Peter W Reiners
- Professor, Geosciences
- Associate Dean, Research
- Ph.D. Geological Sciences
- University of Washington, Seattle, Washington, United States
- Geological Society of America Fellow
- Spring 2016
- GSA Fellow
- Fall 2015
- MIT Crosby Lecturer
- Spring 2015
- Marie Curie Fellowship
- European Union, Fall 2012
No activities entered.
Honors ThesisGEOS 498H (Fall 2020)
ResearchGEOS 900 (Fall 2020)
Honors ThesisGEOS 498H (Spring 2020)
ResearchGEOS 900 (Spring 2020)
Geo & ThermochronologyGEOS 574A (Fall 2019)
Honors ThesisGEOS 498H (Fall 2019)
Independent StudyGEOS 599 (Fall 2019)
Intro To GeochemistryGEOS 400 (Fall 2019)
Intro To GeochemistryGEOS 500 (Fall 2019)
Independent StudyGEOS 499 (Spring 2019)
Physical GeologyGEOS 251 (Spring 2019)
Teaching GeosciencesGEOS 397A (Spring 2019)
DissertationGEOS 920 (Fall 2018)
Independent StudyGEOS 499 (Fall 2018)
Intro To GeochemistryGEOS 400 (Fall 2018)
Intro To GeochemistryGEOS 500 (Fall 2018)
ResearchGEOS 900 (Fall 2018)
DissertationGEOS 920 (Spring 2018)
Geo & ThermochronologyGEOS 474A (Spring 2018)
Geo & ThermochronologyGEOS 574A (Spring 2018)
Master's ReportGEOS 909 (Spring 2018)
DissertationGEOS 920 (Fall 2017)
ResearchGEOS 900 (Fall 2017)
DissertationGEOS 920 (Spring 2017)
DissertationGEOS 920 (Fall 2016)
ResearchGEOS 900 (Fall 2016)
Master's ReportGEOS 909 (Summer I 2016)
DissertationGEOS 920 (Spring 2016)
Geo & ThermochronologyGEOS 474A (Spring 2016)
Geo & ThermochronologyGEOS 574A (Spring 2016)
Independent StudyGEOS 599 (Spring 2016)
Master's ReportGEOS 909 (Spring 2016)
ResearchGEOS 900 (Spring 2016)
- Reiners, P. W. (2017). Geochronology and Thermochronology. New York: Wiley.
- Reiners, P. W. (2017). Reconciling regional continuity with local variability in structure, uplift and exhumation of the Timor orogen. Gondwana Research, 49, 364-386.
- Reiners, P. W. (2017). Thermochronology of sandstone-hosted secondary Fe- and Mn-oxides near Moab, Utah: Record of paleo–fluid flow along a fault. Geological Society of America Bulletin, 130, 93-113. doi:10.1130/B31627.1
- Reiners, P. W. (2017). Thermochronometric and textural evidence for seismicity via asperity flash heating on exhumed hematite fault mirrors, Wasatch fault zone, UT, USA. Earth and Planetary Science Letters, 471, 85-93.
- Reiners, P. W. (2017). Zircon, titanite, and apatite (U-Th)/He ages and age-eU correlations from the Fennoscandian Shield, southern Sweden. Tectonics, 36, 1254–1274. doi:10.1002/2017TC004525
- Ault, A. K., Frenzel, M., Reiners, P. W., Woodcock, N. H., & Thomson, S. N. (2016). Record of paleofluid circulation in faults revealed by hematite (U-Th)/He and apatite fission-track dating: An example from Gower Peninsula fault fissures, Wales. Lithosphere, 8(4), 379--385.
- Guenthner, W. R., Reiners, P. W., & Chowdhury, U. (2016). Isotope dilution analysis of Ca and Zr in apatite and zircon (U-Th)/He chronometry. Geochemistry, Geophysics, Geosystems, 17(5), 1623--1640.
- Loope, D., Kettler, R., Murray, K., Pederson, J., & Reiners, P. (2016). Sandstones and Utah's canyon country: Deposition, diagenesis, exhumation, and landscape evolution. Field Guides, 44, 41--71.
- Murray, K. E., Reiners, P. W., & Thomson, S. N. (2016). Rapid Pliocene--Pleistocene erosion of the central Colorado Plateau documented by apatite thermochronology from the Henry Mountains. Geology, 44(6), 483--486.
- Nasdala, L., Corfu, F., Valley, J. W., Spicuzza, M. J., Wu, F., Li, Q., Yang, Y., Fisher, C., M"unker, C., Kennedy, A. K., & others, . (2016). Zircon M127--A Homogeneous Reference Material for SIMS U--Pb Geochronology Combined with Hafnium, Oxygen and, Potentially, Lithium Isotope Analysis. Geostandards and Geoanalytical Research, 40(4), 457--475.
- Orme, D. A., Guenthner, W. R., Laskowski, A. K., & Reiners, P. W. (2016). Long-term tectonothermal history of Laramide basement from zircon--He age-eU correlations. Earth and Planetary Science Letters, 453, 119--130.
- Zhang, H., Oskin, M. E., Liu-Zeng, J., Zhang, P., Reiners, P. W., & Xiao, P. (2016). Pulsed exhumation of interior eastern Tibet: Implications for relief generation mechanisms and the origin of high-elevation planation surfaces. Earth and Planetary Science Letters, 449, 176--185.
- Ault, A. K., Reiners, P. W., Evans, J. P., & Thomson, S. N. (2015). Linking hematite (U-Th)/He dating with the microtextural record of seismicity in the Wasatch fault damage zone, Utah, USA. Geology, 43, 771--774.
- Guenthner, W. R., Reiners, P. W., DeCelles, P. G., & Kendall, J. (2015). Sevier belt exhumation in central Utah constrained from complex zircon (U-Th)/He data sets: Radiation damage and He inheritance effects on partially reset detrital zircons. Geological Society of America Bulletin, 127, 323--348.
- Long, S. P., Thomson, S. N., Reiners, P. W., & Di Fiori, R. V. (2015). Synorogenic extension localized by upper-crustal thickening: An example from the Late Cretaceous Nevadaplano. Geology, 43, 351--354.
- Orme, D. A., Reiners, P. W., Hourigan, J. K., & Carrapa, B. (2015). Effects of inherited cores and magmatic overgrowths on zircon (U-Th)/He ages and age-eU trends from Greater Himalayan sequence rocks, Mount Everest region, Tibet. Geochemistry, Geophysics, Geosystems, 16, 2499--2507.
- Reiners, P. W., Thomson, S., Vernon, A., Willett, S., Zattin, M., Einhorn, J., Gehrels, G., Quade, J., Pearson, D., Murray, K., & others, . (2015). Low-temperature thermochronologic trends across the central Andes, 21 S--28 S. Geological Society of America Memoirs, 212, 215--249.
- Braun, J., Simon-Labric, T., Murray, K. E., & Reiners, P. W. (2014). Topographic relief driven by variations in surface rock density. Nature Geoscience.
- Carrapa, B., Bywater-Reyes, S., Pour, R. S., Sobel, E. R., Schoenbohm, L. M., DeCelles, P. G., Reiners, P., & Stockli, D. (2014). The effect of inherited paleotopography on exhumation of the Central Andes of NW Argentina (vol 126, pg 66, 2014). GEOLOGICAL SOCIETY OF AMERICA BULLETIN, 126(3-4), 615--615.
- Evenson, N., Reiners, P., Spencer, J., & Shuster, D. (2014). Hematite and Mn oxide (U-Th)/He dates from the Buckskin-Rawhide detachment system, western Arizona: constraining the timing of mineralization and gaining insights into hematite (U-Th)/He systematics. American Journal of Science, 314, 1373-1435.
- Guenthner, W. R., Reiners, P. W., & Tian, Y. (2014). Interpreting date--eU correlations in zircon (U-Th)/He datasets: A case study from the Longmen Shan, China. Earth and Planetary Science Letters, 403, 328--339.
- Guenthner, W., Reiners, P., DeCelles, P., & Kendall, J. (2014). Sevier-belt exhumation in central Utah constrained from complex zircon (U-Th)/He datasets: Radiation damage and He inheritance effects on partially reset detrital zircons. Geological Society of America Bulletin, doi:10.1130/B31032.1.
- McQuarrie, N., Tobgay, T., Long, S. P., Reiners, P. W., & Cosca, M. A. (2014). Variable exhumation rates and variable displacement rates: Documenting recent slowing of Himalayan shortening in western Bhutan. Earth and Planetary Science Letters, 386, 161--174.
- McQuarrie, N., Tobgay, T., Long, S. P., Reiners, P. W., & Cosca, M. A. (2014). Variable exhumation rates and variable displacement rates: Documenting recent slowing of Himalayan shortening in western Bhutan. Earth and Planetary Science Letters, 386, 161-174.More infoAbstract: We link exhumational variability in space and time to the evolving geometry of the Himalayan fold-thrust belt in western Bhutan. By combining new and published geochronologic and thermochronologic data we document the burial age, peak temperatures and complete cooling history from 20 Ma to the present over an across-strike distance of ~125 km. These integrated cooling curves highlight windows of fast exhumation that vary spatially and temporally. We propose that pulses of fast exhumation are a result of structures that facilitate the vertical motion of material, illustrated in sequentially-restored cross sections. Due to a range of permissible geometries at depth, we explore and evaluate the impact of geometry on kinematics and rates of deformation. The linked cooling history and cross sections provide estimates of both magnitude and timing of thrust sheet displacement and highlight temporal variability in potential shortening rates. Structural and chronologic data illustrate a general north to south progression of Himalayan deformation, with emplacement of the Main Central thrust (MCT), Paro thrust and Shumar thrust by 12 to no later than 9 Ma. Two different geometries and kinematic scenarios for the Lesser Himalayan duplex are proposed. A north to south propagating duplex system requires that the southern portion of that system, south of the MCT, deformed and cooled by 9 Ma, leaving only the southernmost thrust sheets, including the Main Boundary and Main Frontal thrusts, to deform between 9 and 0 Ma. This limited post 9 Ma shortening would necessitate a marked slowdown in convergence accommodated on the Main Himalayan thrust. A two-tiered duplex system, which allows for the Paro window duplex and the southern Baxa duplex to form simultaneously, permits duplex formation and accompanying exhumation until 6 Ma. Limited cooling from ~200°C to the surface post 6 Ma suggests either a decrease in shortening rates from 6 to 0 Ma or that duplex formation and exhumation are temporally decoupled. Our combined cooling curves highlight that the youngest cooling ages may not mark the fastest thrusting rates or the window of fastest exhumation. Instead, temporal variations in exhumation are best viewed through identifying transients in exhumation rate. We suggest that the strongest control on exhumation magnitude and variability is fold-thrust belt geometry, particularly the locations and magnitudes of footwall ramps, which can change over 10's of km distance. Balanced cross sections predict the location and magnitude of these ramps and how they vary in space and time, providing an untapped potential for testing permissible cross-section geometries and kinematics against measured cooling histories. © 2013 Elsevier B.V.
- Murray, K. E., Orme, D. A., & Reiners, P. W. (2014). Effects of U-Th-rich grain boundary phases on apatite helium ages. Chemical Geology.
- 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,< sup> 40 Ar/< sup> 39 Ar hornblende,< sup> 40 Ar/< sup> 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.
- Reiners, P. W., Chan, M. A., & Evenson, N. S. (2014). (U-Th)/He geochronology and chemical compositions of diagenetic cement, concretions, and fracture-filling oxide minerals in Mesozoic sandstones of the Colorado Plateau. Geological Society of America Bulletin, 126(9-10).
- Reiners, P., Thomson, S., Vernon, A., Willett, S., Zattin, M., Einhorn, J., Gehrels, G., Quade, J., Pearson, D., Murray, K., & Cavazza, W. (2014). Low-temperature thermochronologic trends across the central Andes 21-28° S. Geological Society of America Memoir, 212, 215-249.
- Simon-Labric, T., Brocard, G. Y., Teyssier, C., Beek, P. A., Reiners, P. W., Shuster, D. L., Murray, K. E., & Whitney, D. L. (2014). Low-temperature thermochronologic signature of range-divide migration and breaching in the North Cascades. Lithosphere, L382--1.
- Tate, G. W., McQuarrie, N., Hinsbergen, D. J., Bakker, R. R., Harris, R., Willett, S., Reiners, P. W., Fellin, M. G., Ganer\od, M., & Zachariasse, J. (2014). Resolving spatial heterogeneities in exhumation and surface uplift in Timor-Leste: constraints on deformation processes in young orogens. Tectonics.
- Toraman, E., Teyssier, C., Whitney, D. L., Fayon, A. K., Thomson, S. N., & Reiners, P. W. (2014). Low-temperature thermochronologic record of Eocene migmatite dome emplacement and late Cenozoic landscape development, Shuswap core complex, British Columbia. Tectonics, 33(8), 1616--1635.
- Wang, C. Y., Campbell, I. H., Reiners, P. W., & Allen, C. M. (2014). Detrital zircon U-Pb-He double dating: A method of quantifying long-and short-term exhumation rates in collisional orogens. Science China Earth Sciences, 1--10.
- Zapata, S., Cardona, A., Montes, C., Valencia, V., Vervoort, J., & Reiners, P. (2014). Provenance of the Eocene Soebi Blanco formation, Bonaire, Leeward Antilles: Correlations with post-Eocene tectonic evolution of northern South America. Journal of South American Earth Sciences, 52, 179--193.
- Zapata, S., Cardona, A., Montes, C., Valencia, V., Vervoort, J., & Reiners, P. (2014). Provenance of the Eocene Soebi Blanco formation, Bonaire, Leeward Antilles: Correlations with post-Eocene tectonic evolution of northern South America. Journal of South American Earth Sciences, 52, 179-193.More infoAbstract: Middle to upper Eocene fluvial strata in the island of Bonaire contain detrital components that were tracked to Precambrian to Triassic massifs in northern Colombia and Venezuela. These detrital components confirm previous hypothesis suggesting that Bonaire and the Leeward Antilles were attached to South American basement massifs (SABM). These are composed of different fragmented South American blocks (Paraguana, Falcon, Maracaibo, Guajira, Perija, and Santa Marta) representing an Eocene, right-laterally displaced tectonic piercing point along the southern Caribbean plate margin. U-Pb LA-ICP-MS from the metamorphic boulders of the Soebi Blanco Formation in Bonaire yield Grenvillian peaks ages (1000-1200. Ma), while detrital zircons recovered from the sandy matrix of the conglomerates contain populations with peaks of 1000 Ma-1200. Ma, 750-950. Ma, and 200-300. Ma. These populations match with geochronological data reported for the northern South American massifs. Thermochronological results from the metamorphic clasts yield Paleocene-middle Eocene ages (65-50. Ma) that confirm a regional-scale cooling event in this time. These data imply a land connection between the SABM and the Leeward Antilles in late Eocene times, followed by a significant strike slip right-lateral displacement and transtensional basin opening starting in latest Eocene times. The succession of Eocene tectonic events recorded by the Soebi Blanco Formation and adjacent basins is a major tracer of the oblique convergence of the Caribbean plate against the South American margin. © 2014 Elsevier Ltd.
- Ketcham, R. A., Guenthner, W. R., & Reiners, P. W. (2013). Geometric analysis of radiation damage connectivity in zircon, and its implications for helium diffusion. American Mineralogist, 98(2-3), 350-360.More infoAbstract: Geometrical modeling of radiation damages zones from α recoil and fission that accounts for their elongate nature provides new estimates of the doses required to reach percolation and full connectivity in zircon. Alpha recoil track damage percolates at doses from 2.5-3.1 × 1016 α/g based on our calculations, about two orders of magnitude lower than previous estimates, with the difference partially due to elongation and partially due to decay chains creating pre-made networks of connected tracks. This dose level is far below that required for metamictization, and suggests that α recoil track percolation has no effect on macroscopic or unit-cell properties, at least as measured to date. However, fission tracks percolate at a dose of approximately 1.9 × 1018 α/g, the approximate level formerly ascribed to α recoil damage percolation and correlating with various transitions in material properties, such as an inflection in the relationship between dose and macroscopic swelling. Consideration of the undamaged regions between damage zones indicates that c-axis-parallel channels are frequently interrupted, at the micrometer scale at very low doses and tens of nanometers at usual doses in natural zircon, with the probable effect of decreasing diffusivity anisotropy. The percolation and further interconnectivity of α recoil damage corresponds with a general minimum in diffusivity and maximum in closure temperature in zircon, indicating that α recoil damage percolation does not make a grain "leaky", but instead quite the opposite. Instead, the onset of poor He retentivity at high damage levels correlates with fission-track percolation. Some of these results are non-intuitive with respect to the trapping model of He diffusivity reduction, and the alternative mechanism of tortuosity is discussed.
- Min, K., Reiners, P. W., & Shuster, D. L. (2013). (U-Th)/He ages of phosphates from St. Séverin LL6 chondrite. Geochimica et Cosmochimica Acta, 100, 282-296.More infoAbstract: We obtained single-grain (U-Th)/He ages from 14 merrillite and five chlorapatite aggregates from St. Séverin to constrain its low-T thermal history. In addition, we performed 3He/4He stepped heating diffusion experiments for merrillite and chlorapatite crystals from Guareña chondrite to better constrain He diffusion properties in chondritic phosphates in general. Among the 19 phosphate grains from St. Séverin, the five oldest merrillites and four oldest chlorapatites yielded weighted mean ages of 4412±75Ma (1σ: MSWD=0.34) and 4152±70Ma (MSWD=0.48), respectively. These weighted mean ages overlap with the peaks of the corresponding probability density plots, thus likely represent the most pristine (U-Th)/He ages of St. Séverin. The radiogenic 4He and proton-induced 3He diffusion experiments on Guareña chondrite resulted in two well-defined linear trends in Arrhenius plot for merrillite (radius=∼59μm) and chlorapatite (∼43μm) grains. Error-weighted linear regressions of the 3He data yielded following diffusion parameters: Ea=135.8±4.8kJ/mol, and ln(Do/a2)=5.83±0.66ln(s-1) for merrillite; and Ea=109.3±9.7kJ/mol, and ln(Do/a2)=8.15±1.93 ln(s-1) for chlorapatite. Assuming the analyzed fragments approximate the diffusion domain, logDo was calculated as (-1.93±0.29) log(cm2/s) and (-1.19±0.84) log(cm2/s) for merrillite and chlorapatite, respectively. These results indicate higher closure T for merrillite than chlorapatite. Assuming these results also apply to St. Séverin and that crystal dimensions define the limiting diffusive length-scale, the closure temperatures of merrillite and chlorapatite in St. Séverin are estimated to be 94-112°C (for cooling rates of 0.3-2.6°C/Ma) and 18°C (for 0.3°C/Ma), respectively. The new single-grain (U-Th)/He ages and 3He/4He stepped heating diffusion results, combined with previously reported Pb/Pb and 40Ar/39Ar (with updated λ-tFCs pairs) data indicate that St. Séverin experienced relatively rapid cooling from 477°C down to ∼100°C at a cooling rate of 2.6°C/Ma. The identified thermal history is generally consistent with the reported cooling path from Pu fission track data. These results suggest that St. Séverin experienced relatively slow cooling compared to H chondrites, which may have resulted from its parent body size being larger than that of the H chondrites. © 2012 Elsevier Ltd.
