretaceous time.
- Butler, R. F., Gehrels, G. E., & Saleeby, J. B. (2001). Paleomagnetism of the Duke Island, Alaska, ultramafic complex revisited. Journal of Geophysical Research B: Solid Earth, 106(B9), 19259-19269.More infoAbstract: The Duke Island ultramafic intrusion was emplaced into the Alexander terrane immediately preceding development of a regional mid-Cretaceous thrust belt. Paleomagnetic samples were collected from exposures of ultramafic rock with cumulate layering northwest of Judd Harbor and northwest of Hall Cove. Thermal demagnetization results were analyzed using principal component analysis to isolate the characteristic remanent magnetization. Site-mean characteristic directions determined from 16 sites fail the fold test at 95% confidence, indicating that cumulate layering attitudes were highly contorted at the time of magnetization, at least on a scale of tens of meters. Variations in cumulate layering attitudes probably resulted from the combined effects of thermal convection phenomena during crystallization and deformation following crystallization but prior to magnetization. Analysis of cumulate layering over larger structural domains indicates that kilometer-scale deformation produced southwest plunging folds within the Hall Cove and Judd Harbor bodies. Bogue et al. [1995] proposed that a compound structural correction involving unplunging of fold axes followed by unfolding of average cumulate layering could restore cumulate layering to horizontal. However, using the full set of 21 site-mean paleomagnetic directions from Duke Island (16 from the current study and 5 from Bogue et al. [1995]), the compound structural correction yields mean paleomagnetic directions from the Judd Harbor and Hall Cove areas that are statistically distinguishable at 99% confidence. This result indicates that even on the kilometer-scale, cumulate layering within the Duke Island ultramafic intrusion was neither coplanar nor horizontal at the time of magnetization. Observations of cumulate layering in other ultramafic intrusive rocks indicate that this layering can significantly depart from horizontal by 10°-20° even on the kilometer scale. Therefore use of cumulate layering of ultramafic rocks as a proxy for paleohorizontal is not justified, and paleomagnetic directions from the Duke Island ultramafic intrusion cannot be used to infer the Cretaceous paleolatitude of the Insular superterrane. Copyright 2001 by the American Geophysical Union.
- Butler, R. F., Gehrels, G. E., Crawford, M. L., & Crawford, W. A. (2001). Paleomagnetism of the Quottoon plutonic complex in the Coast Mountains of British Columbia and southeastern Alaska: Evidence for tilting during uplift. Canadian Journal of Earth Sciences, 38(9), 1367-1385.More infoAbstract: The Quottoon plutonic complex (part of the great tonalite sill) was emplaced into eastern parts of the Coast shear zone along the west flank of the Coast Mountains. U-Pb crystallization ages range from 72.3 to 55.5 Ma. A regional compilation of K-Ar hornblende dates from the tonalite sill indicates a west to east decrease from ~160 Ma to ~154 Ma across the intrusive suite. Paleomagnetic samples were collected at 160 sites (≥ 8 samples per site) along six transects across the Quottoon complex between the Skeena River and Willard Inlet. Directions of characteristic remanent magnetization (ChRM) were successfully determined by principal component analysis of thermal demagnetization data for about half of the samples collected. Site-mean ChRM directions were determined for 56 sites from the Skeena River, Quottoon Inlet, Steamer Passage, and Wales Island transects, using criteria of ≥ 4 sample ChRM directions per site and 95% confidence limit (α95) ≤ 25°. For Filmore Island and Willard Inlet, few sites provided adequate determinations of site-mean ChRM directions, but many sample ChRM directions were well determined and consistent. The mean direction from the Skeena River - Quottoon Inlet collection is nearly concordant with the expected Eocene direction. Mean directions from other transects have clockwise-rotated declinations and inclinations that are shallower than the expected direction by up to 23°. Consistent with expectations for east-side-up tilting of crustal panels, sites from the western part of each transect have mean directions farther from the Eocene expected direction than do sites from the eastern part of the transect. The magnitude of east-side-up tilt varies along strike: The Skeena River to Quottoon Inlet segment experienced little or no tilt; Steamer Passage south of Portland Inlet records 15° tilt; north of Portland Inlet, tilts are 40° for Wales Island and 35° for Filmore Island; farther north, tilt decreases to 12° for Willard Inlet. Tilting was driven by Eocene extension of the Coast Mountains, with tilted crustal panels bounded by northwest-striking, east-side-down normal faults and northeast-striking transfer faults or shear zones. The ACCRETE seismic transect imaged east-dipping reflectors down to ~15 km depth, which likely correspond to the east-side-down normal faults. The most significant of the northeast-striking transfer structures must underlie Portland Inlet. Based on a tilting domino model, about 30% extension is required to produce the 40° tilt of crustal blocks at Wales and Filmore islands.
- Davis, G. A., Zheng, Y., Wang, C., Darby, B. J., Zhang, C., & Gehrels, G. (2001). Mesozoic tectonic evolution of the Yanshan fold and thrust belt, with emphasis on Hebei and Liaoning provinces, northern China. Memoir of the Geological Society of America, 194, 171-197.More infoAbstract: The Yanshan (Yan Mountains) of northern China extend westward at about lat 40°N from Bohai Bay and Liaoning province to the border between Hebei province and Inner Mongolia. It is likely, but not unequivocally demonstrated, that the Archean-floored Yanshan continue farther westward under a cover of Neogene strata to emerge as the Yinshan belt of Inner Mongolia. Mesozoic terrestrial sedimentation, magmatism and deformation-including multiple phases of folding and contractional, extensional, and strike-slip faulting-characterize the Yanshan fold and thrust belt. Field studies and radiometric dating (U-Pb, 40Ar-39Ar) of plutonic and volcanic rock units in Beijing Municipality and northern Hebei and western Liaoning provinces have revealed that the complexity of Mesozoic deformation in these areas is in large part (1) a consequence of profound earlier deformation of Permian(?) to early Mesozoic age, and (2) an unusual younger Mesozoic history of alternating northward and southward tectonic vergence of major structures. Major south-directed low-angle thrust faulting of pre-Middle Jurassic age (7180 Ma) in the Yanshan involved Archean basement rocks and their Proterozoic and Phanerozoic cover, and developed south of a Permian-Triassic magmatic arc. Thrusting could have been (1) a consequence of the collisional suturing of Paleozoic Mongolian arcs against an Andean-style continental arc along the northern margin of the North China plate, or (2) an expression of a backarc, foreland fold and thrust belt of U.S. Cordilleran type formed during southward subduction beneath the North China Archean "craton." This episode of thrusting and folding was followed by widespread erosion of upper plate rocks and subsequent terrestrial deposition of Middle Jurassic volcanic and sedimentary strata across both plates. Our studies indicate that Late Jurassic and Early Cretaceous contractional deformation in the Yanshan was also much more intense than generally believed, and that it followed a heretofore unrecognized phase of Middle Jurassic or early Late Jurassic east-west extension in northern Hebei province. Prior to our studies, popular views of east- to east-northeast- trending Yanshan contractional deformation proposed that (1) involvement of Archean basement rocks in faulting indicates a thickskinned tectonic style analogous to the U.S. Laramide Rocky Mountains; (2) thrust faults steepen downward into the basement; (3) vertical movements predominated in the development of the belt; and (4) Mesozoic contraction across the belt was only a few tens of percent; some workers have believed that the Yanshan developed independently of plate interactions. We question all of these assumptions. We have identified a major, synformally folded, thin-skinned thrust plate just south of Chengde, Hebei province, that is of Late Jurassic age and had a minimum northward displacement of &40 km. In earlier studies in the Yunmeng Shan area of the Yanshan north of Beijing we defined major south-vergent Late Jurassic-Early Cretaceous ductile structures involving Archean basement rocks and their cover. These structures include a recumbent, basement-cored anticlinal nappe and a lower limb ductile thrust fault within a 6-km-thick gneissic shear zone. The Jurassic-Cretaceous Yanshan belt, and its probable western continuation, the Yinshan belt of Inner Mongolia, appear to be reflect regional north-south intraplate shortening. However, some Yanshan patterns of deformation, e.g., ductile nappe formation in the Yunmeng Shan and northeast structural trends of the belt in Liaoning province, were influenced by thermal regimes related to magmatism (≤ 180-190 Ma) accompanying westward or northwestward Pacific plate subduction beneath eastern Asia. We thus believe it likely that two contrasting modes of plate interaction occurred synchronously in the Yanshan segment of the Yinshan-Yanshan belt during Middle Jurassic through Early Cretaceous time. Jurassic-Cretaceous collision of an amalgamated North China-Mongolian plate with the Siberian plate is widely believed to have accompanied closure of a Mongolo- Okhotsk sea more than 800-1100 km north of the Yanshan belt. This collision might have been responsible forYanshan (andYinshan) intraplate contractional deformation farther south, but such a hypothesis is severely complicated by reports of widespread, basin-forming Late Jurassic and Early Cretaceous extension in the terranes between the Mongolo-Okhotsk suture and the Yinshan-Yanshan belt. Following Early Cretaceous contraction in both the Yinshan and Yanshan belts, the northern margin of the Archean-floored plate was also the site of major northwest-southeast regional extension beginning soon after 120 Ma. Subducting plate rollback or postorogenic collapse are only two of several possible explanations for the development of extensional metamorphic core complexes in northern InnerMongolia and theYanshan belt farther east.
- Gehrels, G. E. (2001). Geology of the Chatham Sound region, southeast Alaska and coastal British Columbia. Canadian Journal of Earth Sciences, 38(11), 1579-1599.More infoAbstract: The Coast Mountains orogen is thought to have formed as a result of accretion of the Alexander and Wrangellia terranes against the western margin of the Stikine and Yukon-Tanana terranes, but the nature and age of accretion remain controversial. The Chatham Sound area, which is located along the west flank of the Coast Mountains near the Alaska - British Columbia border, displays a wide variety of relations that bear on the nature and age of the boundary between inboard and outboard terranes. Geologic and U-Pb geochronologic studies in this area reveal a coherent but deformed and metamorphosed sequence of rocks belonging to the Yukon-Tanana terrane, including pre-mid-Paleozoic marble, schist, and quartzite, mid-Paleozoic orthogneiss and metavolcanic rocks, and upper Paleozoic metaconglomerate and metavolcanic rocks. These rocks are overlain by Middle Jurassic volcanic rocks (Moffat volcanics) and Upper Jurassic - Lower Cretaceous strata of the Gravina basin, both of which also overlie Triassic and older rocks of the Alexander terrane. This overlap relationship demonstrates that the Alexander and Wrangellia terranes were initially accreted to the margin of inboard terranes during or prior to mid-Jurassic time. Accretion was apparently followed by Late Jurassic - Early Cretaceous extension-transtension to form the Gravina basin, left-slip along the inboard margin of Alexander-Wrangellia, mid-Cretaceous collapse of the Gravina basin and final structural accretion of the outboard terranes, and early Tertiary dip-slip motion on the Coast shear zone.
- Grasse, S. W., Gehrels, G. E., Lahren, M. M., Schweickert, R. A., & Barth, A. P. (2001). U-Pb geochronology of detrital zircons from the Snow Lake pendant, central Sierra Nevada - implications for Late Jurassic-Early Cretaceous dextral strike-slip faulting. Geology, 29(4), 307-310.More infoAbstract: The Snow Lake pendant is underlain by a thick (>1500 m) sequence of predominantly quartzite, marble, and schist that has previously been correlated with miogeoclinal strata of the western Mojave Desert. In this study, U-Pb analyses of detrital zircons from the Snow Lake pendant have been conducted to test for possible correlations with strata of the Cordilleran miogeocline, as well as nearby rocks of the Roberts Mountains allochthon and the Shoo Fly Complex. Zircons from Snow Lake strata yield dominant age groups that strongly support correlation of Snow Lake strata with miogeoclinal strata in the Mojave Desert. This correlation provides additional support for previous suggestions that the Snow Lake pendant was displaced ∼400 km northward along the Mojave-Snow Lake fault.
- Rusmore, M. E., Gehrels, G., & Woodsworth, G. J. (2001). Southern continuation of the Coast shear zone and Paleocene strain partitioning in British Columbia-Southeast Alaska. Bulletin of the Geological Society of America, 113(8), 961-975.More infoAbstract: This paper documents the newly recognized southern continuation of the early Tertiary Coast shear zone, extending its known length by ~350 km to more than 1200 km. Three sites along the shear zone in British Columbia, Douglas Channel, Bella Coola, and Machmell River, have similar histories during the period ca. 65-55 Ma. The shear zone is 2-11 km thick and is defined by well-developed mylonite zones that strike northwest and dip steeply northeast. Motion on the shear zone was predominantly reverse, with the northeast side up. Synkinematic plutons are common in the shear zone. Lower plate rocks, high-grade gneiss derived from an ancient continental margin assemblage, show little evidence of the extensive deformation and plutonism in the shear zone. North of Bella Coola, high-grade gneiss forms the upper plate, but to the south weakly metamorphosed rocks of Stikinia compose the upper plate. Geochronologic data show that the shear zone was active between ca. 60 and 55 Ma at Douglas Channel, ca. 62 and 56 Ma at Bella Coola, and after 66 and before 56 Ma near the Machmell River. These features match those of the Coast shear zone in southeast Alaska and adjacent British Columbia. Together the shear zones formed a continental-scale reverse ductile fault in Paleocene time. Plate reconstructions show an oblique component to the dominantly dextral transcurrent Paleocene margin. Partitioning of this motion across the continental margin produced a regional system of strike-slip faults and contraction on the Coast shear zone.
- Stewart, J. H., Gehrels, G. E., Barth, A. P., Link, P. K., Christie-Blick, N., & Wrucke, C. T. (2001). Detrital zircon provenance of Mesoproterozoic to Cambrian arenites in the Western United States and Northwestern Mexico. Bulletin of the Geological Society of America, 113(10), 1343-1356.More infoAbstract: U-Pb isotopic dating of detrital zircon from supracrustal Proterozoic and Cambrian arenites from the western United States and northern Mexico reveal three main age groups, 1.90 to 1.62 Ga, 1.45 to 1.40 Ga, and 1.2 to 1.0 Ga. Small amounts of zircons with ages of 3.1 to 2.5 Ga, 1.57 Ga, 1.32 Ga, 1.26 Ga, 0.7 Ga, and 0.5 Ga are also present. Detrital zircons ranging in age from 1.90 to 1.62 Ga and from 1.45 to 1.40 Ga are considered to have been derived from Proterozoic crystalline basement rocks of these known ages, and probably in part from reworked Proterozoic supracrustal sedimentary rocks, of the western United States. The 1.2 to 1.0 Ga detrital zircon ages from California, Arizona, and Sonora are characterized by distinct spikes (1.11 Ga, in particular) in the age-probability plots. These spikes are interpreted to indicate the influx of zircon from major silicic volcanic fields. Igneous rocks such as the Pikes Peak Granite (1.093 Ga) of Colorado, and the Aibo Granite (1.110 Ga) of Sonora, Mexico, may represent the deeply eroded roots of such volcanic fields. Samples from farther north along the Cordilleran margin that contain abundant 1.2-1.0 Ga detrital zircons do not show spikes in the age distribution, but rather ages spread out across the entire 1.2-1.0 Ga range. These age spectra resemble those for detrital zircons from the Grenville province, which is considered their source. Less common detrital zircons had a variety of sources. Zircons ranging in age from 3.36 to 2.31 Ga were apparently derived from inland parts of the North American continent from Wyoming to Canada. Zircons of about 1.577 Ga are highly unusual and may have had an exotic source; they may have come from Australia and been deposited in North America when Australia and North America were juxtaposed as part of the hypothetical Rodinian supercontinent. Detrital zircon of Ο1.320 Ga apparently had the same source as that for tuff (1.320 Ga) in the Pioneer Shale of the Apache Group in Arizona. Detrital zircons of about 1.26 Ga in the Apache Group and Troy Quartzite appear to be related to local, approximately coeval volcanic fields. Zircons of about 0.7 Ga may have had a source in igneous rocks related to rifting of the Proterozoic supercontinent of Rodinia, and 0.5 Ga zircons a source in relatively small areas of granitic rocks of this known, or inferred, age in Oklahoma, Texas, New Mexico, and Colorado.
