Peter G Decelles

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Books

• Decelles, P. G. (2015). Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile. Geological Society of America. doi:10.1130/2015.1212

Chapters

• Carrapa, B., Decelles, P. G., Chapman, J., Kapp, P. A., & He, J. (2018). Structural setting and detrital zircon U-Pb geochronology of Triassic-Cenozoic strata in the eastern Central Pamir, Tajikistan. In Himalayan Tectonics: A Modern Synthesis. London: Geological Society of London Special Publication 483. doi:10.1144/SP483.11
• Kapp, P. A., Leary, R. J., & Decelles, P. G. (2018). Cenozoic basin evolution in the Indus-Yarlung suture zone and High Himalaya. In Tectonics, Sedimentary Basins, and Provenance: A Celebration of William R. Dickinson’s Career(pp 1-34). Boulder: Geological Society of America Special Paper 540. doi:30.1130/2018.2540(30)

Journals/Publications

• Quade, J., Leary, R., & Decelles, P. G. (2017). Evidence from paleosols for low to moderate elevation of the India-Asia suture zone during mid-Cenozoic time.. Geology, 45(5), 399-402.
• Leary, R., Orme, D., Laskowski, A. K., Decelles, P. G., Kapp, P. A., Carrapa, B., & Dettinger, M. (2016). Along-strike diachroneity in the deposition of the Kailas Formation in central southern Tibet: Implications for Indian slab dynamics. Geosphere, 12. doi:10.1130/GEOS01325.1
• Decelles, P. G. (2015). Cyclical Processes in the North American Cordilleran Orogenic System. Geology, 43, 499-502.
• Decelles, P. G. (2015). Tectono-climatic implications of Eocene Paratethys regression in the Tajik basin of central Asia. Earth and Planetary Science Letters, 424, 168-178.
• Carrapa, B., Bywater-Reyes, S., Safipour, R., Sobel, E. R., Schoenbohm, L. M., DeCelles, P. G., Reiners, P., & Stockli, D. (2014). Errata to The effect of inherited paleotopography on exhumation of the Central Andes of NW Argentina [Geological Society of America, 126, 1/2, 66-77] DOI: 10.1130/B30844.1. Bulletin of the Geological Society of America, 126(3-4), 615-.
• Carrapa, B., Mustapha, F. S., Cosca, M., Gehrels, G. E., Schoenbohm, L. M., Sobel, E. R., Decelles, P. G., Russell, J. L., & Goodman, P. J. (2014). Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir. Lithosphere, 6, 443-455.
• Carrapa, B., Reyes-Bywater, S., Safipour, R., Sobel, E. R., Schoenbohm, L. M., DeCelles, P. G., Reiners, P. W., & Stockli, D. (2014). The effect of inherited paleotopography on exhumation of the central andes of NW argentina. Bulletin of the Geological Society of America, 126(1-2), 66-77.
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  Abstract: Differential exhumation in the Puna Plateau and Eastern Cordillera of NW Argentina is controlled by inherited paleostructures and resulting paleotopography related to the Cretaceous Salta Rift paleomargins. The Ceno zoic deformation front related to the development of the Andean retro-arc orogenic system is generally associated with >4 km of exhumation, which is recorded by Cenozoic apatite fission-track (AFT) and (U-Th-[Sm])/He ages (He ages) in the Eastern Cordillera of NW Argentina. New AFT ages from the top of the Nevado de Cachi document Oligocene (ca. 28 Ma) cooling, which, combined with existing data, indicates exhumation of this range between ca. 28 Ma and ca. 14 Ma. However, some of the highest ranges in the Eastern Cordillera preserve Cretaceous ages indicative of limited Cenozoic exhumation. Samples collected from an ~3-km-elevation transect along the northern part of the Sierra de Quilmes paleorift flank (Laguna Brava) show AFT ages between ca. 80 and ca. 50 Ma and He ages between ca. 45 and ca. 10 Ma. Another set of samples from an ~1-km-elevation transect farther to the southwest (La Quebrada) shows Cretaceous AFT ages between ca. 116 Ma and ca. 76 Ma, and mainly Cretaceous He ages, in agreement with AFT data. Analysis of existing AFT and He ages from the area once occupied by the Salta Rift reveals a pattern characterized by Cretaceous ages along paleorift highs and Cenozoic ages within paleorift hanging-wall basins and later foreland basin depocenters. This pattern is interrupted by the Sierras Pampeanas at ~28°S, which record mid-Cenozoic ages. Our data are consistent with a complex inherited pattern of pre-Andean paleostructures, likely associated with paleotopography, which was beveled by the Cenozoic regional foreland basin and reactivated during the late Neogene (ca.
• Decelles, P. G. (2014). Exhumation of the North American Cordillera revealed by multi dating of Upper Jurassic-Upper Cretaceous foreland basin deposits. Geological society of america bulletin.
• Decelles, P. G. (2014). Miocene burial and exhumation of the India-Asia collision zone in southern Tibet: response to slab dynamics and erosion. Geology.
• Leary, R., & Gehrels, G. (2014). Fluvial Deposition During the Transition from Flexural to Dynamic Subsidence in the Cordilleran Foreland Basin: Ericson Formation, Western Wyoming. Basin Research, 26, 1-22.
• Robinson, D. M. (2014). Finding the Lesser Himalayan Duplex in the Himalayan Thrust Belt of Far Western Nepal amidst Forests, Villages, Farming and Leeches. Journal of the Virtual Explorer, 47(paper 2), ISSN 1441-8142.
• barbara, c. (2014). The effect of inherited paleotopography on exhumation of the Central Andes of NW Argentina. Geological Society of America Bulletin, 126, 66-77.
• carrapa, b., F, M., & m, c. (2014). Multidating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir. Lithosphere, 6.
• carrapa, b., robin, c., & Mark, c. (2014). Early Cenozoic uplift of the Puna Plateau, Central Andes, based on stable isotope paleoaltimetry of hydrated volcanic glass. Geology, 42(doi:10.1130/G35239.1), 447-450.
• kapp, p., gehrels, g., & lin, d. (2014). Paleocene-Eocene foreland basin evolution in the Himalaya of southern Tibet and Nepal: Implications for the age of initial India-Asia collision. Tectonics, 33(doi:10.1002/2014TC003522).
• Laskowski, A. K., Decelles, P. G., & Gehrels, G. E. (2013). Detrital zircon geochronology of Cordilleran retroarc foreland basin strata, western North America. Tectonics, 32(5), 1027-1048.
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  Abstract: We present a compilation of 8717 U-Pb analyses from 95 detrital zircon samples of Jurassic-Eocene North American Cordillera foreland basin strata. Of these samples, 30 are new and previously unpublished. Variation in detrital zircon age spectra between samples records erosion or recycling of basement and cover rocks within the Cordilleran orogenic wedge. Each sample can be classified into one of six major provenance groups, whose age spectra suggest derivation from (1) Mesozoic eolianites of the western United States, (2) Paleozoic passive margin strata of the western United States, (3) Paleozoic passive margin strata of western Canada, (4) the Mogollon Highlands, (5) the Cordilleran magmatic arc, or (6) Yavapai-Mazatzal Province crystalline basement rocks. Referencing these provenance interpretations to their location and stratigraphic deposition age produces a detailed spatial and temporal record of sediment dispersal within the foreland basin system. Late Jurassic provenance is dominated by recycling of Mesozoic eolianites from sources in the Sevier thrust belt. Cretaceous-Eocene provenance is dominated by recycling of the passive margin, with increasing complexity upsection. We interpret that this provenance transition records a basin-wide unroofing sequence. A composite age-probability plot of 1539 young (
• Pearson, D. M., Kapp, P., DeCelles, P. G., Reiners, P. W., Gehrels, G. E., Ducea, M. N., & Pullen, A. (2013). Infl uence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude: Northwestern Argentina. Geosphere, 9(6), 1766-1782.
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  Abstract: The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic configuration. These variations have been proposed to explain the considerable southward decrease in the observed magnitude of Cenozoic shortening. Regional mapping, a cross section, and U-Pb and (U-Th)/He age dating of apatite and zircon presented here build upon the preexisting geological framework for the region. At the latitude of the regional transect (24-25°S), results demonstrate that the thrust belt propagated in an overall eastward direction in three distinct pulses during Cenozoic time. Each eastward jump in the deformation front was apparently followed by local westward deformation migration, likely refl ecting a subcritically tapered orogenic wedge. The first eastward jump was at ca. 40 Ma, when deformation and exhumation were restricted to the western margin of the Eastern Cordillera and eastern margin of the Puna Plateau. At 12-10 Ma, the thrust front jumped ~75 km toward the east to bypass the central portion of a horst block of the Cretaceous Salta rift system, followed by initiation of new faults in a subsystem that propagated toward the west into this preexisting structural high. During Pliocene time, deformation again migrated >100 km eastward to a Cretaceous synrift depocenter in the Santa Bárbara Ranges. The sporadic foreland-ward propagation documented here may be common in basement-involved thrust systems where inherited weaknesses due to previous crustal deformation are preferentially reactivated during later shortening. The minimum estimate for the magnitude of shortening at this latitude is ~142 km, which is moderate in magnitude compared to the 250-350 km of shortening accommodated in the retroarc thrust belt of southern Bolivia to the north. This work supports previous hypotheses that the magnitude of shortening decreases significantly along strike away from a maximum in southern Bolivia, largely as a result of the distribution of pre-Cenozoic basins that are able to accommodate a large magnitude of thin-skinned shortening. A major implication is that variations in the pre-orogenic upper-crustal architecture can infl uence the behavior of the continental lithosphere during later orogenesis, a result that challenges geodynamic models that neglect upper-plate heterogeneities. © 2013 Geological Society of America.
• Carrapa, B., Bywater-Reyes, S., Decelles, P. G., Mortimer, E., & Gehrels, G. E. (2012). Late Eocene-Pliocene basin evolution in the Eastern Cordillera of northwestern Argentina (25°-26°S): Regional implications for Andean orogenic wedge development. Basin Research, 24(3), 249-268.
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  Abstract: Important aspects of the Andean foreland basin in Argentina remain poorly constrained, such as the effect of deformation on deposition, in which foreland basin depozones Cenozoic sedimentary units were deposited, how sediment sources and drainages evolved in response to tectonics, and the thickness of sediment accumulation. Zircon U-Pb geochronological data from Eocene-Pliocene sedimentary strata in the Eastern Cordillera of northwestern Argentina (Pucará-Angastaco and La Viña areas) provide an Eocene (ca. 38 Ma) maximum depositional age for the Quebrada de los Colorados Formation. Sedimentological and provenance data reveal a basin history that is best explained within the context of an evolving foreland basin system affected by inherited palaeotopography. The Quebrada de los Colorados Formation represents deposition in the distal to proximal foredeep depozone. Development of an angular unconformity at ca. 14 Ma and the coarse-grained, proximal character of the overlying Angastaco Formation (lower to upper Miocene) suggest deposition in a wedge-top depozone. Axial drainage during deposition of the Palo Pintado Formation (upper Miocene) suggests a fluvial-lacustrine intramontane setting. By ca. 4 Ma, during deposition of the San Felipe Formation, the Angastaco area had become structurally isolated by the uplift of the Sierra de los Colorados Range to the east. Overall, the Eastern Cordillera sedimentary record is consistent with a continuous foreland basin system that migrated through the region from late Eocene through middle Miocene time. By middle Miocene time, the region lay within the topographically complex wedge-top depozone, influenced by thick-skinned deformation and re-activation of Cretaceous rift structures. The association of the Eocene Quebrada del los Colorados Formation with a foredeep depozone implies that more distal foreland deposits should be represented by pre-Eocene strata (Santa Barbara Subgroup) within the region. © 2011 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists.
• Decelles, P. G. (2012). Foreland Basin Systems Revisited: Variations in Response to Tectonic Settings. Tectonics of Sedimentary Basins: Recent Advances, 405-426.
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  Abstract: The four-part districting scheme (wedge-top, foredeep, forebulge, and backbulge depozones) applies to many foreland basin systems worldwide, but significant variations occur in the stratigraphic record. These variations depend on tectonic setting and the nature of the associated fold-thrust belt. Continued growth of the foldthrust belt by horizontal shortening requires foreland lithosphere to migrate toward the fold-thrust belt. The flexural wave set up by the topographic load may migrate ∼1000km sideways through the foreland lithosphere, a distance that is comparable to the flexural wavelength. This extreme lateral mobility results in the vertical stacking of foreland basin depozones in the stratigraphic record. The standard stratigraphic succession consists of a several km-thick upward coarsening sequence, marked in its lower part by a zone of intense stratigraphic condensation or a major disconformity (owing to passage of the forebulge), and in its upper part by coarsegrained proximal facies with growth structures (the wedge-top depozone). Foredeep deposits always reside between the forebulge disconformity/condensation zone and wedge-top deposits, and backbulge deposits may be present in the lowermost part of the succession. Wedge-top deposits are vulnerable to erosion because of their high structural elevation, and preservation of backbulge and forebulge deposits depends in part on tectonic setting. Three main types of fold-thrust belt are recognized: retroarc, collisional (or peripheral), and those associated with retreating collisional subduction zones. Retroarc foreland basin systems (such as the modern Andean) are susceptible to far-field dynamic loading transmitted to the foreland lithosphere by viscous coupling between the subducting oceanic slab and the mantle wedge. This longwavelength subsidence adds to subsidence caused by the topographic flexural wave, allowing for preservation of well-developed forebulge and backbulge depozones. The absence of dynamic subsidence in collisional (peripheral) foreland basin systems (such as the modern Himalayan) renders forebulge and backbulge regions vulnerable to erosion and non-preservation. Retreating collisional foreland basin systems (such as those in the Mediterranean region) are often associated with large subducted slab loads, which produce narrow but very thick accumulations in the foredeep and wedge-top depozones. These foreland basin systems are characterized by very thick foredeep and wedge-top deposits, well beyond what would be expected from topographic loading alone. Changing lithospheric stiffness in collisional settings may affect preservation of the backbulge and forebulge depozones. If these distal foreland basin deposits are not preserved, roughly half the history of the orogenic event (as archived in the stratigraphic record) may be lost. Many foreland stratigraphic successions provide sufficient information to estimate the velocity of migration of the flexural wave through the foreland, which may in turn be decomposed into propagation and shortening velocities in the thrust belt. Foreland basin subsidence curves may be inverted to produce an idealized flexural profile, from which flexural properties of the lithosphere maybe derived. However, spatial changes in flexural rigidity, as well as changes in the size of the orogenic load and rates of propagation and shortening in the thrust belt require that the thrust belt-foreland basin system be palinspastically restored in order to understand the long-term geodynamics. © 2012 Blackwell Publishing Ltd.