- Simon-Labric, T., Brocard, G. Y., Teyssier, C., A., P., Fellin, M. G., Reiners, P. W., & Authemayou, C. (2013). Preservation of contrasting geothermal gradients across the Caribbean-North America plate boundary (Motagua Fault, Guatemala). Tectonics, 32(4), 993-1010.More infoAbstract: Strike-slip plate boundaries juxtapose crustal blocks that may have different geodynamic origins and therefore different thermal structures. Thermo-kinematic modeling of this type of strike-slip plate boundary predicts an asymmetric signature in the low-temperature thermochronologic record across the fault. Age-elevation profiles of zircon (U-Th)/He ages across the Motagua Fault, a 500 km long segment of the transform boundary between the North American and Caribbean plates, document a sharp cooling age discontinuity across the fault. This discontinuity could be interpreted as a difference in denudation history on each side of the fault. However, a low-relief Miocene erosional surface extends across the fault; this surface has been uplifted and incised and provides a geomorphic argument against differential denudation across the fault. By integrating magmatic, volcanic, and heat flow data, age-elevation profiles, and thermo-kinematic modeling, we propose that large horizontal displacement along the Motagua Fault has offset a persistent geothermal asymmetry across the fault and explains both the age discontinuities and the age-elevation patterns. This study illustrates how thermochronology can be used to detect large strike-slip displacements and more generally opens new perspectives in understanding the impact of nonuniform thermal structures on thermochronologic results. ©2013. American Geophysical Union. All Rights Reserved.
- Blondes, M. S., Brandon, M. T., Reiners, P. W., Page, F. Z., & Kita, N. T. (2012). Generation of forsteritic olivine (fo 99·8) by subsolidus oxidation in basaltic flows. Journal of Petrology, 53(5), 971-984.More infoAbstract: We identify olivine grains with compositions up to Fo 99·8, which are found in multiple primitive basaltic lava flows from a monogenetic volcano in the Big Pine Volcanic Field, California, USA. In this study, we show that the forsterite in these basalts formed by subsolidus recrystallization in a high-fO 2 environment. Olivine compositions are bimodal, with flows having either all normal compositions (Fo 74·9-94·4) or highly forsteritic (Fo 97·2-99·8) compositions. In many grains, the subhedral forsteritic olivine has a hematite and clinopyroxene rim, and internal parallel-oriented planes of hematite, clinopyroxene and orthopyroxene. Results of isotopic, chemical, crystallographic, petrographic and mineralogical analyses show that the forsterite formed through subsolidus oxidation of olivine phenocrysts. The forsteritic olivines generally occur in the thinner flows. We infer that a rapidly emplaced sequence of thin, vesicular, spatter-fed flows allowed the original olivine phenocrysts to become repeatedly reheated while exposed to air. Our study required sampling each flow, so it was difficult to avoid the altered portions of the thinner flows. Other studies would tend to avoid such flows, which may account for why such forsteritic olivines have not been more widely recognized. © The Author 2012. Published by Oxford University Press. All rights reserved.
- Braxton, D. P., Cooke, D. R., Dunlap, J., Norman, M., Reiners, P., Stein, H., & Waters, P. (2012). From crucible to graben in 2.3 Ma: A high-resolution geochronological study of porphyry life cycles, boyongan-bayugo copper-gold deposits, Philippines. Geology, 40(5), 471-474.More infoAbstract: The Boyongan and Bayugo porphyry copper-gold deposits are part of a belt of gold-rich copper deposits in the Surigao district of northeast Mindanao, Philippines. The detailed age relationships described in this study provide insight into the geologically short life cycles that characterize porphyry formation in dynamic arc environments. Since their late Pliocene emplacement (2.3-2.1 Ma; SHRIMP [sensitive highresolution ion microprobe] U-Pb zircon dating) at depths of 1.2-2.0 km, these deposits were exhumed, deeply weathered, and buried. Weathering of these deposits led to the development of the world's deepest known porphyry oxidation profile (600 m thick) at Boyongan, and a modest (30-70 m) oxidation profile at adjacent Bayugo. This earlymiddle Pleistocene supergene event followed a period of rapid uplift and exhumation in northeast Mindanao (2.5 km/Ma; [U-Th]/He apatite age-elevation spectrum). Subsequent rapid subsidence (≥0.34 km/Ma; radiocarbon age-elevation spectrum) and burial of these deposits are attributed to a mid-Pleistocene shift from transpressional tectonics to the present-day transtensional setting in northeast Mindanao. During this period, debris flows, volcanic material, and fluviolacustrine sediments accumulating in the actively extending Mainit graben covered the weathered deposits, preserving the supergene profiles beneath 50-500 m of cover. This detailed geochronological study documents the geologically short (
- Long, S. P., McQuarrie, N., Tobgay, T., Coutand, I., Cooper, F. J., Reiners, P. W., Wartho, J., & Hodges, K. V. (2012). Variable shortening rates in the eastern Himalayan thrust belt, Bhutan: Insights from multiple thermochronologic and geochronologic data sets tied to kinematic reconstructions. Tectonics, 31(5).More infoAbstract: We present data on the burial, displacement and exhumation history of the Himalayan fold-thrust belt in eastern Bhutan. These data document the magnitude and timing of displacement of large, discrete structures and highlight temporal variability in shortening rates. Eight new 40Ar/ 39Ar ages from white mica, 32 new zircon (U-Th)/He ages, 7 new apatite fission track ages, and 1 new U-Pb zircon (LA-MC-ICP-MS) metamorphic rim growth age are combined with published cooling ages and deformation temperatures, and incremental shortening magnitudes from restorations of two published balanced cross sections, to illustrate the kinematic and temporal development of the Bhutan thrust belt. Integrating these data from ∼23 Ma to the present illustrates rapid horizontal shortening rates (28-35 mm/yr) between 23-20 Ma and 15-10 Ma, separated by more moderate rates (10-23 mm/yr). Shortening rates decrease significantly to 7-10 mm/yr (and possibly as low as 3-4 mm/yr) from 10 to 0 Ma. This decrease is interpreted to represent the onset of strain partitioning in the eastern part of the Himalayan-Tibetan orogenic system, between shortening in the Bhutan thrust belt, uplift of the Shillong Plateau, and deformation and outward growth of the northern and eastern Tibetan Plateau. Within estimated error, horizontal shortening rates during emplacement of the Main Central thrust sheet and during construction of the upper Lesser Himalayan duplex approached India-Asia tectonic velocities. Thus, for periods of time between ∼23-20 Ma and ∼15-10 Ma, the Bhutan thrust belt may have absorbed nearly all India-Asian convergence at this longitude. © 2012. American Geophysical Union. All Rights Reserved.
- Reiners, P. W. (2012). Paleotopography in the western U.S. Cordillera. American Journal of Science, 312(2), 81-89.
- Riihimaki, C. A., & Reiners, P. W. (2012). Empirical evidence of climate's role in Rocky Mountain landscape evolution. Journal of Geophysical Research F: Earth Surface, 117(2).More infoAbstract: Climate may be the dominant factor affecting landscape evolution during the late Cenozoic, but models that connect climate and landscape evolution cannot be tested without precise ages of landforms. Zircon (U-Th)/He ages of clinker, metamorphosed rock formed by burning of underlying coal seams, provide constraints on the spatial and temporal patterns of Quaternary erosion in the Powder River basin of Wyoming and Montana. The age distribution of 86 sites shows two temporal patterns: (1) a bias toward younger ages because of erosion of older clinker and (2) periodic occurrence of coal fires likely corresponding with particular climatic regimes. Statistical t tests of the ages and spectral analyses of the age probability density function indicate that these episodes of frequent coal fires most likely correspond with times of high eccentricity in Earth's orbit, possibly driven by increased seasonality in the region causing increased erosion rates and coal exhumation. Correlation of ages with interglacial time periods is weaker. The correlations between climate and coal fires improve when only samples greater than 50km from the front of the Bighorn Range, the site of the nearest alpine glaciation, are compared. Together, these results indicate that the interaction between upstream glaciation and downstream erosion is likely not the dominant control on Quaternary landscape evolution in the Powder River basin, particularly since 0.5Ma. Instead, incision rates are likely controlled by the response of streams to climate shifts within the basin itself, possibly changes in local precipitation rates or frequency-magnitude distributions, with no discernable lag time between climate changes and landscape responses. Clinker ages are consistent with numerical models in which stream erosion is driven by fluctuations in stream power on thousand year timescales within the basins, possibly as a result of changing precipitation patterns, and is driven by regional rock uplift on million year timescales. Copyright 2012 by the American Geophysical Union.
- Schoolmeesters, N., Cheadle, M. J., John, B. E., Reiners, P. W., Gee, J., & Grimes, C. B. (2012). The cooling history and the depth of detachment faulting at the Atlantis Massif oceanic core complex. Geochemistry, Geophysics, Geosystems, 13(1).More infoAbstract: Oceanic core complexes (OCCs) are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults. We combine previously reported U-Pb zircon crystallization ages with (U-Th)/He zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the Atlantis Massif OCC (30°N, MAR) and help establish cooling rates, as well as depths of detachment faulting and gabbro emplacement. We present nine new (U-Th)/He zircon ages for samples from IODP Hole U1309D ranging from 40 to 1415mbelow seafloor. These data paired with U-Pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at Atlantis Massif as 2895 (+1276/-1162) °C Myr -1 (from ∼780°C to ∼250°C); the lower 600 m of the borehole cooled more slowly at mean rates of ∼500 (+125/-102) °C Myr -1 (from ∼780°C to present-day temperatures). Rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of ∼300°C Myr -1, possibly due to hydrothermal circulation to ∼4 km depth through the detachment fault zone. Assuming a fault slip rate of 20 mm/yr (from U-Pb zircon ages of surface samples) and a rolling hinge model for the sub-surface fault geometry, we predict that the 780°C isotherm lies at ∼7 km below the axial valley floor, likely corresponding both to the depth at which the semi-brittle detachment fault roots and the probable upper limit of significant gabbro emplacement. © 2012. American Geophysical Union. All Rights Reserved.
- Zattin, M., Andreucci, B., Thomson, S. N., Reiners, P. W., & Talarico, F. M. (2012). New constraints on the provenance of the ANDRILL AND-2A succession (western Ross Sea, Antarctica) from apatite triple dating. Geochemistry, Geophysics, Geosystems, 13(10).More infoAbstract: Apatite triple dating (fission track, U-Pb and U-Th/He techniques) has been applied to detrital grains from the sedimentary core drilled during the ANDRILL 2A project, which documents the Miocene history of the Victoria Land Basin (western Ross Sea). High-temperature cooling ages show two main clusters (about 30 and 500Ma) whereas most of low-temperature data are late Oligocene-Early Miocene in age. These latter data are related to the exhumation of the Transantarctic Mountains south of the Discovery Accommodation Zone. Comparison between low-temperature ages suggests that the Transantarctic Mountains have been in a phase of post-orogenic decay since at least 30Ma. The Oligocene U-Pb data demonstrate the presence of a volcanic event well before the McMurdo volcanic group, whose onset is commonly places at 19Ma. The location of the volcanic centers is unknown, but they could be below the Ross Ice Shelf south of drilling site. As a whole, these data indicate a major flow of sediments from south to north with only minor contributions from nearby outlet glaciers of the East Antarctic Ice Sheet. © 2012. American Geophysical Union. All Rights Reserved.
- Cardona, A., Valencia, V., Weber, M., Duque, J., Montes, C., Ojeda, G., Reiners, P., Domanik, K., Nicolescu, S., & Villagomez, D. (2011). Transient cenozoic tectonic stages in the southern margin of the caribbean plate: U-Th/He thermochronological constraints from eocene plutonic rocks in the Santa Marta massif and Serranía de jarara, northern Colombia. Geologica Acta, 9(3), 445-466.More infoAbstract: We use U-Th/(He) zircon and apatite thermochronology and Al in hornblende geobarometry from Eocene granitoids of the Sierra Nevada de Santa Marta and Guajira uplifted massifs in northern Colombia to elucidate the exhumation history of the northern South America continental margin and its bearing to Cenozoic Caribbean-South American plate interactions. Aluminium in hornblende geobarometry from the Eocene Santa Marta batholith yields pressures between 4.9±0.6kbar and 6.4±0.6kbar, which indicate that at least, 14.7-19.2km of unroofing took place since 56-50Ma in the northwestern Sierra Nevada de Santa Marta. In the Guajira Peninsula, calculated pressures for the Eocene Parashi stock are 2.3±0.6kbar and 3±0.6kbar. Stratigraphic considerations pertaining to Oligocene conglomerates from the Guajira area suggest that 6.9-9km of crust was lost between 50Ma and ca. 26Ma. U-Th/He zircon and apatite thermochronology from granitoids in the Sierra Nevada de Santa Marta shows the existence of major exhumation events in the Late Eocene (ca. 45-40Ma), Late Oligocene (ca. 25Ma) and Miocene (ca. 15Ma). The Guajira region records the Late Eocene to Early Oligocene (35-25Ma) event, but it lacks evidence for the Miocene exhumation phase. These differences reflect isolation of the Guajira region from the Sierra Nevada de Santa Marta and the Andean chain due to extensive block rotation and transtensional tectonics that affected the region during post-Eocene times. The post-Eocene events correlate in time with an increased convergence rate and the frontal approach of North and South America. It is suggested that the two major tectonic mechanisms that govern exhumation in these Caribbean massifs are: 1) subduction of the Caribbean plate, and 2) post Eocene changes in plate convergence obliquity and rates that caused the South American continental margin blocks to override the Caribbean plate. Temporal correlation with other Caribbean and Northern Andean events allows to resolve the regional Cenozoic plate tectonic reorganizations experienced by the South American, Caribbean and Pacific plates at a regional scale.
- Grimes, C. B., Cheadle, M. J., John, B. E., Reiners, P. W., & Wooden, J. L. (2011). Cooling rates and the depth of detachment faulting at oceanic core complexes: Evidence from zircon Pb/U and (U-Th)/He ages. Geochemistry, Geophysics, Geosystems, 12(3).More infoAbstract: Oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid-ocean ridges, but many questions persist about the thermal evolution and depth of faulting. We present new Pb/U and (U-Th)/He zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the Mid-Atlantic Ridge (Ocean Drilling Program (ODP) holes 1270D and 1275D near the 15°20′N Transform, and Atlantis Massif at 30°N). Closure temperatures for the Pb/U (∼800°C-850°C) and (U-Th)/He (∼210°C) isotopic systems in zircon bracket acquisition of magnetic remanence, collectively providing a temperature-time history during faulting. Results indicate cooling to ∼200°C in 0.3-0.5 Myr after zircon crystallization, recording time-averaged cooling rates of ∼1000°C- 2000°C/Myr. Assuming the footwalls were denuded along single continuous faults, differences in Pb/U and (U-Th)/He zircon ages together with independently determined slip rates allow the distance between the ∼850°C and ∼200°C isotherms along the fault plane to be estimated. Calculated distances are 8.4 ± 4.2 km and 5.0 2.1 km from holes 1275D and 1270D and 8.4 ± 1.4 km at Atlantis Massif. Estimating an initial subsurface fault dip of 50 and a depth of 1.5 km to the 200°C isotherm leads to the prediction that the ∼850°C isotherm lies ∼5-7 km below seafloor at the time of faulting. These depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the TAG hydrothermal field and high-temperature fault rocks recovered from many oceanic detachment faults. Copyright 2011 by the American Geophysical Union.
- Hacker, B. R., Kelemen, P. B., Rioux, M., McWilliams, M. O., Gans, P. B., Reiners, P. W., Layer, P. W., Söderlund, U., & Vervoort, J. D. (2011). Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U-Th/He, Sm-Nd, and Lu-Hf dating. Tectonics, 30(1).More infoAbstract: As one of two well-exposed intraoceanic arcs, the Talkeetna arc of Alaska affords an opportunity to understand processes deep within arcs. This study reports new Lu-Hf and Sm-Nd garnet ages, 40Ar/39Ar hornblende, mica and whole-rock ages, and U-Th/He zircon and apatite ages from the Chugach Mountains, Talkeetna Mountains, and Alaska Peninsula, which, in conjunction with existing geochronology, constrain the thermal history of the arc. Zircon U-Pb ages establish the main period of arc magmatism as 202-181 Ma in the Chugach Mountains and 183-153 Ma in the eastern Talkeetna Mountains and Alaska Peninsula. Approximately 184 Ma Lu-Hf and ∼182 Ma Sm-Nd garnet ages indicate that 25-35 km deep sections of the arc remained above ∼700°C for as much as 15 Myr. The 40Ar/39Ar hornblende ages are chiefly 194-170 Ma in the Chugach Mountains and 175-150 Ma in the Talkeetna Mountains and Alaska Peninsula but differ from zircon U-Pb ages in the same samples by as little as 0 Myr and as much as 33 Myr, documenting a spatially variable thermal history. Mica ages have a broader distribution, from ∼180 Ma to 130 Ma, suggesting local cooling and/or reheating. The oldest U-Th/He zircon ages are ∼137 to 129 Ma, indicating no Cenozoic regional heating above ∼180°C. Although the signal is likely complicated by Cretaceous and Oligocene postarc magmatism, the aggregate thermochronology record indicates that the thermal history of the Talkeetna arc was spatially variable. One-dimensional finite difference thermal models show that this kind of spatial variability is inherent to intraoceanic arcs with simple construction histories. Copyright 2011 by the American Geophysical Union.
- Reiners, P. W., Riihimaki, C. A., & Heffern, E. L. (2011). Clinker geochronology, the first glacial maximum, and landscape evolution in the northern Rockies. GSA Today, 21(7), 4-9.More infoAbstract: Late Cenozoic erosion in the Powder River Basin of northern Wyoming and southern Montana has exhumed numerous coal beds to shallow depths where they burn naturally, forming erosion-resistant metamorphic rocks called clinker. Because most clinker forms tens of meters from the surface, its formation age records the timing and rate of exhumation through this depth, which can be used to constrain incision and lateral backwasting rates and the evolution of topographic relief. Zircon (U-Th)/He ages from ∼100 distinct clinker units provide several insights into the geomorphic evolution of the region. Ages of in-situ clinker range from as old as 1.1 Ma to as young as 10 ka, but most formed in one of the last three interglacial periods, reflecting either changes in fluvial incision caused by glacial-interglacial cycles or other climatic effects on rates of natural coal burning. Most clinker older than ca. 200 ka is either detrital or >∼200 m above local base level. Detrital clinker atop a broad strath terrace in the northern part of the basin provides a maximum age of 2.6 ± 0.2 Ma for terrace formation. This corresponds to the onset of major Northern Hemisphere glaciation interpreted from marine records, suggesting that the terrace formed by lateral erosion of the landscape as rivers were overwhelmed with sediment during the earliest Plio-Pleistocene glacial episode. The overall correlation of in-situ clinker ages with elevation above local base level can be interpreted with a simple model for shallow exhumation ages that requires increasing incision and topographic relief over at least the past ∼1 Myr at rates of ∼0.1-0.3 km/Myr, assuming typical clinker formation depths of 20-40 m.
- Chang, Y., Xu, C., Reiners, P. W., & Zhou, Z. (2010). The exhumation evolution of the Micang Shan-Hannan uplift since Cretaceous: Evidence from apatite (U-Th)/He dating. Chinese Journal of Geophysics (Acta Geophysica Sinica), 53(4), 912-919.More infoAbstract: The apatite (U-Th)/He dating results of 13 granitic samples collected in NanjiangNanzheng transect are used to systematically study Micang Shan s exhumation and thermotectonic evolution. While taking into consideration of the influence of the sample s cooling rates and crystal sizes on (U-Th)/He closure temperatures of respective apatites, this research also quantifies the effects of topography, heat conduction, advection and production on regional geothermal gradients. The inversion results reveal a period of rapid exhumation with an average rate of ∼70 m/Ma in the southern sedimentary area of Micang Shan since about 50 Ma B. P., which removed above 3 km-thick sub-surface rock in the region; the Guangwushan Complex in the middle part of Micang Shan-Hannan Uplift experienced a fast exhumation event at -90 Ma B. P. with an exhumation rate of >75 m/Ma; the northern Hannan Uplift was exhumed to the surface with a fast rate above 40 m/Ma before -100 Ma B. P. , and then slowly with an average exhumation rate of -10 m/Ma. The whole research region, Micang Shan-Hannan Uplift experienced a very slow exhumation with an average rate of 10-25 m/Ma during 90-50 Ma B. P.