- Wang, T., Zheng, Y., Gehrels, G. E., & Mu, Z. (2001). Geochronological evidence for existence of South Mongolian microcontinent - A zircon U-Pb age of grantoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex. Chinese Science Bulletin, 46(23), 2005-2008.More infoAbstract: A zircon U-Pb age of (916±16) Ma is measured for grantoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex and represents the crystallization age of the grantoid magma. This age provides evidence for the existence of the South Mongolian microcontinent, which is consistent with the analysis of the regional geology.
- Boghossian, N. D., & Gehrels, G. E. (2000). Nd isotopic signature of metasedimentary pendants in the Coast Mountains between Prince Rupert and Bella Coola, British Columbia. Special Paper of the Geological Society of America, 343, 77-87.More infoAbstract: Three belts of supracrustal metamorphic pendants are preserved within Jurassic to Tertiary plutons of the Coast Mountains between Prince Rupert and Bella Coola, British Columbia. On the basis of their Nd and Sr isotopic signatures, lithologic characteristics, and U-Pb detrital zircon ages, these supracrustal metamorphic rocks are interpreted to represent two belts of continental margin rocks separated by a narrow belt of rocks belonging to the Alexander terrane. The easternmost belt, referred to as the Burke Channel assemblage, and the westernmost belt, referred to as the Banks Island assemblage, consist mainly of interbedded quartzite, marble, metapelite, and metavolcanic rocks. The Burke Channel assemblage represents a heterogeneous crustal fragment containing both juvenile mantle derived and crustally evolved isotopic signatures. The εNd (100 Ma) values of rocks belonging to the Burke Channel assemblage range from -10.3 to +6.6 and 87Sr/86Sr (100 Ma) values range from 0.70418 to 0.70791. The Banks Island assemblage rocks are more systematically continental in their isotopic signatures, with εNd (100 Ma) ranging from +0.5 to -9.9 and 87Sr/86Sr (100 Ma) ranging from 0.71178 to 0.71934. The central belt of supracrustal rocks includes a metaclastic sequence that is dominated by a granitoid clast metaconglomerate that can be traced from the Bella Coola region north into Porcher Island. On the basis of their lithologic characteristic and U-Pb geochronology, the central belt rocks were correlated with the Lower Devonian Karheen Formation of the Alexander terrane. The Nd isotopic signatures of these rocks support this correlation. Their εNd(t) values range from +2.2 to +6.9 and indicate that a significant fraction of these rocks is juvenile and mantle derived.
- Crawford, M. L., Crawford, W. A., & Gehrels, G. E. (2000). Terrane assembly and structural relationships in the eastern Prince Rupert quadrangle, British Columbia. Special Paper of the Geological Society of America, 343, 1-21.More infoAbstract: The eastern part of the Prince Rupert quadrangle, British Columbia, is subdivided into two regions: the western metamorphic belt and the central belt. The western metamorphic belt is underlain by five distinct mappable rock sequences. From west to east these are the Digby, Venn, Delusion Bay, Kaien, and Tsimpsean sequences. Rocks of the Digby sequence seem to correlate with Triassic and upper Paleozoic rocks of the Alexander terrane; the overlying Venn sequence contains rocks of the same age as parts of the Gravina belt. The Tsimpsean sequence is considered to be part of the Yukon-Tanana terrane, a continental margin assemblage. Correlation of the two intervening sequences, Kaien and Delusion Bay, is uncertain; they may belong either to the Taku terrane or to the Yukon-Tanana terrane. We tend to favor the latter interpretation. North- to northwest-striking thrust faults approximately bisect the belt and separate the Digby and Venn sequences on the west from the structurally overlying units to the east. Near Prince Rupert, west-directed, probably mid- Cretaceous, thrusting accompanied emplacement of tonalite and basalt dikes and sills. These thrusts and others to the north juxtapose upper amphibolite facies rocks over greenschist facies schist. The northern thrusts appear to postdate the regional metamorphism. Above and below the thrusts the rocks show intense ductile deformation. The Prince Rupert shear zone, east of the thrusts, records a strong flattening deformation, possibly due to overthrusting by the 91 Ma Ecstall pluton and associated high-grade gneiss. This tectonic emplacement of hot rocks and the associated igneous activity represented by syntectonic tonalite sills emplaced in the shear zone are inferred to be the cause of the 90 Ma amphibolite facies metamorphism that underlies the Prince Rupert shear zone. The western metamorphic belt is bounded on the east by the Coast shear zone, which separates the western belt from the central belt. This shear zone evolved during emplacement of 65-52 Ma plutons of the Paleogene Coast Mountains batholith that underlie much of the central belt. As the Coast Mountains batholith was emplaced, strain within the arc changed from dominantly contractional normal to the batholith and to the orogen, to extensional parallel to the length of the batholith. The latter stages of batholith emplacement apparently accompanied batholith exhumation. The youngest part of the Coast shear zone is a 1-2-km-wide zone of vertical fabric along the western side of the shear zone and is at the westernmost extent of 85-50 Ma plutons. This Work-Behm shear zone formed during the late stages of uplift of the western metamorphic belt by westward tilting about a northwest-trending hinge. After a hiatus with no identified tectonic activity other than gradual exhumation, Miocene and younger felsic and mafic dikes and young brittle faults cut the rocks throughout the eastern part of the quadrangle.
- Darby, B. J., Wyld, S. J., & Gehrels, G. E. (2000). Provenance and paleogeography of the Black Rock terrane, northwestern Nevada: Implications of U-Pb detrital zircon geochronology. Special Paper of the Geological Society of America, 347, 77-87.More infoAbstract: U-Pb ages have been determined for 50 detrital zircon grains from Mississippian and Triassic strata of the Black Rock terrane, northwestern Nevada. The Devonian(?) to Mississippian Pass Creek unit has three broad age groups: 976-1132 Ma (n = 8), 1595-1927 Ma (n = 10), and 2504-266 0 Ma (n = 3). The Triassic Bishop Canyon formation contains a dominant group of grains between 268 and 441 Ma (n = 11), a cluster of ages between 1868 and 1925 Ma (n = 5), and scattered ages between 1184 and 1813 Ma (n = 10) and between 2183 and 3183 Ma (n = 3). Most of the ages in these samples match well with the ages of grains present in basement provinces and off-shelf assemblages in the western United States. Grains in the Pass Creek unit were most likely recycled from lower Paleozoic strata of the Roberts Mountains allochthon and from strata exposed in the Salmon River arch region of Idaho, western Montana, and easternWashington. These provenance links suggest that the Black Rock terrane was located along the northern Nevada-I daho segment of the Cordilleran margin, near its current location, during late Paleozoic time. Because the upper Paleozoic stratigraphy of the Black Rock terrane is similar to that found in more outboard arc assemblages, including those of the Klamath Mountains and Sierra Nevada, this relation provides an indirect but important link between the U.S. continental margin and the more outboard arc assemblages. Zircon grains in the Upper Triassic Bishop Canyon formation were derived from a source containing both upper Paleozoic igneous rocks and clastic strata bearing 1.1-3 .2 Ga detrital zircons. The most likely source for this combination of grains is Paleozoic basement rocks of Mesozoic arc assemblages in the eastern Klamath Mountains and Black Rock terrane. These provenance links provide evidence of uplift and erosion of arc basement in the western Cordillera during early Mesozoic time, and support interpretations that lower Mesozoic arc assemblages in this region of the Cordillera were isolated from the continental margin by a Triassic backarc basin.
- Dickinson, W. R., & Gehrels, G. E. (2000). Sandstone petrofacies of detrital zircon samples from Paleozoic and Triassic strata in suspect terranes of northern Nevada and California. Special Paper of the Geological Society of America, 347, 151-171.More infoAbstract: We collected 35 sandstones and metasandstones for analysis of detrital zircons from Paleozoic and Triassic strata in suspect terranes of northern Nevada and northern California that represent five generic petrofacies: subarkose eroded from uplifted continental basement, quartzarenite (matrix-poor and matrix-rich subfacies) derived from cratonal sources, quartzolithic (quartz-rich, chert-rich, and quartzolithic-volcaniclastic subfacies) derived from recycled orogenic sources, volcaniclastic from volcanic arc structures, and volcanoplutonic from a dissected magmatic arc. Precambrian ages of dominant detrital zircons in the subarkose petrofacies (exclusively Harmony Formation of Cambrian age) and in most samples of the lower to middle Paleozoic quartzarenite petrofacies in the Roberts Mountains allochthon, the Shoo Fly Complex, and the Yreka terrane reflect a provenance in northern Laurentia or a continental block other than Laurentia. Samples from the lower part of the Vinini Formation in central Nevada are the only examples of the quartzarenite petrofacies yielding zircon populations that could have been derived from adjacent southwestern segments of Laurentia. Dominantly Grenville-age zircons in subordinate quartzose sandstones from the mainly subarkosic Harmony Formation suggest derivation from farther south along the Cordilleran continental margin. Zircons in the quartzolithic petrofacies were largely recycled into varied Paleozoic-Mesozoic overlap assemblages and successor basins, including the Havallah sequence of the Golconda allochthon, from older sedimentary sources in lower to middle Paleozoic strata of underlying or nearby terranes. Phanerozoic zircons in the upper Paleozoic to lower Mesozoic volcaniclastic petrofacies of the Sierran-Klamath belt reflect sources in coeval or older volcanic arc assemblages, with homogeneous age populations in each sample. Latest Precambrian and Grenville-age zircons in the volcanoplutonic petrofacies (one sample from the Sierra City mélange) reflect sources in a magmatic arc that was not part of the Cordilleran miogeoclinal margin. The close correlations between petrofacies compositions and the age spectra of detrital zircon populations provide significant and consistent constraints for interpretations of regional tectonic history, the two datasets jointly offering more interpretive insight than either affords alone.
- Gehrels, G. E. (2000). Introduction to detrital zircon studies of Paleozoic and Triassic strata in western Nevada and northern California. Special Paper of the Geological Society of America, 347, 1-17.More infoAbstract: U-Pb geochonologic analyses have been conducted on 648 individual detrital zircon grains from Paleozoic and Triassic strata of western Nevada and northern California. These strata belong to several distinct terranes that are fault bounded and potentially displaced from their sites of origin. The analyses have been conducted in an attempt to provide additional constraints on where these terranes may have formed in relation to each other and to western North America. This paper provides background information that is essential to each of the accompanying chapters presenting detrital zircon data. Main components include (1) an outline of the general tectonic evolution of the region, (2) an overview of the detrital zircon reference for western North America, with a new quantitative analysis of the data that make up this reference, (3) a detailed discussion of the analytical methods used in analyzing detrital zircon grains in our laboratories, and (4) an assessment of the various biases and interpretations involved in this type of study.
- Gehrels, G. E. (2000). Reconnaissance geology and U-Pb geochronology of the western flank of the Coast Mountains between Juneau and Skagway, southeastern Alaska. Special Paper of the Geological Society of America, 343, 213-233.More infoAbstract: Reconnaissance-scale mapping and related structural, stratigraphic, and U-Pb geochronologic studies suggest that the western flank of the Coast Mountains between Juneau and Skagway, Alaska, preserves structural and stratigraphic relations that have not been recognized along other segments of the Coast Mountains orogen. In much of the area southeast of Juneau, the western Coast Mountains area is underlain by moderate- to high-grade metamorphic rocks that are imbricated along mid-Cretaceous thrust faults and intruded by deep-level mid-Cretaceous plutons. Near and just north of Juneau, the metamorphic rocks and associated thrusts are truncated by the latest Cretaceous-early Tertiary Coast shear zone and associated tonalite sills, and mid-Cretaceous plutons have not been recognized. North of this zone of truncation, the western flank is underlain by relatively low grade and only moderately deformed Triassic strata of the Wrangellia terrane and overlying Jurassic-Cretaceous strata of the Gravina belt. Stratigraphic relations preserved in these rocks allow for the recognition of a new unit within the Gravina belt, the Treadwell Formation, and provide evidence that Triassic strata of the Taku terrane are lateral facies equivalents of Triassic basalts of the Wrangellia terrane. The northern Coast Mountains also preserve important relations between the Coast shear zone and the Denali and Chatham Strait faults. All of these structures are known or suspected to have moved during early Tertiary time, with ∼370 km of dextral offset on the Denali fault, ∼150 km of dextral motion on the Chatham Strait fault, and predominantly dip-slip displacement on the Coast shear zone. Where the Denali fault swings abruptly westward into the northeast-striking Chatham Strait fault, the nearby segment of the Coast shear zone records oblique dextral and/or reverse motion. The apparent linkage between these structures supports Lanphere's (1978) suggestion that the difference in offset of the Denali and Chatham Strait faults (∼220 km) is accommodated by dextral slip on the Coast shear zone and related structures along the west flank of the Coast Mountains. This provides an explanation for dextral offset within and adjacent to the Coast shear zone that does not involve large-scale (1500-2000 km) transform motion.