• Fuentes, F., DeCelles, P. G., & Constenius, K. N. (2012). Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana, USA. Geosphere, 8(5), 1104-1128.
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  Abstract: The Cordilleran thrust belt of northwestern Montana (United States) has received much less attention than its counterparts in the western interior of USA and Canada. The structure of the thrust belt in this region is well preserved and has not been strongly overprinted by Cenozoic extension, providing an opportunity to reconstruct its geometry and to relate it to the foreland basin system. The thrust belt in this region consists of a frontal part of highly deformed Paleozoic, Mesozoic, and Paleocene sedimentary rocks, and a western region dominated by a >15-km-thick succession of Proterozoic Belt Supergroup strata underlain by faults of the Lewis thrust system. The frontal part can be subdivided into the foothills and the Sawtooth Range. At the surface, the foothills show deformed Mesozoic and Paleocene rocks; at depth, refl ection seismic data indicate numerous thrust faults carrying Paleozoic strata. The Sawtooth Range, south from the Lewis thrust salient, is defi ned by steeply dipping imbricate thrusts that detach at the basal Cambrian stratigraphic level. The Sawtooth Range plunges northward beneath the Lewis thrust salient and diverges into a pair of independent thrust systems that form the Flathead and Waterton duplexes in Canada. The relatively minor internal deformation in the western part of the thrust belt resulted from the great rheological strength of the Belt Supergroup rocks and initial high taper of the preorogenic stratigraphic wedge. A new ~145-km-long balanced cross section indicates ~135 km of shortening, a value similar to that in the southern part of the Canadian thrust belt. Previous work and new conventional and isotopic provenance data from the foreland basin and U-Pb ages from crosscutting intrusive rocks establish a preliminary kinematic model for this segment of the Cordilleran thrust belt. The emerging pattern is a relatively simple forelandward progression of thrusting events. Most shortening in the Lewis thrust system, Sawtooth Range, and foothills occurred roughly between mid-Campanian and Early Eocene time (ca. 75-52 Ma), yielding a shortening rate of ~5.9 mm/yr. This pattern differs from the pattern of shortening in the better known Sevier thrust belt to the south, where regional far-traveled Proterozoic quartzitebearing thrust sheets were mainly active during Early Cretaceous time. From Middle Eocene to Early Miocene time, this sector of the Cordillera collapsed, generating a number of extensional depocenters. © 2012 Geological Society of America.
• J., D., Lippert, P. C., Dupont-Nivet, G., Kapp, P., Decelles, P. G., & Torsvik, T. H. (2012). Reply to comment by Ali and Aitchison on "restoration of Cenozoic deformation in Asia, and the size of Greater India". Tectonics, 31(4).
• Peyton, S. L., Reiners, P. W., Carrapa, B., & Decelles, P. G. (2012). Low-temperature thermochronology of the northern Rocky Mountains, western U.S.A.. American Journal of Science, 312(2), 145-212.
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  Abstract: We dated 86 borehole and surface samples from basement-cored Laramide uplifts of the northern Rocky Mountain foreland (Wind River, Beartooth, Bighorn and Laramie Ranges) using the apatite (U-Th)/He system, and eleven samples using the apatite fission-track system (Wind River and Bighorn Ranges). Apatite (U-Th)/He ages generally decrease with increasing subsurface depth (decreasing elevation), and typically range from ∼100 to 50 Ma (Cretaceous to Eocene) within ∼1 km of the surface, to ∼20 Ma (Miocene) and younger ages at depths greater than ∼2 to 2.5 km. Most samples display (U-Th)/He age dispersion ranging from tens to hundreds of Ma, and for some samples we find ages that are older than corresponding fission-track ages. At least one sample per range shows a correlation between apatite (U-Th)/He age and effective U concentration (eU ∼ [U] 0.235[Th]) of the crystal, indicating that radiation damage has affected He diffusivity, and hence (U-Th)/He age. Forward modeling of simple Laramide-type thermal histories using a radiation damage diffusion model predicts: 1) fossil apatite fission-track partial annealing and apatite (U-Th)/He partial retention zones over similar elevation ranges, 2) (U-Th)/He age dispersion within a fossil partial retention zone up to hundreds of Ma, and 3) (U-Th)/He ages older than fission-track ages within a fossil partial retention zone if eU ∼ 20 ppm. We observe these features in our data from the Bighorn and Laramie Ranges. Most of our samples, however, do not show the correlation between (UTh)/He age and eU predicted by radiation damage diffusion models. The age dispersion of these samples could be due to the influence of both grain size and eU content, or alternatively due to high U or Th secondary rims around the apatite crystals. (U-Th)/He ages that are older than fission-track ages from Gannett Peak and Fremont Peak in the Wind River Range, and some samples from the Beartooth Range, are most likely the result of He implantation from high eU secondary rims. Best-fit time-temperature paths from inverse modeling of (U-Th)/He age-eU pairs, when extrapolated to other elevations to create model age-elevation plots, reproduce the general distribution and dispersion of (U-Th)/He ages from the Bighorn, Beartooth and Wind River Ranges and suggest that rapid exhumation within the Laramide province likely began earlier in the Bighorn Range (before ∼71 Ma) than the Beartooth Range (before ∼58 Ma). Inverse modeling of borehole data at the northern end of the Laramie Range suggests that the well penetrated a fault sliver at depth. The amount and timing of post-Laramide burial and exhumation cannot be determined from these data.
• Decelles, P. G., Carrapa, B., Horton, B. K., & Gehrels, G. E. (2011). Cenozoic foreland basin system in the central Andes of northwestern Argentina: Implications for Andean geodynamics and modes of deformation. Tectonics, 30(6).
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  Abstract: Cenozoic strata in the central Andes of northwestern Argentina record the development and migration of a regional foreland basin system analogous to the modern Chaco-Paran alluvial plain. Paleocene-lower Eocene fluvial and lacustrine deposits are overlain by middle-upper Eocene hypermature paleosols or an erosional disconformity representing 10-15 Myr. This supersol/disconformity zone is traceable over a 200,000 km 2 area in the Andean thrust belt, and is overlain by 2-6 km of upward coarsening, eastward thinning, upper Eocene through lower Miocene fluvial and eolian deposits. Middle Miocene-Pliocene fluvial, lacustrine, and alluvial fan deposits occupy local depocenters with contractional growth structures. Paleocurrent and petrographic data demonstrate westerly provenance of quartzolithic and feldspatholithic sediments. Detrital zircon ages from Cenozoic sandstones cluster at 470-491, 522-544, 555-994, and 1024-1096 Ma. Proterozoic-Mesozoic clastic and igneous rocks in the Puna and Cordillera Oriental yield similar age clusters, and served as sources of the zircons in the Cenozoic deposits. Arc-derived zircons become prominent in Oligo-Miocene deposits and provide new chronostratigraphic constraints. Sediment accumulation rate increased from ∼20 m/Myr during Paleocene-Eocene time to 200-600 m/Myr during the middle to late Miocene. The new data suggest that a flexural foreland basin formed during Paleocene time and migrated at least 600 km eastward at an unsteady pace dictated by periods of abrupt eastward propagation of the orogenic strain front. Despite differences in deformation style between Bolivia and northwestern Argentina, lithosphere in these two regions flexed similarly in response to eastward encroachment of a comparable orogenic load beginning during late Paleocene time. © 2011 by the American Geophysical Union.
• Fuentes, F., DeCelles, P. G., Constenius, K. N., & Gehrels, G. E. (2011). Evolution of the cordilleran foreland basin system in northwestern montana, U.S.A.. Bulletin of the Geological Society of America, 123(3-4), 507-533.
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  Abstract: New lithostratigraphic and chronostratigraphic, geochronologic, and sedimentary petrologic data illuminate the history of development of the North American Cordilleran foreland basin system and adjacent thrust belt from Middle Jurassic through Eocene time in northwestern Montana. The oldest deposits in the foreland basin system consist of relatively thin, regionally tabular deposits of the marine Ellis Group and fluvial-estuarine Morrison Formation, which accumulated during Bajocian to Kimmeridgian time. U-Pb ages of detrital zircons and sandstone modal petrographic data indicate that by ca. 170 Ma, miogeoclinal strata were being deformed and eroded in hinterland regions. Sandstones of the Swift and Morrison Formations contain detrital zircons derived from the Intermontane belt. The Jurassic deposits probably accumulated in the distal, back-bulge depozone of an early foreland basin, as suggested by the slow rates of tectonic subsidence and tabular geometry. A regional unconformity separates the Jurassic strata from late Barremian(?) foredeep deposits. This unconformity possibly resulted as a combined effect of forebulge migration, decreased dynamic subsidence, and eustatic sea-level fall. The late Barremian(?)-early Albian Kootenai Formation is the first unit that consistently thickens westward, as would be expected in a foredeep depozone. The subsidence curve at this time begins to show the convex-upward pattern characteristic of foredeeps. By Albian time, the fold-and-thrust belt had propagated to the east and incorporated Proterozoic rocks of the Belt Supergroup, as indicated by sandstone compositions, detrital zircon ages in the Blackleaf Formation, and by crosscutting relationships in thrust sheets involving Belt Supergroup rocks in the thrust belt. A major episode of marine inundation and black shale deposition (Marias River Shale) occurred between the Cenomanian and mid-Santonian, and was followed by a regressive succession represented by the Upper Santonian-mid-Campanian Telegraph Creek, Virgelle, and Two Medicine Formations. Provenance data do not resolve the timing of individual thrust displacements during Cenomanian-early Campanian time. The Upper Campanian Bearpaw Formation represents the last major marine inundation in the foreland basin. By latest Campanian time, a major episode of slip on the Lewis thrust system had commenced, as recorded in the foreland by the Willow Creek and St. Mary River Formations in the proximal foredeep depozone. The final stage in the evolution of the Cordilleran fold-and-thrust belt and foreland basin system is recorded by the Paleocene-early Eocene Fort Union and Wasatch Formations, which were preserved in the distal foreland region. Regional extensional faulting along the fold-and-thrust belt began during the middle Eocene. The results presented here enable the establishment of links between previous geological work in Canada and the better known parts of the Cordilleran foreland basin in the United States. © 2011 Geological Society of America.
• Gehrels, G., Kapp, P., Decelles, P., Pullen, A., Blakey, R., Weislogel, A., Ding, L., Guynn, J., Martin, A., McQuarrie, N., & Yin, A. (2011). Detrital zircon geochronology of pre-Tertiary strata in the Tibetan-Himalayan orogen. Tectonics, 30(5).
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  Abstract: Detrital zircon data have recently become available from many different portions of the Tibetan-Himalayan orogen. This study uses 13,441 new or existing U-Pb ages of zircon crystals from strata in the Lesser Himalayan, Greater Himalayan, and Tethyan sequences in the Himalaya, the Lhasa, Qiangtang, and Nan Shan-Qilian Shan-Altun Shan terranes in Tibet, and platformal strata of the Tarim craton to constrain changes in provenance through time. These constraints provide information about the paleogeographic and tectonic evolution of the Tibet-Himalaya region during Neoproterozoic to Mesozoic time. First-order conclusions are as follows: (1) Most ages from these crustal fragments are
• J., D., Kapp, P., Dupont-Nivet, G., Lippert, P. C., Decelles, P. G., & Torsvik, T. H. (2011). Restoration of Cenozoic deformation in Asia and the size of Greater India. Tectonics, 30(5).
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  Abstract: A long-standing problem in the geological evolution of the India-Asia collision zone is how and where convergence between India and Asia was accommodated since collision. Proposed collision ages vary from 65 to 35 Ma, although most data sets are consistent with collision being underway by 50 Ma. Plate reconstructions show that since 50 Ma ∼2400-3200 km (west to east) of India-Asia convergence occurred, much more than the 450-900 km of documented Himalayan shortening. Current models therefore suggest that most post-50 Ma convergence was accommodated north of the Indus-Yarlung suture zone. We review kinematic data and construct an updated restoration of Cenozoic Asian deformation to test this assumption. We show that geologic studies have documented 600-750 km of N-S Cenozoic shortening across, and north of, the Tibetan Plateau. The Pamir-Hindu Kush region accommodated ∼1050 km of N-S convergence. Geological evidence from Tibet is inconsistent with models that propose 750-1250 km of eastward extrusion of Indochina. Approximately 250 km of Indochinese extrusion from 30 to 20 Ma of Indochina suggested by SE Asia reconstructions can be reconciled by dextral transpression in eastern Tibet. We use our reconstruction to calculate the required size of Greater India as a function of collision age. Even with a 35 Ma collision age, the size of Greater India is 2-3 times larger than Himalayan shortening. For a 50 Ma collision, the size of Greater India from west to east is ∼1350-2600 km, consistent with robust paleomagnetic data from upper Cretaceous-Paleocene Tethyan Himalayan strata. These estimates for the size of Greater India far exceed documented shortening in the Himalaya. We conclude that most of Greater India was consumed by subduction or underthrusting, without leaving a geological record that has been recognized at the surface. Copyright © 2011 by the American Geophysical Union.