- Cox, S. E., Thomson, S. N., Reiners, P. W., Hemming, S. R., & Van, T. (2010). Extremely low long-term erosion rates around the Gamburtsev Mountains in interior East Antarctica. Geophysical Research Letters, 37(22).More infoAbstract: The high elevation and rugged relief (>3 km) of the Gamburtsev Subglacial Mountains (GSM) have long been considered enigmatic. Orogenesis normally occurs near plate boundaries, not cratonic interiors, and large-scale tectonic activity last occurred in East Antarctica during the Pan-African (480-600 Ma). We sampled detrital apatite from Eocene sands in Prydz Bay at the terminus of the Lambert Graben, which drained a large pre-glacial basin including the northern Gamburtsev Mountains. Apatite fission-track and (U-Th)/He cooling ages constrain bedrock erosion rates throughout the catchment. We double-dated apatites to resolve individual cooling histories. Erosion was very slow, averaging 0.01-0.02 km/Myr for >250 Myr, supporting the preservation of high elevation in interior East Antarctica since at least the cessation of Permian rifting. Long-term topographic preservation lends credence to postulated high-elevation mountain ice caps in East Antarctica since at least the Cretaceous and to the idea that cold-based glaciation can preserve tectonically inactive topography. © 2010 by the American Geophysical Union.
- Guenthner, W. R., Barbeau, D. L., Reiners, P. W., & Thomson, S. N. (2010). Slab window migration and terrane accretion preserved by low-temperature thermochronology of a magmatic arc, northern Antarctic Peninsula. Geochemistry, Geophysics, Geosystems, 11(3).More infoAbstract: Existing paleogeographic reconstructions indicate that the northern Antarctic Peninsula was central to several Mesozoic and Cenozoic tectonic events that have implications for ocean circulation and continental margin evolution. To evaluate the exhumational record of these processes, we collected new samples and measured fission track and (U-Th)/He cooling ages of apatite and zircon from 13 Jurassic and Cretaceous granitoids in western Graham Land between the northern tip of the peninsula and the Antarctic Circle. Apatite He data reveal distinct ages and systematic age patterns north and south of Anvers Island, near the midpoint of the study area: To the south, apatite He ages range from 16 to 8 Ma and young northward, whereas to the north they range between 65 and 24 Ma (with one exception at 11 Ma) and young southward. Thermal histories inferred from the ages and closure temperatures of multiple thermochronometers in single samples indicate distinct histories for northern and southern Graham Land. Northern sites reveal a Late Cretaceous pulse of rapid cooling (>7C/Myr) followed by very slow cooling (∼1C/Myr) to the Recent, whereas southern sites record either a pulse of rapid mid-Miocene cooling (∼8C/Myr) or steady and moderate cooling (∼3C/Myr) from the Late Cretaceous to the Recent. We interpret the Late Cretaceous rapid cooling in the northern part of the study area as a possible manifestation of terrane accretion associated with the Palmer Land event. We interpret the systematic spatial trends in apatite He ages and contrasting thermal histories along the peninsula as recording progressive Late Cenozoic northward opening of a slab window south of Anvers Island. This is consistent with a time transgressive pulse of ∼2-3 km of rock uplift and exhumation in the upper plate following ridge-trench collision, cessation of subduction, and opening of the slab window, presumably caused by increased asthenospheric upwelling beneath the overriding plate. Copyright 2010 by the American Geophysical Union.
- Ouimet, W., Whipple, K., Royden, L., Reiners, P., Hodges, K., & Pringle, M. (2010). Regional incision of the eastern margin of the Tibetan Plateau. Lithosphere, 2(1), 50-63.More infoAbstract: New (U-Th)/He analyses from three elevation transects collected within river gorges that dissect the eastern margin of the Tibetan Plateau provide constraints on the rates and timing of accelerated river incision into the high-elevation, low-relief topography of the region. Apatite He data from the easternmost transect (Dadu River), ̃120 km from the plateau margin adjacent to the Sichuan Basin, indicate that rapid river incision of ~0.33 ± 0.04 km/m.y. began at ca. 10 Ma and has continued to the present. Apatite He and zircon He data from the middle transect (Yalong River), collected ~225 km SSW of the Dadu data, indicate that rapid river incision of ~0.34 ± 0.02 km/m.y. began prior to ca. 14 Ma and continued until the early Quaternary, when it increased, likely in response to local uplift and erosion associated with active faults nearby. Apatite He and zircon He data from the westernmost transect (Yangtze River), ~210 km W of the Yalong data, indicate that rapid river incision of ~0.38 ± 0.04 km/m.y. began at or prior to ca. 10 Ma, though likely not prior to ca. 15 Ma, and has continued to the present. The regional consistency of these data indicates that the entire eastern margin of the Tibetan Plateau was being dissected by 10 Ma and that incision has been relatively constant and uniform in the region since that time; this pattern is consistent with an erosional response to broad regional uplift at or before 10 Ma. The larger amount, and earlier onset, of exhumation observed in the Yalong River gorge shows that certain areas on the eastern margin deviate from a simple regional pattern of epeirogenic uplift and subsequent river incision and probably refl ect the superposition of local upper-crustal deformation and uplift on the broad regional pattern. © 2010 Geological Society of America.
- Thomson, S. N., Brandon, M. T., Reiners, P. W., Zattin, M., Isaacson, P. J., & Balestrieri, M. L. (2010). Thermochronologic evidence for orogen-parallel variability in wedge kinematics during extending convergent orogenesis of the northern Apennines, Italy. Bulletin of the Geological Society of America, 122(7-8), 1160-1179.More infoAbstract: Analysis of 146 new apatite (U-Th)/He ages, six new apatite fission-track ages, and 165 previously published apatite fissiontrack (AFT) ages from the northern Apennine extending convergent orogen reveals a significant along-strike change in post-late Miocene wedge kinematics and exhumation history. East of ~11°30'E, age patterns and age-elevation relationships are diagnostic of ongoing frontal accretion and slab retreat consistent with a northeastward-migrating "orogenic wave." Enhanced erosion rates of ~1 mm/yr over a period of ~3-5 Ma are recorded on the contractional pro-side of the orogen and ~0.3 mm/yr on the extending retro-side. West of ~11°30'E, ongoing exhumation has been restricted to the range core since at least ca. 8 Ma at rates of ~0.4 mm/yr increasing to ~1 mm/yr in the Pliocene (ca. 3 Ma) accompanied by post-Pliocene tilting and associated faulting. This pattern can be attributed to either continued convergence (but a switch in the transfer of material into the wedge to a regime dominated by underplating or out-of-sequence shortening), or a slowdown or cessation of frontal accretion and slab retreat with enhanced Pliocene uplift and erosion triggered by a deeper seated process such as lithospheric delamination, complete slab detachment, or slab tear. These findings emphasize that no single model of wedge kinematics is likely appropriate to explain long-term northern Apennine orogenesis and synconvergent extension, but rather that different lithospheric geodynamic processes have acted at different times in different lateral segments of the orogen. © 2010 Geological Society of America.
- Thomson, S. N., Brandon, M. T., Tomkin, J. H., Reiners, P. W., Vásquez, C., & Wilson, N. J. (2010). Glaciation as a destructive and constructive control on mountain building. Nature, 467(7313), 313-317.More infoPMID: 20844534;Abstract: Theoretical analysis predicts that enhanced erosion related to late Cenozoic global cooling can act as a first-order influence on the internal dynamics of mountain building, leading to a reduction in orogen width and height. The strongest response is predicted in orogens dominated by highly efficient alpine glacial erosion, producing a characteristic pattern of enhanced erosion on the windward flank of the orogen and maximum elevation controlled by glacier equilibrium line altitude, where long-term glacier mass gain equals mass loss. However, acquiring definitive field evidence of an active tectonic response to global climate cooling has been elusive. Here we present an extensive new lowerature thermochronologic data set from the Patagonian Andes, a high-latitude active orogen with a well-documented late Cenozoic tectonic, climatic and glacial history. Data from 38°S to 49°S record a marked acceleration in erosion 7 to 5Myr ago coeval with the onset of major Patagonian glaciation and retreat of deformation from the easternmost thrust front. The highest rates and magnitudes of erosion are restricted to the glacial equilibrium line altitude on the windward western flank of the orogen, as predicted in models of glaciated critical taper orogens where erosion rate is a function of ice sliding velocity. In contrast, towards higher latitudes (49°S to 56°S) a transition to older bedrock cooling ages signifies much reduced late Cenozoic erosion despite dominantly glacial conditions here since the latest Miocene. The increased height of the orogenic divide at these latitudes (well above the equilibrium line altitude) leads us to conclude that the southernmost Patagonian Andes represent the first recognized example of regional glacial protection of an active orogen from erosion, leading to constructive growth in orogen height and width. © 2010 Macmillan Publishers Limited. All rights reserved.
- Zattin, M., Cavazza, W., Okay, A. I., Federici, I., Fellin, M. G., Pignalosa, A., & Reiners, P. (2010). A precursor of the North Anatolian Fault in the Marmara Sea region. Journal of Asian Earth Sciences, 39(3), 97-108.More infoAbstract: Apatite (U-Th)/He and fission-track analyses of both basement and sedimentary cover samples collected around the Marmara Sea point to the existence of a system of major E-W-trending structural discontinuities active at least from the Late Oligocene. In the Early Pliocene, inception of the present-day North Anatolian Fault (NAF) system in the Marmara region occurred by reactivation of these older tectonic structures. This is particularly evident across the Ganos fault in southern Thrace, as exhumation south of it occurred during the latest Oligocene and north of it during the mid-Miocene. In this area, large tectonic structures long interpreted as the results of Plio-Quaternary NAF-related transpressional deformation (i.e. the Ganos monocline, the Korudaĝ anticline, and the Gelibolu folds) were in fact produced during the Late Oligocene - Early Miocene. The overall lack of significant (U-Th)/He age differences across the NAF indicates that the Early Pliocene inception of strike-slip motion in the Marmara region represents a relatively minor episode. At the scale of the entire Marmara region, the geographic pattern of exhumation ages shown in this study results instead from the complex superposition of older tectonic events including: (i) the amalgamation of Sakarya and Anatolide-Tauride terranes and (ii) Aegean-related extension. © 2010 Elsevier Ltd.
- Hourigan, J. K., Brandon, M. T., Soloviev, A. V., Kirmasov, A. B., Garver, J. I., Stevenson, J., & Reiners, P. W. (2009). Eocene arc-continent collision and crustal consolidation in Kamchatka, Russian Far East. American Journal of Science, 309(5), 333-396.More infoAbstract: The age and origin of high-grade metamorphic rocks of the Sredinnyi Range, Kamchatka have been the subject of a long and controversial debate. Based on geochronologic data and its association with a collage of accreted oceanic terranes, leading interpretations argue that the Sredinnyi Range metamorphic rocks represent an accreted Precambrian or Mesozoic microcontinent. In this contribution, we present new data that indicate that these metamorphic rocks were formed from the Cretaceous-Paleocene sedimentary margin of northeast Russia when it was overridden during Eocene obduction of the Olyutorsky arc, a far-travelled oceanic island arc. Our data include new mapping and structural observations along the northern and eastern flanks of the Sredinnyi Range, and SHRIMP zircon and monazite U-Th-Pb age data from 15 key samples. These new isotopic data demonstrate that paragneissic units were formed from sediments with depositional ages locally no older than Late Cretaceous to Paleocene. Furthermore, the statistical similarity of zircon U-Pb grain-ages from the Kamchatka Schist with very low-grade turbidite sandstones of the Ukelayat and Khozgon Groups indicate that metasediments of the Sredinnyi Range are upgraded stratigraphic equivalents of northeast Russian marginal strata. SHRIMP U-Pb ages of zircon overgrowths and metamorphic monazite extracted from migmatite and gneiss indicate that peak metamorphism occurred at 52 Ma, which is synchronous with the onset of the Olyutorsky arc-continent collision. Heating and cooling occurred rapidly, at rates approaching 80°C/m.y. Rapid heating is attributed to syncolhsional, subduction-related magmatism. Thermochronology and structural observations indicate that exhumation was due to a combination of ductile and brittle thinning of the crust. We speculate that this thinning was caused by diapiric ascent of a low-density low-viscosity continental material beneath a dense structural lid of the obducted island arc.
- Qiu, N., Peter, R., Mei, Q., Jiang, G., Nicolesco, S., & Tao, C. (2009). Application of the (U-Th)/He thermochronometry to the tectono-thermal evolution of sedimentary basin - A case history of well KQ1 in the Tarim basin. Chinese Journal of Geophysics (Acta Geophysica Sinica), 52(7), 1825-1835.More infoAbstract: The (U-Th)/He thermochronometry of apatite or zircon has been used as a new technique to study the structural uplift and thermal history of sedimentary basins in recent years. Based on the tested apatite and zircon He ages data from drilling wells samples, an evolution model of apatite He ages with depth and/or temperature is built, which illustrates that the He closure temperature in apatite is about 85°C in the Tarim basin. However, the zircon He ages reveals that these samples hadn t undergone its higher closure temperature. The thermal history since Ordovician in Well KQ1 has been modeled by using the He ages, AFT and Ro data. The modeling result shows that the thermal gradient was about 35. 5°C/km in the end of Ordovician, and 33. 3∼34. 5°C/km during the period of Silurian to Devonian, and it decreased to 27. 6°C/km in the end of Cretaceous. Therefore, the (U-Th)/He ages may provide a newly tool to rebuilt the thermal history of sedimentary basins, especially to the Low Paleozoic carbonate stratum in Tarim basin which is lack of normal thermal indicators.
- Reiners, P. W. (2009). Nonmonotonic thermal histories and contrasting kinetics of multiple thermochronometers. Geochimica et Cosmochimica Acta, 73(12), 3612-3629.More infoAbstract: Thermochronologic data are typically interpreted as point- or path-wise constraints on monotonic cooling histories. This is at least partly because allowing for the possibility of nonmonotonic thermal histories both precludes straightfoward use of closure temperature (Tc) concepts and introduces ambiguities in modeling continuous time-temperature paths from multiple thermochronometers or closure profile, multi-domain, or fission-track length data. However, the monotonic cooling assumption severely limits the ability to elucidate reheating episodes with potentially important geologic significance. Here I show that in some cases multiple thermochronometers with contrasting kinetic properties can be used to both diagnose reheating events and constrain their duration and temperature. Thermochronometric systems with varying activation energies display "kinetic crossovers," whereby relative diffusivities are reversed at certain temperatures. For reheating events of certain durations and temperatures, this results in "inverted" ages, whereby systems with higher nominal Tcs have younger ages than systems with lower Tcs. However, even if reheating does not cause age inversion, in cases where two systems are partially reset and constraints on the timing of a reheating event are available, the relative fractional resetting extents can be inverted to estimate a square-pulse equivalent duration and temperature of the reheating event. Here I outline this approach for diagnosing nonmonotonic thermal histories and for deducing features of reheating events from thermochronometric data, and review several examples from previously published data that illustrate its use and potential in a range of applications. © 2009 Elsevier Ltd. All rights reserved.
- Reiners, P. W., & Shuster, D. L. (2009). Thermochronology and landscape evolution. Physics Today, 62(9), 31-36.
- Riihimaki, C. A., Reiners, P. W., & Heffern, E. L. (2009). Climate control on quaternary coal fires and landscape evolution, powder river basin, Wyoming and Montana. Geology, 37(3), 255-258.More infoAbstract: Late Cenozoic stream incision and basin excavation have strongly influenced the modern Rocky Mountain landscape, but constraints on the timing and rates of erosion are limited. The geology of the Powder River basin provides an unusually good opportunity to address spatial and temporal patterns of stream incision. Numerous coal seams in the Paleocene Fort Union and Eocene Wasatch Formations within the basin have burned during late Cenozoic incision, as coal was exposed to dry and oxygen-rich near-surface conditions. The topography of this region is dominated by hills capped with clinker, sedimentary rocks metamorphosed by burning of underlying coal beds. We use (U-Th)/He ages of clinker to determine times of relatively rapid erosion, with the assumption that coal must be near Earth's surface to burn. Ages of 55 in situ samples range from 0.007 to 1.1 Ma. Clinker preferentially formed during times in which eccentricity of the Earth's orbit was high, times that typically but not always correlate with interglacial periods. Our data therefore suggest that rates of landscape evolution in this region are affected by climate fluctuations. Because the clinker ages correlate better with eccentricity time series than with an oxygen isotope record of global ice volume, we hypothesize that variations in solar insolation modulated by eccentricity have a larger impact on rates of landscape evolution in this region than do glacial-interglacial cycles. © 2009 The Geological Society of America.
- Schwartz, J. J., John, B. E., Cheadle, M. J., Reiners, P. W., & Baines, A. G. (2009). Cooling history of atlantis bank oceanic core complex: Evidence for hydrothermal activity 2.6 Ma off axis. Geochemistry, Geophysics, Geosystems, 10(8).More infoAbstract: We report 26 (U-Th)/He zircon ages from Atlantis Bank, Southwest Indian Ridge, which constrain time scales and rates of lower crustal cooling in ultraslow spreading oceanic crust in this setting. Samples from the detachment fault surface indicate that denuded oceanic crust cooled rapidly (>1 Ma), yielding cooling rates >1200°C/Ma, consistent with existing models for the cooling of oceanic crust. (U-Th)/He zircon ages from samples collected along N-S and E-W trending faults scarps record young ages inconsistent with standard cooling models for lower oceanic crust. These samples have a mean (U-Th)/He zircon age 2.6 Ma younger than their corresponding igneous crystallization ages and record cooling through 200°C well outside the rift valley. Similar anomalously young ages are recorded by zircon, sphene, and apatite fission track data from ODP Hole 735B. We interpret these young ages as recording an off-axis thermal/heating event associated with localized high-temperature (>300°C) hydrothermal fluid flow resulting from underplated mafic magmas. © 2009 by the American Geophysical Union.
- Van, P., Melle, J. V., Guillot, S., Pcher, A., Reiners, P. W., Nicolescu, S., & Latif, M. (2009). Eocene Tibetan plateau remnants preserved in the northwest Himalaya. Nature Geoscience, 2(5), 364-368.More infoAbstract: The northwest Himalaya shows strongly contrasting relief. Deeply incised mountain ranges that are characterized by extremely rapid exhumation and some of the highest peaks in the world are in contrast with high-elevation, low-relief areas such as the Deosai plateau in northern Pakistan, which lies at an altitude of 4,000 m. The origin and evolution of such plateau regions at the convergence of the most active continental collision in the world remain elusive. Here we report low-temperature thermochronology data from the Deosai plateau and use thermal history modelling to show that the plateau has undergone continuous slow denudation at rates below 250 m Myr 1 for the past 35 Myr at least. This finding suggests tectonic and morphologic stability of the plateau since at least Eocene times, only 15-20 Myr after the onset of the India-Asia collision. Our work contradicts the hypothesis that widespread low-relief surfaces in the northwest Himalaya result from efficient kilometre-scale glacial erosion during Quaternary times. We show that similarly stable surfaces exist throughout the entire northwest Himalaya and share common morphologic characteristics and denudation histories, which are comparable to those of the western Tibetan plateau. Our results suggest that these surfaces are preserved remnants of an Eocene southwestern Tibetan plateau that was more extensive than today. © 2009 Macmillan Publishers Limited.