- Gehrels, G. E., & Boghossian, N. D. (2000). Reconnaissance geology and U-Pb geochronology of the west flank of the Coast Mountains between Bella Coola and Prince Rupert, coastal British Columbia. Special Paper of the Geological Society of America, 343, 61-75.More infoAbstract: Metamorphic pendants within and west of the Coast Mountains between Bella Coola and Prince Rupert British Columbia belong to three northwest-trending belts of metasedimentary and metavolcanic rocks. These belts separate Mesozoic and older strata of the Stikine terrane on the northeast from Wrangellia terrane strata on the southwest. The northeastern belt, herein referred to as the Burke Channel assemblage, consists primarily of interlayered quartzite, marble, metapelite, and metavolcanic rocks that are interpreted to have accumulated along a continental margin with one or more phases of arc-type(?) volcanism. Similarities in protoliths and detrital zircon ages indicate that this assemblage correlates with components of the Yukon-Tanana, Nisling, and/or Stikine terranes of northern British Columbia, Yukon, and eastern Alaska. The southwestern belt, referred to as the Banks Island assemblage, consists mainly of quartzite, marble, and metapelite that may also have accumulated in a continental margin setting. The tectonic affinity of these rocks is uncertain. Separating these assemblages is a belt of metasedimentary and metavolcanic rocks of the Alexander terrane and possibly the Gravina belt. Structures bounding the three sets of pendants are not well known, but are interpreted to include: (1) an early Tertiary reverse fault (part of the Coast shear zone) that juxtaposes Stikine strata westward over the Burke Channel assemblage in the Bella Coola area; (2) a mid-Cretaceous thrust fault that juxtaposes the Burke Channel assemblage over Alexander and possibly Gravina strata; and (3) the sinistral Kitkatla fault, which separates the Banks Island assemblage from the Alexander terrane.
- Gehrels, G. E., & Dickinson, W. R. (2000). Detrital zircon geochronology of the Antler overlap and foreland basin assemblages, Nevada. Special Paper of the Geological Society of America, 347, 57-63.More infoAbstract: U-Pb geochronologic analyses were conducted on 44 detrital zircon grains from two samples from Mississippian sandstones of the Inskip and Tonka formations of central Nevada. The Inskip Formation represents the westernmost exposure of clastic strata that accumulated on top of the Roberts Mountains allochthon following mid-Paleozoic emplacement, whereas the Tonka accumulated in a foreland basin inboard of the Antler orogen. This study complements a previous detrital zircon analysis of the Pennsylvanian Battle Formation, a unit that unconformably overlies central portions of the Roberts Mountains allochthon. The main groups of ages in the Inskip and Tonka samples are ca. 1750-2070 Ma. The Tonka Formation also yielded a few grains that are 2.2 Ga. The detrital zircon ages from these two units and from the Battle Formation are similar to the ages of detrital zircons in the main units of the Roberts Mountains allochthon. This result is consistent with stratigraphic evidence suggesting that detritus in all three upper Paleozoic units was derived primarily from the Antler orogen. The age spectra for the three samples accordingly provide a detrital zircon reference for the ages of zircon grains accumulating along the Cordilleran margin in proximity to the Antler orogen during Carboniferous time.
- Gehrels, G. E., & Miller, M. M. (2000). Detrital zircon geochronologic study of upper Paleozoic strata in the eastern Klamath terrane, northern California. Special Paper of the Geological Society of America, 347, 99-107.More infoAbstract: U-Pb analyses have been conducted on individual detrital zircon grains from Upper Devonian(?)-Carboniferous strata of the Bragdon Formation and Upper Carboniferous-L ower Permian strata of the Baird Formation. These are the two most important clastic units in the Redding section of the eastern Klamath terrane of northern California. There are 31 grains from the Bragdon Formation that yield mainly concordant to slightly discordant ages ranging from ca. 363 Ma to ca. 3.12 Ga. Grains with ages of ca. 363-572 Ma in this sample were apparently shed primarily from the nearby Trinity complex, which contains igneous rocks of these ages. Older grains, ranging from 1.07 to 3.12 Ga, but mostly between 1.76 and 1.99 Ga, were likely recycled from lower Paleozoic strata of the Yreka terrane or the Roberts Mountains allochthon. Six detrital zircons from the Baird Formation are all apparently concordant, with interpreted ages between 320 and 326 Ma. These grains are locally derived and probably record igneous activity represented by the widespread volcanic and volcaniclastic rocks within the Baird Formation.
- Gehrels, G. E., Dickinson, W. R., Darby, B. J., Harding, J. P., Manuszak, J. D., Riley, B. C., Spurlin, M. S., Finney, S. C., Girty, G. H., Harwood, D. S., Miller, M. M., Satterfield, J. I., Smith, M. T., Snyder, W. S., Wallin, E. T., & Wyld, S. J. (2000). Tectonic implications of detrital zircon data from Paleozoic and Triassic strata in western Nevada and northern California. Special Paper of the Geological Society of America, 347, 133-150.More infoAbstract: U-Pb analyses of detrital zircons from various allochthonous assemblages of Paleozoic and early Mesozoic age in western Nevada and northern California yield new constraints on the sediment dispersal patterns and tectonic evolution of western North America. During early Paleozoic time, a large submarine fan system formed in slope, rise, basinal, and perhaps trench settings near the continental margin, west of continental shelf deposits of the Cordilleran miogeocline. Our detrital zircon data suggest that most of the detritus in this fan system along the western U.S. segment of the margin was derived from the Peace RiverArch region of northwestern Canada, and some detritus was shed from basement rocks of the southwestern United States or westernMexico. In most cases, the detritus in the allochthonous assemblages was recycled through platformal and/or miogeoclinal sedimentary units prior to accumulating in offshelf environments. Lower Paleozoic rocks of the Roberts Mountains allochthon, Shoo Fly Complex, and Yreka terrane are interpreted to have been parts of this fan complex that accumulated along the central U.S. segment of the continental margin, probably within 1000 km of the miogeocline. During the mid-Paleozoic Antler orogeny, parts of the lower Paleozoic fan complex were deformed and uplifted, and strata of the Roberts Mountains allochthon were tectonically emplaced onto the continental margin. This orogeny was apparently driven at least in part by convergence of the Sierra-Klamath arc with the continental margin, as has been proposed by many previous workers, because these arc terranes are overlain by Mississippian clastic strata derived from the Roberts Mountains allochthon. Our data are not sufficient, however, to determine the polarity of the arc, or whether the arc formed along the continental margin or was exotic to western North America. Detrital zircon data indicate that following the Antler orogeny, clastic sediments derived from the Roberts Mountains allochthon were deposited both on the continental margin to the east and within intra-arc and backarc basins to the west. The occurrence of this detritus in terranes of western Nevada and northern California indicates that they were proximal to each other and to the continental margin during late Paleozoic time. The presence of upper Paleozoic volcanic and plutonic rocks and arc-derived detrital zircons in strata of the northern Sierra, eastern Klamath, and Black Rock terranes records the existence of a west-facing magmatic arc near the continental margin during late Paleozoic time. Our data are not supportive of scenarios in which these arc terranes were located farther north or thousands of kilometers offshore of the Nevada continental margin during late Paleozoic time. Following a second phase of uplift, erosion, and allochthon emplacement during the Permian-Early Triassic Sonoma orogeny, Middle and Upper Triassic strata now preserved in west-central Nevada accumulated in a backarc basin. Our data indicate that the basinal assemblages contain detritus from arc terranes to the west as well as the craton to the east.
- Gehrels, G. E., Dickinson, W. R., Riley, B. C., Finney, S. C., & Smith, M. T. (2000). Detrital zircon geochronology of the Roberts Mountains allochthon, Nevada. Special Paper of the Geological Society of America, 347, 19-42.More infoAbstract: U-Pb geochronologic analyses have been conducted on 205 individual detrital zircon grains from Cambrian through Devonian sandstones of the Roberts Mountains allochthon in central Nevada. These strata were tectonically emplaced onto the Cordilleran margin during the mid-Paleozoic Antler orogeny, but their original depositional settings and provenance have been controversial. Our data, combined with previous detrital zircon studies, define four different age signatures for the eugeoclinal strata: (1) 690-7 15 and 1065-1350 Ma grains in a minor group of sandstones in the Upper Cambrian(?) Harmony Formation, (2) 1745-1790, 1820-1860, and 2595-2700 Ma grains for most of the Harmony Formation, (3) 1410-1445, 1665-1690, and 1705-1740 Ma grains for lower Middle Ordovician sandstones of the Vinini Formation, and (4) 1020-1045, 1815-1860, 1905-1940, and 2645-2740Ma ages for lower Upper Ordovician sandstones in the Vinini, Valmy, Snow Canyon, and McAfee Formations, for the Silurian Elder Sandstone, and for the Devonian Slaven Chert. Comparison of these data with the detrital zircon reference for the Cordilleran miogeocline and with ages of basement provinces in cratonal North America indicates that sandstones in the lower Vinini and parts of the Harmony Formations were derived from 1.0-1.3, ∼1.43, and 1.6-1.8 Ga provinces of southwestern North America. In contrast, most older and younger units contain few grains of the appropriate ages to have come from the southwestern part of North America, and instead have strong similarities with the Peace River arch region of western Canada. We propose that detritus in most of the Harmony Formation was shed from off-shelf basement rocks exposed along the Canadian continental margin, perhaps as a western continuation of the Peace River arch or as extensional fault blocks. In contrast, detritus in the lower Upper Ordovician through Devonian strata is interpreted to have been recycled from platformal strata exposed along the flanks of the Peace River arch. Transport of the detritus is interpreted to have been largely via turbidity currents flowing in offshelf basins or trenches, rather than by longshore currents on the shelf. These provenance links provide new insights into the paleodispersal history along the Cordilleran margin, and indicate that sandstones of the Roberts Mountains allochthon received detritus from, and therefore accumulated near, western North America.
- Harding, J. P., Gehrels, G. E., Harwood, D. S., & Girty, G. H. (2000). Detrital zircon geochronology of the Shoo Fly Complex, northern Sierra terrane, northeastern California. Special Paper of the Geological Society of America, 347, 43-55.More infoAbstract: U-Pb analyses have been conducted on 92 individual detrital zircon grains from 4 of the main thrust sheets of the Shoo Fly Complex. Samples from the Culbertson Lake allochthon, Duncan Peak allochthon, and Lang sequence yield mainly 1.80- 2.10, 2.20- 2.45, and 2.55- 2.70 Ga ages, which suggests that sediments in these units originated in a cratonal region containing Paleoproterozoic and Archean igneous rocks. These ages match those of basement provinces from the northwestern Canadian shield, suggesting a provenance link with northwestern North America during early Paleozoic time. The Sierra City melange, however, has significantly different zircon ages of 551-635 and 1170-131 9 Ma, with only a subordinate population of >1.8 Ga grains. These grains apparently were derived originally from an outboard Neoproterozoic- Cambrian(?) volcanic arc and from 1.0-1 .7 Ga basement rocks of southwestern North America. The occurrence of all three sets of ages in a sandstone that accumulated outboard of the Lang, Culbertson Lake, and Duncan Peak thrust sheets indicates that most rocks of the Shoo Fly Complex formed inboard of a volcanic arc located in proximity to the southern portion of the Cordilleran margin.
- Manuszak, J. D., Satterfield, J. I., & Gehrels, G. E. (2000). Detrital zircon geochronology of Upper Triassic strata in western Nevada. Special Paper of the Geological Society of America, 347, 109-118.More infoAbstract: Several distinct assemblages of basinal, volcanic arc, and shelfal strata in western Nevada are separated from each other and from adjacent assemblages by known and inferred thrust and strike-slip faults. We analyzed 84 detrital zircon grains from Upper Triassic strata in three assemblages to constrain their paleogeography and tectonic history. Basinal strata that probably belong to the Lovelock assemblage yielded grains with ages between 950-11 40 Ma, three grains of ca. 1417 Ma, and grains with a scattering of ages between 227 and 683 Ma. Shelfal strata of the Luning assemblage yield grains with ages mainly of 218-229 and 1643-1966 Ma; additional grains have ages of ca. 272, 1058, 1089, 1438, 1444, and 2325 Ma. All grains analyzed from the volcanic-rich Pine Nut assemblage yielded ages of ca. 231 Ma. The detrital zircon grains were apparently derived from sources both to the east and the west. The 1.40-1 .45 and 1.63-1 .74 Ga grains most likely originated in Precambrian basement of the southwestern United States, which contains igneous rocks of these ages. Grains >1.8 Ga could have been derived from the Golconda or Roberts Mountains allochthons to the east, or the northern Sierra and eastern Klamath terranes to the west. Grains of Paleozoic and Triassic age probably originated in preexisting and active magmatic arcs to the west. These provenance ties are consistent with stratigraphic and regional tectonic arguments suggesting that Triassic assemblages in western Nevada formed in a backarc basin between the Sierra-Klamath arc terranes to the west and the Cordilleran continental margin to the east.
- Miller, L. D., Stowell, H. H., & Gehrels, G. E. (2000). Progressive deformation associated with mid-Cretaceous to Tertiary contractional tectonism in the Juneau gold belt, Coast Mountains, southeastern Alaska. Special Paper of the Geological Society of America, 343, 193-212.More infoAbstract: The Juneau gold belt in northern southeastern Alaska is composed of a disparate assemblage of lithotectonic terranes ranging in age from Paleozoic and perhaps older to Cretaceous. Four progressive deformational events (D 1-D4) associated with metamorphism and contractional tectonism began in mid-Cretaceous time, and continued well into the Tertiary. Structures associated with these events are overprinted by Eocene vein systems (D5) that contain gold mineralization. The final recognized deformation event (D6) formed brittle contractional structures and strike-slip faults. The geometry of deformed sedimentary and volcanic clasts outside of shear zones indicates that a flattening style of strain developed during contraction. Clast orientations, associated mineral lineations, folds, and asymmetric fabrics are interpreted to indicate flattening and top-to-the-west shear associated with D1-D4. The distinct D5 faulting and fluid-flow event occurred over a short time period (56.5-52.8 Ma) and produced a series of economically important auriferous quartz vein deposits that characterize the 160-km-long Juneau gold belt. Gold vein mineralization may have been initiated by changes in the far-field stress regime and/or rapid exhumation. Structural relations are interpreted to indicate that the deformation regime initiated as contractional and ultimately developed into transpressional.
- Riley, B. C., Snyder, W. S., & Gehrels, G. E. (2000). U-Pb detrital zircon geochronology of the Golconda allochthon, Nevada. Special Paper of the Geological Society of America, 347, 65-75.More infoAbstract: The Golconda allochthon consists of deformed and imbricated deep-marine strata and volcanic rocks of late Paleozoic age that structurally overlie lower Paleozoic rocks of the Roberts Mountains allochthon and overlying upper Paleozoic strata. We analyzed 86 detrital zircon grains from 4 sandstone units of the Golconda allochthon in an effort to help reconstruct the paleogeographic and tectonic setting of the allochthon prior to Permian(?)-Triassic thrusting onto the continental margin. Of these, 81 grains yielded concordant to moderately discordant ages that define three main groups: 338-35 8Ma (n = 7), 1770-1922 Ma (n = 38), and 2474-2729 Ma (n = 19). Comparison of these ages with detrital zircon age spectra from adjacent terranes indicates that most of the detritus in Golconda sandstones was probably shed from rocks of both the Roberts Mountains allochthon to the east and the northern Sierra terrane to the west. Some grains may also have been shed from basement rocks of the southwestern United States. These relations support previously proposed tectonic models in which strata in the Golconda allochthon were deposited between the Sierra-Klamath magmatic arc to the west and the previously emplaced Roberts Mountains allochthon to the east.