• Pullen, A., Kapp, P., DeCelles, P. G., Gehrels, G. E., & Ding, L. (2011). Cenozoic anatexis and exhumation of Tethyan Sequence rocks in the Xiao Gurla Range, Southwest Tibet. Tectonophysics, 501(1-4), 28-40.
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  Abstract: In order to advance our understanding of the suturing process between continental landmasses, a geologic and geochronologic investigation was undertaken just south of the India-Asia suture in southwestern Tibet. The focus of this study, the Xiao Gurla Range, is located near the southeastern terminus of the active, right-lateral strike-slip Karakoram fault in southwestern Tibet. The range exposes metasandstone, phyllite, schist (locally of sillimanite facies), calc-gneiss and marble, paragneiss (± pyroxene), quartzite, metagranite, and variably deformed leucogranite. These metamorphic rocks are exposed in the footwall of a domal, top-to-the-west low-angle normal (detachment) fault, structurally beneath Neogene-Quaternary basin fill and serpentinized ultramafic rocks of the Kiogar-Jungbwa ophiolite. The detachment is interpreted to be the northeastern continuation of the Gurla Mandhata detachment fault system that bounds metamorphic rocks of the Gurla Mandhata Range ~. 60. km to the southwest. U-Pb geochronology on five detrital zircon samples of schist, phyllite, and quartzite yielded maximum depositional ages that range from 644-363. Ma and age probability distributions that are more similar to Tethyan sequence rocks than Lesser Himalayan sequence rocks. A felsic gneiss yielded a metamorphic zircon age of 35.3 ± 0.8. Ma with a significant population of early Paleozoic xenocrystic core ages. The gneiss is interpreted to be the metamorphosed equivalent of the Cambro-Ordovician gneiss that is exposed near the top of the Greater Himalayan sequence. Leucogranitic bodies intruding metasedimentary footwall rocks yielded two distinct U-Pb zircon ages of ~. 23. Ma and ~. 15. Ma. Locally, rocks exposed in the hanging wall of this fault and of the southward-dipping, northward-verging Great Counter thrust to the north consist of serpentinite-bearing mélange and conglomerate of inferred Paleogene age dominated by carbonate clasts. The mélange is intruded by a 44. Ma granite and the stratigraphically highest conglomerate unit yielded detrital zircon U-Pb ages similar to Tethyan sequence rocks. We attribute the middle Eocene magmatism south of the suture to break-off of the Neo-Tethyan oceanic slab. In addition, our observations are consistent with the late Eocene shortening and crustal thickening within the Tethyan Himalayan sequence, early-middle Miocene leucogranite emplacement being related to underthrusting and melting of the Greater and possibly Lesser Himalayan sequences, and late Miocene arc-parallel extension in the hinterland of the southward propagating Himalayan thrust belt. © 2011 Elsevier B.V.
• Martin, A. J., Ganguly, J., & DeCelles, P. G. (2010). Metamorphism of greater and lesser himalayan rocks exposed in the Modi Khola valley, central Nepal. Contributions to Mineralogy and Petrology, 159(2), 203-223.
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  Abstract: Thermobarometric estimates for Lesser and Greater Himalayan rocks combined with detailed structural mapping in the Modi Khola valley of central Nepal reveal that large displacement thrust-sense and normal-sense faults and ductile shear zones mostly control the spatial pattern of exposed metamorphic rocks. Individual shear zone- or fault-bounded domains contain rocks that record approximately the same peak metamorphic conditions and structurally higher thrust sheets carry higher grade rocks. This spatial pattern results from the kinematics of thrust-sense faults and shear zones, which usually place deeper, higher grade rocks on shallower, lower grade rocks. Lesser Himalayan rocks in the hanging wall of the Ramgarh thrust equilibrated at about 9 kbar and 580°C. There is a large increase in recorded pressures and temperatures across the Main Central thrust. Data presented here suggest the presence of a previously unrecognized normal fault entirely within Greater Himalayan strata, juxtaposing hanging wall rocks that equilibrated at about 11 kbar and 720°C against footwall rocks that equilibrated at about 15 kbar and 720°C. Normal faults occur at the structural top and within the Greater Himalayan series, as well as in Lesser Himalayan strata 175 and 1,900 m structurally below the base of the Greater Himalayan series. The major mineral assemblages in the samples collected from the Modi Khola valley record only one episode of metamorphism to the garnet zone or higher grades, although previously reported ca. 500 Ma concordant monazite inclusions in some Greater Himalayan garnets indicate pre-Cenozoic metamorphism. © Springer-Verlag 2009.
• Pelletier, J. D., DeCelles, P. G., & Zandt, G. (2010). Relationships among climate, erosion, topography, and delamination in the Andes: A numerical modeling investigation. Geology, 38(3), 259-262.
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  Abstract: Cordilleran orogenic systems such as the Andes are controlled by shortening rates, climatically-controlled erosion rates, and, in some cases, eclogite production and delamination. All of these processes are coupled, however, making it difficult to uniquely determine the relative importance of each process and the feedbacks among them. In this paper we develop a massbalanced numerical model that couples an actively-shortening orogen and crustal root with eclogite production, delamination, and climatically controlled erosion. The model provides a first-order quantification of the sources (shortening) and sinks (erosion and eclogite production and delamination) of crustal volume during the Cenozoic in the Andes as a function of latitude and time. Given reasonable estimates for the rates of eclogite production and the threshold size of the eclogitic root required for delamination, the model suggests that, in the central Andes between 5° S and 32° S, the orogen has grown to a sufficient height to produce and maintain eclogite, which in turn has promoted delamination in the lower crust and mantle. In this region, climatically controlled erosion rates influence the size of the orogen through two separate mechanisms: by exporting mass via surface processes and by controlling the lithostatic pressure in the lower crust, which modulates the rates of eclogite production and/or delamination. To the north and south of the central Andes, relatively low shortening rates and high precipitation and erosion rates have slowed eclogite production such that delamination likely has not occurred during the Cenozoic. © 2010 Geological Society of America.
• Saylor, J., DeCelles, P., Gehrels, G., Murphy, M., Zhang, R., & Kapp, P. (2010). Basin formation in the High Himalaya by arc-parallel extension and tectonic damming: Zhada basin, southwestern Tibet. Tectonics, 29(1).
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  Abstract: The late Miocene-Pleistocene Zhada basin in southwestern Tibet provides a record of subsidence and basin formation within an active collisional thrust belt. The >800 m thick basin fill is undeformed and was deposited along an angular unconformity on top of Tethyan strata that were previously shortened in the Himalayan fold-thrust belt. Modal sandstone petrographic data, conglomerate clast count data, and detrital zircon U-Pb age spectra indicate a transition from detritus dominated by a distal, northern source to a local, southern source. This transition was accompanied by a change in paleocurrent directions from uniformly northwestward to basin-centric. At the same time the depositional environment in the Zhada basin changed from a large, braided river to a closed-basin lake. Sedimentation in the Zhada basin was synchronous with displacement on the Qusum and Gurla Mandhata detachment faults, which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin, respectively. These observations indicate that accommodation for Zhada basin fill was produced by a combination of tectonic subsidence and damming, as midcrustal rocks were evacuated from beneath the Zhada basin in response to arc-parallel slip on crustal-scale detachment faults. Copyright 2010 by the American Geophysical Union.
• Carrapa, B., DeCelles, P. G., Reiners, P. W., Gehrels, G. E., & Sudo, M. (2009). Apatite triple dating and white mica⁴⁰Ar/³⁹Ar thermochronology of syntectonic detritus in the Central Andes: A multiphase tectonothermal history. Geology, 37(5), 407-410.
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  Abstract: We applied apatite U-Pb, fission track, and (U-Th)/He triple dating and white mica 40Ar/39Ar thermochronology to syntectonic sedimentary rocks from the central Andean Puna plateau in order to determine the source-area geochronology and source sedimentary basin thermal histories, and ultimately the timing of multiple tectonothermal events in the Central Andes. Apatite triple dating of samples from the Eocene Geste Formation in the Salar de Pastos Grandes basin shows late Precambrian-Devonian apatite U-Pb crystallization ages, Eocene apatite fission track (AFT), and Eocene-Miocene (U-Th)/He (ca. 8-47 Ma) cooling ages. Double dating of cobbles from equivalent strata in the Arizaro basin documents early Eocene (46.2 ± 3.9 Ma) and Cretaceous (107.6 ± 7.6, 109.5 ± 7.7 Ma) AFT and Eocene-Oligocene (ca. 55-30 Ma) (U-Th)/He ages. Thermal modeling suggests relatively rapid cooling between ca. 80 and 50 Ma and reheating and subsequent diachronous basin exhumation between ca. 30 Ma and 5 Ma. The 40Ar/39Ar white mica ages from the same samples in the Salar de Pastos Grandes area are mainly 400-350 Ma, younger than apatite U-Pb ages, suggesting source-terrane cooling and exhumation during the Devonian-early Carboniferous. Together these data reveal multiple phases of mountain building in the Paleozoic and Cenozoic. Basin burial temperatures within the plateau were limited to
• Decelles, P. G., Ducea, M. N., Kapp, P., & Zandt, G. (2009). Cyclicity in Cordilleran orogenic systems. Nature Geoscience, 2(4), 251-257.
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  Abstract: Cordilleran orogenic systems, such as the modern Andes, are long belts of deformation and magmatism that are associated with the subduction of oceanic plates beneath continental ones. Although the oceanic plates have been thought to control the evolution of such systems, a number of processes operating in the upper continental plates have not been fully accounted for. The western American Cordilleras, for example, display a 25-50 million year (Myr) cycle of linked upper-plate processes. In a typical cycle, as the two plates converge and a magmatic arc forms, most of the continental crust shortens by thrusting behind the arc, whereas the lowermost continental lithosphere is shoved beneath the arc a process that fuels episodic high-flux magmatism in the arc and simultaneously generates dense melt residues. On reaching a critical mass, these residues sink into the mantle, creating space beneath the arc and setting the stage for renewal of the cycle. This alternative model explains key features of Cordilleran systems, such as cyclical trends in the flux and composition of magma supplied to the upper plate, and the foundering of arc roots. © 2009 Macmillan Publishers Limited.
• Fuentes, F., DeCelles, P. G., & Gehrels, G. E. (2009). Jurassic onset of foreland basin deposition in northwestern Montana, USA: Implications for along-strike synchroneity of Cordilleran orogenic activity. Geology, 37(4), 379-382.
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  Abstract: Stratigraphic, provenance, and subsidence analyses suggest that by the Middle to Late Jurassic a foreland basin system was active in northwestern Montana (United States). U-Pb ages of detrital zircons and detrital modes of sandstones indicate provenance from accreted terranes and deformed miogeoclinal rocks to the west. Subsidence commenced ca. 170 Ma and followed a sigmoidal pattern characteristic of foreland basin systems. Thin Jurassic deposits of the Ellis Group and Morrison Formation accumulated in a backbulge depozone. A regional unconformity and/or paleosol zone separates the Morrison from Early Cretaceous foredeep deposits of the Kootenai Formation. The model presented here is consistent with regional deformation events registered in hinterland regions, and challenges previous interpretations of a strongly diachronous onset of Cordilleran foreland basin deposition from northwestern Montana to southern Canada. © 2009 The Geological Society of America.
• Carrapa, B., & DeCelles, P. G. (2008). Eocene exhumation and basin development in the Puna of northwestern Argentina. Tectonics, 27(1).
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  Abstract: The Puna is part of the larger Puna-Altiplano Plateau (also known as the Central Andean Plateau), characterized by high elevation, low relief, and aridity, located in the central Andes of Bolivia and Argentina. Tertiary sedimentary rocks preserved within the Puna contain a unique archive of information regarding the paleogeography, depositional environments, and timing of sediment source exhumation during the early stages of Andean mountain building. The Eocene Geste Formation in the Salar de Pastos Grandes area (within the central Puna of northwestern Argentina) consists of deposits that are the result of confined to unconfined flows in a sandy to gravelly, braided fluvial system and alluvial fans proximal to the source terrane. Paleocurrent data document an overall eastward flow direction. Up-section coarsening of the Geste Formation suggests that topographic relief in the source area increased through time, possibly owing to enhanced tectonic activity and source terrane unroofing. Sandstone petrography and conglomerate clast-count data document quartzose and phyllitic compositions typical of Ordovician rocks preserved just west of the Salar de Pastos Grandes area. Paleocene-Eocene detrital apatite fission track age populations (P1: ∼35-52 Ma; P2: ∼52-65 Ma) of the Geste Formation and their consistent trends up-section suggest moderate to rapid (∼0.4 mm/a to >1 mm/a) exhumation of western sediment sources during the early to mid-Tertiary stages of Andean mountain building. Sedimentation rates increase up-section from ∼0.1 mm/a to 1 mm/a. Our data, when combined with other structural, stratigraphic and seismic evidence from surrounding regions, suggest that the Geste Formation was deposited in response to crustal shortening and resulting erosion and sedimentation, which started as early as Cretaceous in the Chilean Cordillera de Domeyko and in the Salar de Pastos Grandes area by Eocene time. The Geste Formation could be interpreted either as a local wedge-top accumulation on the eastward propagating central Andean orogenic wedge, or as a local intermontane basin. The similarities between wedgetop deposits preserved in Bolivia and Eocene deposits in northwestern Argentina, south of ∼25°S, lead us to favor the wedge-top scenario for the Geste Formation. If correct, this implies that the deformation front of the Andean orogenic wedge incorporated both thin- and thick-skinned structures as it migrated, possibly unsteadily, from the Cordillera de Domeyko during the Cretaceous-Paleocene to areas within the Puna and Eastern Cordillera by mid-late Eocene time. Contemporaneously, a regional-scale foreland basin system developed over an along-strike distance of at least 650 km. Copyright 2008 by the American Geophysical Union.