- Bryan, S. E., Ferrari, L., Reiners, P. W., Allen, C. M., Petrone, C. M., Ramos-rosique, A., & Campbell, I. H. (2008). New insights into crustal contributions to large-volume rhyolite generation in the mid-Tertiary Sierra Madre Occidental province, Mexico, revealed by U-Pb geochronology. Journal of Petrology, 49(1), 47-77.More infoAbstract: Voluminous (≥3.9 × 105 km3), prolonged (∼18 Myr) explosive silicic volcanism makes the mid-Tertiary Sierra Madre Occidental province of Mexico one of the largest intact silicic volcanic provinces known. Previous models have proposed an assimilation-fractional crystallization origin for the rhyolites involving closed-system fractional crystallization from crustally contaminated andesitic parental magmas, with 33 to ∼100% of the dated population; most antecrysts range in age between ∼20 and 32 Ma. A sub-population of the antecrystic zircons is chemically distinct in terms of their high U (>1000 ppm to 1.3 wt %) and heavy REE contents; these are not present in the Oligocene ignimbrites in the northeastern sector of the Sierra Madre Occidental. The combination of antecryst zircon U-Pb ages and chemistry suggests that much of the zircon in the youngest rhyolites was derived by remelting of partially molten to solidified igneous rocks formed during preceding phases of Sierra Madre Occidental volcanism. Strong Zr undersaturation, and estimations for very rapid dissolution rates of entrained zircons, preclude coeval mafic magmas being parental to the rhyolite magmas by a process of lower crustal assimilation followed by closed-system crystal fractionation as interpreted in previous studies of the Sierra Madre Occidental rhyolites. Mafic magmas were more probably important in providing a long-lived heat and material flux into the crust, resulting in the remelting and recycling of older crust and newly formed igneous materials related to Sierra Madre Occidental magmatism. © The Author 2007. Published by Oxford University Press. All rights reserved.
- Colgan, J. P., Shuster, D. L., & Reiners, P. W. (2008). Two-phase Neogene extension in the northwestern basin and range recorded in a single thermochronology sample. Geology, 36(8), 631-634.More infoAbstract: We use a combination of apatite 4He/3He, (U-Th)/ He, and fission-track thermochronology to date slip on the Surprise Valley fault in northeastern California by analyzing a single sample from the Warner Range in the footwall of the fault. This sample, a granitic clast from a conglomerate, yielded a fission-track age of 11.6 ± 2.8 Ma and a (U-Th)/He age of 3.02 ± 0.52 Ma. Geologic relationships indicate that this sample was buried to a depth of ∼3.3 km prior to exhumation during slip on the Surprise Valley fault. Fission-track age and length data indicate that the sample was fully reset (>120 °C) prior to exhumation, which began sometime after 14 Ma. A single aliquot of nine apatite grains was step-heated for 4He/3He analysis; modeling of the resulting 4He distribution indicates that cooling from
- Harris, A. C., Dunlap, W. J., Reiners, P. W., Allen, C. M., Cooke, D. R., White, N. C., Campbell, I. H., & Golding, S. D. (2008). Multimillion year thermal history of a porphyry copper deposit: Application of U-Pb, 40Ar/39Ar and (U-Th)/He chronometers, Bajo de la Alumbrera copper-gold deposit, Argentina. Mineralium Deposita, 43(3), 295-314.More infoAbstract: Application of multiple chronometers (including U-Pb and 40Ar/ 39Ar geochronology and zircon and apatite (U-Th)/He thermochronology) to porphyry intrusions at the Bajo de la Alumbrera porphyry copper-gold deposit, Argentina, reveals a complex history of reheating that spans millions of years. Previous U-Pb geochronology, combined with our new 40Ar/39Ar data, shows that the multiple porphyritic intrusions at Bajo de la Alumbrera were emplaced during two episodes, the first at about 8.0 Ma (P2 and associated porphyries) and the second about a million years later (Early and Late P3 porphyries). Complex overprinting alteration events have obscured the earliest hydrothermal history of the deposit. By contrast, 40Ar/39Ar data reveal the close temporal relationship of ore-bearing potassic alteration assemblages (7.12±0.13 Ma; biotite) to the emplacement of the P3 intrusions. Consistent with low closure temperatures, younger ages have been determined for associated hydrothermal alkali feldspar (6.82±0.05 Ma and 6.64±0.09 Ma). The temperature-sensitive Ar data also record an unexpected prolonged cooling history (to below 200°C) extending to 5.9 Ma. Our data suggest that the Bajo de la Alumbrera system underwent protracted cooling, after the collapse of the main hydrothermal system, or that one or more low-temperature (∼100-200°C) reheating events occurred after emplacement of the porphyritic intrusions at Bajo de la Alumbrera. These have been constrained in part by our new 40Ar/39Ar data (including multidomain diffusion modeling) and (U-Th)/He ages. Single-grain (U-Th)/He ages (n = 5) for phenocrystic zircon from P2 and P3 intrusive phases bracket these thermal events to between 6.9 (youngest crystallization of intrusion) and 5.1 Ma. Multidomain modeling of alkali feldspar data (from both igneous and hydrothermal crystals) is consistent with the deposit cooling rapidly from magmatic temperatures to below about 300°C, with a more protracted history down to 150°C. We conclude that the late-stage low-temperature (150 to 200°C) thermal anomaly localized at Bajo de la Alumbrera resulted from radiation of heat and/or fluids sourced from deeper-seated magma bodies, emplaced beneath the deposit. To produce the observed thermal longevity of the porphyry system, magma bodies underlying the Bajo de la Alumbrera deposit must have been repeatedly replenished by new magma batches. Without replenishment, crystallization of the source magma will occur, and heat release will stop, leading to rapid cooling (in less than ten thousand years). The influx of deep-seated magma may have caused the development of late low-temperature hydrothermal alteration assemblages at Bajo de la Alumbrera, at the same time that mineralization formed at Agua Rica, some 25 km away. All available chronologic data for the Bajo de la Alumbrera deposit suggest that the hydrothermal system was active episodically over at least a three-million and possibly up to a four-million-year period. © Springer-Verlag 2007.
- Muceku, B., Van, P., Bernet, M., Reiners, P., Mascle, G., & Tashko, A. (2008). Thermochronological evidence for Mio-Pliocene late orogenic extension in the north-eastern Albanides (Albania). Terra Nova, 20(3), 180-187.More infoAbstract: New apatite and zircon (U-Th)/He and apatite fission-track (FT) data allow constraining the timing of Miocene-Pliocene extensional exhumation that affected the central part of the Dinarides-Albanides-Hellenides orogen. Apatite (U-Th)/He ages in the northern and western Internal Albanides range from 57 to 17Ma, contrasting to younger ages of 5.2-9.3Ma in the eastern Internal Albanides. Eastward younging is also reflected in zircon (U-Th)/He ages varying from 101Ma in the north-western Internal Albanides to 19-50Ma in the east, as well as in recently published apatite FT ages. Thermal history predictions with the new data point to a phase of rapid exhumation of the eastern Internal Albanides around 6-4Ma, while the western Internal Albanides record slower continuous exhumation since the Eocene. This asymmetric exhumation pattern is most likely linked to extensional reactivation of NE-SW-trending thrusts east of the Mirdita zone and within the Korabi zone of the eastern Internal Albanides. © 2008 Blackwell Publishing Ltd.
- Nasdala, L., Hofmeister, W., Norberg, N., Martinson, J. M., Corfu, F., Dörr, W., Kamo, S. L., Kennedy, A. K., Kronz, A., Reiners, P. W., Frei, D., Kosler, J., Wan, Y., Götze, J., Häger, T., Kröner, A., & Valley, J. W. (2008). Zircon M257 - A homogeneous natural reference material for the ion microprobe U-Pb analysis of zircon. Geostandards and Geoanalytical Research, 32(3), 247-265.More infoAbstract: We introduce and propose zircon M257 as a future reference material for the determination of zircon U-Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS-determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U-Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ∼ 840 μg g-1 U (Th/U ∼ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit-cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the "Sri Lankan trends", with actinide concentrations, U-Pb age, and the calculated alpha fluence of 1.66 × 1018 g-1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9‰ VSMOW suggesting a metamorphic genesis in a marble or calc-silicate skarn. © 2008 The Authors. Journal compilation © 2008 International Association of Geoanalysts.
- Barbero, L., Teixell, A., Arboleya, M., Río, P. d., Reiners, P. W., & Bougadir, B. (2007). Jurassic-to-present thermal history of the central High Atlas (Morocco) assessed by low-temperature thermochronology. Terra Nova, 19(1), 58-64.More infoAbstract: Apatite fission track (AFT) and (U-Th)/He data from the High Atlas have been obtained for the first time to constrain the tectono-thermal evolution of the central part of the chain. Results from Palaeozoic basement massifs indicate long residence at low temperatures, consistently with their original location out of the deepest Mesozoic rift troughs and indicating minor exhumation. The best rocks for extracting the Alpine history of the Atlas Mountains are Jurassic intrusives, which yield AFT ages centred on c. 80 Ma; thermal models based on AFT data and constrained by (U-Th)/He suggest that these ages are included in a slow cooling trend from intrusion age to c. 50 Ma ago that we attribute to post-rift thermal relaxation. This is followed by a stability period of c. 30 Ma and then by a final exhumational cooling until present exposure. Eocene intrusives yield AFT ages similar to those of Rb-Sr and K-Ar suggesting rapid emplacement in the uppermost crust. © 2007 Blackwell Publishing Ltd.
- Blondes, M. S., Reiners, P. W., Edwards, B. R., & Biscontini, A. (2007). Dating young basalt eruptions by (U-Th)/He on xenolithic zircons. Geology, 35(1), 17-20.More infoAbstract: Accurate ages for young (e.g., Pleistocene) volcanic eruptions are important for geomorphic, tectonic, climatic, and hazard studies. Existing techniques can be time-consuming and expensive when many ages are needed, and in the case of K/Ar and 40Ar/39Ar dating, extraneous Ar often can limit precision, especially for continental basalts erupted through old lithosphere. We present a new technique for dating young basaltic eruptions by (U-Th)/He dating of zircons (ZHe) from crustal xenoliths. Single-crystal ZHe dates generally have lower precision than typical 40Ar/39Ar dates, but can be determined relatively easily on multiple replicate grain aliquots. We dated zircons from xenoliths from four volcanic centers in western North America: Little Bear Mountain, British Columbia (157 ± 3.5 [2.2%] ka weighted 95% confidence interval [CI], mean square of weighted deviates [MSWD] = 1.7) and Prindle Volcano, Alaska (176 ± 16 [8.9%] ka, MSWD = 13), in the northern Cordilleran volcanic province, and Fish Springs (273 ± 23 [8.6%], MSWD = 43) and Oak Creek (179 ± 8.1 [4.5%] ka, MSMID = 12), in the Big Pine Volcanic Field, California. All ZHe ages are either equivalent to or younger than previously determined K/Ar or 40Ar/39Ar ages, indicating the possibility of inherited 40Ar in some of the previous measurements. Zircons from upper crustal xenoliths in the Oak Creek and Fish Springs vents show poorer reproducibility and multiple apparent age distribution peaks, consistent with either intracrystatline U-Th zonation or
- Edgar, C. J., Wolff, J. A., Olin, P. H., Nichols, H. J., Pittari, A., Cas, R. A., Reiners, P. W., Spell, T. L., & Martí, J. (2007). The late Quaternary Diego Hernandez Formation, Tenerife: Volcanology of a complex cycle of voluminous explosive phonolitic eruptions. Journal of Volcanology and Geothermal Research, 160(1-2), 59-85.More infoAbstract: The Diego Hernandez Formation (DHF; 600-ca. 180 ka) represents the products of the most recent complete cycle of phonolitic explosive volcanism on Tenerife (Canary Islands, Spain). We provide a revised and detailed stratigraphy, new 40Ar/39Ar and (U-Th)/He age determinations for major eruptive units, a summary of new chemical data and an overview of the key characteristics of the cycle, including volume estimates, dispersal patterns, eruption styles, phreatomagmatic influences and caldera collapse episodes. The complex stratigraphy of the DHF is divided into 20 named members, each representing a major eruption, as well as numerous unnamed members of limited present-day exposure. The major eruptions are represented by the Fortaleza (370 ka), Roque (347 ka, 3 km3), Aldea (319 ka, 3 km3), Fasnia (309 ka, 13 km3), Poris (268 ka, 3.5 km3), Arafo (4 km3), Caleta (223 ka, 3.5 km3) and Abrigo (between 196 and 171 ka, 20 km3) Members. The Aldea, Fasnia and Poris Members consist of highly complex successions of plinian fall, surge and flow deposits and several of the eruptions produced widespread and internally complex ignimbrite sheets. Phreatomagmatism occurred most frequently in the opening phase of the eruptions but also recurred repeatedly throughout many of the sequences. Inferred sources of water include a shallow caldera lake and groundwater, and intermittent phreatomagmatic activity was an important influence on eruption style. Another important factor was conduit and vent instability, which frequently loaded the eruption column with dense lithic debris and occasionally triggered column collapse and ignimbrite formation. Most of the major DHF eruptions were triggered by injection of mafic magma into existing phonolitic magma bodies. Two phonolitic magma types were available for eruption during the lifetime of the DHF, but each was dominant at different times. The results presented here support a caldera collapse rather than a landslide model for the origin of the Las Cañadas Caldera, although the evolution of the caldera is evidently more complex and incremental than first thought. © 2006 Elsevier B.V. All rights reserved.
- Fellin, M. G., Reiners, P. W., Brandon, M. T., Wüthrich, E., Balestrieri, M. L., & Molli, G. (2007). Thermochronologic evidence for the exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy. Tectonics, 26(6).More infoAbstract: The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. The Alpi Apuane massif in the northern Apennines exposes synorogenic metamorphic rocks, and provides information about exhumation processes associated with accretion and retreat. (U-Th)/He and fission-track ages on zircon and apatite are used to resolve exhumational histories for the Apuane metamorphic rocks and the structurally overlying, very low grade Macigno Formation. Stratigraphic, metamorphic, and thermochronologic data indicate that the Apuane rocks were structurally buried to 15-30 km and ∼400°C at about 20 Ma. Exhumation to 240°C and 9 km depth (below sea level) occurred at 10-13 Ma. By 5 Ma the Apuane rocks were exhumed to 70°C and ∼2 km. The Macigno, and associated Tuscan nappe were also structurally buried and the Macigno reached its maximum depth of 7 km at ∼15 to 20 Ma. Stratigraphic evidence indicates that the Apennine wedge was submarine at this time. Thus we infer that initial exhumation of the Apuane was coeval with tectonic thickening higher in the wedge, as indicated by synchronous structural burial of the Tuscan nappe. From 6 to 4 Ma, thinning at shallow depth is indicated by continued differential exhumation between the Apuane and the Tuscan nappe at high rates. After 4 Ma, differential exhumation ceased and the Apuane and the Tuscan nappe were exhumed at similar rates (∼0.8 km/Ma), which we attribute to erosion of the Apennines, following their emergence above sea level. Copyright 2007 by the American Geophysical Union.
- Min, K., & Reiners, P. W. (2007). High-temperature Mars-to-Earth transfer of meteorite ALH84001. Earth and Planetary Science Letters, 260(1-2), 72-85.More infoAbstract: Martian meteorites provide crucial insights into Martian evolution and interplanetary mass transfer, including the potential for exogenesis. ALH84001 is the oldest Martian meteorite discovered so far, and has been used to derive important conclusions about Martian surface temperatures and very low-temperature Mars-to-Earth transfer. To better constrain the thermal evolution and shock metamorphic history of ALH84001, we applied (U-Th)/He thermochronometry to single grains of phosphate (merrillite) from ALH84001. The (U-Th)/He ages of individual phosphate grains in ALH84001 range from 60 Ma to 1.8 Ga, with a weighted mean of ~830 Ma. This broad age distribution reflects multiple diffusion domains, and requires a relatively high-temperature resetting event younger than ∼ 60 Ma. These new data are combined with the published whole-rock (maskelynite as a main Ar reservoir) 40Ar/39Ar age spectra which show 5-8% fractional loss of radiogenic 40Ar since 4.0 Ga. He diffusion in both terrestrial and extraterrestrial apatite has a significantly higher activation energy (138 ∼ 184 kJ/mol) than Ar diffusion in maskelynite (75 kJ/mol), leading to an important "kinetic crossover" in fractional loss contours for these systems. Taken together, the phosphate (U-Th)/He and whole-rock 40Ar/39Ar ages require both very low surface temperatures on Mars, and one or more short-lived, high-temperature, shock events after 4.0 Ga. We suggest that the last shock event occurred with ejection of ALH84001 from Mars, and reached a peak temperature of approximately 400 °C. These results undermine the proposed low-temperature ejection hypothesis for ALH84001, but support long-lived extremely cold Martian surface temperatures. © 2007 Elsevier B.V. All rights reserved.
- Peppe, D. J., & Reiners, P. W. (2007). Conodont (U-Th)/He thermochronology: Initial results, potential, and problems. Earth and Planetary Science Letters, 258(3-4), 569-580.More infoAbstract: We performed He diffusion experiments and (U-Th)/He age determinations on conodonts from a variety of locations to explore the potential of conodont (U-Th)/He thermochronology to constrain thermal and exhumation histories of some sedimentary-rock dominated terrains. Based on two diffusion experiments and age results from some specimens, He diffusion in conodont elements appears to be similar to that in Durango apatite fragments of similar size, and closure temperatures are approximately 60-70 °C (for cooling rates of ∼ 10 °C/m.y.). (U-Th)/He ages of conodonts from some locations yield reproducible ages consistent with regional thermal history constraints and, in at least two cases, require a closure temperature lower than ∼ 80 °C. Other samples however, yield irreproducible ages, and in one case yield ages much younger than expected based on regional geologic considerations. These irreproducible samples show inverse correlations between parent nuclides and age consistent with late-stage open-system U-Th behavior. © 2007 Elsevier B.V. All rights reserved.
- Reiners, P. W. (2007). Thermochronologic approaches to paleotopography. Reviews in Mineralogy and Geochemistry, 66, 243-267.More infoAbstract: Although thermochronology cannot directly constrain paleoelevation, it can provide estimates of the form, location, and scale of paleotopographic relief, i.e., paleotopography, and its change through time. Unique thermochronologic perspectives on paleotopography come from 1) spatial patterns of surface and subsurface cooling ages or cooling histories that reflect either the influence of topography on subsurface isotherm warping, or spatially focused erosion (including incision), and 2) the age-elevation relationship in paleolandscapes that may be preserved in detrital cooling age distributions. This chapter reviews the fundamental theory and results of these approaches and several example applications. Case studies show examples of both decreasing and increasing topographic relief through time, at the orogen scale and across short ridge-valley wavelengths, and significant modification of local topographic features in glaciated and fluvial settings. In some cases, thermochronologic evidence for fluvial incision at short wavelengths has also been argued to be the result of surface uplift at very long wavelengths. Although not yet used for such purposes, detrital approaches also have the potential to reconstruct paleotopographic relief and paleohypsometry in paleocatchments. In all cases, paleotopographic interpretations from thermochronology require important assumptions or case-by-case support from other lines of evidence. Central issues pertinent to thermochronologic interpretations of paleotopography are the nature of the shallow crustal thermal field through which the samples cooled (including the influence of fluid flow) and the role of rock uplift gradients in modifying simple relationships between erosion and topography. Copyright © Mineralogical Society of America.
- Reiners, P. W., Thomson, S. N., McPhillips, D., Donelick, R. A., & Roering, J. J. (2007). Wildfire thermochronology and the fate and transport of apatite in hillslope and fluvial environments. Journal of Geophysical Research F: Earth Surface, 112(4).More infoAbstract: Wildfire heating of the outer few centimeters of exposed rock or soil generates short-duration, high-temperature thermal events that produce characteristic thermochronologic signatures in minerals. Contrasting activation energies of fission track annealing and He diffusion in apatite lead to a kinetic crossover whereby wildfire heating resets fission track (FT) ages much faster than (U-Th)/He ages, resulting in "inverted" FT-He ages in single grains. This can be used to trace wildfire-affected detritus at the Earth's surface. We show that in exposed bedrock, inverted apatite FT-He ages vary systematically with depth to ∼3 cm, and detrital clasts on hillslopes also show strong but heterogeneous wildfire-resetting signatures. In soils, colluvium, and low-order channel sediments, strongly wildfire-reset apatite grains are abundant, and in some cases dominate the population of detrital apatite, to depths at least as great as 10 cm. Wildfire-reset apatite is rare, however, in fluvial sediments sampled from larger basins, indicating a strong fractionation of apatite populations from hillslopes to rivers. Characteristic dissolution features in hillslope apatite and slower relative dissolution rates of other common minerals suggest that wildfire-reset apatite grains are rare or absent in rivers because they dissolve relatively rapidly in soil profiles. Apatite that does contribute to fluvial sediments is likely to be dominantly derived from bedrock landslides in steep regions or from large clasts containing grains protected from both wildfire heating and dissolution. This means that apatite in fluvial sediment is spatially fractionated with respect to its sources in the catchment, even if catchment erosion rates are spatially uniform. Copyright 2007 by the American Geophysical Union.