- Rusmore, M. E., Woodsworth, G. J., & Gehrels, G. E. (2000). Late Cretaceous evolution of the eastern Coast Mountains, Bella Coola, British Columbia. Special Paper of the Geological Society of America, 343, 89-105.More infoAbstract: Structural and stratigraphic data from the eastern Coast Mountains, British Columbia, point to the presence of a Late Cretaceous thrust belt on the western margin of Stikinia. In the Bella Coola region, a fragment of this belt is preserved as the Sheemahant shear zone and its lower plate strata, the Early Cretaceous Monarch volcanics and Taylor Creek Group. The Sheemahant shear zone strikes northwest (∼300), dips moderately (∼55°) southwest, and verges to the northeast. Tonalitic protomylonites and mylonites within the shear zone constitute the Mt. Daunt orthogneiss. Fabrics within the orthogneiss and metamorphic patterns suggest that thrusting occurred during or soon after emplacement of the Mt. Daunt orthogneiss. U-Pb dating yields a crystallization age of 91 ± 3 Ma for the orthogneiss, suggesting that the Sheemahant shear zone was active in Late Cretaceous time. After thrusting, the upper plate of the Sheemahant shear zone was cut by the early Tertiary Coast shear zone and intruded by the Sheemahant pluton. The Sheemahant pluton has a biotite cooling age of 54 Ma, placing a younger limit on the age of the Sheemahant shear zone. Reconstruction of the upper plate of the shear zone suggests that amphibolite facies gneiss of the Burke Channel assemblage composed the highest parts of the upper plate. This assemblage was metamorphosed and deformed prior to 82 Ma and appears to belong to a suite of Precambrian to Paleozoic volcanic-rich continental margin assemblages present in the core of the central and northern Coast Mountains. The Sheemahant shear zone is probably coeval with and kinematically linked to the eastern Mt. Waddington thrust belt and coeval thrusts near Whitesail Lake. The Monarch volcanics and Taylor Creek Group are correlated with Lower Cretaceous units in these areas and are interpreted as a coherent volcanic arc built on the western edge of Stikinia. Continuity of the thrust belt and arc strengthens the view that a northeast-vergent thrust belt formed the western margin of Stikinia in mid-Cretaceous time. This conclusion reinforces the interpretation that middle to Late Cretaceous arc magmatism in the Coast Mountains was coeval with regionally extensive contractional deformation. Existence of a coherent thrust belt along the western margin of Stikinia is difficult to reconcile with the ∼3000 km of northward transport of western British Columbia suggested by paleomagnetic data. If this interpretation of the paleomagnetic data is correct, either the thrust belt was not continuous, or parts of Stikinia had different transport histories.
- Spurlin, M. S., Gehrels, G. E., & Harwood, D. S. (2000). Detrital zircon geochronology of upper Paleozoic and lower Mesozoic strata of the northern Sierra terrane, northeastern California. Special Paper of the Geological Society of America, 347, 89-98.More infoAbstract: U-Pb analyses of 56 individual detrital zircon grains from mid-Paleozoic through Lower Jurassic clastic strata of the northern Sierra terrane yield two distinct sets of ages: 1.7-2 .8 Ga grains mainly in Upper Devonian-Mississippian strata of the Picayune Valley Formation, and ca. 370-18 5 Ma grains in strata of Permian through Jurassic age. The older ages are most similar to the ages of grains in the underlying Shoo Fly Complex and in the Roberts Mountains allochthon in Nevada. This age similarity, combined with stratigraphic relations that record a provenance link with the Roberts Mountains allochthon, are consistent with models in which the northern Sierra terrane was located in proximity to the Nevada continental margin during Late Devonian- Mississippian time. The ca. 370-18 5Ma ages of detrital grains in Permian, Triassic, and Jurassic strata are an excellent match for the ages of volcanic rocks within the northern Sierra terrane. The clastic detritus in these units was presumably derived from these intraterrane volcanic rocks.
- Wallin, E. T., Noto, R. C., & Gehrels, G. E. (2000). Provenance of the Antelope Mountain Quartzite, Yreka terrane, California: Evidence for large-scale late Paleozoic sinistral displacement along the North American Cordilleran margin and implications for the mid-Paleozoic fringing-arc model. Special Paper of the Geological Society of America, 347, 119-131.More infoAbstract: The Antelope Mountain Quartzite is a coarse feldspathic siliciclastic unit in the Yreka terrane of the eastern Klamath Mountains. U-Pb geochronology of single detrital zircons from the Antelope Mountain indicates a source to the north of the 1.7-1 .8 Ga mobile belt that transects the United States. When compared to cratonic and miogeoclinal detrital zircon ages, the signature of the Antelope Mountain is most compatible with derivation from a " northern British Columbia" source. The manner in which that detritus reached its present position is explained best by catastrophic failure of the continental margin in northern British Columbia during the early Paleozoic, incorporation of olistoliths into melange of the Yreka terrane by an indeterminate amount of subduction-related transport before the Middle Devonian, and southward sinistral offset of the Yreka terrane during the late Paleozoic. This inferred tectonic transport of detritus in the Antelope Mountain Quartzite conflicts with models in which the eastern Klamath province is parautochthonous relative to the Paleozoic United States.
- Crawford, M. L., Klepeis, K. A., Gehrels, G., & Isachsen, C. (1999). Batholith emplacement at mid-crustal levels and its exhumation within an obliquely convergent margin. Tectonophysics, 312(1), 57-78.More infoAbstract: Emplacement of the central part of the Coast Mountains batholith of northern coastal British Columbia occurred within a regime characterized by oblique convergence between the Farallon/Kula and North American plates. We use new structural, kinematic and U-Pb isotopic data to show that the locations, geometry, and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones. These data also show that the most active period of pluton emplacement, from ~67 to ~51 Ma, coincided with a change in style of deformation within the batholith. Prior to ~67 Ma plutons were emplaced within an arc dominated by regional-scale contractional shear zones. In contrast, emplacement of 67-51 Ma plutons occurred in an arc increasingly dominated by normal faults with arc-parallel to oblique displacement and by sinistral transtensional shear zones. We have identified and mapped the structure of three plutonic complexes composed of 67 to 51 Ma plutons: the Khyex sill complex, Arden Lake plutonic complex and Quottoon plutonic complex. Shear-zone-controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes. During and after this latest Cretaceous-Paleogene period of intense plutonic activity and accompanying deformation, the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 Ma.
- Gehrels, G. E., Johnsson, M. J., & Howell, D. G. (1999). Detrital zircon geochronology of the Adams Argillite and Nation River Formation, east-central Alaska, U.S.A. Journal of Sedimentary Research, 69(1), 135-144.More infoAbstract: The Cambrian Adams Argillite and the Devonian Nation River Formation are two sandstone-bearing units within a remarkably complete Paleozoic stratigraphic section in east-central Alaska. These strata, now foreshortened and fault-bounded, were originally contiguous with miogeoclinal strata to the east that formed as a passive-margin sequence along the northwestern margin of the North American continent. Seventy-five detrital zircon grains from the Adams Argillite and the Nation River Formation were analyzed in an effort to provide constraints on the original sources of the grains, and to generate a detrital zircon reference for miogeoclinal strata in the northern Cordillera. Thirty-five single zircon grains from a quartzite in the Adams Argillite yield dominant age clusters of 1047-1094 (n = 6), 1801-1868 (n = 10), and 2564-2687 (n = 5) Ma. Forty zircons extracted from a sandstone in the Nation River Formation yield clusters primarily of 424-434 (n = 6), 1815-1838 (n = 6), 1874-1921 (n = 7), and 2653-2771 (n = 4) Ma. The Early Proterozoic and Archean grains in both units probably originated in basement rocks in a broad region of the Canadian Shield. In contrast, the original igneous sources for mid-Protcrozoic grains in the Adams Argillite and ∼ 430 Ma grains in the Nation River Formation are more difficult to identify. Possible original sources for the mid-Proterozoic grains include: (1) the Grenville Province of eastern Laurentia, (2) the Pearya terrane along the Arctic margin, and (3) mid-Proterozoic igneous rocks that may have been widespread along or outboard of the Cordilleran margin. The ∼ 430 Ma grains may have originated in: (1) arc-type sources along the Cordilleran margin, (2) the Caledonian orogen, or (3) a landmass, such as Pearya, Siberia, or crustal fragments now in northern Asia, that resided outboard of the Innuitian orogen during mid-Paleozoic time. Copyright © 1999, SEPM (Society for Sedimentary Geology).
- Johnston, S. T., Mihalynuk, M. G., Brew, D. A., J., C., Erdmer, P., & Gehrels, G. E. (1999). Paleozoic and Mesozoic rocks of Stikinia exposed in northwestern British Columbia: Implications for correlations in the northern Cordillera: Discussion. Bulletin of the Geological Society of America, 111(7), 1103-1104.
- Davis, G. A., Cong, W., Yadong, Z., Jinjiang, Z., Changhou, Z., & Gehrels, G. E. (1998). The enigmatic Yinshan fold-and-thrust belt of northern China: New views on its intraplate contractional styles. Geology, 26(1), 43-46.More infoAbstract: The east- to east-northeast-trending Yinshan belt lies within North China, extending westward at least 1100 km from China's eastern coast to Inner Mongolia. This intraplate Jurassic-Cretaceous belt underwent contractional and normal faulting, folding, and contemporaneous terrestrial sedimentation and magmatism. Current views on its contractional deformational style favor relatively limited "thick-skinned" faulting of Archean basement and cover units. These views are challenged, however, by recent discoveries in the eastern part of the belt of south-directed ductile nappe formation and large-displacement (>40-45 km) "thin-skinned" northward thrust faulting, both involving Archean and younger rock units. Collision of the Siberian and North China plates upon closure of a Jurassic and Early Cretaceous Mongolo-Okhotsk ocean more than 800-1100 km to the north may have been responsible for Yinshan north-south contraction. Some patterns of contraction, e.g., Jurassic-Cretaceous ductile nappe formation, appear to have been influenced by a superposed magmatic regime related to westward subduction of a Pacific basin plate beneath the North China plate.
- Gehrels, G. E., & Kapp, P. A. (1998). Detrital zircon geochronology and regional correlation of metasedimentary rocks in the Coast Mountains, southeastern Alaska. Canadian Journal of Earth Sciences, 35(3), 269-279.More infoAbstract: U-Pb ages have been determined for 55 detrital zircon grains from a metasedimentary sequence along the west flank of the Coast Mountains in southeastern Alaska. These rocks belong to the Port Houghton assemblage, which consists of upper Paleozoic pelitic and psammitic schist, metaconglomerate, metabasalt and marble. The Port Houghton assemblage rests unconformably(?) on metamorphosed and deformed mid-Paleozoic arc-type volcanics (Endicott Arm assemblage), which gradationally overlie upper Proterozoic(?)-lower Paleozoic continental margin strata (Tracy Arm assemblage). Three main clusters of ages are present: 330-365 Ma (19 grains), 1710-2000 Ma (27 grains) and 2450-2680 Ma (6 grains). Additional grains are approximately 2334, 2364 and 3324 Ma. Comparison of these ages with detrital zircon ages in other Cordilleran assemblages supports previous interpretations that metasedimentary rocks in the Coast Mountains (i) for a southwestern continuation of the Yukon-Tanana terrane of eastern Alaska and Yukon, (ii) are not correlative with strata of the Alexander terrane, and (iii) contain detritus that was probably shed from cratonal rocks in the Canadian Shield to the east. Several scenarios exist to explain the occurrence of these continental margin rocks west (outboard) of arc-type and ocean-floor assemblages such as the Stikine, Cache Creek, Quesnel and Slide Mountain terranes.
- Gehrels, G. E., & Ross, G. M. (1998). Detrital zircon geochronology of Neoproterozoic to Permian miogeoclinal strata in British Columbia and Alberta. Canadian Journal of Earth Sciences, 35(12), 1380-1401.More infoAbstract: U-Pb ages have been determined on 250 detrital zircon grains from Neoproterozoic through Permian miogeoclinal strata in British Columbia and Alberta. Most of the grains in these strata are >1.75 Ga and are interpreted to have been derived from nearby basement provinces (although most grains were probably cycled though one or more sedimentary units prior to final deposition). Important exceptions are Ordovician sandstones that contain grains derived from the Peace River arch, and upper Paleozoic strata with detrital zircons derived from the Franklinian orogen, Salmon River arch (northwestern U.S.A.), and (or) Grenville orogen. These provenance changes resulted in average detrital zircon ages that become progressively younger with time, and may also be reflected by previously reported shifts in the Nd isotopic signature of miogeoclinal strata. In addition to the grains that have identifiable sources, grains of ~1030, ~1053, 1750-1774, and 2344-2464 Ma are common in our samples, but igneous rocks of these ages have not been recognized in the western Canadian Shield. We speculate that unrecognized plutons of these ages may be present beneath strata of the western Canada sedimentary basin. Collectively, our data provide a record of the ages of detrital zircons that accumulated along the Canadian Cordilleran margin during much of Paleozoic time. Comparisons between this reference and the ages of detrital zircons in strata of potentially displaced outboard terranes may help reconstruct the paleogeography and accretionary history of the Cordilleran orogen.
- Gehrels, G. E., & Stewart, J. H. (1998). Detrital zircon U-Pb geochronology of Cambrian to Triassic miogeoclinal and eugeoclinal strata of Sonora, Mexico. Journal of Geophysical Research B: Solid Earth, 103(B2), 2471-2487.More infoAbstract: One hundred and eighty two individual detrital zircon grains from Cambrian through Permian miogeoclinal strata, Ordovician eugeoclinal rocks, and Triassic post-orogenic sediments in northwestern Sonora have been analyzed. During Cambrian, Devonian, Permian, and Triassic time, most zircons accumulating along this part of the Cordilleran margin were shed from 1.40-1.45 and 1.62-1.78 Ga igneous rocks that are widespread in the southwestern United States and northwestern Mexico. Zircons with ages of approximately 1.11 Ga are common in Cambrian strata and were apparently shed from granite bodies near the sample site. The sources of 225-280 Ma zircons in our Triassic sample are more problematic, as few igneous rocks of these ages are recognized in northwestern Mexico. Such sources may be present but unrecognized, or the grains could have been derived from igneous rocks of the appropriate ages to the northwest in the Mojave Desert region, to the east in Chihuahua and Coahuila, or to the south in accreted(?) arc-type terranes. Because the zircon grains in our Cambrian and Devonian to Triassic samples could have accumulated in proximity to basement rocks near their present position or in the Death Valley region of southern California, our data do not support or refute the existence of the Mojave-Sonora megashear. Ordovician strata of both miogeoclinal and eugeoclinal affinity are dominated by >1.77 Ga detrital zircons, which are considerably older than most basement rocks in the region. Zircon grains in the miogeoclinal sample were apparently derived from the Peace River arch area of northwestern Canada and transported southward by longshore currents. The eugeoclinal grains may also have come from the Peace River arch region, with southward transport by either sedimentary or tectonic processes, or they may have been shed from off-shelf slivers of continents (perhaps Antarctica?) removed from the Cordilleran margin during Neoproterozoic rifting. It is also possible that the Ordovician eugeoclinal strata are far traveled and exotic to North America.