• Pullen, A., Kapp, P., Gehrels, G. E., DeCelles, P. G., Brown, E. H., Fabijanic, J. M., & Ding, L. (2008). Gangdese retroarc thrust belt and foreland basin deposits in the Damxung area, southern Tibet. Journal of Asian Earth Sciences, 33(5-6), 323-336.
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  Abstract: Geologic mapping and U-Pb detrital zircon geochronologic studies of (meta)sedimentary rocks in the Damxung area (∼90 km north of Lhasa) of the southern Lhasa terrane in Tibet provide new insights into the history of deformation and clastic sedimentation prior to late Cenozoic extension. Cretaceous nonmarine clastic rocks ∼10 km southeast of Damxung are exposed as structural windows in the footwall of a thrust fault (the Damxung thrust) that carries Paleozoic strata in the hanging wall. To the north of Damxung in the southern part of the northern Nyainqentanglha Range (NNQTL), metaclastic rocks of previously inferred Paleozoic age are shown to range in depositional age from Late Cretaceous to Eocene. The metaclastic rocks regionally dip southward and are interpreted to have been structurally buried in the footwall of the Damxung thrust prior to being tectonized during late Cenozoic transtension. Along the northern flank of the NNQTL, Lower Eocene syncontractional redbeds were deposited in a triangle zone structural setting. All detrital zircon samples of Cretaceous-Eocene strata in the Damxung area include Early Cretaceous grains that were likely sourced from the Gangdese arc to the south. We suggest that the that newly recognized Late Cretaceous to Early Eocene (meta)clastic deposits and thrust faults represent the frontal and youngest part of a northward directed and propagating Gangdese retroarc thrust belt and foreland basin system that led to significant crustal thickening and elevation gain in southern Tibet prior to India-Asian collision. © 2008 Elsevier Ltd. All rights reserved.
• Cavazza, W., Decelles, P. G., Fellin, M. G., & Paganelli, L. (2007). The miocene saint-florent basin in northern Corsica: Stratigraphy, sedimentology, and tectonic implications. Basin Research, 19(4), 507-527.
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  Abstract: Late early-early middle Miocene (Burdigalian-Langhian) time on the island of Corsica (western Mediterranean) was characterized by a combination of (i) postcollisional structural inversion of the main boundary thrust system between the Alpine orogenic wedge and the foreland, (ii) eustatic sealevel rise and (iii) subsidence related to the development of the Ligurian-Provençal basin. These processes created the accommodation for a distinctive continental to shallow-marine sedimentary succession along narrow and elongated basins. Much of these deposits have been eroded and presently only a few scattered outcrop areas remain, most notably at Saint-Florent and Francardo. The Burdigalian-Langhian sedimentary succession at Saint-Florent is composed of three distinguishing detrital components: (i) siliciclastic detritus derived from erosion of the nearby Alpine orogenic wedge, (ii) carbonate intrabasinal detritus (bioclasts of shallow-marine and pelagic organisms), and (iii) siliciclastic detritus derived from Hercynian-age foreland terraines. The basal deposits (Fium Albino Formation) are fluvial and composed of Alpine-derived detritus, with subordinate foreland-derived volcanic detritus. All three detrital components are present in the middle portion of the succession (Torra and Monte Sant'Angelo Formations), which is characterized by thin transitional deposits evolving vertically into fully marine deposits, although the carbonate intrabasinal component is predominant. The Monte Sant'Angelo Formation is characteristically dominated by the deposits of large gravel and sandwaves, possibly the result of current amplification in narrow seaways that developed between the foreland and the tectonically collapsing Alpine orogenic wedge. The laterally equivalent Saint-Florent conglomerate is composed of clasts derived from the late Permian Cinto volcanic district within the foreland. The uppermost unit (Farinole Formation) is dominated by bioclasts of pelagic organisms. The Saint-Florent succession was deposited during the last phase of the counterclockwise rotation of the Corsica-Sardinia-Calabria continental block and the resulting development of the Provençal oceanic basin. The succession sits at the paleogeographic boundary between the Alpine orogenic wedge (to the east), its foreland (to the west), and the Ligurian-Provençal basin (to the northwest). Abrupt compositional changes in the succession resulted from the complex, varying interplay of post-collisional extensional tectonism, eustacy and competing drainage systems. © 2007 Blackwell Publishing Ltd.
• Coogan, J. C., & DeCelles, P. G. (2007). Regional structure and kinematic history of the Sevier fold-and-thrust belt, central Utah: Reply. Bulletin of the Geological Society of America, 119(3-4), 508-512.
• DeCelles, P. G., Carrapa, B., & Gehrels, G. E. (2007). Detrital zircon U-Pb ages provide provenance and chronostratigraphic information from Eocene synorogenic deposits in northwestern Argentina. Geology, 35(4), 323-326.
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  Abstract: Paleogene elastic sedimentary rocks in the Puna plateau of northwestern Argentina contain valuable information about the timing and location of early mountain building in the central Andes. Because these rocks generally lack tuffaceous facies, only paleontological ages have been available. We present U-Pb ages from detrital zircons in the conglomeratic Eocene Geste Formation of the central Puna plateau. The zircon ages indicate that the Geste Formation was derived from nearby high-relief ranges composed of Ordovician metasedimentary rocks. A small population of ca. 37-35 Ma grains also confirms the late Eocene stratigraphic age of the Geste Formation, and suggests that U-Pb detrital zircon ages may provide a new tool for determining depositional ages and provenance of widespread Paleogene deposits in the central Andes. © 2007 The Geological Society of America.
• DeCelles, P. G., Kapp, P., Ding, L., & Gehrels, G. E. (2007). Late Cretaceous to middle Tertiary basin evolution in the central Tibetan Plateau: Changing environments in response to tectonic partitioning, aridification, and regional elevation gain. Bulletin of the Geological Society of America, 119(5-6), 654-680.
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  Abstract: Located in the Bangong suture zone between the Lhasa and Qiangtang terranes of central Tibet, the Nima basin records Cretaceous-late Miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation. The Bangong suture zone originally formed during Late Jurassic-Early Cretaceous time as the Lhasa terrane collided with the Qiangtang terrane. During Early to middle Cretaceous time, the region lay at northern near-equatorial paleolatitudes, near sea level. By Aptian time the Nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated Bangong suture zone. In the southern Nima basin, an ∼50 m.y. (Late Cretaceous-Eocene) depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the Lhasa terrane. In the northern Nima basin, deposition continued during latest Cretaceous time, recording arid paleoclimate in evaporitic lacustrine and eolian dune-field deposits. By Oligocene time the Nima basin comprised two independent depocenters that accumulated coarse-grained alluvial, fluvial, lacustrine (evaporitic), and fan-delta deposits in close association with reactivated thrusts in the Bangong suture zone. Carbon and oxygen isotope data from Oligocene paleosol carbonate, reported elsewhere, indicate that regional paleoelevation during the late Oligocene was >4.6 km, as it is today. Overall, the depositional record of the Nima basin, combined with ongoing structural and geochronologic studies, demonstrates that the Bangong suture zone was reactivated during middle Cretaceous and middle Tertiary time, that the intervening ∼50 m.y. interval was a time of regional upper crustal shortening in the Lhasa terrane followed by regional ignimbrite eruptions, and that arid paleoclimate and high paleoelevation were established by the Late Cretaceous and late Oligocene, respectively. Within the context of other data sets from the Lhasa terrane, the record of deposition in the Nima basin is consistent with low-angle subduction of Neotethyan oceanic lithosphere and reactivation of the Bangong suture zone during the Early Cretaceous, followed by shortening within the Gangdese retroarc and northern Lhasa terrane thrust belts during the Late Cretaceous, lithospheric delamination-dripping and regional magmatic flare-up during latest Cretaceous through early Tertiary time, and underthrusting of Indian lower crust and lithosphere as far north as the Bangong suture zone during late Oligocene time. © 2007 Geological Society of America.
• Garzanti, E., Vezzoli, G., Andò, S., Lavé, J., Attal, M., France-Lanord, C., & DeCelles, P. (2007). Quantifying sand provenance and erosion (Marsyandi River, Nepal Himalaya). Earth and Planetary Science Letters, 258(3-4), 500-515.
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  Abstract: We use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the Marsyandi basin of the central Himalaya, a privileged natural laboratory in which a series of multidisciplinary geomorphological, sedimentological, geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic, climatic and sedimentary processes in high-relief orogenic belts. Although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the Marsyandi River, uncertainties are caused by potential compositional variation between the monsoon and post-monsoon season. Estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin. Our results support focused erosion of the southern, tectonically-lower part of the Greater Himalaya in the hangingwall of the MCT Zone, where the summer monsoon reaches its peak intensity (up to 5 m/a), and sediment yields and erosion rates reach 14,100 ± 3400 t/km2 and 5.1 ± 1.2 mm/a. Erosion rates sharply decrease southward in low-relief Lesser Himalayan units (1.6 ± 0.6 mm/a), and also progressively decrease northwards in the high-altitude, tectonically-upper part of the Greater Himalaya, where rainfall decreases rapidly to < 2 m/a. Even areas of extreme topography such as the Manaslu Granite are characterized by relatively low erosion rates (2.4 ±0.9 mm/a), because precipitations become too scarce to feed significant ice flux and glacial activity. Monsoonal rainfall decreases further to < 0.5 m/a in the Tethys Himalayan zone farther north, where erosion rates are ∼ 1 mm/a. Coupling between erosion and peak monsoonal rainfall along the southern front of the Greater Himalaya is consistent with both channel-flow models of tectonic extrusion and tectonic uplift above a mid-crustal ramp. Altitude and relief are not the principal factors controlling erosion, and the central Nepal eight-thousanders may be viewed as topographic anomalies in cold desert climate at the southern edge of the Tibetan rain shadow. © 2007 Elsevier B.V. All rights reserved.
• Kapp, P., DeCelles, P. G., Gehrels, G. E., Heizler, M., & Ding, L. (2007). Geological records of the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of central Tibet. Bulletin of the Geological Society of America, 119(7-8), 917-932.
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  Abstract: A geological and geochronologic investigation of the Nima area along the Jurassic-Early Cretaceous Bangong suture of central Tibet (∼32°N, ∼87°E) provides well-dated records of contractional deformation and sedimentation during mid-Cretaceous and mid-Tertiary time. Jurassic to Lower Cretaceous (≤125 Ma) marine sedimentary rocks were transposed, intruded by granitoids, and uplifted above sea level by ca. 118 Ma, the age of the oldest nonmarine strata documented. Younger nonmarine Cretaceous rocks include ca. 110-106 Ma volcanic-bearing strata and Cenomanian red beds and conglomerates. The Jurassic-Cretaceous rocks are unconformably overlain by up to 4000 m of Upper Oligocene to Lower Miocene lacustrine, nearshore lacustrine, and fluvial red-bed deposits. Paleocurrent directions, growth stratal relationships, and a structural restoration of the basin show that Cretaceous-Tertiary nonmarine deposition was coeval with mainly S-directed thrusting in the northern part of the Nima area and N-directed thrusting along the southern margin of the basin. The structural restoration suggests >58 km (>47%) of N-S shortening following Early Cretaceous ocean closure and ∼25 km shortening (∼28%) of Nima basin strata since 26 Ma. Cretaceous magmatism and syncontractional basin development are attributed to northward low-angle subduction of the Neotethyan oceanic lithosphere and Lhasa-Qiangtang continental collision, respectively. Tertiary syncontractional basin development in the Nima area was coeval with that along the Bangong suture in westernmost Tibet and the Indus-Yarlung suture in southern Tibet, suggesting simultaneous, renewed contraction along these sutures during the Oligocene-Miocene. This suture-zone reactivation immediately predated major displacement within the Himalayan Main Central thrust system shear zone, raising the possibility that Tertiary shortening in Tibet and the Himalayas may be interpretable in the context of a mechanically linked, composite orogenic system. © 2007 Geological Society of America.
• Kapp, P., DeCelles, P. G., Leier, A. L., Fabijanic, J. M., He, S., Pullen, A., Gehrels, G. E., & Ding, L. (2007). The Gangdese retroarc thrust belt revealed. GSA Today, 17(7), 4-9.