- Schildgen, T. F., Hodges, K. V., Whipple, P. W., Reiners, P. W., & Pringle, M. S. (2007). Uplift of the western margin of the Andean plateau revealed from canyon incision history, southern Peru. Geology, 35(6), 523-526.More infoAbstract: We explore canyon incision history of the western margin of the Andean (Altiplano-Puna) plateau in the central Andes as a proxy for surface uplift. (U-Th)/He apatite data show rapid cooling beginning at ca. 9 Ma and continuing to ca. 5.1 Ma in response to incision. A minimum of 1.0 km of incision took place during that interval. The youngest apatite date and a volcanic flow perched 125 m above the present valley floor dated at 2.261 ± 0.046 Ma (40Ar/39Ar) show that an additional ∼1.4 km of incision occurred between ca. 5.1 and 2.3 Ma. Thus, we infer that a total of at least 2.4 km, or 75% of the present canyon depth was incised after ca. 9 Ma. (U-Th)/He zircon data collected along the same transect imply that the western margin of the plateau was warped upward into its present monoclinal form, rather than uplift being accommodated on major surface-breaking faults. © 2007 The Geological Society of America.
- Boyce, J. W., Hodges, K. V., Olszewski, W. J., Jercinovic, M. J., Carpenter, B. D., & Reiners, P. W. (2006). Laser microprobe (U-Th)/He geochronology. Geochimica et Cosmochimica Acta, 70(12), 3031-3039.More infoAbstract: A new analytical method had been developed to enable high-spatial-resolution (U-Th)/He dating of accessory minerals. It involves the use of a focused ArF excimer to ablate pits in a polished grain surface, with the evolved gases spiked for isotope-dilution measurement of radiogenic 4He. These data are converted to concentrations by precise measurement of each pit using an optical interferometric microscope. U, Th, and Sm concentration measurements are made using one of several alternative microanalytical techniques (e.g., wavelength-dispersive electron microprobe analysis or laser-ablation, inductively coupled plasma mass spectrometry). By way of illustration, we present both conventional and laser microprobe (U-Th)/He dating results for a Brazilian monazite sample. Laser microprobe data (28 measurements on two crystal fragments) yield a weighted mean (U-Th)/He date of 455.3 ± 3.7 Ma (2SE). This result is statistically indistinguishable from the mean conventional (U-Th)/He date for three separate grain fragments: 449.6 ± 9.8 Ma (2SE). The agreement of conventional and laser ablation dates should encourage a wide variety of applications of the technique, including: (1) detrital mineral dating for provenance and unroofing studies; (2) the dating of broken, included, highly zoned, or irregular grains which are not easily corrected for α-ejection; and (3) measuring 4He loss profiles that can be inverted to determine cooling histories. © 2006 Elsevier Inc. All rights reserved.
- Colgan, J. P., Dumitru, T. A., Reiners, P. W., Wooden, J. L., & Miller, E. L. (2006). Cenozoic tectonic evolution of the basin and range province in northwestern Nevada. American Journal of Science, 306(8), 616-654.More infoAbstract: A regional synthesis of new and existing geologic and thermochronologic data document late Cretaceous - early Cenozoic regional erosion, Oligocene - Miocene volcanism, and subsequent late Miocene extension of the Basin and Range Province in northwestern Nevada and northeastern California. Across an ∼220-km-wide region between the Santa Rosa and Warner Ranges, conformable sequences of 35 to 15 Ma volcanic rocks are cut by only a single generation of high-angle normal faults that accommodated ∼23 km of total east-west extension (∼12%). Fission-track, (U-Th)/He, geologic, and structural data from the Pine Forest Range show that faulting there began at 11 to 12 Ma, progressed at a relatively constant rate until at least 3 Ma, and has continued until near the present time. Extension in the Santa Rosa Range to the east took place during the same interval, although the post-6 Ma part of this history is less well constrained. Less complete constraints from adjacent ranges permit a similar timing for faulting, and we infer that extensional faulting in northwestern Nevada began everywhere at 12 Ma and has continued up to the present. Faulting in the Warner Range in northeastern California can only be constrained to have begun between 14 and 3 Ma, but may represent westward migration of Basin and Range extension during the Pliocene. Compared to the many parts of the Basin and Range in central and southern Nevada, extension in northwestern Nevada began more recently, is of lesser total magnitude, and was accommodated entirely by high-angle normal faults. Fission-track data document Late Cretaceous unroofing of Cretaceous (115 - 100 Ma) granitic basement rocks in northwestern Nevada, followed by a long period of relative tectonic quiescence that persisted through Oligocene and Miocene volcanism until the onset of Basin and Range extension at ∼12 Ma. The low magnitude of extension (12%) and early Tertiary stability suggest that the modern ∼31 km thick crust in northwestern Nevada was only slightly thicker (∼35 km) prior to extension at 12 Ma, and was no thicker than ∼38 km in the Late Cretaceous. This stands in contrast to other parts of the Basin and Range, where the crust was thickened to at least 45 to 50 km by Cretaceous thrusting and subsequently thinned to ∼30 km by large magnitude (>50%) extension.
- Flowers, R. M., Bowring, S. A., & Reiners, P. W. (2006). Low long-term erosion rates and extreme continental stability documented by ancient (U-Th)/He dates. Geology, 34(11), 925-928.More infoAbstract: Zircon and apatite crystal the western Canadian shield yield (U-Th)/He dates that are the oldest yet reported for terrestrial rocks. Zircon dates from 1.73 to 1.58 Ga are consistent with independent geological and thermochronological constraints, and indicate that the rocks were at temperatures ≤180 °C and crustal depths ≤7-10 km since ca. 1.7 Ga. Apatite dates from 0.95 to 0.55 Ga suggest residence of rocks at temperatures ≤40-50 °C and crustal depths ≤1.5-2 km for ∼1.0-0.6 b.y., when interpreted using conventionally accepted apatite He diffusion kinetics and considering the proposed effect of radiation damage on apatite He retentivity. Our analysis implies long-term integrated unroofing rates of ≤2.5 μm/yr since ca. 1.7 Ga. These rates are significantly lower than the long-term rates suggested by previous thermochronological data sets in continental interiors, but are within the range of short-term erosional estimates. The results are consistent with the extreme stability of this region since the Proterozoic. © 2006 Geological Society of America.
- Hansen, K., & Reiners, P. W. (2006). Low temperature thermochronology of the southern East Greenland continental margin: Evidence from apatite (U-Th)/He and fission track analysis and implications for intermethod calibration. Lithos, 92(1-2), 117-136.More infoAbstract: The southern coast of East Greenland is characterized by high topographic relief and deep fjords, but the evolution of the landscape and the low-temperature thermochronology of the region is not well understood. Here we present apatite fission-track and (U-Th)/He ages that suggest several important features of the long-term geomorphic history of this region, but which also illustrate an important discrepancy in the thermal histories derived from each technique. Apatite from bedrock of the southern coast of East Greenland between 62°N and 67°N has fission-track ages ranging from 60 to 840 Ma and (U-Th)/He ages ranging from 21 to 250 Ma. The ages generally increase with elevation and distance from the coast, and fission-track analyses show significant differences in thermal histories for the different regions. In the Kangertittivatsiaq area (c. 66-67°N) apatite fission track data and models suggest two separate periods of slow cooling: prior to c. 200 Ma and between c. 160 Ma and the late Cenozoic (more recently than c. 20 Ma), each of which was followed by a period of rapid cooling and inferred exhumation. Apatite He data in the Kangertittivatsiaq region, including crystal-size-age correlations in low-elevation samples, are most simply interpreted as recording an incision event of at least 1.5 km later than 20 Ma near the coast. This may have been caused by glacial erosion. The (U-Th)/He data also indicate an earlier phase of rapid exhumation at c. 250 Ma. In the Skjoldungen/Kap Møsting area (c. 62-64°N) approximately 200 km south of the Kangertittivatsiaq, apatite fission-track data suggest slow exhumation from c. 200 Ma into the Neogene followed by fast exhumation. The similarity of fission track ages (200 Ma) at sea level in the fjords in the Skjoldungen area (c. 62-64°N) do not suggest tilting in the hinterland related to the breakup of the East Greenland continental margin. Furthermore, the Cenozoic fission track ages and modeling fission track data suggest that pre-breakup basins may have covered the outer coast. Despite the broadly similar topographic implications of the fission-track and (U-Th)/He data, the thermal histories derived from these systems are inconsistent. Fission-track data require thermal histories that predict He ages younger than observed, and He data require thermal histories that predict fission-track ages older than observed. Similar discrepancies have also been observed in other settings characterized by long-term low-temperature thermal histories, and may reflect changes in annealing or diffusion behavior (or both) that either develop or become more apparent in such cases. © 2006 Elsevier B.V. All rights reserved.
- Min, K., Reiners, P. W., Wolff, J. A., Mundil, R., & Winters, R. L. (2006). (U-Th)/He dating of volcanic phenocrysts with high-U-Th inclusions, Jemez Volcanic Field, New Mexico. Chemical Geology, 227(3-4), 223-235.More infoAbstract: We measured (U-Th)/He ages of fayalite and hedenbergite phenocrysts with britholite and chevkinite inclusions from the ∼1.6 Ma Otowi (Lower) Member of the Bandelier Tuff, Jemez Volcanic Field. U and Th, the major parent nuclides of radiogenic 4He, are concentrated in britholite and chevkinite inclusions and are essentially absent from the host grains. We analyzed 36 fayalite (21 non-abraded + 15 abraded) and 13 hedenbergite (7 non-abraded + 6 abraded) phenocrysts, and applied alpha-ejection corrections assuming uniform intracrystalline distributions of inclusions and parent nuclides in the phenocrysts. A subset of phenocrysts was subjected to abrasion to remove the alpha-ejection affected regions. The alpha-ejection correction factors (FT) of the non-abraded samples range between 0.90 and 0.96, whereas those of the abraded samples are virtually unity. The fayalite and hedenbergite crystals yield mean ages of 1.60 ± 0.10 Ma (S.D.) and 1.69 ± 0.13 Ma, respectively. The overall mean age of 1.62 ± 0.11 Ma (S.D.) is indistinguishable from previously published 40Ar/39Ar ages of 1.61 ± 0.01 Ma (1σ). Our results suggest that other inclusion-bearing volcanic phenocrysts and their host units may be dateable by the (U-Th)/He method. © 2005 Elsevier B.V. All rights reserved.
- Reiners, P. W., & Brandon, M. T. (2006). Using thermochronology to understand orogenic erosion. Annual Review of Earth and Planetary Sciences, 34, 419-466.More infoAbstract: Erosion of orogenic mountain ranges exhumes deeply buried rocks and controls weathering, climate and sediment production and transport at a variety of scales. Erosion also affects the topographic form and kinematics of orogens, and it may provide dynamic feedbacks between climate and tectonics by spatially focused erosion and rock uplift. Thermochronology measures the timing and rates at which rocks approach the surface and cool as a result of exhumation. Relatively well-understood noble gas and fission-track thermochronometric systems have closure temperatures ranging from ∼60 to ∼550°C, making them sensitive to exhumation through crustal depths of about one to tens of kilometers. Thus, thermochronology can constrain erosion rates and their spatial-temporal variations on timescales of ∼105-107 years, commensurate with orogenic growth and decay cycles and possible climate-tectonic feedback response times. Useful methods for estimating erosion rates include inverting ages for erosion rates using crustal thermal models, vertical transects, and detrital approaches. Spatial-temporal patterns of thermochronometrically determined erosion rates help constrain flow of material through orogenic wedges, orogenic growth and decay cycles, paleorelief, and relationships with structural, geomorphic, or climatic features. Copyright © 2006 by Annual Reviews. All rights reserved.
- Shengbiao, H. u., Raza, A., Min, K., Kohn, B. P., Reiners, P. W., Ketcham, R. A., Wang, J., & J., A. (2006). Late mesozoic and cenozoic thermotectonic evolution along a transect from the north China craton through the Qinling orogen into the Yangtze craton, central China. Tectonics, 25(6).More infoAbstract: Cretaceous and Cenozoic reactivation of the Triassic Qinling-Dabie orogen between the north China and Yangtze cratons resulted from the combined effects of Pacific subduction - back-arc extension in east China and collisions in west China. We report new apatite fission track and apatite and zircon (U-Th)/He data from east Qinling along a >400-km-long N-S transect from Huashan through the Qinling orogen to Huangling. The ages show a general pattern of younging northward. Three major cooling phases are defined by modeling the multiple thermochronologic data sets. The first phase occurred locally in the North and South Qinling during the late Triassic to early Jurassic, following heating associated with the Triassic Yangtze subduction and exhumation of the Wudang metamorphic core complex on the cratonal edge. A second phase represents regional exhumation between 100 and 60 Ma, coeval with rifting marked by the Late Cretaceous-Eocene (K2-E) red bed deposition in eastern China and possibly indicating a link with Pacific subduction-back-arc extension in eastern China; however, it may also have been superimposed by eastward tectonic escape resulting from the Lhasa-West Burma-Qiangtang-Indochina collision. The third cooling phase was initiated at ∼45 Ma exclusively in the north Qinling and in the footwall of the graben-bounding normal fault of the Weihe graben in the Lesser Qinling. We suggest the third phase was related to reactivation of the Qinling fault system as a result of eastward tectonic escape imposed by the India-Asia collision at ∼50 Ma. Copyright 2006 by the American Geophysical Union.
- Campbell, I. H., Reiners, P. W., Allen, C. M., Nicolescu, S., & Upadhyay, R. (2005). He-Pb double dating of detrital zircons from the Ganges and Indus Rivers: Implication for quantifying sediment recycling and provenance studies. Earth and Planetary Science Letters, 237(3-4), 402-432.More infoAbstract: He-Pb double dating of detrital zircons is more reliable than conventional U-Pb dating for tracing the source of detritus in sediments and can be used to constrain the percentage of recycled material in sediments. Conventional U-Pb dating can be used to constrain the provenance of sediments if the U-Pb zircon age pattern for potential source regions is known but can only be used to trace the source of individual zircons if they are first-cycle grains. The advantages of He-Pb double dating are demonstrated using examples from the Indus and Ganges rivers, and previously published data from the Navajo sandstone. Conventional U-Pb dating can unambiguously identify only 2.5% of the Ganges zircons, and 18% of the Indus zircons as coming from the Himalayan Mountains or Tibet Plateau and only 23% of the Navajo zircons as coming from the Appalachian Mountains. The correct figure, as determined from double dating, is over 95% from the Himalayan Mountains or Tibet Plateau in the case of the Indus and Ganges rivers and at least 70% from the Appalachian Mountains in the case of the Navajo Sandstone. This result casts doubt on the reliability of the U-Pb method when used in the absence of other techniques, such as He dating, to identify the true provenance of sediments, as opposed to the ultimate source of the zircons. Double dating also shows that at least 60% of the Indus and 70% of the Ganges and Navajo sandstone zircons have been recycled from earlier sediments. Exhumation rates, estimated from the He dates, reveal that ∼ 75% of the Indus and Ganges zircons were derived from areas where the exhumation rate exceeds 1.5 km/Myr. These rates are higher and more varied than those calculated from detrital muscovites. These results imply that ∼ 75% of the eroded material in the Himalayan Mountains is derived from areas of anomalously high erosion where the short-term exhumation rate exceeds the long-term average. © 2005 Elsevier B.V. All rights reserved.
- Changhai, X. u., Zhou, Z., Van, P., Donelick, R. A., Grave, J. D., Changqian, M., & Reiners, P. W. (2005). Apatite-fission-track geochronology and its tectonic correlation in the Dabieshan orogen, central China. Science in China, Series D: Earth Sciences, 48(4), 506-520.More infoAbstract: AFT data of granitoid and metamorphic samples from the Dabieshan orogen have an age range between 96.4 Ma and 41.9 Ma coupled with confined track lengths of 11.5-14.0 μm, reflecting the cooling time of rocks differentially through the 100°C± isotherm. The Jurassic-Eogene sediments from the North Huaiyang and Hefei basin, however, yield their AFT ages of 128.8 - 62.0 Ma with mean track lengths of 8.6 - 11.9 μm, recording a complicated thermal development of the basin. These AFT data structurally allowing the coupling between the orogen and the basin are to a great extent considered as the result from the control of NNE-trending regional tectonics. It took place approximately at 95 - 90 Ma that the orogen developed with an abrupt shift from the doming extension to the differential transtension, which is assumed to be related with the changes of the convergence between the Western Pacific and Eurasia plates. The differentially sinistral strike-slipping from the transtension between Tanlu and Shangcheng-Macheng faults brought the doming extension of the orogen shrunk sharply in Late Cretaceous. In Eogene, the full-grown transtension that dominated in East Asia was caused mainly by a rapid decrease of the rates of the Western Pacific convergence, which can be therefore regarded as a major mechanism creating the zonal anomalies of 60 - 40 Ma in the orogen. The thermal anomaly of 70 - 40 Ma zonally along the Tanlu fault cooled more slowly than the anomaly of 60 - 55 Ma zoning along the Shangcheng-Macheng fault, but the 60 - 40 Ma anomaly trending E-W in hinterlands of the orogen seems to be relicts of the doming extension as a result of intensive rebuilding by the transtension. In addition, the orogen was also reactivated by the far-field effect from India-Asia collision in Eogene, where major NWW-trending faults played an important role in facilitating such far-field tectonic transferring. In Oligocene to Miocene, the Dabieshan orogen remained a few thermal disturbances with their AFT ages particularly in coincidence with the time of opening and closing of the South China Sea. Copyright by Science in China Press 2005.
- Fellin, M. G., Zattin, M., Picotti, V., Reiners, P. W., & Nicolescu, S. (2005). Relief evolution in northern Corsica (western Mediterranean): Constraints on uplift and erosion on long-term and short-term timescales. Journal of Geophysical Research F: Earth Surface, 110(1).More infoAbstract:  The long-term erosional evolution of relief is revealed by low-temperature thermochronometric data, whereas geomorphic features give constraints on the short-term evolution. We discuss the topographic evolution of northeastern Corsica by fission track and (U-Th)/He dating of apatite, the provenance of Neogene alluvial sediments, an analysis of uplifted strath terraces, and the shape of river profiles. Cooling ages indicate high cooling rates of up to 40°C/Myr during the early-middle Miocene, when this region was affected by extensional tectonics. Cooling rates are less than 7.5°C/Myr from the middle Miocene to the present, when topographic perturbations are detected by variations in the source areas of middle-to-late Miocene alluvial sediments and by the presence of large-scale irregularities along modern river longitudinal profiles. We suggest that the contrast of slow long-term erosion rates and the high local relief in the present Corsican landscape require that the landscape be in a transient state in response to a recent tectonic event. Copyright 2005 by the American Geophysical Union.