- Kapp, P. A., & Gehrels, G. E. (1998). Detrital zircon constraints on the tectonic evolution of the Gravina Belt, southeastern Alaska. Canadian Journal of Earth Sciences, 35(3), 253-268.More infoAbstract: Upper Jurassic-Lower Cretaceous marine clastic strata and mafic to intermediate volcanic rocks of the Gravina belt are part of a complex suture zone separating the Alexander and Wrangellia terranes on the west from the Yukon-Tanana and Stikine terranes to the east. U-Pb ages have been determined on 118 single detrital zircon grains from Gravina strata in an effort to determine the tectonic setting of the Gravina belt and the paleoposition of outboard terranes prior to their Late Cretaceous juxtaposition against inboard terranes. Samples from five stratigraphic units yield ages of 105-120 (n = 5), 140-165 (n = 56), 310-380 (n = 17), 400-450 (n = 19), 520-560 (n = 5), 920-1210 (n = 5) and 1755-1955 Ma (n = 5). The 105-120 and 140-165 Ma grains were shed primarily from arc-related plutons that lie outboard of the Gravina belt. The lack of 120-140 Ma ages coincides with a lull in magmatism in the outboard arc and in the western United States, which suggests that Gravina strata accumulated during major changes in plate motion along the Cordilleran margin. The 400-560 Ma zircons were derived from rocks of the Alexander terrane which also lie to the west. In contrast, the 310-380 and >900 Ma grains were apparently shed from inboard regions. Likely sources include the Yukon-Tanana and Stikine terranes in the northern Cordillera and assemblages in the northern California region which contain igneous rocks and detrital zircons of the appropriate ages. Our data accordingly support models in which the Gravina basin formed in narrow rift or transtensional basins, whereas the outboard Alexander and Wrangellia terranes were located along the California-Oregon-Washington-British Columbia-Alaska margin. Our data are less supportive of models in which the Gravina strata and underlying Alexander and Wrangellia terranes were separated from western North America by a large ocean basin, or were located along the coast of Mexico.
- Klepeis, K. A., Crawford, M. L., & Gehrels, G. (1998). Structural history of the crustal-scale Coast shear zone north of Portland Canal, southeast Alaska and British Columbia. Journal of Structural Geology, 20(7), 883-904.More infoAbstract: Structural, metamorphic and U-Pb geochronologic data reveal how a steep, crustal-scale shear zone influenced the evolution of the Paleogene Coast Mountains batholith during and since its emplacement. We document two distinct stages of deformation (DCSZ3 and DCSZ4) that produced the Coast shear zone north of Portland Inlet. Between 65 Ma and 57 Ma, deformation now preserved within the eastern side of the Coast shear zone (DCSZ3) produced a moderately to gently, north-northeast-dipping foliation and north-east-plunging mineral lineations. DCSZ3 involved dominantly east-side-up, top-to-the-southwest displacements during the after the intrusion of tabular tonalite and granodiorite plutons. Widespread crustal thickening followed by rapid exhumation, east-side-up tilting of the batholith, and decompression of rocks equilibrating at 5.6±0.4 kbars, 710±30°C occurred at this time. Prior to DCSZ3, deformation (DWTB1-2) now preserved west of the Coast shear zone resulted in tectonic imbrication of lithologically distinctive crustal fragments at 809 kbars, and west- to southwest-vergent ductile thrust faults before ~92 Ma. From ~57 Ma to 55 Ma, deformation in the western Coast shear zone (DCSZ4) produced a narrow, 1-2 km wide, zone comprised of a steeply-dipping to subvertical folidation that overprints and transposes all DWTB1/2 and DCSZ3 structures. DCSZ4 involved bulk east-side-down displacements parallel to a steeply-plunging, down-dip sillimanite lineation and regional tilting of the batholith. This east-side-down displacement may reflect a final period of crustal readjustment and collapse following an earlier period of crustal thickening during batholith construction. The variable history of motion within the Coast shear zone appears to reflect a response to different periods of batholith development within a convergent to obliquely-convergent continental margin.
- Patchett, P. J., & Gehrels, G. E. (1998). Continental influence on Canadian Cordilleran terranes from Nd isotopic study, and significance for crustal growth processes. Journal of Geology, 106(3), 269-280.More infoAbstract: Nd isotopic data are presented for rocks of the terrane assembly that lies inboard of the Sikine terrane in the Canadian Cordillera of British Columbia and Yukon. These are, from most inboard outward: Cassiar, Kootenay, Slide Mountain, Quesnel, and Cache Creek terranes. They are regarded as documenting a transition from terranes whose evolution was closely tied to that of the North American continental margin out to far-traveled oceanic terranes. the results emphasize sedimentary rocks as indicators of tectonic position of the crustal fragments. Sedimentary rocks of the Cassiar and Kootenay terranes show a strong connection to miogeoclinal sediment sources. argillites of Pennsylvanian-Permian age from a paleonotologically controlled section in the Slide Mountain terrane are also consistent with sediment sources in the North American miogeocline. Igneous rocks of the slide Mountain, Quesnel, and Cache Creek terranes show juvenile oceanic or arc origins based on εNd values between +3 and +10, and are essentially identical with published results. Cache Creek and Quesnel terranes also contain sediments with positive εNd values, suggesting a juvenile, ultimately volcanogenic, origin. Both terranes, however, also contain some Triassic and apparently Pennsylvanian-Permian sediment rocks with negative εNd values between -5 and -7, like those of Devonian to Jurassic sedimentary rocks of the North American miogeocline. Possible explanations include proximity to sources of North American terrigenous sediment, expected intriassic time, or very far-traveled fine-grained sediment in the form of hemipelagic clay or eolian dust for older samples. The addition of a continental sedimentary component affected the isotopic signatures, but usually cannot be separately identified due to intense reprocessing during orogenesis.
- Patchett, P. J., Gehrels, G. E., & Isachsen, C. E. (1998). Nd isotopic characteristics of metamorphic and plutonic rocks of the Coast Mountains near Prince Rupert, British Columbia. Canadian Journal of Earth Sciences, 35(5), 556-561.More infoAbstract: Nd isotopic data are presented for a suite of metamorphic and plutonic rocks from a traverse across the Coast Mountains between Terrace and Prince Rupert, British Columbia, and for three contrasting batholiths in the Omineca Belt of southern Yukon. A presumed metamorphic equivalent of Jurassic volcanic rocks of the Stikine terrane gives ε(Nd)=+6, and a number of other metaigneous and metasedimentary rocks in the core of the Coast Mountains give ε(Nd) values from +3 to +7. A single metasedimentary rock approximately 3 km east of the Work Channel shear zone gives a ε(Nd) value of -9. Coast Belt plutons in the traverse yield ε(Nd) from -1 to +2. The Omineca Belt plutons give ε(Nd) from -10 to -17. All results are consistent with published data in demonstrating that (i) juvenile origins for both igneous and metamorphic rocks are common in the Coast Belt; (ii) representatives of a continental-margin sedimentary sequence with Precambrian crustal Nd are tectonically interleaved in the Coast Mountains; (iii) Coast Mountains plutons can be interpreted as derived from a blend of metamorphic rocks like those seen at the surface, or as arc-type melts contaminated with the older crustal component; and (iv) Omineca Belt plutons are dominated by remelted Precambrian crustal rocks.
- Butler, R. F., Gehrels, G. E., & Bazard, D. R. (1997). Paleomagnetism of Paleozoic strata of the Alexander terrane, southeastern Alaska. Bulletin of the Geological Society of America, 109(10), 1372-1388.More infoAbstract: Paleomagnetic samples were collected from 180 sites (sedimentary horizons or igneous flows) in nine Paleozoic formations of the Alexander terrane on or near Prince of Wales Island in southeastern Alaska. Analysis of samples collected from 68 sites within 2 stratigraphic sections of red sedimentary rocks from the Lower Devonian Karheen Formation allows determination of a characteristic component with unblocking temperatures between 550 °C and 680 °C in many samples. Site-mean characteristic directions from 29 sites pass fold and reversal tests and define polarity zones correlative between the two stratigraphic sections. These paleomagnetic data indicate an Early Devonian paleolatitude of 14° ± 4° (north or south). Geochronologic analysis of detrital zircon grains from the Karheen Formation requires that the Alexander terrane was adjacent to a continent with crust distributed within 1.6-1.8 Ga and 1.45-1.6 Ga age intervals. Early Paleozoic locations as part of the Australian paleo-Pacific margin of Gondwana or the Scandinavian margin of Baltica are consistent with the paleomagnetic, geologic, and geochronologic data, and available paleontologic data favor the Baltica paleoposition. The Lower Permian Halleck Formation volcanic rocks contain a stable paleomagnetism indicating a paleolatitude of ∼25°N, consistent with the 25°N-30°N Permian paleolatitudes determined from three other formations of the Alexander- Wrangellia terranes. The 10°-20° paleolatitudes determined from numerous paleomagnetic studies of Late Triassic igneous rocks from Alexander-Wrangellia are almost certainly Northern Hemisphere paleolatitudes. Available evidence indicates: (1) early Paleozoic development of the Alexander terrane as a volcanic arc without significant incorporation of continental crust; (2) mid-Paleozoic juxtaposition with a continent containing 1.6-1.8 Ga and 1.45-1.6 Ga crust, probably the Scandinavian margin of Baltica; (3) rifting from that margin in Devonian time followed by tectonic transport to 25°-30° latitude in the northern paleo-Pacific by Permian time; and (4) southward motion to a Late Triassic paleolatitude of 10°-20° followed by accretion to North America with subsequent dispersal of fragments from northern Oregon to southern Alaska. The Ordovician Descon Formation, Silurian Heceta Limestone, Devonian Port Refugio Formation, and Pennsylvanian Ladrones Formation did not yield primary magnetizations. Upper Paleozoic carbonate rocks and siliciclastic strata of the Devonian Wadleigh Limestone and the Mississippian Peratrovich and Pennsylvanian Klawak Formations have characteristic magnetizations that are complex and, in part, demonstrably secondary. Rock-magnetic investigations of the Peratrovich Formation and the Wadleigh Limestone reveal wasp-waisted hysteresis loops with parameters closely matching those of other carbonate rocks known to have undergone chemical remagnetization.
- Restrepo-Pace, P. A., Ruiz, J., Gehrels, G., & Cosca, M. (1997). Geochronology and Nd isotopic data of Grenville-age rocks in the Colombian Andes: New constraints for Late Proterozoic-Early Paleozoic paleocontinental reconstructions of the Americas. Earth and Planetary Science Letters, 150(3-4), 427-441.More infoAbstract: New U-Pb zircon crystallization ages and 40Ar/39Ar cooling ages from the Colombian Andes confirm the existence of rocks metamorphosed during the Orinoquian Orogenic Event (ca. 1.0 Ga) of northern South America. ∈Nd (t = 1.1 Ga) for these rocks range from -3.9 to +0.91, which is interpreted as a mixture of Late Archean-Early Proterozoic crust with juvenile material produced during the 1.1 Ga erogenic event. The Colombian Grenville age rocks are part of a much longer metamorphic pericratonal belt, sporadically exposed along the Andes, in western-central Peru, southern Bolivia and northern Argentina. In addition, Nd model (TDM) ages for the Colombian rocks range from 1.9 to 1.45 Ga, similar to those obtained in the Grenville Province of the eastern U.S. and in the Mexican basement, placing constraints on Late Proterozoic-Early Paleozoic paleocontinental reconstructions. © 1997 Elsevier Science B.V.
- Ross, G. M., Gehrels, G. E., & Patchett, P. J. (1997). Provenance of Triassic strata in the Cordilleran miogeocline, western Canada. Bulletin of Canadian Petroleum Geology, 45(4), 461-473.More infoAbstract: Siliciclastic strata of Triassic age that are exposed in the Rocky Mountains have been sampled for isotopic studies in order to determine their provenance. Sm-Nd analyses of whole rock shale and sandstone samples from northeast British Columbia and the Bow Corridor of western Alberta are isotopically homogeneous with εNd values of -6.7 to -10.5 at the time of deposition. These values are part of a distinct isotopic provenance signature that characterizes the Devonian through Jurassic of the miogeocline of western Canada and thus the sources of Triassic sediments are part of a larger provenance signature within the miogeocline. These isotopic data do not support sediment derivation from the exposed Canadian Shield and our present working model is that most of the sediment is detrital material reworked from the Innuitian (Ellesmerian) clastic wedge. No evidence has been found in the Nd data to suggest that magmatic arc rocks of Triassic age to the west of the miogeocline (e.g., Quesnellia) made any contribution to the sediment record. U-Pb geochronology of detrital zircons from Triassic sandstones in the Pine Pass and Kananaskis areas are dominated by Precambrian grains with only a few Silurian grains. Comparison with detrital zircons dated from the Devonian Ellesmerian clastic wedge of Arctic Canada show a similar pattern and strengthen the contention that sedimentary strata of the western miogeocline were derived from reworking of Innuitian clastic strata which must have been formerly more widespread and covered the Canadian Shield.
- Boghossian, N. D., Patchett, P. J., Ross, G. M., & Gehrels, G. E. (1996). Nd isotopes and the source of sediments in the miogeocline of the Canadian Cordillera. Journal of Geology, 104(3), 259-277.More infoAbstract: Nd isotopes in clastic 600-75 Ma sedimentary rocks from the miogeocline of Alberta and British Columbia are used to (1) constrain the importance of proximal and distal North-American basement and juvenile Cordilleran sedimentary sources; (2) document large-scale changes in provenance; and (3) develop a North American miogeoclinal reference for Nd isotopes as a baseline for continental input to Cordilleran terranes. Initial ∈Nd for Neoproterozoic to Upper Ordovician samples range from -14 to -22 and can be explained by derivation from proximal Precambrian basement. A positive shift of six ∈Nd units occurs between Late Ordovician and Late Devonian time and persists until foreland basin formation in the Late Jurassic. ∈Nd for this 370-170 Ma period ranges from -6 to -9 and requires involvement of a more juvenile source. Cretaceous foreland basin sediments show extreme heterogeneity, with ∈Nd from zero to -12. High values can be explained by incorporation of volcanic detritus, either from Triassic and Jurassic units in the Quesnellia terrane, or as airborne material from magmatic activity in the Coast Belt of the Cordillera. Sediments with negative ∈Nd may have come from Proterozoic or lower Paleozoic miogeoclinal sediments of the fold and thrust belt, or from the Omineca Belt. Three hypotheses can explain this Ordovician-Devonian ∈Nd shift. First, the detritus could represent a mix of Cordilleran juvenile components plus Precambrian basement. The ∈Nd shift is approximately coeval with Devono-Mississippian igneous and tectonic activity along the western margin of North America. Any western juvenile sources should have ceased to supply sediments by mid-Mississippian time, so this hypothesis requires sufficient detritus in Devono-Mississippian time so that cannibalistic recycling maintained the same isotopic signature until the Late Jurassic. Second, the detritus was possibly transported across the craton from Appalachian sources, maybe a multi-step journey. The Appalachian belt (which includes Grenville-age basement and sediments ultimately derived from it) provides the appropriate isotopic signature. It was the main mountain belt in North America during most of the Paleozoic and was a major source of detritus to the craton. Third, detritus may have come from the Innuitian Orogen in the Canadian Arctic. A large amount of clastic material was shed southward from the Innuitian Orogen in mid-Late Devonian time. This hypothesis requires that sediment delivered from the North in Devono-Mississippian time was cannibalistically recycled in the miogeocline until Late Jurassic time; however, isotopic signatures of Innuitian Belt rocks and of sediments derived from them are presently unknown.