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  Abstract: The Cretaceous-early Tertiary Gangdese arc in southern Tibet is generally attributed to the northward subduction of Neotethyan oceanic lithosphere prior to Indo-Asian collision. However, the history and tectonic significance of deformation and sedimentation in Tibet during this time interval have remained enigmatic. We show that contractional structures and clastic rocks near the city of Lhasa can be attributed to the development of a northward-propagating retroarc thrust belt that was active between 105 and 53 Ma. A kinematic model shows that the thrust belt could have accommodated >230 km (>55%) of N-S shortening. An episode of large magnitude (>160 km) and rapid (>8 mm/yr) shortening predated the onset of a magmatic flare-up ca. 69 Ma, which is linked to removal of overthickened mantle lithosphere. This tectonic history implies that southern Tibet underwent substantial crustal thickening and elevation gain prior to the Indo-Asian collision.
• Leier, A. L., DeCelles, P. G., Kapp, P., & Ding, L. (2007). The Takena Formation of the Lhasa terrane, southern Tibet: The record of a Late Cretaceous retroarc foreland basin. Bulletin of the Geological Society of America, 119(1-2), 31-48.
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  Abstract: Our understanding of the processes involved in the Indo-Asian collision and the construction of the Tibetan Plateau are, in part, predicated on our understanding of the tectonic setting and crustal conditions of southern Tibet in the time period immediately preceding the Indo-Asian collision (Late Cretaceous). Several hypotheses have been proposed that describe the middle to Late Cretaceous tectonic and paleogeographic evolution of southern Tibet, each with different implications for when and how the Tibetan Plateau was uplifted. We examined the mid-upper Cretaceous Takena Formation of the Lhasa terrane in southern Tibet in order to reconstruct the middle to Late Cretaceous tectono-sedimentary history of this area and test the competing tectonic hypotheses. The Takena Formation consists of >2 km of sedimentary strata that include a lower marine limestone member (Penbo Member) and an upper member of clastic red beds (Lhunzhub Member). The Aptian-Albian Penbo Member consists of ∼250 m of orbitolinid-limestone beds that were deposited in a shallow-marine seaway during a time when there was little regional subsidence. The overlying Lhunzhub Member (>1500 m) consists primarily of fluvial strata that were deposited during a period of increased subsidence. Overall, the Lhunzhub Member coarsens upward from meandering and anastomosing stream deposits interbedded with numerous paleosols near the base, to multistory-multilateral braided stream deposits near the top. The fluvial sandstone units are lithic-rich and contain abundant plagioclase and volcanic grains, and paleo-current data record northwest-directed transport; these data indicate that the sediment was derived from the Gangdese volcanic arc that developed along the southern margin of the Lhasa terrane. Following deposition, but prior to ca. 70 Ma, the beds of the Takena Formation were folded and partially eroded. The sedimentary and stratigraphic characteristics of the Takena Formation are most consistent with deposition in a retroarc foreland basin setting. The limestone beds of the Penbo Member were deposited in a shallow-marine seaway that was eventually infilled by clastic sediment derived principally from the Gangdese volcanic arc. The low subsidence rate recorded in the lowermost strata of the Takena Formation, including the Penbo Member and the paleosol-rich interval of the Lhunzhub Member, is interpreted to be associated with the passage of a flexural forebulge. The overlying, upward-coarsening fluvial strata were deposited in progressively more proximal locations within the foredeep of a foreland basin. The Late Cretaceous folding of the Takena Formation indicates that the foreland basin strata were eventually incorporated into a fold-and-thrust belt. The upper Cretaceous sedimentary strata of the Lhasa terrane of southern Tibet predict that a north-verging fold-and-thrust belt existed along the southern margin of the Lhasa terrane during Late Cretaceous time, prior to the Indo-Asian collision. Collectively, the evidence indicates that the southern margin of the Lhasa terrane had thickened crust and was likely to have been at high elevations immediately before the Indo-Asian collision. © 2006 Geological Society of America.
• Leier, A. L., DeCelles, P. G., Kapp, P., & Gehrels, G. E. (2007). Lower Cretaceous strata in the Lhasa Terrane, Tibet, with implications for understanding the early tectonic history of the Tibetan Plateau. Journal of Sedimentary Research, 77(10), 809-825.
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  Abstract: Sedimentary strata in southern Tibet indicate that upper crustal deformation occurred throughout the region during Early Cretaceous time, suggesting that construction of the Tibetan plateau commenced tens of millions of years before the Late Cretaceous-early Tertiary Indo-Asian collision. Lower Cretaceous strata in the northern portion of the Lhasa terrane are characterized by lithic-rich conglomerate beds deposited in shallow marine and meandering-river fluvial environments. Sediments in these units were derived from two primary sources: volcanic rocks associated with Early Cretaceous intrusions, and sedimentary strata eroded from the northern Lhasa and southern Qiangtang terranes. The majority of detrital zircons from Lower Cretaceous fluvial conglomerate beds in northern Lhasa have U-Pb ages between 125 and 140 Ma and provide a maximum depositional age for these units of 125 ± 2 Ma. Lower Cretaceous strata in the southern portion of the Lhasa terrane consist of mudstone, quartzose sandstone, and subordinate quartzite-pebble conglomerate beds that were deposited in shallow marine and fluvial environments. Populations of detrital zircons in Lower Cretaceous conglomerate beds in southern Lhasa have U-Pb ages between 140 and 150 Ma, 500 and 600 Ma, and 850 and 950 Ma, and provide a maximum depositional age for these units of 143 ± 2 Ma. Both the modal composition and detrital-zircon U-Pb ages of the Lower Cretaceous conglomerate exposed in northern and southern Lhasa suggest different source areas, diachronous deposition, and possibly distinct genetic histories. Throughout most of the Lhasa terrane, the Lower Cretaceous clastic strata are overlain by a widespread limestone of Aptian-Albian age that was deposited in a shallow carbonate sea containing rudist patch reefs and muddy inter-reef zones. With respect to the tectono-sedimentary setting of the Lhasa terrane during Early Cretaceous time, the sedimentological and stratigraphic data are most consistent with a peripheral foreland basin model, which is interpreted to have resulted from the collision between the northern margin of the Lhasa terrane and the southern margin of Asia (the Qiangtang terrane). Several characteristics of the Aptian-Albian succession can be attributed to a peripheral foreland basin setting, although deposition within the region may have been influenced by a combination of mechanisms. Sedimentary characteristics of Lower Cretaceous rocks in the Lhasa terrane are consistent with recent ideas suggesting that portions of southern and central Tibet were deformed and above sea level before the Indo-Asian collision. Copyright © 2007, SEPM (Society for Sedimentary Geology).
• Leier, A. L., Kapp, P., Gehrels, G. E., & Decelles, P. G. (2007). Detrital zircon geochronology of Carboniferous-Cretaceous strata in the Lhasa terrane, southern Tibet. Basin Research, 19(3), 361-378.
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  Abstract: Sedimentary strata in the Lhasa terrane of southern Tibet record a long but poorly constrained history of basin formation and inversion. To investigate these events, we sampled Palaeozoic and Mesozoic sedimentary rocks in the Lhasa terrane for detrital zircon uranium-lead (U-Pb) analysis. The >700 detrital zircon U-Pb ages reported in this paper provide the first significant detrital zircon data set from the Lhasa terrane and shed new light on the tectonic and depositional history of the region. Collectively, the dominant detrital zircon age populations within these rocks are 100-150, 500-600 and 1000-1400 Ma. Sedimentary strata near Nam Co in central Lhasa are mapped as Lower Cretaceous but detrital zircons with ages younger than 400 Ma are conspicuously absent. The detrital zircon age distribution and other sedimentological evidence suggest that these strata are likely Carboniferous in age, which requires the existence of a previously unrecognized fault or unconformity. Lower Jurassic strata exposed within the Bangong suture between the Lhasa and Qiangtang terranes contain populations of detrital zircons with ages between 200 and 500 Ma and 1700 and 2000 Ma. These populations differ from the detrital zircon ages of samples collected in the Lhasa terrane and suggest a unique source area. The Upper Cretaceous Takena Formation contains zircon populations with ages between 100 and 160 Ma, 500 and 600 Ma and 1000 and 1400 Ma. Detrital zircon ages from these strata suggest that several distinct fluvial systems occupied the southern portion of the Lhasa terrane during the Late Cretaceous and that deposition in the basin ceased before 70 Ma. Carboniferous strata exposed within the Lhasa terrane likely served as source rocks for sediments deposited during Cretaceous time. Similarities between the lithologies and detrital zircon age-probability plots of Carboniferous rocks in the Lhasa and Qiangtang terranes and Tethyan strata in the Himalaya suggest that these areas were located proximal to one another within Gondwanaland. U-Pb ages of detrital zircons from our samples and differences between the geographic distribution of igneous rocks within the Tibetan plateau suggest that it is possible to discriminate a southern vs. northern provenance signature using detrital zircon age populations. © 2007 Blackwell Publishing Ltd.
• Martin, A. J., Gehrels, G. E., & DeCelles, P. G. (2007). The tectonic significance of (U,Th)/Pb ages of monazite inclusions in garnet from the Himalaya of central Nepal. Chemical Geology, 244(1-2), 1-24.
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  Abstract: Textural observations, chemical zoning, and 196 in situ LA-ICP-MS 232Th/208Pb and 238U/206Pb dates of monazite inclusions in Greater Himalayan garnets demonstrate that inclusions are frequently composite grains composed of multiple generations of monazite. Cenozoic prograde and retrograde monazite are commonly intergrown, and both of these components are frequently intergrown with lower Paleozoic monazite. Because zones in most inclusions are too small to date, and because some inclusions show little zoning of Y and Th, chemical characterization alone is often insufficient to guide interpretation of the crystallization significance of Th/Pb dates. Comparison of 238U/206Pb and 232Th/208Pb dates thus is critical for interpreting the tectonic significance of dates derived from these composite inclusions. Most Cenozoic monazite crystallized between 42-29 and 22-12 Ma, and these age ranges may record prograde and retrograde metamorphism of Greater Himalayan rocks, respectively. Dates younger than ∼ 12 Ma are discordant, without exception. The < 12 Ma dates come from the eastern side of the Annapurna Range and record limited monazite growth during retrograde metamorphism and metasomatism of these Greater Himalayan rocks. Microcracks allow communication between the interior and exterior of garnet, affecting monazite inclusions by facilitating Pb loss, dissolution, recrystallization, and intergrowth of younger monazite. © 2007 Elsevier B.V. All rights reserved.
• Shundong, H. e., Kapp, P., DeCelles, P. G., Gehrels, G. E., & Heizler, M. (2007). Cretaceous-Tertiary geology of the Gangdese Arc in the Linzhou area, southern Tibet. Tectonophysics, 433(1-4), 15-37.
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  Abstract: Knowledge of the Cretaceous-Tertiary history of upper crustal shortening and magmatism in Tibet is fundamental to placing constraints on when and how the Tibetan plateau formed. In the Lhasa terrane of southern Tibet, the widely exposed angular unconformity beneath uppermost Cretaceous-lower Tertiary volcanic-bearing strata of the Linzizong Formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs. during the Indo-Asian collision. In the Linzhou area, located ∼ 30 km north of the city of Lhasa, a > 3-km-thick section of the Linzizong Formation lies unconformably on Cretaceous and older rocks that were shortened by both northward- and southward-verging structures during the Late Cretaceous. The Linzizong Formation dips northward in the footwall of a north-dipping thrust system that involves Triassic-Jurassic strata and a granite intrusion in the hanging wall. U-Pb zircon geochronologic studies show that the Linzizong Formation ranges in age from 69 Ma to at least 47 Ma and that the hanging wall granite intrusion crystallized at ∼ 52 Ma, coeval with dike emplacement into footwall Cretaceous strata. 40Ar/39Ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 Ma, followed by an episode of accelerated cooling to upper crustal levels beginning at ∼ 42 Ma. The onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern Tibet and is interpreted be a consequence of either (1) Tertiary thrusting in this region or (2) regional rock uplift and erosion following removal of overthickened Gangdese arc lower crust and upper mantle or break-off of the Neo-Tethyan oceanic slab. © 2007 Elsevier B.V. All rights reserved.
• DeCelles, P. G., & Coogan, J. C. (2006). Regional structure and kinematic history of the Sevier fold-and-thrust belt, central Utah. Bulletin of the Geological Society of America, 118(7-8), 841-864.
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  Abstract: The Canyon Range, Pavant, Paxton, and Gunnison thrust systems in central Utah form the Sevier fold-and-thrust belt in its type area. The Canyon Range thrust carries an ∼12-km-thick succession of Neoproterozoic through Triassic sedimentary rocks and is breached at the surface by the Neogene extensional Sevier Desert detachment fault. The Pavant, Paxton, and Gunnison thrusts carry Lower Cambrian through Cretaceous strata and have major footwall detachments in weak Jurassic rocks. The Canyon Range thrust was active during latest Jurassic-Early Cretaceous time. The Pavant thrust sheet was emplaced in Albian time, formed an internal duplex beneath the Canyon Range during the Cenomanian, and then developed a frontal duplex during the Turonian. The Paxton thrust sheet was initially emplaced during the Santonian, and subsequently formed the Paxton duplex during the early to mid-Campanian. Some slip on the Paxton system was fed into a frontal triangle zone along the Sanpete Valley antiform. The Gunnison thrust system became active in late Campanian time and continued to feed slip into the frontal triangle zone through the early Paleocene. The Canyon Range and main Pavant thrust sheets experienced long-distance eastward transport (totaling >140 km) mainly because they are composed of relatively strong rocks, whereas the eastern thrust sheets accommodated less shortening and formed multiple antiformal duplexes in order to maintain sufficient taper for continued forward propagation of the fold-and-thrust belt. Total shortening was at least 220 km. Upper crustal thickening of ∼16 km produced crust that was >50 km thick and a likely surface elevation >3 km in western Utah. Shortening across the entire Cordilleran retroarc thrust belt at the latitude of central Utah may have exceeded 335 km. The Late Cretaceous paleogeography of the fold-and-thrust belt and foreland basin was similar to the modern central Andean fold-and-thrust belt, with a high-elevation, low-relief hinterland plateau and a rugged topographic front. The frontal part of the Sevier belt was buried by several kilometers of nonmarine and shallow-marine sediments in the wedge-top depozone of the foreland basin system. The Canyon Range thrust sheet dominated sediment supply throughout the history of shortening in the Sevier belt. Westward underthrusting of a several hundred-kilometer-long panel of North American lower crust beneath the Cordilleran magmatic arc is required to balance upper-crustal shortening in the thrust belt, and may be petrogenetically linked to a Late Cretaceous flare-up of the magmatic arc as preserved in the Sierra Nevada Batholith. © 2006 Geological Society of America.