- Garver, J. I., Reiners, P. W., Walker, L. J., Ramage, J. M., & Perry, S. E. (2005). Implications for timing of Andean uplift from thermal resetting of radiation-damaged zircon in the Cordillera Huayhuash, northern Peru. Journal of Geology, 113(2), 117-138.More infoAbstract: The Cordillera Huayhuash is a north-south-oriented range along the drainage divide of the northern Peruvian Andes. The range has high topography with peaks in excess of 5500 m and the second-highest peak in Peru, Nevados Yerupaja (6617 m). Bedrock is dominated by folded Mesozoic miogeoclinal rocks unconformably overlain by mid-Tertiary volcanics intruded by Late Tertiary granitic rocks and silicic dikes. Zircon fission track (ZFT) and (U-Th)/He (ZHe) dating of zircons along a west-east transect elucidates the thermal evolution of exhumed and uplifted rocks. The stability of fission tracks in zircons is a function of single-grain radiation damage. In samples with grain-to-grain variability in radiation damage, resetting results in variable resetting and multiple age populations. Low retentive zircons (LRZs), which have a partly disordered crystalline structure, have significant radiation damage and a low temperature of annealing (ca. 180°-200°C). High retentive zircons (HRZs), which are nearly crystalline, fully anneal at temperatures in excess of ca. 280°-300°C. Partly reset samples are those where LRZs are reset and HRZs are not reset, and therefore the cooling age is not concordant, but the young population of grain ages records the youngest thermal event. Full resetting of both LRZs and HRZs results in cooling ages that are concordant or nearly so. Lower Cretaceous quartzites show ZFT ages with a wide range of cooling ages, but most have LRZ reset ages at ca. 27 and 63 Ma. The ZFT ages from three quartzites and two granites from the core of the range yielded a single mean reset age of 11.4 ± 1 Ma. The ZHe ages from four samples in these rocks ranged from 10 to 7 Ma, with older ages away from the high topography. Together, the ZFT and ZHe cooling ages near the core of the range indicate moderate to rapid postintrusive cooling in the Miocene and a high Miocene geothermal gradient (ca. 40°-50°C /km). This widespread cooling age represents a falling geotherm, not a period of significant exhumation. Estimations of the thickness of preexhumation cover rock suggest that nearly 5 km of unroofing has occurred since the eruption of the Puscanturpa Formation (Huayllay Formation) at ca. 6.2 Ma. Exhumation was driven by valley incision initiated by uplift of this part of the Andes between 5 and 6 Ma. The high topography may have been formed by isostatic response to canyon incision. Therefore, the thermochronologic record of uplift and canyon incision is not yet apparent in the low-temperature thermochronology (for zircons) of these rocks. © 2005 by The University of Chicago. All rights reserved.
- Hourigan, J. K., Reiners, P. W., & Brandon, M. T. (2005). U-Th zonation-dependent alpha-ejection in (U-Th)/He chronometry. Geochimica et Cosmochimica Acta, 69(13), 3349-3365.More infoAbstract: Both theoretical and empirical evidence shows that intracrystalline U-Th heterogeneity in zircon can lead to biases in (U-Th)/He ages if not accurately accounted for in α-ejection corrections. We present a model for age correction for U-Th zoned crystals. We apply this to spherical and prismatic grains with bipyramidal terminations. The spherical calculation is simplistic but allows rapid calculation of the approximate effects of a wide variety of U-Th zoning patterns. The bipyramidal calculation is computationally intensive but permits an more complete estimate of the combined effects of crystal morphology and source zoning as relevant to zircon. Our principle findings are: (1) the assumption of U-Th homogeneity can result in errors of up to ∼30% (in rare cases, higher) for ejection-corrected ages for typical grain sizes and realistic zonation. (2) Tetragonal prisms with bipyramidal terminations, which are typical of most zircons, exhibit bulk retentivities that can differ by several percent from models considering crystals with pinacoidal terminations. When extreme cases, such as dipyramids, are considered, the bias can exceed 10% or more. (3) Morphologic effects can be accounted for to better than 1% precision by using new second-order polynomial parameters that describe retentivity as a function of surface-area-to-volume ratio calculated through more complete analysis of crystal dimensions including the height of pyramidal crystal sections. We illustrate application of our model using U-Th concentration profiles determined from single zircons by laser ablation ICP-MS from zoned Tardree Rhyolite zircons. Copyright © 2005 Elsevier Ltd.
- Reiners, P. W. (2005). Zircon (U-TH)/He thermochronometry. Reviews in Mineralogy and Geochemistry, 58, 151-179.
- Reiners, P. W., & Ehlers, T. A. (2005). Preface. Reviews in Mineralogy and Geochemistry, 58, vii-ix.
- Reiners, P. W., Campbell, I. H., Nicolescu, S., Allen, C. M., Hourigan, J. K., Garver, J. I., Mattinson, J. M., & Cowan, D. S. (2005). (U-Th)/(He-Pb) double dating of detrital zircons. American Journal of Science, 305(4), 259-311.More infoAbstract: Geochronology and thermochronology on detrital material provides unique constraints on sedimentary provenance, depositional ages, and orogenic evolution of source terrains. In this paper we describe a method and case-studies of measurement of both U/Pb and (U-Th)/He ages on single crystals of zircon that improves the robustness of constraints in each of these areas by establishing both formation and cooling ages of single detrital grains. Typically these ages correspond to crystallization and exhumation or eruption ages, and their combination can be used to more confidently resolve candidate source terrains, establish maximum depositional ages, and constrain the thermal histories of orogenic source regions. U/Pb dating is accomplished by laser-ablation ICP-MS in a small pit on the exterior of the crystal, and He dates are then determined on the bulk grain by conventional laser-heating and dissolution techniques. We present examples from Mesozoic aeolian sandstones, both modern and Paleogene fluvial sediments, and active margin turbidite assemblages from the Cascadia and Kamchatka margins. Important results include the fact that detritus from ancient orogens may dominate sediments thousands of kilometers away, crustal melting and exhumation appear to be spatially-temporally decoupled in at least two orogens, and first-cycle volcanic zircons older than depositional age are surprisingly rare in most settings except in the continental interior. In the case of the Kamchatkan, and possibly Olympic, turbidites, zircon He ages are partially reset. We present a method for estimating the extent of resetting of each grain and the thermal history of the sample, based on coupled (U-Th)/(He-Pb) age patterns among all the grains.
- Reiners, P. W., Ehlers, T. A., & Zeitler, P. K. (2005). Past, present, and future of thermochronology. Reviews in Mineralogy and Geochemistry, 58, 1-18.
- Hourigan, J. K., Solov'ev, A., Ledneva, G. V., Garver, J. I., Brandon, M. T., & Reiners, P. W. (2004). Timing of syenite intrusions on the eastern slope of the Sredinnyi Range, Kamchatka: Rate of accretionary structure exhumation. Geochemistry International, 42(2), 97-105.More infoAbstract: The isotopic thermochronological study included the U-Pb (SHRIMP) and fission-track dating of zircon and fission-track and (U-Th)-He dating of apatite from syenite intrusions cropping out in the eastern slope of the Sredinnyi Range of Kamchatka. The emplacement time of the intrusion was determined, and its exhumation velocity was evaluated. According to U-Pb (SHRIMP) zircon dating, the age of syenite crystallization (63.0 ± 0.6 and 70.4 ± 0.7 Ma) corresponds to the Late Cretaceous and Early Paleogene. Starting from 25 Ma, the cooling rate of the intrusion, which was likely controlled by exhumation, varied from 4 °C/Ma to 20 °C/Ma. If the geothermal gradient is assumed to be 30 °C/km, the exhumation rate of the intrusion from the Late Oligocene and until now ranged from 0.18 km/Ma to 0.67 km/Ma. Copyright © 2004 by MAIK "Nauka/Interperiodica" (Russia).
- Hourigan, J. K., Solov'ev, A., Ledneva, G. V., Garver, J. I., Brandon, M. T., & Reiners, P. W. (2004). Timing of syenite intrusions on the eastern slope of the Sredinnyi Range, Kamchatka: Rate of accretionary structure exhumation. Geokhimiya, 42(2), 131-141.
- Kogiso, T., Hirschmann, M. M., & Reiners, P. W. (2004). Length scales of mantle heterogeneities and their relationship to ocean island basalt geochemistry. Geochimica et Cosmochimica Acta, 68(2), 345-360.More infoAbstract: The upper mantle is widely considered to be heterogeneous, possibly comprising a "marble-cake" mixture of heterogeneous domains in a relatively well-mixed matrix. The extent to which such domains are capable of producing and expelling melts with characteristic geochemical signatures upon partial melting, rather than equilibrating diffusively with surrounding peridotite, is a critical question for the origin of ocean island basalts (OIB) and mantle heterogeneity, but is poorly constrained. Central to this problem is the characteristic length scale of heterogeneous domains. If radiogenic osmium signatures in OIB are derived from discrete domains, then sub-linear correlations between Os isotopes and other geochemical indices, suggesting melt-melt mixing, may be used to constrain the length scales of these domains. These constraints arise because partial melts of geochemically distinct domains must segregate from their sources without significant equilibration with surrounding peridotite. Segregation of partial melts from such domains in upwelling mantle is promoted by compaction of the domain mineral matrix, and must occur faster than diffusive equilibration between the domain and its surroundings. Our calculations show that the diffusive equilibration time depends on the ratios of partition and diffusion coefficients of the partial melt and surrounding peridotite. Comparison of time scales between diffusion and melt segregation shows that segregation is more rapid than diffusive equilibration for Os, Sr, Pb, and Nd isotopes if the body widths are greater than tens of centimeter to several meters, depending on the aspect ratio of the bodies, on the melt fraction at which melt becomes interconnected in the bodies, and on the diffusivity in the solid. However, because Fe-Mg exchange occurs significantly more rapidly than equilibration of these isotopes under solid-state and partially molten conditions, it is possible that some domains can produce melts with Fe/Mg ratios reflecting that of the surrounding mantle but retaining isotopic signatures of heterogeneous domains. Although more refined estimates on the rates of, and controls on, Os mobility are needed, our preliminary analysis shows that heterogeneous domains large enough to remain compositionally distinct in the mantle (as solids) for ∼109 yr in a marble-cake mantle, can produce and expel partial melts faster than they equilibrate with surrounding peridotite. © 2003 Elsevier Ltd.
- Min, K., Reiners, P. W., Nicolescu, S., & Greenwood, J. P. (2004). Age and temperature of shock metamorphism of Martian meteorite Los Angeles from (U-Th)/He thermochronometry. Geology, 32(8), 677-680.More infoAbstract: Mineralogic features attributed to impact-induced shock metamorphism are commonly observed in meteorites and terrestrial impact craters. Partly because the duration of shock metamorphism is very short, constraining the timing and temperature of shock events has been problematic. We measured (U-Th)/He ages of single grains of merrillite and chlorapatite from the Martian meteorite Los Angeles (LA). Merrillite and chlorapatite ages cluster at 3.28 ± 0.15 Ma (2σ) and 2.18 ± 0.19 (2σ) Ma, respectively. The mean age of the merrillites, which are larger than chlorapatites, is indistinguishable from cosmic-ray exposure ages (3.1 ± 0.2 Ma), suggesting that impact-induced shock metamorphism was coeval with ejection of the LA precursor from Mars. To constrain the initial temperature of shock metamorphism in the LA precursor body, we modeled diffusive loss of He from merrillite as a function of diffusion domain size, LA precursor body size, and ablation depth. From these calculations, we suggest that the metamorphic temperature of the shock event was higher than 450 °C. These results support the idea that shock pressures of the Martian meteorite Shergotty were higher than 45 GPa, as inferred from the presence of post-stishovite SiO2 polymorphs. Single-grain (U-Th)/He dating of phosphates may provide unique constraints on the timing and pressure-temperature dynamics of shock metamorphism in a wide variety of extraterrestrial materials. © 2004 Geological Society of America.
- Nasdala, L., Reiners, P. W., Garver, J. I., Kennedy, A. K., Stern, R. A., Balan, E., & Wirth, R. (2004). Incomplete retention of radiation damage in zircon from Sri Lanka. American Mineralogist, 89(1), 219-231.More infoAbstract: A suite of 18 zircon gemstones from placers in the Highland/Southwestern Complex, Sri Lanka, were subjected to a comprehensive study of their radiation damages and ages. The investigation included X-ray diffraction, Raman and PL spectroscopy, electron microprobe, PIXE and HRTEM analysis, as well as (U-Th)/He and SHRIMP U-Th-Pb age determinations. Zircon samples described in this study are virtually homogeneous. They cover the range from slightly metamict to nearly amorphous. Generally concordant U-Th-Pb ages averaging 555 ± 11 Ma were obtained. Late Ordovician zircon (U-Th)/He ages scattering around 443 ± 9 Ma correspond reasonably well with previously determined biotite Rb-Sr ages for rocks from the HSWC. Slightly to moderately metamict zircon has retained the radiogenic He whereas only strongly radiation-damaged zircon (calculated total fluences exceeding ∼3.5 × 1018 α-events/g) has experienced significant He loss. When compared to unannealed zircon from other localities, Sri Lanka zircon is about half as metamict as would correspond to complete damage accumulation over a ∼555 m.y. lasting self-irradiation period, suggesting significant annealing of the structural radiation damage. Insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon. We suggest to estimate "effective α-doses" for Sri Lanka zircon by multiplying total α-fluences, which were calculated using the zircon U-Th-Pb age, by a correction factor of 0.55. This conversion may be applied to literature data as well, because all gem-zircon samples from Sri Lanka (this work and previous studies) seem to reveal the same general trends of property changes depending on the radiation damage. The use of "effective α-doses" for Sri Lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation-damaged zircon.
- Reiners, P. W., Spell, T. L., Nicolescu, S., & Zanetti, K. A. (2004). Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with 40Ar/39Ar dating. Geochimica et Cosmochimica Acta, 68(8), 1857-1887.More infoAbstract: (U-Th)/He chronometry of zircon has a wide range of potential applications including thermochronometry, provided the temperature sensitivity (e.g., closure temperature) of the system be accurately constrained. We have examined the characteristics of He loss from zircon in a series of step-heating diffusion experiments, and compared zircon (U-Th)/He ages with other thermochronometric constraints from plutonic rocks. Diffusion experiments on zircons with varying ages and U-Th contents yield Arrhenius relationships which, after about 5% He release, indicate Ea = 163-173 kJ/mol (39-41 kcal/mol), and D0 = 0.09-1.5 cm2/s, with an average Ea of 169 ± 3.8 kJ/mol (40.4 ± 0.9 kcal/mol) and average D0 of 0.46+0.87-0.30 cm2/s. The experiments also suggest a correspondence between diffusion domain size and grain size. For effective grain radius of 60 μm and cooling rate of 10°C/myr, the diffusion data yield closure temperatures, Tc, of 171-196°C, with an average of 183°C. The early stages of step heating experiments show complications in the form of decreasing apparent diffusivity with successive heating steps, but these are essentially absent in later stages, after about 5-10% He release. These effects are independent of radiation dosage and are also unlikely to be due to intracrystalline He zonation. Regardless of the physical origin, this non-Arrhenius behavior is similar to predictions based on degassing of multiple diffusion domains, with only a small proportion (
- Spotila, J. A., Bank, G. C., Reiners, P. W., Naeser, C. W., Naeser, N. D., & Henika, B. S. (2004). Origin of the Blue Ridge escarpment along the passive margin of Eastern North America. Basin Research, 16(1), 41-63.More infoAbstract: The Blue Ridge escarpment is a rugged landform situated within the ancient Appalachian orogen. While similar in some respects to the great escarpments along other passive margins, which have evolved by erosion following rifting, its youthful topographic expression has inspired proposals of Cenozoic tectonic rejuvenation in eastern North America. To better understand the post-orogenic and post-rift geomorphic evolution of passive margins, we have examined the origin of this landform using low-temperature thermochronometry and manipulation of topographic indices. Apatite (U-Th)/He and fission-track analyses along transects across the escarpment reveal a younging trend towards the coast. This pattern is consistent with other great escarpments and fits with an interpretation of having evolved by prolonged erosion, without the requirement of tectonic rejuvenation. Measured ages are also comparable specifically to those measured along other great escarpments that are as much as 100 Myr younger. This suggests that erosional mechanisms that maintain rugged escarpments in the early post-rift stages may remain active on ancient passive margins for prolonged periods. The precise erosional evolution of the escarpment is less clear, however, and several end-member models can explain the data. Our preferred model, which fits with all data, involves a significant degree of erosional escarpment retreat in the Cenozoic. Although this suggests that early onset of topographic stability is not required of passive margin evolution, more data are required to better constrain the details of the escarpment's development. © 2003 Blackwell Publishing Ltd.
- Spotila, J. A., Buscher, J. T., Meigs, A. J., & Reiners, P. W. (2004). Long-term glacial erosion of active mountain belts: Example of the Chugach- St. Elias Range, Alaska. Geology, 32(6), 501-504.More infoAbstract: An emerging paradigm that equates glaciers to "buzz saws" of exceptional erosional efficiency has been strengthened by short-term (
- Balestrieri, M. L., Bernet, M., Brandon, M. T., Picotti, V., Reiners, P., & Zattin, M. (2003). Pliocene and Pleistocene exhumation and uplift of two key areas of the Northern Apennines. Quaternary International, 101-102(1), 67-73.More infoAbstract: Application of different thermochronological methods to two areas of pronounced relief (Apuane Alps and Mt. Falterona) of the Northern Apennines chain documents an average exhumation rate of about 0.7 mm/yr. This result suggests that the general exhumation of the chain is driven mainly by erosion. Nevertheless, the long-term exhumation trend seems to be perturbed by short-term events at higher exhumation rates. The time of increased exhumation rates coincides with onset of intramontane sedimentation, documenting a contribution of tectonics (local normal faulting?) to the surface uplift. The data indicate an eastward shifting of the onset of topographic relief, Early Pliocene in the Apuane Alps and Early Pleistocene in the Mt. Falterona area. © 2002 Elsevier Science Ltd and INQUA. All rights reserved.
- Chakurian, A. M., Arehart, G. B., Donelick, R. A., Zhang, X., & Reiners, P. W. (2003). Timing constraints of gold mineralization along the Carlin trend utilizing apatite fission-track, 40Ar/39Ar, and apatite (U-Th)/He methods. Economic Geology, 98(6), 1159-1171.More infoAbstract: Apatite fission-track analysis is used to demonstrate that the timing of gold mineralization at the Carlin East and Betze-Post deposits is 37.3 ± 1.5 Ma, based on the weighted mean of measured fission-track ages, and 41.6 ± 1.6 Ma, based on the weighted mean of the ages of the oldest fission tracks retained. Regional measured apatite fission-track ages cluster between 28 and 41 Ma to the south of the Carlin East deposit whereas measured ages are greater than ∼56 Ma to the north. The regional trend of the apatite fission-track ages and the pattern of gravity and magnetic anomalies suggest that the thermal center for gold mineralization along the Carlin trend was south of the Carlin East deposit, near the 37 Ma Welches Canyon stock. Modeling of apatite fission-track-age data on both sides of the Post fault indicates that the Little Boulder stock was approximately 35°C cooler at 40 Ma relative to similarly positioned samples in the Goldstrike stock. It is unclear whether this thermal history difference across the Post fault is due to post-40 Ma differential exhumation or differential heating across the fault at the time of gold mineralization. Comparison of apatite fission-track and whole-rock 40Ar/39Ar ages from altered dikes in the ore zone at Carlin East demonstrates that 40Ar/39Ar ages do not reflect the age of gold mineralization, whereas the apatite fission-track ages were completely reset at the time of hydrothermal activity. (U-Th)/He ages are younger than the inferred mineralization age and may reflect weak hydrothermal activity associated with Miocene volcanism.