- Gehrels, G. E., Butler, R. F., & Bazard, D. R. (1996). Detrital zircon geochronology of the Alexander terrane, southeastern Alaska. Bulletin of the Geological Society of America, 108(6), 722-734.More infoAbstract: U-Pb analyses of 101 detrital zircon grains from Paleozoic and Triassic clastic strata of the Alexander terrane indicate that most detritus in the terrane was derived from intraterrane igneous rocks, although the occurrence of 1.0-3.0 Ga grains in a Lower Devonian sandstone indicates proximity to a continental region during the Silurian-Devonian Klakas orogeny. Because these Precambrian grains are not the same age as grains that were accumulating along the western margin of North America, it appears unlikely that the terrane was in proximity to the Cordilleran margin during Early Devonian time. Continental regions that could have shed grains of the appropriate ages, and that also record Silurian-Devonian orogenic activity, include the paleo-Pacific margin of Australia and the Scandinavian portion of Baltica. Existing paleomagnetic data are consistent with either paleoposition, whereas previous biogeographic studies suggest closer ties with Baltica.
- Riggs, N. R., Lehman, T. M., Gehrels, G. E., & Dickinson, W. R. (1996). Detrital zircon link between headwaters and terminus of the upper triassic Chinle-Dockum paleoriver system. Science, 273(5271), 97-100.More infoAbstract: New detrital-zircon geochronologic data reveal that a through-going paleoriver connected Texas with Nevada in Late Triassic time. Sandstone from the Upper Triassic Santa Rosa Sandstone (Dockum Group) from northwestern Texas contains a detrital zircon suite nearly identical to that found in western Nevada in the Upper Triassic Osobb Formation (Auld Lang Syne Group, correlative with the Chinle Formation). The Santa Rosa Sandstone was derived in large part from the eroded Cambrian core of the Amarillo-Wichita uplift, as evidenced by abundant zircons with ages of 515 to 525 million years. Other zircon grains in the sandstone are Permian, Devonian, Proterozoic, and Archean in age and, with the exception of the Archean grain, are also matched by the population in the Nevada strata.
- Vervoort, J. D., Patchett, P. J., Gehrels, G. E., & Nutman, A. P. (1996). Constraints on early Earth differentiation from hafnium and neodymium isotopes. Nature, 379(6566), 624-627.More infoAbstract: INFERENCES about the early evolution of the Earth's crust and mantle have come largely from the study of isotope systematics - in particular, those of neodymium1-5. Neodymium isotope data from the oldest preserved rocks have been interpreted4,6-8 as reflecting early large-scale chemical depletion of the mantle (presumably resulting from the extraction of continental crust), but these data have remained controversial, in view of the potential for disturbances to the samarium-neodymium system during these rocks' long history9-11. Here we provide an independent evaluation of the Nd isotope compositions of ten early Archaean (3.6-3.8 Gyr old) gneisses, by investigating the hafnium isotope systematics of zircons from these rocks. The Hf data are consistent with the Nd record in indicating early depletion of the mantle, but fail to verify the scale and variability of this depletion. We conclude that Nd isotopes of early Archaean gneisses do not faithfully record isotopic variations in the early Earth, and therefore that these data need to be examined more critically before they can be used to constrain the early history of crust-mantle differentiation.
- Bazard, D. R., Butler, R. F., Gehrels, G., & Soja, C. M. (1995). Early Devonian paleomagnetic data from the Lower Devonian Karheen Formation suggest Laurentia-Baltica connection for the Alexander Terrane. Geology, 23(8), 707-710.More infoAbstract: Paleomagnetic data from 29 sites within the Lower Devonian Karheen Formation pass fold and reversal tests, and define two polarity zones that are correlative between two separate stratigraphic sections. Ths magnetization is proposed to be Early Devonian and thus a paleopole for the Alexander terrane is proposed that places the terrane at about 14° (north or south). Due to uncertainties of the Early Devonian parts of the North American and Gondwana apparent polar wander paths, several interpretations of the Early Devonian position of the Alexander terrane relative to either North America (Laurentia) or Gondwana are permissible. A combined data set of palaeomagnetic, detrital zircon, and palaeontological observations is used to narrow the paleogeographic options. -from Authors
- Fillipone, J. A., Yin, A., Harrison, T. M., Gehrels, G., Smith, M., & Sample, J. C. (1995). Age and magnitude of dip-slip faulting deduced from differential cooling histories: an example from the Hope Fault, northwest Montana. Journal of Geology, 103(2), 199-211.More infoAbstract: A new method is described in which the age and magnitude of dip-slip faulting are estimated from contrasting cooling histories of footwall and hanging wall rocks adjacent to the Hope fault, northwest Montana. The Hope fault has been interpreted in the past as a mostly right-slip fault. New kinematic data, 40Ar/39Ar thermochronometry, and geobarometry indicate that cooling of footwall rocks at ~40 Ma resulted from dip-slip movement. This movement caused vertical separation of about 3 to 5 km between footwall and hanging wall rocks, suggesting that a minimum dip-slip component of 4 km developed during the Late Eocene. These results indicate that the Hope fault experienced substantial normal slip in the Late Eocene, making it coeval with other normal and detachment-style faults in the northern US Cordillera. -from Authors
- Gehrels, G. E., Dickinson, W. R., Ross, G. M., Stewart, J. H., & Howell, D. G. (1995). Detrital zircon reference for Cambrian to Triassic miogeoclinal strata of western North America. Geology, 23(9), 831-834.More infoAbstract: U-Pb analyses of 656 single zircon grains from Cambrian to Triassic miogeoclinal strata provide a latitudinal and temporal reference for the ages of grains that accumulated along the western margin of North America. Comparisons between this detrital zircon reference and the ages of grains in potentially displaced terranes outboard (west) of the miogeocline should help establish when the terranes first arrived in sedimentary proximity to western North America. North-south variations in the ages of grains in Cambrian and Devonian to Triassic strata, which reflect the north-south changes in the age of cratonal rocks near the margin, should also help place constraints on a terrane's paleolatitude during these time periods. -from Authors
- Greig, C. J., & Gehrels, G. E. (1995). U-Pb zircon geochronology of Lower Jurassic and Paleozoic Stikinian strata and Tertiary intrusions, northwestern British Columbia. Canadian Journal of Earth Sciences, 32(8), 1155-1171.More infoAbstract: New U-Pb zircon ages are reported from western Stikinia. Devonian and Pennsylvanian ages of volcanic rocks at Oweegee dome confirm the presence of pre-Permian strata, and with Paleozoic and Triassic detrital zircons from Lower Jurassic sandstone, they help to demonstrate pre-Lower Jurassic deformation and uplift. The absence of pre-Paleozoic inherited zircon from all samples is consistent with Nd-Sr isotopic data which suggest that Stikinia consists mainly of juvenile crust. U-Pb ages for posttectonic intrusions suggest that structures in Skeena Fold Belt in the Kinskuch area formed prior to Eocene time. Five ages for felsic volcanic rocks suggest near-contemporaneity for cessation of volcanism in the areas studied. -from Authors
- Long, K. B., Baldwin, S. L., & Gehrels, G. E. (1995). Tectonothermal evolution of the Pinaleno-Jackson Mountain core complex, southeast Arizona. Geological Society of America Bulletin, 107(10), 1231-1240.More infoAbstract: Petrologic data, U-Pb analyses of zircon, and 40Ar/39Ar analyses were used to document the tectonothermal history of two mylonite zones and the temporal relationship of mylonite formation to movement on detachment faults in the Pinaleno-Jackson Mountain metamorphic core complex of southeastern Arizona. Heterogeneous age distributions documented over relatively short distances in the lower plate of The Pinaleno-Jackson Mountain metamorphic core complex cannot be readily explained by a model of simple unroofing through a steady-state geotherm. Multiple recrystallization events during mylonite formation, and/or transient thermal events are required to explain the partial and complete isotopic resetting of thermochronometers in this area. -from Authors
- Miller, L. D., Goldfarb, R. J., Gehrels, G. E., & Snee, L. W. (1994). Genetic links among fluid cycling, vein formation, regional deformation, and plutonism in the Juneau gold belt, southeastern Alaska. Geology, 22(3), 203-206.More infoAbstract: Gold-bearing quartz vein systems in the Juneau gold belt formed within a 160-km-long by 5- to 8-km-wide zone along the western margin of the Coast Mountains, Alaska. Vein systems are spatially associated with shear zones adjacent to terrane-bounding, mid-Cretaaceous thrust faults. Analysis of vein orientations and sense of shear data define a stress configuration with greatest and least principal axes oriented subhorizontally with northeast-southwest trends and subvertically, respectively. This local stress configuration is compatible with the far-field plate configuration during Eocene time. Isotopic ages of vein formation indicate that fluid cycling occured between 56.5 and ≥52.8 Ma, and are consistent with a genetic link between veining and a change in plate motion in early Eocene time. -from Authors
- Smith, M., & Gehrels, G. (1994). Detrital zircon geochronology and the provenance of the Harmony and Valmy Formations, Roberts Mountains allochthon, Nevada. Geological Society of America Bulletin, 106(7), 968-979.More infoAbstract: A long-standing debate over the provenance of lower Palaeozoic sedimentary rocks in the Roberts Mountains allochthon, Nevada, is addressed through abraded single-grain U-Pb detrital zircon analysis of the Harmony and Valmy Formations. The data are consistent with ultimate derivation of the Valmy Formation from the north and suggest a link between the Valmy Formation and the coeval Eureka quartzite on the adjacent shelf and miogeoclinal prism. The ~700 Ma and ~1100 Ma ages (which represent >80% of the grain sampled from the Harmony Formation) have no known western North American basement source of reasonable proximity and age, suggesting derivation from an unknown western or southern source, or perhaps from rocks that presently underlie the Roberts Mountains allochthon. A growing body of U-Pb isotopic evidence indicates that 1000-1270 Ma and 670-750 Ma zircon ages are relatively common in offshelf continental margin strata. The presence of widespread mid- and late Proterozoic felsic magmatic rocks along the Cordilleran margin is not easily accommodated by popular pre-570 Ma plate reconstructions that place Nevada and the central Cordillera opposite the Transantarctic Mountains or eastern Australia. -from Authors
- Gehrels, G. E., & Barker, F. (1993). Reconnaissance geochemistry of Permian and Triassic basalts of the Taku and Wrangellia terranes, southeastern Alaska. US Geological Survey Bulletin, 2068, 218-227.More infoAbstract: Major and minor element geochemical data are presented for Permian and Triassic basalts that belong to the Taku terrane, and for Triassic basalts of the Wrangellia terrane. The Permian rocks are high-magnesian alkaline basalts, whereas the Upper Triassic rocks are similar to midocean ridge or within-plate tholeiites. The data indicate that the Triassic rocks of the Taku and Wrangellia terranes are very similar chemically, which supports previous stratigraphic correlations between these two sequences. The Wrangellia and Taku terranes either evolved in close proximity to each other during Late Triassic time, or the Taku terrane of the Juneau-Port Houghton area actually is a facies of the Wrangellia terrane. -from Authors
- Smith, M. T., Dickinson, W. R., & Gehrels, G. E. (1993). Contractional nature of Devonian-Mississippian Antler tectonism along the North American continental margin. Geology, 21(1), 21-24.More infoAbstract: Regional tectonic analysis suggests that the entire Cordilleran continental margin, from Nevada to the Yukon, underwent contractional orogeny involving emplacement of deep basinal, submarine-fan, and mafic volcanic strata over authochthonous continental margin strata during Late Devonian to Mississippian time (Antler orogeny). Displaced parautochthonous strata are present from Nevada to east-central British Columbia, and perhaps within the Yukon-Tanana terrane of Yukon and eastern Alaska. Manifestations of the orogenic event in the northern Cordillera are preserved primarily by normal faults and related coarse clastic strata of the Selwyn basin. These features formed due to flexural extension during foreland deformation, rather than mid-Paleozoic rifting or transtension. -Authors
- Cole, F., Butler, R. F., & Gehrels, G. E. (1992). Paleomagnetism of late Paleozoic rocks in the northern Cache Creek terrane near Atlin, British Columbia. Canadian Journal of Earth Sciences, 29(3), 486-498.More infoAbstract: Samples were collected from layered Paleozoic rocks at 22 sites (≥6 samples/site) on the Alfred Butte. Principal component analysis of detailed thermal demagnetization data allowed clear isolation of a characteristic remanent magnetization (ChRM) from 17 of these sites. Blocking temperatures to 680°C indicate that this magnetization is carried by hematite, and site-mean ChRM directions are determined with α95 < 10° for the majority of sites. On Sentinel Mountain, samples were collected from 16 sites in layered Paleozoic volcanic and chert rocks and from a diabase sill. Thermal demagnetization revealed a ChRM in the chert and volcanic rocks with blocking temperatures to 680°C, whereas alternating-field demagnetization to 40 mT successfully isolated ChRM in the diabase sill. ChRM directions from four sites involved in a mesoscopic S-fold at Alfred Butte fail the fold test, indicating that the ChRM is a postfolding secondary remagnetization. Rock-magnetic and geologic constraints argue for a chemical remagnetization of these Paleozoic rocks in Late Triassic to Middle Jurassic time. -from Authors
- Gehrels, G. E., McClelland, W. C., Samson, S. D., Patchett, P. J., & Orchard, M. J. (1992). Geology of the western flank of the Coast Mountains between Cape Fanshaw and Taku Inlet, southeastern Alaska. Tectonics, 11(3), 567-585.More infoAbstract: The western flank of the Coast Mountains batholith between Cape Fanshaw and Taku Inlet is underlain primarily by Jura-Cretaceous strata of the Gravina belt; pre-Permian(?); Permian, and Triassic strata of the Taku terrane, and mid-Proterozoic(?) to upper Paleozoic rocks of continental margin affinity. It is suggested that these three assemblages are correlative with components of the Yukon-Tanana terrane, which underlies a large region of Yukon and eastern Alaska. The continental margin rocks in the northern Coast Mountains may be part of the in situ North American margin that has been overthrust by the Stikine and adjacent terranes. Alernatively, these rocks may have 1) rifted from and then returned to the North American margin, 2) moved >800 km along left-lateral or right-lateral faults from elsewhere along the North American margin, or 3) originated near a continent other than North America. -from Authors
- McClelland, W. C., Gehrels, G. E., & Saleeby, J. B. (1992). Upper Jurassic-Lower Cretaceous basinal strata along the Cordilleran margin: implications for the accretionary history of the Alexander- Wrangellia-Peninsular terrane. Tectonics, 11(4), 823-835.More infoAbstract: Upper Jurassic and Lower Cretaceous basinal strata are preserved in a discontinuous belt along the inboard margin of the Alexander-Wrangellia-Peninsular terrane (AWP). On the basis of the general similarities between the basinal sequences, it is proposed that they record involvement of the AWP in the Late Jurassic-Early Cretaceous evolution of the Cordilleran margin. A geologically reasonable scenario for the accretion of the AWP includes (1) Middle Jurassic accretion to the Cordilleran margin, (2) Late Jurassic-Early Cretaceous overall northward translation of the AWP and evolution of a series of transtensional basins within a complex dextral strike-slip system along the Cordilleran margin, and (3) mid-Cretaceous structural imbrication of the AWP and inboard terranes that either terminated or resulted in a change in the character of deposition in the marginal basins. -from Authors
- McClelland, W. C., Gehrels, G. E., Samson, S. D., & Patchett, P. J. (1992). Protolith relations of the Gravina belt and Yukon-Tanana terrane in central southeastern Alaska. Journal of Geology, 100(1), 107-123.More infoAbstract: Metamorphic rocks west of the Coast Mountains batholith in central southeastern Alaska are divided into the Gravina belt, Taku terrane, and newly defined Ruth assemblage. The Ruth assemblage is similar in age and protolith to, and thus correlated with, the Yukon-Tanana terrane. The Gravina belt is characterized by upper Jurassic and lower Cretaceous mafic volcanic rocks and tuffaceous turbiditic clastic strata that unconformably overlie the Alexander terrane. Metamorphic rocks that structurally underlie the Taku terrane and Ruth assemblage are included in this assemblage. Trace element geochemistry and the abundance of pyroclastic flows associated with tuffaceous turbidites suggest that the Gravina belt evolved in an intra-arc basinal setting. -from Authors
- McClelland, W., Gehrels, G., Samson, S., & Patchett, P. (1992). Structural and geochronologic relations along the western flank of the coast mountains batholith: Stikine river to Cape Fanshaw, central southeastern Alaska. Journal of Structural Geology, 14(4), 475-489.More infoAbstract: Geologic and U-Pb geochronologic studies in central southeastern Alaska provide constraints on the mid-Cretaceous to mid-Tertiary deformation west of the Coast Mountains batholith. The NE-dipping Sumdum and Fanshaw faults record the W-directed emplacement of the Ruth assemblage (Yukon-Tanana terrane) and Taku terrane over the subjacent Gravina belt and Alexander terrane. Ductile fabrics of the Sumdum-Fanshaw fault system truncate early-formed foliation and thrust faults in Albian and older Gravina belt strata. U-Pb age data from syn- and post-tectonic plutons suggest that deformation was ongoing at 92.9 ± 3.0 Ma but had mostly ceased by 90 Ma. The Sumdum-Fanshaw fault system marks the tectonic boundary between the Alexander-Wrangellia terrane and inboard Yukon-Tanana and Stikine terranes and lies within a thrust belt that extends from southern Alaska to northern Washington. Mid-Cretaceous structures are truncated to the east by NE-dipping ductile fabrics of the Coast shear zone. Sheet-like Paleocene tonalites were emplaced into and deformed within the shear zone. Undeformed mid-Eocene dikes cross-cut the tonalites. Kinematic relations in the Coast shear zone suggest a complex displacement history that includes both east-side-up (reverse) and west-side-up (normal) shear. The shear zone probably accommodated the collapse of overthickened crust developed during mid-Cretaceous shortening. © 1992.