• Gehrels, G. E., DeCelles, P. G., Ojha, T. P., & Upreti, B. N. (2006). Geologic and U-Pb geochronologic evidence for early Paleozoic tectonism in the Dadeldhura thrust sheet, far-west Nepal Himalaya. Journal of Asian Earth Sciences, 28(4-6), 385-408.
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  Abstract: The Dadeldhura thrust sheet inm western Nepal consists of Proterozoic-Lower Paleozoic sedimentary and plutonic rocks, and their metamorphic equivalents, that rest structurally on Proterozoic strata of the Lesser Himalayan sequence. Although regional metamorphism and ductile deformation were widespread during Tertiary thrust emplacement, relicts of early Paleozoic tectonism are preserved locally. New field and geochronologic studies, together with the findings of previous workers, indicate that this early Paleozoic tectonism included: (1) regional metamorphism to at least garnet grade, (2) regional folding of a thick metamorphic sequence into a broad east-west trending syncline, (3) outcrop-scale folding of metasedimentary rocks, (4) emplacement of Cambro-Ordovician granitic bodies during and after the metamorphism and deformation, (5) uplift and erosion of the metamorphic sequence, with garnet-grade rocks locally exposed at the surface, and (6) derivation of Ordovician conglomeratic sandstones from the early Paleozoic orogen. Similar records of metamorphism, deformation, and uplift/erosion have been found in other regions of the Himalaya, indicating that rocks of the Dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system. Future tectonic models of Himalayan orogenesis must accommodate this early Paleozoic event. © 2005 Elsevier Ltd. All rights reserved.
• Gehrels, G. E., DeCelles, P. G., Ojha, T. P., & Upreti, B. N. (2006). Geologic and U-Th-Pb geochronologic evidence for early Paleozoic tectonism in the Kathmandu thrust sheet, central Nepal Himalaya. Bulletin of the Geological Society of America, 118(1-2), 185-198.
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  Abstract: The Kathmandu thrust sheet consists of Upper Proterozoic through mid-Paleozoic rocks that were emplaced over Lesser Himalayan strata (part of India's cratonal cover) during middle to late Tertiary time. Primary components of the thrust sheet include Upper Proterozoic metasedimentary rocks of the Bhimphedi Group, Cambrian-Ordovician granite bodies, and Ordovician (through Devonian?) conglomerate, sandstone, shale, and limestone of the Phulchauki Group. Deformation, metamorphism, uplift, and erosion accompanied Tertiary emplacement of the thrust sheet, but there is also evidence of widespread early Paleozoic tectonism, such as: (1) Rocks of the Bhimphedi Group were metamorphosed at ca. 490 Ma, as indicated by Th-Pb ages of monazite inclusions in garnet crystals. (2) Bhimphedi rocks are interpreted to have been imbricated along a south-vergent(?) thrust system during early Paleozoic time, with U-Pb ages recording ductile deformation through ca. 484 Ma but ending (at least locally) by ca. 473 Ma. (3) Cambrian-Ordovician granite bodies may have been generated by crustal melting during thrust loading, and at least some are interpreted to have been emplaced as syntectonic sills along the early Paleozoic thrust faults. (4) Ordovician conglomeratic strata were shed from the uplifted Bhimphedi Group and Cambrian-Ordovician granite bodies and are interpreted to have accumulated in a foreland basin setting with respect to the early Paleozoic thrust system. Our findings, together with evide nce for early Paleozoic ductile deformation and metamorphism in adjoining regions, indicate that early Paleozoic tectonism has played an important role in shaping the Himalayan orogen. This early Paleozoic tectonism has been overlooked in most current models for the formation of the Himalayan orogen. © 2006 Geological Society of America.
• Robinson, D. M., DeCelles, P. G., & Copeland, P. (2006). Tectonic evolution of the Himalayan thrust belt in western Nepal: Implications for channel flow models. Bulletin of the Geological Society of America, 118(7-8), 865-885.
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  Abstract: We present a new geologic map of western Nepal and three balanced regional cross sections in the Himalayan thrust belt. The minimum shortening between the South Tibetan detachment and the Main Frontal thrust is 485-743 km and suggests that total Himalayan shortening may exceed 900 km. All rocks involved in the thrust belt are of upper crustal affinity, implying that a comparable length of Indian lower crust and mantle lithosphere was subducted beneath Tibet. Major structural features are the Subhimalayan thrust system, Lesser Himalayan imbricate zone, Dadeldhura thrust sheet, Lesser Himalayan duplex, Ramgarh thrust sheet, Main Central thrust sheet, and a north-dipping normal-sense shear zone, possibly related to the South Tibetan detachment. These structures are continuous along the entire Nepalese portion of the Himalayan thrust belt. New 40Ar/39Ar ages from the Ramgarh thrust zone, Greater Himalayan rocks, and the lower part of the Tethyan sequence support a kinematic model in which major thrust systems in Nepal propagated southward from early Miocene time onward. The geometry and kinematic history of the thrust belt in western Nepal are not compatible with recent models for southward ductile extrusion of Greater Himalayan rocks in a mid-crustal channel. Instead, the thrust belt in western Nepal behaved like a typical forward propagating thrust system, involving unmetamorphosed, brittlely deformed rocks in its frontal part and ductilely deformed, higher-grade metamorphic rocks in its hinterland region. Although our results do not support published versions of the channel flow model, they provide additional geological and geochronological data that will assist future attempts to develop geodynamic models for the Himalayan-Tibetan orogenic system. © 2006 Geological Society of America.
• Szulc, A. G., Najman, Y., Sinclair, H. D., Pringle, M., Bickle, M., Chapman, H., Garzanti, E., Andò, S., Huyghe, P., Mugnier, J. L., Ojha, T., & DeCelles, P. (2006). Tectonic evolution of the Himalaya constrained by detrital ⁴⁰Ar-³⁹Ar, Sm-Nd and petrographic data from the Siwalik foreland basin succession, SW Nepal. Basin Research, 18(4), 375-391.
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  Abstract: 40Ar-39Ar dating of detrital white micas, petrography and heavy mineral analysis and whole-rock geochemistry has been applied to three time-equivalent sections through the Siwalik Group molasse in SW Nepal [Tinau Khola section (12-6 Ma), Surai Khola section (12-1 Ma) and Karnali section (16-5Ma)]. 40Ar-39Ar ages from 1415 single detrital white micas show a peak of ages between 20 and 15 Ma for all the three sections, corresponding to the period of most extensive exhumation of the Greater Himalaya. Lag times of less than 5 Myr persist until 10Ma, indicating Greater Himalayan exhumation rates of up to 2.6 mm year-1, using one-dimensional thermal modelling. There are few micas younger than 12Ma, no lag times of less than 6 Myr after 10 Ma and whole-rock geochemistry and petrography show a significant provenance change at 12 Ma indicating erosion from the Lesser Himalaya at this time. These changes suggest a switch in the dynamics of the orogen that took place during the 12-10 Ma period whereby most strain began to be accommodated by structures within the Lesser Himalaya as opposed to the Greater Himalaya. Consistent data from all three Siwalik sections suggest a lateral continuity in tectonic evolution for the central Himalayas. © 2006 Blackwell Publishing Ltd.
• Leier, A. L., DeCelles, P. G., & Pelletier, J. D. (2005). Mountains, monsoons, and megafans. Geology, 33(4), 289-292.
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  Abstract: In certain cases, the rivers draining mountain ranges create unusually large fan-shaped bodies of sediment that are referred to as fluvial megafans. We combine information from satellite imagery, monthly discharge and precipitation records, digital elevation models, and other sources to show that the formation of fluvial megafans requires particular climatic conditions. Specifically, modern fluvial megafans in actively aggrading basins are produced by rivers that undergo moderate to extreme seasonal fluctuations in discharge that result from highly seasonal precipitation patterns. The global distribution of modern megafans is primarily restricted to 15°-35° latitude in the Northern and Southern Hemispheres, corresponding to climatic belts that fringe the tropical climatic zone. No relationship exists between megafan occurrence and drainage-basin relief or area. The tendency of rivers with large fluctuations in discharge to construct megafans is related to the instability of channels subject to such conditions. Because of the correlation between seasonal precipitation and megafan occurrence, the recognition of fluvial megafan deposits in ancient stratigraphic successions may provide critical information for paleoclimate reconstructions. © 2005 Geological Society of America.
• Martin, A. J., DeCelles, P. G., Gehrels, G. E., Patchett, P. J., & Isachsen, C. (2005). Isotopic and structural constraints on the location of the Main Central thrust in the Annapurna Range, central Nepal Himalaya. Bulletin of the Geological Society of America, 117(7-8), 926-944.
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  Abstract: Five isotope-enhanced geologic transects in the southern Annapurna Range of central Nepal elucidate structural geometries near the Main Central thrust. Whole-rock Nd isotopes and U-Pb ages of detrital zircons unambiguously distinguish Greater Himalayan (hanging wall) and Lesser Himalayan (footwall) metasedimentary rocks. εNd(0) values for lower Lesser Himalayan rocks typically range from -20 to -26, whereas Greater Himalayan rocks usually have εNd(0) values of -19 to -12. Lower Lesser Himalayan rocks yield detrital zircons with an age peak at ca. 1880 Ma and no detrital zircons younger than ca. 1550 Ma. In contrast, Greater Himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca. 1050 Ma and no detrital zircons younger than ca. 600 Ma. The protolith boundary between Greater and Lesser Himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology. Field and microstructural observations indicate the presence of a top-to-the-south ductile shear zone superimposed on this boundary, confirming this shear zone as the Main Central thrust. No evidence exists for large-scale structural mixing of Greater and Lesser Himalayan rocks along the Main Central thrust in the Annapurna Range. © 2005 Geological Society of America.
• McQuarrie, N., Horton, B. K., Zandt, G., Beck, S., & DeCelles, P. G. (2005). Lithospheric evolution of the Andean fold-thrust belt, Bolivia, and the origin of the central Andean plateau. Tectonophysics, 399(1-4 SPEC. ISS.), 15-37.
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  Abstract: We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 18° and 20° S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by ∼70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold-thrust belt at 40 Ma through the eastward propagation of a 200-400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera-Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24-23 Ma, and further volcanism recorded by 12-7 Ma crustal ignimbrites. 4) After ∼20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold-thrust belt indicate that the average shortening rate has remained fairly constant (∼8-10 mm/year) through time with possible slowing (∼5-7 mm/year) in the last 15-20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera-Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination. © 2005 Elsevier B.V. All rights reserved.
• Pearson, O. N., & DeCelles, P. G. (2005). Structural geology and regional tectonic significance of the Ramgarh thrust, Himalayan fold-thrust belt of Nepal. Tectonics, 24(4), 1-26.
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  Abstract: The Ramgarh thrust is one of the major fault systems of the Himalayan thrust belt in Nepal and northern India. The Ramgarh thrust sheet is ∼0.2-2.0 km thick and can be traced along strike the entire length of the Himalaya in Nepal. The fault generally places the oldest Paleoproterozoic rocks in the Lesser Himalayan series upon younger Lesser Himalayan rocks or lower Miocene foreland basin deposits. Regional balanced cross sections suggest that the Ramgarh thrust had at least ∼120 km of initial south vergent displacement. Subsequently, the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying Lesser Himalayan rocks. Ramgarh hanging wall strata are greenschist-grade phyllite, quartzite, and augen gneiss, all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top-to-the-south sense of shear. Structural fabrics in the Ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet. Regional and local mapping of the Ramgarh thrust in Nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat, except where local lateral ramps complicate its geometry. Similarly, the structurally overlying Main Central thrust always places a hanging wall flat in Greater Himalayan series rocks upon the regionally flat Ramgarh thrust sheet. These geometric relationships preclude kinematic and thermal models that elevate Greater Himalayan and lower Lesser Himalayan rocks along high-angle thrust ramps in the vicinity of the present traces of the Ramgarh and Main Central thrust faults. Instead, the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the Main Central thrust. The present steep dips of the Ramgarh and Main Central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the Lesser Himalayan zone. The Ramgarh thrust sheet overlaps a bed length of at least 100 kin in lower Miocene foreland basin deposits, indicating that a significant amount of displacement on the thrust must have occurred after ∼15 Ma. Growth of the Lesser Himalayan duplex and additional slip on the frontal part of the Ramgarh thrust occurred from ∼12 to 5 Ma. The presence of a major greenschist-grade metasedimentary thrust sheet composed of Lesser Himalayan rocks directly below the Main Central thrust suggests that the famous "inverted metamorphism" in this region is a result of structural inversion. Similarly, the concept of a broad zone of intense shear strain related exclusively to emplacement of the Main Central thrust sheet is probably invalid in Nepal. Copyright 2005 by the American Geophysical Union.