- Mitchell, S. G., & Reiners, P. W. (2003). Influence of wildfires on apatite and zircon (U-Th)/He ages. Geology, 31(12), 1025-1028.More infoAbstract: Low closure temperatures of the apatite and zircon (U-Th)/He thermochronometers allow valuable constraints on timing and rates of bedrock exhumation through shallow crustal depths, but raise the possibility that shallow-level processes other than exhumation- related cooling may also influence He ages. A simple He diffusion model predicts that wildfires can completely or partially reset apatite He ages as much as 3 cm below rock surfaces and partially reset zircon He ages in the outermost 1 cm. Measured He ages in bedrock and sediments from the Washington Cascades that were exposed to extensive wildfires in 2001 show strong agreement with these model predictions. Apatite He ages decrease from a regionally consistent age of 19.5 ± 1.2 Ma at a distance >3 cm from the rock surface to as low as 1.9 Ma in the outermost 1 cm, whereas zircon He ages decrease from 65 to 55 Ma over the same distance. Thin (
- Rahl, J. M., Reiners, P. W., Campbell, I. H., Nicolescu, S., & Allen, C. M. (2003). Combined single-grain (U-Th)/He and U/Pb dating of detrital zircons from the Navajo Sandstone, Utah. Geology, 31(9), 761-764.More infoAbstract: Radioisotopic dating of detrital minerals in sedimentary rocks can constrain sediment sources (provenance), elucidate episodes and rates of ancient orogenesis, and give information on paleogeography and sediment-dispersal patterns. Previous approaches have been restricted to the application of a single technique, such as U/Pb or fission-track dating, to detrital grains. These methods provide crystallization and cooling ages, respectively, of sediment sources (terranes). However, evidence for source regions from a single technique can be ambiguous because candidate source terranes often have similar ages for a given radioisotopic system. This ambiguity can be avoided by applying multiple radioisotopic systems to individual detrital grains. Here we present a method for measuring both (U-Th)/He and U/Pb ages of single crystals of detrital zircon, providing both formation and cooling ages (through ∼ 180 °C). We applied this technique to zircons from the Lower Jurassic Navajo Sandstone, which represents one of the largest erg deposits in the geologic record. A large fraction of these zircons was derived from crust that formed between 1200 and 950 Ma, but cooled below ∼ 180 °C ca. 500-250 Ma. This history is characteristic of Grenvillian-age crust involved in Appalachian orogenesis (and subsequent rifting) in eastern North America. Our finding requires the existence of a transcontinental sediment-dispersal system capable of moving a large volume of detritus westward (modern coordinates) throughout the late Paleozoic and early Mesozoic.
- Reiners, P. W., Ehlers, T. A., Mitchell, S. G., & Montgomery, D. R. (2003). Coupled spatial variations in precipitation and long-term erosion rates across the Washington Cascades. Nature, 426(6967), 645-647.More infoPMID: 14668859;Abstract: Past studies of tectonically active mountain ranges have suggested strong coupling and feedbacks between climate, tectonics and topography. For example, rock uplift generates topographic relief, thereby enhancing precipitation, which focuses erosion and in turn influences rates and spatial patterns of further rock uplift. Although theoretical links between climate, erosion and uplift have received much attention, few studies have shown convincing correlations between observable indices of these processes on mountain-range scales. Here we show that strongly varying long-term (>106-10 7 yr) erosion rates inferred from apatite (U-Th)/He cooling ages across the Cascades mountains of Washington state closely track modern mean annual precipitation rates. Erosion and precipitation rates vary over an order of magnitude across the range with maxima of 0.33 mm yr-1 and 3.5 m yr-1, respectively, with both maxima located 50 km west (windward) of the topographic crest of the range. These data demonstrate a strong coupling between precipitation and long-term erosion rates on the mountain-range scale. If the range is currently in topographic steady state, rock uplift on the west flank is three to ten times faster than elsewhere in the range, possibly in response to climatically focused erosion.
- Reiners, P. W., Zhou, Z., Ehlers, T. A., Changhai, X. u., Brandon, M. T., Donelick, R. A., & Nicolescu, S. (2003). Post-orogenic evolution of the Dabie-Shan, eastern China, from (U-Th)/He and fission-track thermochronology. American Journal of Science, 303(6), 489-518.More infoAbstract: The Dabie Shan of eastern China is a ∼200 kilometers wide mountain range with nearly 2 kilometers of relief and is an archetype of deep ultrahigh-pressure metamorphic rock exhumation. Despite its regional and petrologic importance, little is known about the low-temperature and post-orogenic evolution of the Dabie Shan. Here we present apatite and zircon (U-Th)/He (AHe and ZHe, respectively) and apatite fission-track (AFT) cooling ages from the Dabie Shan that constrain the patterns and history of exhumation over the last ∼115 myr. On the scale of the whole orogen, ZHe and AHe ages are inversely correlated with mean elevation and are systematically younger in the core of the range. These cooling ages were converted to exhumation rates assuming steady-state erosion and accounting for topographic effects. These results indicate that, since the Eocene, flanks of the range have eroded at rates as low as 0.02 km/myr, while the range core has eroded at about 0.06 km/myr. Even in the core of the range, these recent exhumation rates are at least 10 to 20 times slower than those estimated for the initial stages of exhumation in the Triassic-Jurassic. In a 1.4 kilometer vertical transect in the core of the range, all ages are positively correlated with elevation, with ZHe ages increasing from 76 to 112 Ma, AFT from 44 to 70 Ma, and AHe from 24 to 43 Ma. We present a simple model for topographic correction of thermochronometric ages in vertical transects, using the admittance ratio (ratio of isotherm relief to topographic relief). Applied to the AHe age-elevation relationship, this yields Tertiary exhumation rates of 0.05 to 0.07 km/myr in the core of the Dabie Shan, in good agreement with regional exhumation rate patterns. Finally, age-elevation relationships for all three chronometers in the vertical transect are consistent with a constant exhumation rate of 0.06 ± 0.01 km/myr since the Cretaceous, with a possible modest increase in exhumation rates (as high as 0.2 km/myr) between 80 to 40 Ma. These data show no evidence for significant variations in exhumation rates over the last ∼115 myr, as might be expected for decay of old topography or tectonic reactivation of old structures.
- Zhou, Z., Changhai, X. u., Reiners, P. W., Yang, F., & Donelick, R. A. (2003). Late Cretaceous-Cenozoic exhumation history of Tiantangzhai region of Dabieshan Orogen: Constraints from (U-Th)/He and fission track analysis. Chinese Science Bulletin, 48(11), 1151-1156.More infoAbstract: Integrated fission track and (U-Th)/He analysis is carried out on 6 apatite and 6 zircon samples from a near vertical section in The Tiantangzhai region at the core of the present Dabieshan orogen. The result shows that the region experienced cooling/exhumation during the Late Cretaceous and Early Tertiary period. Age-elevation relationships for different dating systems and different minerals suggest a pulse of rapid exhumation at ∼110 Ma before present, preserved in the structurally highest samples. At lower elevations, ages begin to decrease with decreasing elevation, suggesting lower exhumation rates since 90 Ma. Two periods of different exhumation rates are identified since 90 Ma. The average apparent exhumation rate for the period of 43.4-22.5 is 0.062 km/Ma, whereas that for the period of 76.4-47.4 Ma is 0.039 km/Ma.
- Changhai, X. u., Zhou, Z., Changqian, M. a., & Reiners, P. W. (2002). Geochronological constraints on 140-85 Ma thermal doming extension in the Dabie orogen, central China. Science in China, Series D: Earth Sciences, 45(9), 801-817.More infoAbstract: Regional architecture of geochronology and differential cooling pattern show that the Dabie orogen underwent a thermal doming extension during 140-85 Ma. This extension resulted in widespread re-melting of the Dabie basement, intense volcanic activities in North Huaiyang and the formation of fault-controlled depressions in the Hefei basin. This thermal doming extension can be further divided into two consecutive evolving stages, i.e. the intensifying stage (140-105 Ma) and the declining stage (105-85 Ma). In the first stage (140-105 Ma), the thermal doming mainly was concentrated in the Dabie block, and to a less degree, in the Hongan block. The thermal doming structure of the Dabie block is configured with Macheng-Yuexi thermal axis, Yuexi/Luotian thermal cores and their downslide flanks. The orientation of thermal axis is dominantly parallel to the strike of orogen, and UHP/HP units together with metamorphic rocks of North Huaiyang constitute the downslide flanks. The Yuexi core differs from the Luotian core in both the intensity and the shaping time. To some extent, the Hongan block can be regarded as part of downslide systems of the Dabie doming structure. The doming process is characterized by thermal-center's migration along the Macheng-Yuexi thermal axis; consequently, it is speculated to be attributed to the convective removal of thickened orogenic root, which is a process characterized by intermittance, migration, large-scale and differentiation. During the declining stage (105-85 Ma), the domeshaped figure still structurally existed in the Dable orogen, but orogenic units cooled remarkably slow and magmatic activities stagnated gradually. Study on the thermal doming of Dabieshan Mountains can thus provide detailed constraints on the major tectonic problems such as the UHP/HP exhumation model, the boundary between North Dabie and South Dabie, and the orogenesis mechanism.
- Crowley, P. D., Reiners, P. W., Reuter, J. M., & Kaye, G. D. (2002). Laramide exhumation of the Bighorn Mountains, Wyoming: An apatite (U-Th)/He thermochronology study. Geology, 30(1), 27-30.More infoAbstract: We report (U-Th)/He apatite ages from the crystalline core of the Bighorn Mountains and compare the exhumation history derived from those ages with the exhumation history determined from sedimentary rocks in the adjacent Powder River and Bighorn basins. Our (U-Th)/He apatite ages range from 62 to 369 Ma and represent a pre-Laramide He partial retention zone that was deformed and uplifted at ca. 65 ± 5 Ma. The geometry of the He partial retention zone indicates that the basement in the Bighorn Mountains is deformed into a doubly plunging anticline. The preservation of a pre-Laramide partial retention zone in the upper few hundred meters of Precambrian basement indicates that in general, the temperature at the Cambrian unconformity did not exceed the apatite He closure temperature. This is difficult to reconcile with evidence from adjacent basins for thick sequences of sedimentary rocks (3-4 km) prior to 65 Ma, and normal modern geothermal gradients. Either the range was never deeply buried (
- Kirby, E., Reiners, P. W., Krol, M. A., Whipple, K. X., Hodges, K. V., Farley, K. A., Tang, W., & Chen, Z. (2002). Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U-th)/He thermochronology. Tectonics, 21(1), 1-1-1-20.More infoAbstract: High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperature-time paths inferred from 40Ar/39Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar40Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling (
- Reiners, P. W. (2002). Editorial: AGU online journals: An open letter to librarians. Eos, 83(3), 21+25.
- Reiners, P. W., Ehlers, T. A., Garver, J. I., Mitchell, S. G., Montgomery, D. R., Vance, J. A., & Nicolescu, S. (2002). Late Miocene exhumation and uplift of the Washington Cascade Range. Geology, 30(9), 767-770.More infoAbstract: The Washington Cascade Range is a complex, polygenetic mountain range that dominates the topographic, climatic, and cultural configurations of Washington State. Although it has been the locus of ongoing are magmatism since the Eocene, most of the range is distinct from the southern part of the arc in Oregon and California in that bedrock uplift has produced high surface elevations and topographic relief, rather than volcanic burial or edifice construction. (U-Th)/He and fission-track ages of bedrock samples on the east flank of the range record relatively rapid cooling in the early Tertiary, but slow exhumation rates (∼0.2 km/m.y.) through most of the Oligocene. Samples on the west flank suggest rapid cooling in the late Miocene (8-12 Ma), and age variations in vertical transects are consistent with a pulse of rapid exhumation (0.5-1.0 km/m.y.) at that time. Apatite He ages as young as 1-5 Ma in several areas suggest that high cooling and possibly exhumation rates persist locally. Accelerated exhumation rates ca. 10 Ma are also observed in the Coast Mountains of British Columbia and southeast Alaska, ∼1500 km to the north, suggesting a large-scale mechanism for the exhumation pulse at that time.
- Reiners, P. W., Farley, K. A., & Hickes, H. J. (2002). He diffusion and (U-Th)/He thermochronometry of zircon: Initial results from Fish Canyon Tuff and Gold Butte. Tectonophysics, 349(1-4), 297-308.More infoAbstract: To evaluate the potential of (U-Th)/He geochronometry and thermochronometry of zircon, we measured He diffusion characteristics in zircons from a range of quickly and slowly cooled samples, (U-Th)/He ages of zircons from the quickly cooled Fish Canyon Tuff, and age-paleodepth relationships for samples from 15 to 18 km thick crustal section of the Gold Butte block, Nevada. (U-Th)/He ages of zircons from the Fish Canyon Tuff are consistent with accepted ages for this tuff, indicating that the method can provide accurate ages for quickly cooled samples. Temperature- dependent He release from zircon is not consistent with thermally activated volume diffusion from a single domain. Instead, in most samples apparent He diffusivity decreases and activation energy (Ea) increases as cycled step-heating experiments proceed. This pattern may indicate a range of diffusion domains with distinct sizes and possibly other characteristics. Alternatively, it may be the result of ongoing annealing of radiation damage during the experiment. From these data, we tentatively suggest that the minimum Ea for He diffusion in zircon is about 44 kcal/mol, and the minimum closure temperature (Tc, for a cooling rate of 10 °C/myr) is about 190 °C. Age-paleodepth relationships from the Gold Butte block suggest that the base of the zircon He partial retention zone is at pre-exhumation depths of about 9.5 - 11 km. Together with constraints from other thermochronometers and a geothermal gradient derived from them in this location, the age-depth profile suggests a He Tc of about 200 °C for zircon, in reasonable agreement with our interpretation of the laboratory measurements. A major unresolved question is how and when radiation damage effects become significant for He loss from this mineral. © 2002 Elsevier Science B.V. All rights reserved.
- Zhou, Z., Reiners, P. W., Changhai, X. u., Liao, Z., & Yang, F. (2002). Zircon (U-Th)/He thermochronological constraints on Cretaceous thermal extension of Dabieshan orogen. Progress in Natural Science, 12(7), 524-.
- Changhai, X. u., Zhou, Z., Changqian, M. a., & Reiners, P. W. (2001). Geochronological Constraints on the Post-Collisional (150-75 Ma) Thermal Extension in the Dabieshan Orogen, Central China. Gondwana Research, 4(4), 829-831.
- Reiners, P. W., & Farley, K. A. (2001). Influence of crystal size on apatite (U-Th)/He thermochronology: An example from the Bighorn Mountains, Wyoming. Earth and Planetary Science Letters, 188(3-4), 413-420.More infoAbstract: Near-surface tectonic and geomorphic processes involve cooling of rocks through low temperatures (50-200°C). Because rates of helium diffusion in apatite, titanite, and zircon are sensitive to temperature variations in this range, uranium-thorium/helium thermochronometry ((U-Th)/Hedating is well-suited to to establishing the timing and rates of these processes in the geologic record. However,because fractional loss of He is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic HE, (U-Th)/He ages must vary not only with thermal history but also with crystal size. Here we present crystal size-correlated He ages from co-existing apatites from the Bighorn Mountains, Wyoming that range from 100 to 350 Ma. These correlations are a sensitive indicator of the rock's thermal history in a temperature range below the system's nominal closure temperature (Tc̃ 70°C for apatite), and are consistent with a thermal history involving residence in the upper 2-3 km of crust since the Precambrian, with maximum temperatures of 65-80°C just prior to Laramide orogenic echumation. The influence of crystal size on He ages will be most apparent in rocks where temperatures have been in the range of partial He retention for long periods of time (̃30-70°C for > 10 7 years). In such cases, accurate interpretation of (U-Th)/he ages must incorporate the effect of crystal size, and this method may provide insights tothermal histories of rocks in previously inaccessible low-temperature ranges. © 2001 Elsevier science B.V. All rights reserved.
- Spotila, J. A., Farley, K. A., Yule, J. D., & Reiners, P. W. (2001). Near-field transpressive deformation along the San Andreas fault zone in southern California, based on exhumation constrained by (U-Th)/He dating. Journal of Geophysical Research B: Solid Earth, 106(B12), 30909-30922.More infoAbstract: Low-temperature thermochronometry reveals that a narrow crustal sliver trapped within strands of the San Andreas fault zone in southern California has experienced recent, rapid exhumation. Eight apatite (U-Th)/He ages from a 1-km-relief section along Yucaipa Ridge in the San Bernardino Mountains range from 1.4 to 1.7 Ma. The minimal change in age with elevation implies exhumation of ∼5-7 mm yr-1, sustained for at least several hundred thousand years. Three titanite helium ages from the ridge are much older, ranging from 57 to 82 Ma. These show a steep gradient with elevation, representing either an exhumed, partial retention zone or slow cooling through much of the Tertiary. These data imply that a total exhumation of ∼3 to 6 km has occurred since 1.8 Ma. It is uncertain whether this exhumation terminated as early as 1 Ma or has continued up to the present at a decelerated rate. We surmise that this exhumation represents rock uplift in the absence of major surface uplift, in that it kept pace with tectonic uplift as the narrow fault block maintained steady state relief. The record of sedimentation in adjacent basins is consistent with the implied magnitude of erosion. Such rapid, large-magnitude exhumation within the strands of the San Andreas fault zone is important for models of transpressional tectonics. It is consistent with a strain partitioning model which predicts that pure shear dominated fault zones experience significant vertical strain. However, it is inconsistent with a stress-partitioning model which predicts that fault zone weakness limits pure shear deformation to the borderlands of the master strike-slip fault. In addition, a concentration of secondary contraction within the fault zone may require modification of coupling models between strong upper mantle and brittle upper crust via the weak lower crust. These models predict that transpressional deformation will either be uniformly distributed across the plate boundary or be limited to the far-field borderlands, rather than concentrated in the near field. Alternatively, the exhumation of Yucaipa Ridge may have been driven by the nearby restraining bend in the San Andreas fault at San Gorgonio Pass, in which case it represents local fault geometry rather than accommodation of far-field plate motion. Copyright 2001 by the American Geophysical Union.
- Holcomb, R. T., Nelson, B. K., Reiners, P. W., & Sawyer, N. -. (2000). Overlapping volcanoes: The origin of Hilo Ridge, Hawaii. Geology, 28(6), 547-550.More infoAbstract: The submarine Hilo Ridge has been interpreted as a part of Mauna Kea volcano, but is crossed at ~1100 m depth by a submerged shoreline terrace composed of basalts that are isotopically distinct from those of Mauna Kea and similar to those of Kohala volcano. This terrace evidently is a product of Kohala instead of Mauna Kea. Almost all of Hilo Ridge below the terrace therefore must predate the principal growth of Mauna Kea, which has superficially isolated the ridge from its Kohala source by overlapping its proximal segment. The Mauna Kea section penetrated by the Hawaii Scientific Drilling Project is predicted to be thinner than expected previously, owing to the overlap. Similar overlaps are suspected among other volcanoes and may cause significant changes in the understanding of Hawaiian volcanism.
- Lassiter, J. C., Hauri, E. H., Reiners, P. W., & Garcia, M. O. (2000). Generation of Hawaiian post-erosional lavas by melting of a mixed lherzolite/pyroxenite source. Earth and Planetary Science Letters, 178(3-4), 269-284.More infoAbstract: Melting of mafic veins in a marble-cake mantle may play an important role in generating isotopic and chemical heterogeneities in mid-ocean ridge and ocean island basalts. Mafic veins have lower solidi than mantle peridotite and will be preferentially sampled during partial melting, particularly at low melt fractions. However, the abundance of mafic components in the mantle or their role during melt generation has been difficult to quantify because most isotopic systems (e.g. Rb-Sr, Sm-Nd, U-Th-Pb) are not diagnostic of the presence or absence of mafic components. The compatible behavior of Os during mantle melting combined with the incompatible behavior of Re makes the Re-Os isotopic system uniquely well suited for distinguishing mafic and ultramafic contributions to melt generation. Almost all peridotites have low 187Os/188Os (e.g. chondritic to subchondritic). In contrast, mafic rocks have much higher Re/Os than peridotites, which results in the rapid ingrowth of 187Os and the development of large isotopic contrasts between mafic and ultramafic components within the mantle. In this paper, we show that Os-isotopes in Hawaiian post-erosional lavas extend to more radiogenic values than are found in Hawaiian lherzolites, abyssal peridotites or most other ultramafic samples. Os-isotopes are not correlated with other isotopic tracers, in contrast with plume-derived Hawaiian shield-stage lavas. The lack of correlation between Os-isotopes and Sr-, Nd- or Pb-isotopes and the more 'depleted' or MORB-like Sr-Nd isotopic signature of the post-erosional lavas relative to other Hawaiian lavas precludes significant melt input from the Hawaiian plume. However, Os-isotopes are correlated with major and trace elements. Lavas with more radiogenic Os-isotope compositions have higher silica and alumina and lower calcium and incompatible trace element abundances than lavas with less radiogenic Os-isotopes. These correlations result from mixing of pyroxenite- and peridotite-derived melts, both likely derived from the ~100 Ma Pacific lithospheric mantle. (C) 2000 Elsevier Science B.V. All rights reserved.