- Smith, M. T., & Gehrels, G. E. (1992). Stratigraphic comparison of the Lardeau and Covada Groups: implications for revision of stratigraphic relations in the Kootenay Arc. Canadian Journal of Earth Sciences, 29(6), 1320-1329.More infoAbstract: The Lardeau Group is a heterogeneous assemblage of lower Paleozoic, outer continental margin strata present in the Kootenay Arc in southeastern British Columbia. A correlation between the Lardeau Group and the Covada Group and Bradeen Hill assemblage, both in north-eastern Washington is proposed. The latter contain the same stratigraphic elements, in the same structural order, as those of the Lardeau Group. However, the sense of facing, and hence the stratigraphic sequence in the Covada Group and Bradeen Hill assemblage, is reversed in relation to the Lardeau Group, with the quartzo-feldspathic wacke unit the oldest and slate and argillate the youngest. Because the degree of preservation of the units in northeastern Washington is superior to that of the Lardeau Group, it is suggested that: 1) the Lardeau Group may be inverted relative to the sequence as originally defined; 2) the Lardeau Group may range from Late Cambrian to Devonian; and 3) further work is warranted to test this hypothesis. -from Authors
- Smith, M. T., & Gehrels, G. E. (1992). Stratigraphy and tectonic significance of Lower Paleozoic continental margin strata in northeastern Washington. Tectonics, 11(3), 607-620.More infoAbstract: Lower Paleozoic eugeoclinal strata in the Kootenay Arc in northeastern Washington and southeastern British Columbia are transitional between autochthonous lower Paleozoic miogeoclinal strata and outboard volcanic arc terranes of uncertain paleogeographic affinity. They provide a record of lower Paleozoic continental margin depositional and tectonic processes oceanward of the continental shelf. The Covada Group and Bradeen Hill assemblage can be correlated with other stratigraphic units in the Kootenay arc and resemble units as far north as the Selwyn basin in northern Canada and as far south as the Roberts Mountains allochthon in central Nevada. -from Authors
- Smith, M. T., & Gehrels, G. E. (1992). Structural geology of the Lardeau Group near Trout Lake, British Columbia: implications for the structural evolution of the Kootenay Arc. Canadian Journal of Earth Sciences, 29(6), 1305-1319.More infoAbstract: The Lardeau Group is a heterogeneous assemblage of lower Paleozoic eugeoclinal strata present in the Kootenay Arc in southeastern British Columbia. It is in fault contact with lower Paleozoic miogeoclinal strata for all or some of its length along a structure termed the Lardeau shear zone. The Lardeau Group was deformed prior to mid-Mississippian time. Regional constraints indicate probable Devono-Mississippian timing of orogeny, and possible juxtaposition of the Lardeau Group over miogeoclinal strata along the Lardeau shear zone at this time. Further ductile deformation during the Middle Jurassic Columbian orogeny produced large folds with subhorizontal axes, northwest-striking foliation and faults, and orogen-parallel stretching lineations. This deformation was apparently not everywhere synchronous, and may have continued through Late Jurassic time northeast of Trout Lake. This was followed by Cretaceous(?) dextral strike-slip and normal movement on the Lardeau shear zone and other parallel faults. -from Authors
- Butler, R. F., Dickinson, W. R., & Gehrels, G. E. (1991). Palaeomagnetism of coastal California and Baja California: alternatives to large-scale northward transport. Tectonics, 10(3), 561-576.More infoAbstract: Paleomagnetic data from the Santa Lucia-Orocopia (SLOA) and Baja-Borderland (BBA) allochthons of coastal California and Baja California have been interpreted to indicate up to 2500 km of post-mid-Cretaceous northward transport of these regions with respect to interior North America. However, with Neogene strike-slip offsets taken into account, geological interpretations correlate basement rocks of the coastal allochthons with continental basement rocks directly across the San Andreas and related fault systems. We have examined paleomagnetic data from SLOA and BBA and conclude that apparent discordances can be explained without large-scale pre-Neogene tectonic transport. Coupled with recent documentations of compaction shallowing of paleomagnetic inclination in other marine sedimentary rocks, inconsistencies suggest that paleolatitudes derived from most of the marine sedimentary rocks of SLOA and BBA are biased towards low paleolatitudes by compaction shallowing. -from Authors
- Gehrels, G. E., & Smith, C. H. (1991). U-Pb geochronologic constraints on the age of thrusting, crustal extension, and peraluminous plutonism in the Little Rincon Mountains, southern Arizona. Geology, 19(3), 238-241.More infoAbstract: The Little Rincon thrust fault is a mylonitic shear zone that juxtaposes Middle Proterozoic Continental Granodiorite over metasedimentary rocks of Proterozoic and early Paleozoic age. This fault is structurally beneath the San Pedro detachment fault and associated ductile deformational fabrics, which formed during early Oligocene to early Miocene time. A syntectonic leucogranite sill within the Little Rincon shear zone yields a U-Pb concordia-intercept age of 66 ± 10 Ma for zircon and a concordant age of 51 ± 2 Ma for fractions composed of monazite and xenotime. This demonstrates that compressional deformation in the Catalina and Rincon mountains is generally coeval with Laramide thrust faults that extend at least from southeastern California to southeastern Arizona. A peraluminous granite pluton that truncates the shear zone but displays extension-related fabrics yields a lower-intercept age of 24 ± 12 Ma for zircon and an age of 30 ± 6 Ma for monazite. This indicates that some peraluminous plutons in the region were emplaced during regional crustal extension. -Authors
- Gehrels, G. E., McClelland, W. C., Samson, S. D., & Patchett, P. J. (1991). U-Pb geochronology of detrital zircons from a continental margin assemblage in the northern Coast Mountains, southeastern Alaska. Canadian Journal of Earth Sciences, 28(8), 1285-1300.More infoAbstract: Metamorphic rocks within and west of the northern Coast Mountains in southeastern Alaska consists of an Upper Proterozoic(?) to upper Paleozoic continental margin assemblage that we interpret to belong to the Yukon-Tanana terrane. U-Pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of ~495 Ma, ~750 Ma, 1.05-1.40 Ga, 1.75-2.00 Ga, ~2.3 Ga, 2.5-2.7 Ga, and ~3.0 Ga. Results suggest that metaclastic rocks in the northern Coast Mountains accumulated in proximity to western North America. The younger zircon populations were likely shed from mid-Proterozoic to early Paleozoic igneous rocks that now occur locally (but may have been widespread) along the Cordilleran margin. -from Authors
- Gehrels, G. E., McClelland, W. C., Samson, S. D., Jackson, J. L., & Patchett, P. J. (1991). U-Pb geochronology of two pre-Tertiary plutons in the Coast Mountains Batholith near Ketchikan, southeastern Alaska. Canadian Journal of Earth Sciences, 28(6), 894-898.More infoAbstract: U-Pb geochronologic studies indicate that the central part of the Coast Mountains Batholith in the Ketchikan area contains plutons of Triassic and possibly mid-Paleozoic age in addition to the previously recognized early Tertiary plutons. The presence of these Triassic and possibly mid-Paleozoic plutons supports previous interpretations that pre-Tertiary rocks in this part of the batholith may belong to the Yukon-Tanana terrane. -from Authors
- Gehrels, G. E., McClelland, W. C., Samson, S. D., Patchett, P. J., & Brew, D. A. (1991). U-Pb geochronology of late Cretaceous and early Tertiary plutons in the northern Coast Mountains Batholith. Canadian Journal of Earth Sciences, 28(6), 899-911.More infoAbstract: U-Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains Batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic-granodioritic bodies in central portions of the batholith yield ages of 59-58 Ma, coeval with younger phases of the tonalitic sheets. Large granite-granodiorite bodies in central and eastern portions of the batholith were emplaced at 51-48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons. -from Authors
- Jackson, J. L., Gehrels, G. E., Patchett, P. J., & Mihalynuk, M. G. (1991). Stratigraphic and isotopic link between the northern Stikine terrane and an ancient continental margin assemblage, Canadian Cordillera. Geology, 19(12), 1177-1180.More infoAbstract: Geologic and isotopic data strongly imply a Late Triassic depositional link between a juvenile volcanic arc (northern Stikine terrane) and an outboard ancient continental margin assemblage (Nisling terrane) in the Canadian Cordillera. Two sandstone samples and a schist clast from a conglomerate layer at the base of the Upper Triassic Stuhini Group (northern Stikine terrane) have Nd-depleted mantle model ages of 1400-1430 and 1600 Ma, respectively; other Stuhini Group rocks have model ages of 390, 660, and 690 Ma. Three samples of Nisling terrane schist and gneiss yield Nd model ages of 910, 1770 and 2450 Ma and highly radiogenic 87Sr/86Sr ratios. These isotopic data corroborate stratigraphic evidence that detritus at the base of northern Stikine was shed from the Nisling terrane and strengthen interpretations that these terranes became linked by Late Triassic time. Thus, Upper Triassic strata of the northern Stikine terrane may have accumulated on top of or adjacent to an exotic continental fragment, a rifted fragment of the North American margin, or the in situ North American margin. -Authors
- McClelland, W. C., Anovitz, L. M., & Gehrels, G. E. (1991). Thermobarometric constraints on the structural evolution of the Coast Mountains Batholith, central southeastern Alaska. Canadian Journal of Earth Sciences, 28(6), 912-928.More infoAbstract: Thermobarometric data from amphibolite-facies metamorphic rocks west of the Coast Mountain Batholith provide important constraints on the structural evolution of the mid-Cretaceous Sumdum-Fanshaw fault system and Late Cretaceous-Paleocene Le Conte Bay shear zone in central southeastern Alaska. -from Authors
- Samson, S. D., Patchett, P. J., McClelland, W. C., & Gehrels, G. E. (1991). Nd and Sr isotopic constraints on the petrogenesis of the west side of the northern Coast Mountains Batholith, Alaskan and Canadian Cordillera. Canadian Journal of Earth Sciences, 28(6), 939-946.More infoAbstract: Nd and Sr isotopic ratios are reported from 15 samples of plutons of the northern Coast Mountains Batholith (CMB), between the Alexander-Wrangellia terrane and the Stikine terrane of southeastern Alaska. Samples of plutons that are part of the Late Cretaceous-Eocene CMB suite have a range in initial εNd of -3.0 to -0.2 and 87Sr/86Sr of 0.70494-0.70607. Two plutons that are probably older than the bulk of the CMB plutons have present-day εNd values of -6.8 and -2.6. The Late Cretaceous-Eocene plutons have Nd depleted-mantle model ages (tDM) of 620-1070 Ma. These data indicate that the northern CMB must contain a significant component of old, evolved continental crust. The mid to Late Proterozoic tDM ages of the CMB plutons are probably a result of a mixture of Early Proterozoic crustal material with younger, juvenile crust. -from Authors
- Samson, S. D., Patchett, P. J., McClelland, W. C., & Gehrels, G. E. (1991). Nd isotopic characterization of metamorphic rocks in the Coast Mountains, Alaskan and Canadian cordillera: ancient crust bounded by juvenile terranes. Tectonics, 10(4), 770-780.More infoAbstract: Nd isotopic data are reported for 52 samples from the crustal region between the Alexander-Wrangellia terrane and the Stikine terrane of the Alaskan and Canadian Cordillera. This region is composed of the Gravina belt, a Jurassic-Cretaceous assemblage of volcanic and clastic sedimentary rocks, the Taku terrane, and four assemblages of metamorphic rocks. The isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from Precambrian continental crust. Possible tectonic scenarios that can explain the present geometry of the YTT with respect to the Alexander-Wrangellia and Stikine terranes include: 1) the YTT is the upturned stratigraphic basement of the Stikine terrane, 2) part of the YTT was structurally emplaced beside the Stikine terrane in a transpressive tectonic regime, 3) the Stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the YTT before the final accretion of the Alexander-Wrangellia terrane. -from Authors
- Smith, M. T., & Gehrels, G. E. (1991). Detrital zircon geochronology of Upper Proterozoic to lower Paleozoic continental margin strata of the Kootenay Arc: implications for the early Paleozoic tectonic development of the eastern Canadian Cordillera. Canadian Journal of Earth Sciences, 28(8), 1271-1284.More infoAbstract: The Kootenay Arc in Northeastern Washington and southeastern British Columbia contains the transition between autochthonous Upper Proterozoic to lower Paleozoic miogeoclinal strata and outboard Paleozoic to Mesozoic eugeoclinal terranes of uncertain paleogeographic affinity. To better understand the nature of this transition, U-Pb detrital zircon geochronologic studies were carried out on Upper Proterozoic and lower Paleozoic sedimentary units in the Kootenay Arc. -from Authors
- Yanez, P., Ruiz, J., Patchett, P. J., Ortega-Gutierrez, F., & Gehrels, G. E. (1991). Isotopic studies of the Acatlan Complex, southern Mexico: implications for Paleozoic North American tectonics. Geological Society of America Bulletin, 103(6), 817-828.More infoAbstract: The Paleozoic Acatlan Complex and Grenville-age Oaxaca terrane of southern Mexico have been suggested to be the southern continuation of North American orogenic systems. The Paleozoic Acatlan Complex consists of multiply deformed metasedimentary rocks, schists, granitoids, and eclogites and have been compared to rocks of the Appalachian belt. The northward extension of both the Oaxaca terrane and Acatlan Complex, however, are obscurred by younger rock cover as they enter central Mexico. Isotopic studies show that the Acatlan Complex records three tectonothermal events. Neodymium model ages of the Acatlan Complex are unlike those of some accreted crustal blocks of the Pacific margin, which have little signs of crustal recyling. It is suggested that the Devonian metamorphic event that affected these two areas was the result of a Laurentia-Gondwana collision. A later collision in late Carboniferous time caused deformation in the Acatlan Complex, plutonic activity in southern Mexico, and deformation in the Ouachita, Marathon, and Appalachian belts. Both the Acatlan and Oaxaca terranes would have been continuous with South America until the break-up of Pangea in the Mesozoic era. -from Authors
- Gehrels, G. E. (1990). Late Proterozoic-Cambrian metamorphic basement of the Alexander terrane on Long and Dall Islands, southeast Alaska. Bulletin of the Geological Society of America, 102(6), 760-767.More infoAbstract: Geologic and U-Pb (zircon) geochronologic studies demonstrate that Long Island and southern and central Dall Island are underlain in large part by upper Proterozoic-Lower Cambrian metamorphic rocks of the Wales Group. The group comprises a lower assemblage of predominantly metadacite, metamorphosed to amphibolite and greenschist facies, and an upper assemblage of lower-grade basaltic metavolcanic rocks,thick marble layers, and interlayered calcareous metadacite, marble, metapelite, and metagraywacke. Rocks of both assemblages are intruded by metagranodiorite-diorite-gabbro suites, one of which (Kaigani orthogneiss) yields a concordant U-Pb (zircon) age of 554 ± 4 m.y. (Early Cambrian). Rocks of the upper Wales assemblage were thrust over Ordovician(?)-Silurian strata of the Descon Formation, possibly during the Middle Silurian-earliest Devonian Klakas orogeny. A younger normal(?) fault juxtaposes rocks of the upper Wales Group, and locally the Descon Formation and Heceta Limestone, over higher-grade rocks of the lower Wales Group. This poorly known structure may have been active during a Late Permian(?)-Triassic rifting event recorded along the northeastern margin of the terrane. The youngest major unit in the area is the Stripe Mountain granodiorite, which yields a concordant U-Pb (zircon) age of 114 ± 2 m.y. © 1990 Geological Society of America.