• Ross, G. M., Patchett, P. J., Hamilton, M., Heaman, L., DeCelles, P. G., Rosenberg, E., & Giovanni, M. K. (2005). Evolution of the Cordilleran orogen (southwestern Alberta, Canada) inferred from detrital mineral geochronology, geochemistry, and Nd isotopes in the foreland basin. Bulletin of the Geological Society of America, 117(5-6), 747-763.
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  Abstract: The provenance of sedimentary strata that accumulate in foreland basins record the growth and denudation of the adjacent orogen. We use U-Pb geochronology of detrital zircon and monazite, Sm-Nd isotope geochemistry, trace- and rare earth element geochemistry, and petrographic data from synorogenic clastic sedimentary rocks in the Cordilleran foreland basin of southwestern Alberta to provide new perspectives on the evolution of the orogen. Foreland basin clastic rocks comprise three major pulses of sediment delivery: (1) upper Fernie Formation-Kootenay Group (154-142 Ma), (2) Blairmore Group (115-103 Ma), and (3) Milk River Group-Porcupine Hills Formation (78-58 Ma). Nd isotope data are dominated by εNd values of -7 to -12, interpreted to represent a well-mixed provenance from Devonian through Triassic strata and subordinate contributions from thrust-imbricated pre-Devonian strata of the Cordilleran miogeocline. Significant deviations to less negative (more juvenile) values between -5 and +1 represent periods when the foreland was flooded by juvenile detritus from oceanic are sources such as Quesnel terrane and from syndepositional continental magmatic arcs of mid-and Late Cretaceous ages. Detrital zircon and monazite from the Fer nie-Kootenay clastic pulse (pulse 1) indicate derivation from Triassic-Ordovician sandstones imbricated within the thrust-and-fold belt, consistent with the Nd tracer results and petrography. U-Pb zircon ages from the Blairmore Group (pulse 2) confirm a provenance from Triassic and Jurassic arc rocks of Quesnel terrane with only minor contributions from older miogeoclinal rocks; they also record the presence of syndepositional magmatic material. The upper part of the Blairmore Group shows a transition to less juvenile Nd isotopic signatures and the reappearance of detrital zircons of miogeoclinal derivation. A similar pattern occurs in the Milk River-Porcupine interval (pulse 3) with juvenile material occurring early in the sequence, accompanied by zircon grains from syndepositional volcanic sources and by more continental material in the upper part of the sequence. Pulse 1 records the erosion of thrust-imbricated miogeoclinal rocks during the creation and erosion of the foreland thrust-and-fold belt with no detectable material derived from the deeper parts of the hinterland. A significant unconformity of ∼27 m.y. duration led to redistribution of the foreland basin fill and erosion of the adjacent thrust-and-fold belt and corresponds to a period of magmatic and tectonic quiescence in the southern Canadian Cordillera. Renewed contraction within the erosionally modified thrust wedge led to development of out-of-sequence thrust structures which allowed juvenile terranes (Quesnel terrane) to become the dominant source for foreland (pulse 2) to the exclusion of miogeoclinal material. Reappearance of the miogeoclinal signature in upper pulse 2 is interpreted to record eastward propagation of the thrust-and-fold belt into miogeoclinal strata. The third pulse of sediment records significant input of windblown ash from juvenile sources in the Coast Belt mixed with bedload components derived from more local sources in the eastern Cordillera. The youngest deposits in the basin (ca. 58 Ma) are characterized by a cosmopolitan provenance that likely records cannibalization of older parts of the foreland as previously deposited foreland strata became important components of the thrust wedge. © 2005 Geological Society of America.
• DeCelles, P. G. (2004). Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S.A. American Journal of Science, 304(2), 105-168.
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  Abstract: Geochronological, structural, and sedimentological data provide the basis for a regional synthesis of the evolution of the Cordilleran retroarc thrust belt and foreland basin system in the western U.S.A. In this region, the Cordilleran orogenic belt became tectonically consolidated during Late Jurassic time (∼ 155 Ma) with the closure of marginal oceanic basins and accretion of fringing arcs along the western edge of the North American plate. Over the ensuing 100 Myr, contractile deformation propagated approximately 1000 kilometers eastward, culminating in the formation of the Laramide Rocky Mountain ranges. At the peak of its development, the retroarc side of the Cordillera was divided into five tectonomorphic zones, including from west to east the Luning-Fencemaker thrust belt; the central Nevada (or Eureka) thrust belt; a high-elevation plateau (the "Nevadaplano"); the topographically rugged Sevier fold-thrust belt; and the Laramide zone of intraforeland basement uplifts and basins. Mid-crustal rocks beneath the Nevadaplano experienced high-grade metamorphism and shortening during Late Jurassic and mid- to Late Cretaceous time, and the locus of major, upper crustal thrust faulting migrated sporadically eastward. By Late Cretaceous time, the middle crust beneath the Nevadaplano was experiencing decompression and cooling, perhaps in response to large-magnitude ductile extension and isostatic exhumation, concurrent with ongoing thrusting in the frontal Sevier belt. The tectonic history of the Sevier belt was remarkably consistent along strike of the orogenic belt, with emplacement of regional-scale Proterozoic and Paleozoic megathrust sheets during Early Cretaceous time and multiple, more closely spaced, Paleozoic and Mesozoic thrust sheets during Late Cretaceous-Paleocene time. Coeval with emplacement of the frontal thrust sheets, large structural culminations in Archean-Proterozoic crystalline basement developed along the basement step formed by Neoproterozoic rifting. A complex foreland basin system evolved in concert with the orogenic wedge. During its early and late history (∼ 155 - 110 Ma and ∼ 70 - 55 Ma) the basin was dominated by nonmarine deposition, whereas marine waters inundated the basin during its midlife (∼ 110 - 70 Ma). Late Jurassic basin development was controlled by both flexural and dynamic subsidence. From Early Cretaceous through early Late Cretaceous time the basin was dominated by flexural subsidence. From Late Cretaceous to mid-Cenozoic time the basin was increasingly partitioned by basement-involved Laramide structures. Linkages between Late Jurassic and Late Cretaceous Cordilleran arc-magmatism and westward underthrusting of North American continental lithosphere beneath the arc are not plainly demonstrable from the geological record in the Cordilleran thrust belt. A significant lag-time (∼ 20 Myr) between shortening and coeval underthrusting, on the one hand, and generation of arc melts, on the other, is required for any linkage to exist. However, inferred Late Jurassic lithospheric delamination may have provided a necessary precondition to allow relatively rapid Early Cretaceous continental underthrusting, which in turn could have catalyzed the Late Cretaceous arc flare-up.
• DeCelles, P. G., Gehrels, G. E., Najman, Y., Martin, A. J., Carter, A., & Garzanti, E. (2004). Detrital geochronology and geochemistry of Cretaceous-Early Miocene strata of Nepal: Implications for timing and diachroneity of initial Himalayan orogenesis. Earth and Planetary Science Letters, 227(3-4), 313-330.
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  Abstract: The onset of mountain building in the western part of the Himalayan orogenic belt has been documented in the synorogenic stratigraphic record of northern Pakistan and India as Early to Middle Eocene (∼52 Ma). Eocene strata in the Tethyan portion of the central part of the Himalayan orogenic belt consist of shallow marine carbonate rocks that lack evidence for initial Himalayan orogenesis, thus leaving open the possibility that the onset of orogeny was significantly diachronous along strike. We report U-Pb ages of detrital zircons and Nd-isotopic and trace element data from associated mudrocks in Cretaceous-Paleocene(?), Eocene, and lower Miocene strata of the southern Lesser Himalayan zone of central Nepal. The Cretaceous-Paleocene(?) Amile Formation is dominated by zircons with Archean-Early Proterozoic U-Pb ages. An abrupt influx of Cambrian-Ordovician and Middle to Late Proterozoic zircons marks the transition into the Eocene Bhainskati Formation, and indicates the onset of erosion of Tethyan rocks in the nascent Himalayan thrust belt. An increased proportion of Late Proterozoic zircons in fluvial litharenites of the lower Miocene Dumri Formation signals initial erosion of Greater Himalayan protoliths. The Nd-isotopic and trace element data support the unroofing history documented by the U-Pb zircon ages. The fact that middle Eocene strata in Nepal were derived from the Himalayan thrust belt reduces the maximum time lag between the onset of orogenesis in the northwest and central Himalaya to no more than ∼2 My. © 2004 Elsevier B.V. All rights reserved.
• Ducea, M. N., Valencia, V. A., Shoemaker, S., Reiners, P. W., DeCelles, P. G., Campa, M. F., Morán-Zenteno, D., & Ruiz, J. (2004). Rates of sediment recycling beneath the Acapulco trench: Constraints from (U-Th)/He thermochronology. Journal of Geophysical Research B: Solid Earth, 109(9), B09404 1-11.
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  Abstract: The Sierra Madre del Sur mountain range is an uplifted forearc associated with the subduction of the Cocos plate along the Acapulco trench beneath mainland southern Mexico. The shallow subduction angle, the truncation of geologic features along the modern Acapulco trench, and direct seismic and drill hole observations in the trench through deep sea drilling data suggest that subduction erosion is an important process during the evolution of this margin. Turbidites derived from the uplifted forearc are the predominant sedimentary input into this trench, while pelagic sediments are subordinate. Apatite (U-Th)/He ages were obtained on 23 samples from two transects across the Sierra Madre del Sur (Acapulco and Puerto Escondido) and reveal slow cooling during the Miocene. (U-Th)/He ages range between ∼25 and 8 Ma and correlate inversely with elevation. Long-term erosional exhumation rates inferred from these ages range from 0.11 to 0.33 km/m.y., with higher rates in the range core, and suggest that the Sierra Madre del Sur has been a slowly decaying mountain range, since at least the early Miocene. Apparent Miocene-Pliocene sedimentation ("preservation") rates in the Acapulco trench derived from Deep Sea Drilling Project data are about an order of magnitude smaller than the Miocene forearc erosion rates estimated from (U-Th)/He ages, suggesting that the terrigenous input to the trench was almost entirely recycled via subduction erosion, at least during the Miocene. The Miocene subducted flux per unit length of the margin is about 30 km3/(km m.y.), or a subducted volume per unit time of 44 × 103 km3/m.y., when integrated over the length of the trench. Copyright 2004 by the American Geophysical Union.
• Horton, B. K., Constenius, K. N., & DeCelles, P. G. (2004). Tectonic control on coarse-grained foreland-basin sequences: An example from the Cordilleran foreland basin, Utah. Geology, 32(7), 637-640.
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  Abstract: Newly released reflection seismic and borehole data, combined with sedimentological, provenance, and biostratigraphic data from Upper Cretaceous-Paleocene strata in the proximal part of the Cordilleran foreland-basin system in Utah, establish the nature of tectonic controls on stratigraphic sequences in the proximal to distal foreland basin. During Campanian time, coarse-grained sand and gravel were derived from the internally shortening Charleston-Nebo salient of the Sevier thrust belt. A rapid, regional Campanian progradational event in the distal foreland basin (>200 km from the thrust belt in
• Conder, J., Butler, R. F., DeCelles, P. G., & Constenius, K. (2003). Paleomagnetic determination of vertical-axis rotations within the Charleston-Nebo salient, Utah. Geology, 31(12), 1113-1116.
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  Abstract: The Charleston-Nebo salient is highly convex toward the foreland of the Sevier fold-thrust belt in Utah. To determine the importance of vertical-axis rotations to the kinematic history of the salient, paleomagnetic samples were collected from red Triassic strata of the Woodside Shale and Ankareh Formation from 68 sites distributed around the curvature of the salient. The paleomagnetic data indicate counterclockwise rotation of 33.3° ± 4.8° in the northern part of the salient and clockwise rotation of 66.7° ± 5.1° in the southern part of the salient. These data support divergent-flow models for thrust-salient development, augmented by relatively minor contributions from original basin geometry and local strike-slip along salient margins.
• DeCelles, P. G., & Horton, B. K. (2003). Early to middle Tertiary foreland basin development and the history of Andean crustal shortening in Bolivia. Bulletin of the Geological Society of America, 115(1), 58-77.