- Reiners, P. W., Brady, R., Farley, K. A., Fryxell, J. E., Wernicke, B., & Lux, D. (2000). Helium and argon thermochronometry of the Gold Butte block, south Virgin Mountains, Nevada. Earth and Planetary Science Letters, 178(3-4), 315-326.More infoAbstract: One of the largest exposures of Precambrian crystalline rock in the Basin and Range province of the southwestern USA is the Gold Butte block of the south Virgin Mountains, about 15 km west of the Colorado Plateau. It has been interpreted as a largely continuous crustal cross-section about 15-20 km thick that was exhumed by a deeply penetrating normal fault during Miocene extension. To test this interpretation as well as the use of the newly developed titanite (U-Th)/He thermochronometer, we examined the low temperature thermal history of the Gold Butte block with the apatite and titanite (U-Th)/He and muscovite 40Ar/39Ar thermochronometers. Apatite He ages average 15.2±1.0 (2σ) Ma throughout the block, indicating that the entire section was warmer than 70°C prior to Miocene exhumation. Titanite He ages increase from 18.6±1.5 Ma near the paleobottom (west) end of the block, to 195±15 Ma near the paleotop (east) end. A rapid change from mid-Tertiary to increasingly older titanite He ages to the east is observed at about 9.3 km paleodepth, and is interpreted as a fossil He partial retention zone for titanite. Assuming a pre-exhumation geotherm of 20°C/km (consistent with earlier apatite fission track work), this depth would have corresponded to 196°C prior to exhumation, indicating that laboratory-derived He diffusion characteristics for titanite that yield a closure temperature of about 200°C are applicable and correct. Muscovite 40Ar/39Ar ages are 1.0-1.4 Ga near the paleotop of the block, and 90 Ma near the paleobottom. Together with 207Pb/206Pb ages on apatite and titanite, and an earlier apatite fission track transect across the Gold Butte block, our data indicate that the continental crust at the western edge of the Colorado Plateau resided at moderate geothermal gradients (and slowly declined in temperature) from 1.4 Ga to about 100-200 Ma. A 90 Ma cooling event clearly affected the mid-crust (deepest portions of Gold Butte), which may reflect accelerated cooling or a brief heating and cooling cycle at this time, after which gradients returned to about 20°C/km prior to rapid exhumation in the Miocene. This work thus supports previous structural and thermochronologic studies that suggest that the Gold Butte block is the thickest largely continuous cross-section of crust exposed in the southwestern USA. (C) 2000 Elsevier Science B.V. All rights reserved.
- Reiners, P. W., Hammond, P. E., McKenna, J. M., & Duncan, R. A. (2000). Young basalts of the central Washington Cascades, flux melting of the mantle, and trace element signatures of primary arc magmas. Contributions to Mineralogy and Petrology, 138(3), 249-264.More infoAbstract: Basaltic lavas from the Three Sisters and Dalles Lakes were erupted from two isolated vents in the central Washington Cascades at 370-400 ka and 2.2 Ma, respectively, and have distinct trace element compositions that exemplify an important and poorly understood feature of arc basalts. The Three Sisters lavas are calc-alkaline basalts (CAB) with trace element compositions typical of most arc magmas: high ratios of large-ion-lithophile to high-field-strength elements (LILE/HFSE), and strong negative Nb and Ta anomalies. In contrast, the Dalles Lakes lavas have relatively low LILE/HFSE and no Nb or Ta anomalies, similar to ocean-island basalts (OIB). Nearly all Washington Cascade basalts with high to moderate incompatible element concentrations show this CAB or OIB-like compositional distinction, and there is pronounced divergence between the two magma types with a large compositional gap between them. We show that this trace element distinction can be easily explained by a simple model of flux-melting of the mantle wedge by a fluid-rich subduction component (SC), in which the degree of melting (F) of the peridotite source is correlated with the amount of SC added to it. Distinctive CAB and OIB-like trace element compositions are best explained by a flux-melting model in which dF/dSC decreases with increasing F, consistent with isenthalpic (heat-balanced) melting. In the context of this model, CAB trace element signatures simply reflect large degrees of melting of strongly SC-fluxed peridotite along relatively low dF/dSC melting trends, consistent with derivation from relatively cold mantle. Under other conditions (i.e., small degrees of melting or large degrees of melting of weakly SC-fluxed peridotite [high dF/dSC]), either OIB- or MORB (mid-ocean ridge basalt)-like compositions are produced. Trace element and isotopic compositions of Washington Cascade basalts are easily modeled by a correlation between SC and F across a range of mantle temperatures. This implies that the dominant cause of arc magmatism in this region is flux melting of the mantle wedge.
- Farley, K. A., Reiners, P. W., & Nenow, V. (1999). An apparatus for high-precision helium diffusion measurements from minerals. Analytical Chemistry, 71(10), 2059-2061.More infoAbstract: We describe a simple and low-cost apparatus for in-vacuum helium diffusion measurements that reduces temperature gradients, set point overshoot, and ramping times compared with conventional resistance furnace techniques. The sample, suspended by a thin wire in a vacuum chamber, is heated by radiation from an Al-coated projector bulb passed through a sapphire viewport. Because the total mass of the sample package being heated is small (typically
- Reiners, P. W., & Farley, K. A. (1999). Helium diffusion and (U-Th)/He thermochronometry of titanite. Geochimica et Cosmochimica Acta, 63(22), 3845-3859.More infoAbstract: We have explored the diffusivity characteristics of radiogenic He in titanite (sphene) and have developed analytical techniques for (U-Th)/He dating of this mineral. Results of incremental He outgassing experiments performed on titanites from a variety of geological environments suggest a thermally activated volume diffusion mechanism with an activation energy of 44.6 ± 3.4 (2σ) kcal/mol and a frequency factor of ~60 cm2/s. Diffusivity is highly linearly correlated with the inverse square of the grain size, indicating that the He diffusion domain in titanite is the crystal itself. For typical titanite grains of 200- to 800-μm minimum dimension, the He closure temperature is in the range 191 to 218°C (for a cooling rate of 10°C/Myr). There is no indication in the titanites we studied that radiation damage plays a major role in He diffusion. (U-Th)/He ages of titanites from quickly cooled rocks yield ages (with ~5-8% 1 σ reproducibility) that are concordant with known ages, and (U-Th)/He ages of titanites from slowly cooled rocks are consistent with independently established cooling paths, supporting the closure temperature estimates. These experiments suggest that titanite (U-Th)/He ages may be useful for constraining cooling histories at temperatures near the lower limit of those accessed by feldspar 40Ar/39Ar dating but higher than apatite fission track or (U-Th)/He dating.
- Reiners, P. W., Nelson, B. K., & Izuka, S. K. (1999). Structural and petrologic evolution of the Lihue basin and eastern Kauai, Hawaii. Bulletin of the Geological Society of America, 111(5), 674-685.More infoAbstract: The topography of the eastern part of the Hawaiian island of Kauai is dominated by the Lihue basin, a large (∼110 km2) semicircular depression bounded by steep cliffs and partly filled by late rejuvenated-stage (or post-erosional stage) volcanic material. As with other large, semicircular basins on ocean-island volcanoes, the subsurface geology and origin (e.g., structural collapse vs. fluvial erosion) of the Lihue basin are poorly understood. New analyses of samples collected from eastern Kauai and drill holes within the basin document several important features of the late-stage geologic evolution of Kauai. First, thick (>300 m) sequences of rejuvenated-stage Koloa Volcanics in the Lihue basin show systematic, basin-wide geochemical trends of increasingly incompatible elements with time, indicating a gradual decrease in the extent of partial melting of mantle sources with time. Second, beneath the rejuvenated-stage volcanics in the basin, a thin layer of postshield alkalic stage lavas (e.g., hawaiites and mugearites) overlies older shield-stage tholeiitic lavas of the Napali Member, indicating that the Lihue basin formed by structural collapse, not fluvial erosion. Third, a large (∼2-5 km3) matrix-supported breccia, interpreted as deposits of one or more debris flows, is within the rejuvenated-stage volcanics throughout the basin, and correlates with surficial exposures of the Palikea Breccia west of the basin. Isotopic compositions of the bulk breccia are similar to those of tholeiites from the east side of Kauai, and distinct from those of west Kauai tholeiites. Clasts within the breccia are dominantly hawaiite and alkali gabbro. The source region of the breccia in the steep cliffs and highlands of the central massif to the west of the basin must contain magmatic products of an extensive postshield alkalic stage, including hawaiite flows and one or more large intrusive bodies or ponded sequences of alkali gabbro.
- Reiners, P. W. (1998). Reactive melt transport in the mantle and geochemical signatures of mantle-derived magmas. Journal of Petrology, 39(5), 1039-1061.More infoAbstract: Modeling of equilibrium reactive melt transport in the mantle, including the effects of mineralogic reactions and changes in porosity in the solid matrix, provides a series of robust predictions of the consequences of reactive melt transport on magma trace element compositions. The composition of the leading melt batch through a column of reactive mantle (the melt front) will be shifted towards that of an incipient partial melt of the mantle matrix. Successive melt batches migrating through and emerging from the column will show a temporal-compositional trend reflecting exhaustion of the reactive capacity of the mantle, and will eventually return to the original input melt composition. In cases where the melt source and column matrix are similar in composition, the melt front will be enriched in incompatible elements, and the temporal-compositional trend will be one of decreasing incompatible elements in erupted melt batches with time. Cogenetic melt batches should show some type of chromatographic decoupling in trace element and/or isotopic variations. However, mineralogic reaction in the mantle column, in the form of changing matrix mode, can smooth or mask chromatographic effects on trace element abundances, though not on isotopic compositions. Thus lack of chromatographic decoupling in magma trace element abundances alone does not preclude significant melt-mantle reaction. Mineralogic reactions within the column may also impart distinctive trace element variations to melts. In particular, lherzolite-to-dunite reaction in the matrix produces large variations in heavy rare earth elements in emerging melts, whereas lherzolite-to-pyroxenite reaction produces a series of subparallel rare earth element patterns with decreasing overall abundances with time. Although these models assume end-member conditions and maximum extents of melt-mantle reaction, if reactive melt transport is an important petrogenetic process that strongly influences magma trace element compositions these chemical effects should be observed to some extent in carefully chosen sample suites. Certain magma types show chemical characteristics that are broadly similar to these predicted effects, but more complete sample suites yielding detailed temporal-compositional variations of clearly cogenetic lavas are needed to test the petrogenetic significance of reactive melt transport on erupted magmas.
- Reiners, P. W., & Nelson, B. K. (1998). Temporal-compositional-isotopic trends in rejuvenated-stage magmas of Kauai, Hawaii, and implications for mantle melting processes. Geochimica et Cosmochimica Acta, 62(13), 2347-2368.More infoAbstract: Primitive, low-silica and high-alkali magmas that erupt late in the evolution of most ocean island volcanoes are highly enriched in incompatible trace elements, yet their isotopic compositions require a time-integrated mantle source history of incompatible-element depletion. Reconciling these observations has traditionally required either extremely low degrees of partial melting of depleted mantle (commonly less than 0.2%), invoking an unusual mantle source recently enriched in incompatible elements or extensive melt-mantle interaction. Analyses of stratigraphic sequences of rejuvenated-stage volcanics of Kauai, Hawaii show previously unrecognized isotopic-trace element correlations, as well as temporal variations within monogenetic lava sequences that provide evidence of variable degrees of melting of several distinct mantle sources. Inter-element and isotopic-trace-element correlations indicate little or no chromatographic effects on melt compositions, inconsistent with the expected effects of significant melt-mantle reaction as the source of their incompatible-element enrichment. Trace-element compositions of rejuvenated-stage magmas can be produced by melting of typical depleted mantle sources only if they are mixtures of small- and large-degree (0.1% and 2-15%, depending on source mineralogy) melts of isotopically distinct sources. The simplest model for the Koloa Volcanics, however, consistent with previous interpretations of other Hawaiian lavas, is that they are derived from a range of incompatible-element enriched mantle sources variably metasomatized by small-degree melts of depleted mantle. Isotopic-trace-element trends in the Koloa magmas (of the opposite sense as the overall Hawaiian trend) are best explained by a positive correlation between the extent of source metasomatism and degree of melting to produce the Koloa magmas. Systematic decreases in incompatible element concentrations within individual eruption sequences probably represent sequential eruption of progressively larger-degree melt, possibly caused by vertical zonation in extent of melting in the source regions, or extraction of low-degree melts from surrounding mantle by early-ascending magma batches.
- Holcomb, R. T., Reiners, P. W., Nelson, B. K., & Sawyer, N. E. (1997). Evidence for two shield volcanoes exposed on the island of Kauai, Hawaii. Geology, 25(9), 811-814.More infoAbstract: The island of Kauai has always been interpreted as a single shield volcano, but lavas of previously correlated reversed-to-normal magnetic-polarity transitions on opposite sides of the island differ significantly in isotopic composition. Samples from west Kauai have 87Sr/86Sr < 0.7037, εNd ≥ 6.14, and 206Pb/204Pb > 18.25; samples from east Kauai have 87Sr/86Sr > 0.7037, εNd ≤ 6.14, and 206Pb/204Pb < 18.25. Available data suggest that a younger eastern shield grew on the collapsed flank of an older western one.
- Reiners, P. W., Nelson, B. K., & Nelson, S. W. (1996). Evidence for multiple mechanisms of crustal contamination of magma from compositionally zoned plutons and associated ultramafic intrusions of the Alaska Range. Journal of Petrology, 37(2), 261-292.More infoAbstract: Models of continental crustal magmagenesis commonly invoke the interaction of mafic mantle-derived magma and continental crust to explain geochemical and petrologic characteristics of crustal volcanic and plutonic rocks. This interaction and the specific mechanisms of crustal contamination associated with it are poorly understood. An excellent opportunity to study the progressive effects of crustal contamination is offered by the composite plutons of the Alaska Range, a series of nine early Tertiary, multiply intruded, compositionally zoned (peridotite to granite) plutons. Large initial Sr and Nd isotopic contrasts between the crustal country rock and likely parental magmas allow evaluation of the mechanisms and extents of crustal contamination that accompanied the crystallization of these ultramafic through granitic rocks. Three contamination processes are distinguished in these plutons. The most obvious of these is assimilation of crustal country rock concurrent with magmatic fractional crystallization (AFC), as indicated by a general trend toward crustal-like isotopic signatures with increasing differentiation. Second, many ultramafic and mafic rocks have late-stage phenocryst reaction and orthocumulate textures that suggest interaction with felsic melt. These rocks also have variable and enriched isotopic compositions that suggest that this felsic melt was isotopically enriched and probably derived from crustal country rock. Partial melt from the flysch country rock may have reacted with and contaminated these partly crystalline magmas following the precipitation and accumulation of the cumulus phenocrysts but before complete solidification of the magma. This suggests that in magmatic mush (especially of ultramafic composition) crystallizing in continental crust, a second distinct process of crustal contamination may be super-imposed on AFC or magma mixing involving the main magma body. Finally, nearly all rocks, including mafic and ultramafic rocks, have (87Sr/86Sr)i that are too high, and ε(T)Nd that are too low, to represent the expected isotopic composition of typical depleted mantle. However, gabbro xenoliths with typical depleted-mantle isotopic compositions are found in the plutons. This situation requires either an additional enriched mantle component to provide the parental magma for these plutons, or some mechanism of crustal contamination of the parent magma that did not cause significant crystallization and differentiation of the magma to more felsic compositions. Thermodynamic modeling indicates that assimilation of alkali- and water-rich partial melt of the metapelite country rock by fractionating, near-liquidus basaltic magma could cause significant contamination while suppressing significant crystallization and differentiation.
- Reiners, P. W., Nelson, B. K., & Ghiorso, M. S. (1995). Assimilation of felsic crust by basaltic magma: thermal limits and extents of crustal contamination of mantle-derived magmas. Geology, 23(6), 563-566.More infoAbstract: Using the MELTS software package, isenthalpic (heat-balanced) AFC between two common types of basaltic magma and several types of felsic crust at 1 kbar was modelled. The simulations show that during the early stages of isenthalpic AFC, the ratio of rates of assimilation to crystallization (r) may be substantially greater than one (2.0-2.7), allowing assimilation of a mass of country rock up to 5%-18% of the initial magma mass with only 3%-7% crystallization. The second stage of AFC, beginning with plagioclase and/or pyroxene saturation, is characterized by lower r (0.5-1.0). The initial high-r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses. Under certain conditions, even small degrees of crystallization of olivine alone, coupled with cooling of the magma, can accommodate relatively large amounts of crustal assimilation, and cause large shifts in isotopic and trace element geochemical indicates with little differentiation. -from Authors
- Bice, D. M., Newton, C. R., McCauley, S., Reiners, P. W., & McRoberts, C. A. (1992). Shocked quartz at the Triassic-Jurassic boundary in Italy. Science, 255(5043), 443-446.More infoPMID: 17842896;Abstract: Quartz grains that appear to have been shock-metamorphosed occur within three closely spaced shale beds from the uppermost Triassic ("Rhaetian") Calcare a Rhaetavicula in the Northern Apennines of Italy. The upper shale coincides with the abrupt termination of the distinctive, uppermost Triassic Rhaetavicula fauna and is overlain by the Hettangian (Lower Jurassic) Calcare Massiccio; no extinctions appear to be associated with the two lower layers, which occur 1.2 and 2.4 meters below the boundary shale. Approximately 5 to 10% of the quartz grains within these layers exhibit one or more sets of planar deformational features whose orientations cluster around the rational crystallographic planes (basal, ω, and π) most commonly observed in shocked quartz. Textural and stratigraphic observations support an interpretation of at least three closely spaced impacts at the end of the Triassic.
- Guenthner, W., Orme, D., Reiners, P., & Laskowski, A. (2016). Billion year thermal histories constrained by zircon (U-Th)/He age-eU correlations: Examples from the Laramide and Sevier Provinces of the western US. In EGU General Assembly Conference Abstracts, 18.
- Lefebvre, C., Thomson, S. N., Reiners, P. W., Whitney, D. L., & Teyssier, C. (2015). Thermochronologic evaluation of the Arabia-Anatolia collision: new results from Apatite (U-Th) He and Fission Track. In EGU General Assembly Conference Abstracts, 17.
- Reiners, P. (2015). Formation ages and thermal histories of fracture-filling hematite and Mn-oxide in Precambrian basement from (U-Th)/He dating and 4 He/3 He diffusion experiments. In 2015 AGU Fall Meeting.
- Reiners, P. (2015). Secondary Mineralization in Bedrock: Diagenesis, Cementation, Hydrothermal Precipitation, and the Interpretation of Paleofluid Flow II Posters. In 2015 AGU Fall Meeting.
- Reiners, P. W. (2014, Summer). Keynote Presentation to Session: Response of Diffusion, Kinetics, and Related Phenomena to Lattice Damage in Minerals. Goldschmidt 2014.