- Gehrels, G. E., McClelland, W. C., Samson, S. D., Patchett, P. J., & Jackson, J. L. (1990). Ancient continental margin assemblage in the northern Coast Mountains, southeast Alaska and northwest Canada. Geology, 18(3), 208-211.More infoAbstract: Geologic relations indicate that quartz-rich metasedimentary rocks in the northern Coast Mountains separate strata to the east that belong to the Stikine terrane from strata to the west of the Alexander,Wrangellia, and Taku terranes. The quartz-rich rocks structurally overlie western terranes along a mid-Cretaceous thrust fault and are overlain structurally (originally stratigraphically?) by strata of the Stikine terrace. These rocks are interpreted to be a continental margin assemblage that belongs to the Yukon Crystalline terrane. U-Pb and Nd isotopic data indicate that the metasedimentary rocks were shed from a source terrane consisting at least in part of Proterozoic rocks. © 1990 Geological Society of America.
- McClelland, W. C., & Gehrels, G. E. (1990). Geology of the Duncan Canal Shear Zone: evidence for Early to Middle Jurassic deformation of the Alexander Terrane, southeastern Alaska. Geological Society of America Bulletin, 102(10), 1378-1392.More infoAbstract: The Duncan Canal Shear Zone, central southeastern Alaska, disrupts and penetratively deforms Devonian, Pennsylvanian, and Upper Triassic strata of the Alexander Terrane. The deformation is interpreted to have occurred within a regime of right-lateral shear during Early or Middle Jurassic time. Clastic and volcanic rocks correlated with the Upper Jurassic and Lower Cretaceous strata of the Gravina Belt lack the fabrics characteristic of the deformed Alexander Terrane strata and are inferred to unconformably overlie the shear zone. Deformational fabrics of mid-Cretaceous age observed in the Gravina rocks locally truncate and deform the shear-zone strata. Dextral stike-slip faults of Late Cretaceous and(or) Tertiary age that disrupt and modify fabrics of the shear zone are interpreted to accommodate some of the displacement observed on the Clarence Strait fault. The Duncan Canal shear zone extends southward to the Mosman Inlet region on Etolin Island where it projects beneath Jura-Cretaceous Gravina strata. In Duncan Canal and Mosman Inlet, the shear zone evolved within the Alexander Terrane and does not coincide with any recognized terrane boundaries. Since the Duncan Canal Shear Zone is restricted to the currently exposed eastern margin of the Alexander Terrane, the shear zone is interpreted to reflect deformation along the eastern margin of the Alexander Terrane. -Authors
- Miller, R. B., Johnson, S. Y., McDougall, J. W., Butler, R. F., Dickinson, W. R., Gehrels, G. E., McClelland, W. C., May, S. R., & Klepacki, D. (1990). Comment and Reply on "Discordant paleomagnetic poles from the Canadian Coast Plutonic Complex: Regional tilt rather than large displacement?". Geology, 18(11), 1164-1166.
- Samson, S. D., Patchett, P. J., Gehrels, G. E., & Anderson, R. G. (1990). Nd and Sr isotopic characterization of the Wrangellia terrane and implications for crustal growth of the Canadian Cordillera. Journal of Geology, 98(5), 749-762.More infoAbstract: Nd and Sr isotopic data are reported from samples of most of the major lithologic units of the Wrangellia terrane exposed on Vancouver Island and the Queen Charlotte Islands. Initial εNd values range from +1.0 to +7.3 and initial 87Sr/86Sr ratios range from 0.70323 to 0.70481. These ratios are similar to those of modern island arcs and demonstrate the unevolved, juvenile nature of this terrane. These data indicate that Wrangellia probably resided in an intra-oceanic environment until its accretion to North America. Early Proterozoic crustal components cannot comprise more than about 6% of Wrangellia; therefore the terrane consists of ~94% new, mantle-derived crust. The amount of new crust in Wrangellia is very similar to the amount determined for two other large Cordilleran terranes, Alexander and Stikine. The accretion of juvenile terranes to a continent appears to be an important mechanism of crustal growth. -from Authors
- Umhoefer, P. J., Magloughlin, J. F., Butler, R. F., Gehrels, G. E., McClelland, W. C., May, S. R., & Klepacki, D. (1990). Comment and Reply on "Discordant paleomagnetic poles from the Canadian Coast Plutonic Complex: Regional tilt rather than large-scale displacement". Geology, 18(8), 800-802.
- Butler, R. F., Gehrels, G. E., McClelland, W. C., May, S. R., & Klepacki, D. (1989). Discordant paleomagnetic poles from the Canadian Coast Plutonic Complex: regional tilt rather than large-scale displacement?. Geology, 17(8), 691-694.More infoAbstract: Stratigraphic, petrologic, and isotopic data indicate that parts of the Coast Plutonic Complex and the North Cascade Range have been tilted northeast-side-up by angles of ~30° about north-northwest-trending axes. These tilts can account for discordant paleomagnetic directions observed in mid-Cretaceous plutons from these regions without large-scale displacement relative to North America. -Authors
- Samson, S. D., McClelland, W. C., Patchett, P. J., Gehrels, G. E., & Anderson, R. G. (1989). Evidence from neodymium isotopes for mantle contributions to Phanerozoic crustal genesis in the Canadian Cordillera. Nature, 337(6209), 705-709.More infoAbstract: Sm-Nd and Rb-Sr isotopic studies of several Phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked, pre-existing continental crust. In contrast, isotopic data collected from two of the largest terranes in the Canadian Cordilleran orogenic belt, the Alexander and Stikine terranes, show that these tectonic fragments are composed of mantle-derived continental crust. In many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some Proterozoic crustal regions © 1989 Nature Publishing Group.
- Gehrels, G. E., & Saleeby, J. B. (1987). GEOLOGY OF SOUTHERN PRINCE OF WALES ISLAND, SOUTHEASTERN ALASKA.. Bulletin of the Geological Society of America, 98(2), 123-137.More infoAbstract: Geologic mapping on southern Prince of Wales Island combined with U-Pb (zircon) geochronologic, geochemical, and conodont studies yield new information about the early Paleozoic geologic framework and evolution of the southern Alexander terrane. The oldest rocks recognized consist of arc-type(? ) metavolcanic and metasedimentary rocks (Wales metamorphic suite) which were derived from late Proterozoic(? )-Cambrian protoliths. The middle Silurian-earliest Devonian Klakas orogeny marked the end of arc-type magmatism; it is manifest on southern Prince of Wales Island by regional imbrication on southwest-vergent thrust faults, penetrative brecciation and pervasive hydrothermal alteration of pre-Devonian rocks in some areas, and at least several kilometers of structural uplift. Post-Devonian rocks on southern Prince of Wales Island include the Bokan Mountain Granite (Jurassic), basaltic to dacitic(? ) dike swarms, and mid-Cretaceous granodiorite and diorite bodies.
- Gehrels, G. E., & Saleeby, J. B. (1987). Geologic framework, tectonic evolution, and displacement history of the Alexander terrane ( USA Canada).. Tectonics, 6(2), 151-173.More infoAbstract: The Alexander terrane consists of upper Proterozoic(?)-Cambrian through Middle(?) Jurassic rocks that underlie much of SE Alaska and parts of E Alaska, W British Columbia, and SW Yukon Territory. A variety of geologic, paleomagnetic, and paleontologic evidence indicates that these rocks have been displaced considerable distances from their sites of origin and were not accreted to western North America until Late Cretaceous-early Tertiary time. Our geologic and U-Pb geochronologic studies in southern SE Alaska and the work of others to the N indicate that the terrane evolved through 3 distinct tectonic phases; these are discussed. -from Authors
- Gehrels, G. E., & Saleeby, J. B. (1987). Geology of southern Prince of Wales Island, southeastern Alaska ( USA).. Geological Society of America Bulletin, 98(2), 123-137.More infoAbstract: Geologic mapping on southern Prince of Wales Island combined with U-Pb (zircon) geochronologic, geochemical, and conodont studies yield new information about the early Palaeozoic geologic framework and evolution of the southern Alexander terrane. -from Authors
- Gehrels, G. E., & Smith, M. T. (1987). "Antler" allochthon in the Kootenay arc?. Geology, 15(8), 769-770.
- Gehrels, G. E., Saleeby, J. B., & Berg, H. C. (1987). Geology of Annette, Gravina, and Duke islands, southeastern Alaska ( USA).. Canadian Journal of Earth Sciences, 24(5), 866-881.More infoAbstract: Geological mapping, U-Pb (zircon) geochronometry, and conodont studies indicate that the major pre-Jurassic assemblages on Annette, Gravina, Duke, and adjacent small islands include pre-Middle Ordovician metavolcanic and metasedimentary rocks (Wales metamorphic suite): Cambrian metaplutonic rocks; Ordovician - Early Silurian volcanic (Descon Formation), dioritic, and gabbroic rocks; Silurian trondhjemitic plutons; Early Devonian sedimentary (Karheen Formation) and volcanic rocks; Late Triassic sedimentary and volcanic rocks (Hyd Group); and a large body of Late Triassic pyroxene gabbro.-from Authors
- Savage, N. M., & Gehrels, G. E. (1984). Early Devonian conodonts from Prince of Wales Island, southeastern Alaska.. Canadian Journal of Earth Sciences, 21(12), 1415-1425.More infoAbstract: Early Devonian conodonts from 11 localities on Prince of Wales Island and adjacent small islands, SE Alaska, include Pandorinellina optima (Moskalenko), Pandorinellina exigua philipi (Klapper), Pandorinellina exigua exigua (Philip), Pandorinellia? boucoti (Klapper), Pelekysgnathus serratus Jentzsch, Pelekysgnathus cf. P. klamathensis Savage, Pelekysgnathus n. sp. Eognathodus sulcatus sulcatus Philip, Eognathodus sulcatus kindlei Lune and Ormiston, Eognathodus sulcatus n.ssp. A, Eognathodus sucatus n.ssp. B. Ozarkodina eberleini Savage, Kimognathus alexeii Mashkova, Kimognathus n.sp. A, and Kimognathus n.sp. B. The occurrences indicate that Lower Devonian deposits are widespread on Prince of Wales Island. On Wadleigh Island, Zlichovian, and probably older, deposits underlie Eifelian beds of the Wadleigh Limestone, previously thought to include limestone only of Middle and Late Devonian age. The Early Devonian faunas of this part of SE Alaska contain species characteristic of Cordilleran N America. Regional variations in conodont color alteration indices suggest that the S half of Prince of Wales Island has a different post-Devonian thermal history from the N half.-Authors