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  Abstract: A >2.5-km-thick succession of Tertiary strata in the Eastern Cordillera of southern central Bolivia consists of predominantly fluvial and lacustrine deposits. The age of the base of the succession is middle Paleocene, and its upper part (which is erosionally truncated) is probably late Oligocene-early Miocene. The lower 50-130 m of the succession consists of interbedded fluvial sandstone and mudstone (including minor paleosols) of the Santa Lucía Formation. These strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the Santa Lucía Formation and lower part of the Impora Formation that may represent much of late Paleocene-Eocene time. Above the paleosol zone is a thin zone (∼10 m) of lacustrine carbonate and clastic rocks in the uppermost Impora Formation, and this in turn is overlain by a >2000-m-thick, upward- coarsening succession of clastic fluvial deposits in the Cayara, Camargo, and Suticollo Formations. Paleocurrent data indicate that fluvial channels that deposited the Santa Lucía, Impora, and Cayara Formations flowed mainly westward, whereas channels responsible for the Camargo and Suticollo Formations flowed generally eastward. Modal sandstone petrographic data show a long-term evolution from subarkosic (during Santa Lucía deposition) to quartz-arenitic (during Impora and Cayara deposition) to sublitharenitic (during Camargo and Suticollo deposition) compositions. We argue that the lithofacies and sediment-accumulation history of this succession are most consistent with deposition in an eastward-migrating foreland basin system. Critical to this interpretation is the zone of extreme stratigraphic condensation in the lower Impora Formation, the inferred upper Paleocene-Eocene part of the succession. The abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous, postrift thermal subsidence resulted in a gradual, continuous decay of early Tertiary sediment- accumulation rates. Stratigraphic condensation is, however, consistent with passage of the forebulge through the region, an interpretation that also can be reconciled with the record of foreland basin development in the Altiplano to the west and gross sediment accumulation patterns in the Subandean zone and modern foreland basin to the east. Simple flexural modeling and palinspastic reconstruction of the complete Cenozoic migration history of the foreland basin system suggest that ∼1000 km of foreland lithosphere has been underthrust westward beneath the Andean orogenic belt at the latitude of southern Bolivia. The amount of middle and lower crust that would have been added to the Andean infrastructure is sufficient to explain the present crustal thicknesses in Bolivia.
• Garzione, C. N., DeCelles, P. G., Hodkinson, D. G., Ojha, T. P., & Upreti, B. N. (2003). East-west extension and miocene environmental change in the southern Tibetan plateau: Thakkhola graben, central Nepal. Bulletin of the Geological Society of America, 115(1), 3-20.
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  Abstract: East-west extensional basins are distributed across the southern half of the Tibetan plateau at an elevation of ∼4 km. These basins have generated much interest because of their potential implications for the regional tectonics and force distribution in the plateau. This study documents the sedimentology of the Miocene-Pliocene Thakkhola graben fill in order to reconstruct basin evolution and paleoenvironment. Analysis of depositional systems, paleodrainage patterns, and conglomerate clast provenance of the >1-km-thick graben fill sets limits on the timing of activity of the basin-bounding faults and the development of southward axial drainage in the basin. During the deposition of the oldest basin fill (Tetang Formation, ca. 11-9.6 Ma), probably in a restricted basin, minor motion occurred on the basin-bounding fault systems. An angular unconformity separates the Tetang and overlying Thakkhola Formations, where this contact can be observed in the southern part of the basin. Southward axial drainage was established by ca. 7 Ma with the onset of deposition of the Thakkhola Formation. Several episodes of damming of this drainage system are recorded by widespread lacustrine deposits in the southern part of the basin. Facies distribution and the progressive rotation of strata in the Thakkhola Formation indicate that the Dangardzong fault on the western edge of the basin was active at this time, and drainage ponding may have been related to displacement on normal faults associated with the South Tibetan detachment system to the south of Thakkhola graben. Contrasts between deposits of the Tetang and Thakkhola Formations provide evidence for environmental change in the basin. In the Tetang Formation, the abundance of lacustrine facies, the pollen record, and the absence of paleosol carbonate suggest that conditions were more humid than during subsequent deposition of the Thakkhola Formation. Environmental change in the Thakkhola graben coincided with environmental change observed in the Siwalik foreland basin sequence, Arabian Sea, and Bay of Bengal at ca. 8-7 Ma. Although this climate change event has been previously attributed to intensification of the Asian monsoon in response to uplift of the Tibetan plateau, paleoaltimetry data indicate that this region had already attained a high elevation by ca. 11 Ma. Thus, the Thakkhola graben stratigraphic record suggests that uplift of the southern Tibetan plateau and the onset of the Asian monsoon as inferred from paleoclimatic indicators were not directly related in a simple way.
• Gehrels, G. E., DeCelles, P. G., Martin, A., Ojha, T. P., Pinhassi, G., & Upreti, B. N. (2003). Initiation of the Himalayan orogen as an early Paleozoic thin-skinned thrust belt. GSA Today, 13(9), 4-9.
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  Abstract: Research by many workers in various regions of the Himalaya, combined with our recent geologic and geochronologic studies in Nepal, indicate that fundamental aspects of the Himalayan orogen originated in an early Paleozoic thrust belt and are unrelated to Tertiary India-Asia collision. Manifestations of early Paleozoic tectonism include ductile deformation, regional moderate- to high-grade metamorphism, large-scale south-vergent thrusting, crustal thickening and the generation of granitic crustal melts, uplift and erosion of garnet-grade rocks, and accumulation of thick sequences of synorogenic strata. Determining the relative contributions of early Paleozoic versus Tertiary tectonism constitutes a significant challenge in understanding the Himalayan orogen.
• Robinson, D. M., DeCelles, P. G., Garzione, C. N., Pearson, O. N., Harrison, T. M., & Catlos, E. J. (2003). Kinematic model for the Main Central thrust in Nepal. Geology, 31(4), 359-362.
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  Abstract: We present a kinematic model for the Himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late Miocene-Pliocene geochronologic and thermochronologic ages from rocks in the Main Central thrust zone in central Nepal. At its current exposure level, the Main Central thrust juxtaposes a hanging-wall flat in Greater Himalayan rocks with a footwall flat in Lesser Himalayan rocks of the Ramgarh thrust sheet, which is the roof thrust of a large Lesser Himalayan duplex. Sequential emplacement of the Main Central (early Miocene) and Ramgarh (middle Miocene) thrust sheets was followed by insertion of thrust sheets within the Lesser Himalayan duplex and folding of the Main Central and Ramgarh thrusts during the Miocene-Pliocene time. Thorium-lead (Th-Pb) ages of monazite inclusions in garnets from central Nepal record the timing of coeval, progressive metamorphism of Lesser Himalayan rocks in the footwall of the Main Central thrust. Although this model does not rule out minor, late-stage reactivation of the Main Central thrust, major late Miocene reactivation is not required.
• Robinson, D. M., DeCelles, P. G., Pearson, O. N., Garzione, C. N., Harrison, T. M., & Catlos, E. J. (2003). Kinematic model for the Main Central thrust in Nepal: Reply. Geology, 31(1), e41.
• DeCelles, P. G., Robinson, D. M., & Zandt, G. (2002). Implications of shortening in the Himalayan fold-thrust belt for uplift of the Tibetan Plateau. Tectonics, 21(6), 12-1 - 12-25.
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  Abstract: Recent research in the Himalayan fold-thrust belt provides two new sets of observations that are crucial to understanding the evolution of the Himalayan- Tibetan orogenic system. First, U-Pb zircon ages and Sm-Nd isotopic studies demonstrate that (1) Greater Himalayan medium- to high-grade metasedimentary rocks are much younger than true Indian cratonic basement; and (2) these rocks were tectonically mobilized and consolidated with the northern margin of Gondwana during early Paleozoic orogenic activity. These observations require that Greater Himalayan rocks be treated as supracrustal material in restorations of the Himalayan fold-thrust belt, rather than as Indian cratonic basement. In turn, this implies the existence of Greater Himalayan lower crust that is not exposed anywhere in the fold-thrust belt. Second, a regional compilation of shortening estimates along the Himalayan arc from Pakistan to Sikkim reveals that (1) total minimum shortening in the fold-thrust belt is up to ∼670 km; (2) total shortening is greatest in western Nepal and northern India, near the apex of the Himalayan salient; and (3) the amount of Himalayan shortening is equal to the present width of the Tibetan Plateau measured in an arc-normal direction north of the Indus-Yalu suture zone. This information suggests that a slab of Greater Indian lower crust (composed of both Indian cratonic lower crust and Greater Himalayan lower crust) with a north-south length of ∼700 km may have been inserted beneath the Tibetan crust during the Cenozoic orogeny. We present a modified version of the crustal underthrusting model for Himalayan-Tibetan orogenesis that integrates surface geological data, recent results of mantle tomographic studies, and broadband seismic studies of the crust and upper mantle beneath the Tibetan Plateau. Previous studies have shown that incremental Mesozoic and early Cenozoic shortening had probably thickened Tibetan crust to ∼45-55 km before the onset of the main Cenozoic orogenic event. Thus, the insertion of a slab of Greater Indian lower crust with maximum thickness of ∼20 km (tapering northward) could explain the Cenozoic uplift of the Tibetan Plateau. The need for Tibetan crust to stretch laterally as the Greater Indian lower crust was inserted may explain the widespread east-west extension in the southern half of the Plateau. Our reconstruction of the Himalayan fold-thrust belt suggests that Indian cratonic lower crust, of presumed mafic composition and high strength, should extend approximately halfway across the Tibetan Plateau, to the Banggong suture. From there northward, we predict that the Tibetan Plateau is underlain by more felsic, and therefore weaker, lower crust of Greater Himalayan affinity. Two slab break-off events are predicted by the model: the first involved Neotethyan oceanic lithosphere that foundered ∼45-35 Ma, and the second consisted of Greater Indian lithosphere (most likely composed of Greater Himalayan material) that delaminated and foundered ∼20-10 Ma. Asthenospheric upwelling associated with the break-off events may explain patterns of Cenozoic volcanism on the Tibetan Plateau. Although the model predicts a northward migrating topographic front due solely to insertion of Greater Indian lower crust, the actual uplift history of the Plateau was complicated by early-middle Tertiary shortening of Tibetan crust.
• DeCelles, P. G., & DeCelles, P. C. (2001). Rates of shortening, propagation, underthrusting, and flexural wave migration in continental orogenic systems. Geology, 29(2), 135-138.
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  Abstract: The rate of horizontal shortening in an orogenic wedge is the rate at which the length of undeformed crust decreases as it is incorporated into the orogen. This rate equals the rate of convergence of the foreland lithosphere toward the central surface of the orogenic belt and the rate of subduction of foreland lithosphere beneath the central surface. The rate of propagation of an orogenic wedge is the rate at which it elongates in the direction of horizontal shortening. This rate is controlled by the rates of mass accretion to the orogenic wedge and erosion. The orogenic belt drives a flexural isostatic wave through the foreland lithosphere at a velocity equal to the rate of propagation plus the rate of subduction (or convergence or shortening). In orogenic belts where the total amount of shortening cannot be reliably estimated from balanced regional cross sections, it may be possible to determine total shortening from the distance of flexural wave migration in the foreland basin and the width of the orogenic wedge. In addition, orogenic wedges may accelerate solely in response to a reduction in taper.
• Horton, B. K., & Decelles, P. G. (2001). Modern and ancient fluvial megafans in the foreland basin system of the Central Andes, Southern Bolivia: Implications for drainage network evolution if foldthrust belts. Basin Research, 13(1), 43-63.
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  Abstract: Fluvial megafans chronicle the evolution of large mountains drainage networks, providing a record of erosional denudation in adjacent mountain belts. An actualistic investigation of the development of fluvial megafans is presented here by comparing active fluvial megafans in the proximal foreland basin of the central Andes to Tertiary foreland-basin deposits exposed in the interior of the mountain belt. Modern fluvial megafans of the Chaco Plain of southern Bolivia are large (5800-22 600 km2), fan-shaped masses of dominantly sand and mud deposited by major transverse rivers (Rio Grande, Rio Parapeti, and Rio Pilcomayo) emanating from the central Andes. The rivers exit the mountain belt and debouch onto the low-relief Chaco Plain at fixed points along the mountain front. On each fluvial megafan, the presently active channel is straight in plan view and dominated by deposition of mid-channel and bank-attached sand bars. Overbank areas are characterized by crevasse-splay and paludal deposition with minor soil development. However, overbank areas also contain numerous relicts of recently abandoned divergent channels, suggesting a long-term distributary drainage pattern and frequent channel avulsions. The position of the primary channel on each megafan is highly unstable over short time scales. Fluvial megafans of the Chaco Plain provide a modern analogue for a coarsening-upward, >2-km-thick succession of Tertiary strata exposed along the Camargo syncline in the Eastern Cordillera of the central Andean fold-thrust belt, about 200 km west of the modern megafans. Lithofacies of the mid-Tertiary Camargo Formation include: (1) large channel and small channel deposits interpreted, respectively, as the main river stem on the proximal megafan and distributary channels on the distal megafan; and (2) crevasse-splay, paludal and palaeosol deposits attributed to sedimentation in overbank areas. A reversal in palaeocurrents in the lowermost Camargo succession and an overall upward coarsening and thickening trend are best explained by progradation of a fluvial megafan during eastward advance of the fold-thrust belt. In addition, the present-day drainage network in this area of the Eastern Cordillera is focused into a single outlet point that coincides with the location of the coarsest and thickest strata of the Camargo succession. Thus, the modern drainage network may be inherited from an ancestral mid-Tertiary drainage network. Persistence and expansion of Andean drainage networks provides the basis for a geometric model of the evolution of drainage networks in advancing fold-thrust belts and the origin and development of fluvial megafans. The model suggests that fluvial megafans may only develop once a drainage network has reached a particular size, roughly 104km2 - a value based on a review of active fluvial megafans that would be affected by the tectonic, climatic and geomorphologic processes operating in a given mountain belt. Furthermore, once a drainage network has achieved this critical size, the river may have sufficient stream power to prove relatively insensitive to possible geometric changes imparted by growing frontal structures in the fold-thrust belt.
• McQuarrie, N., & DeCelles, P. (2001). Geometry and structural evolution of the Central Andean backthrust belt, Bolivia. Tectonics, 20(5), 669-692.
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  Abstract: The central Andean backthrust belt is a large-scale west vergent thrust system along the western side of the Eastern Cordillera in the generally east vergent Andean fold-thrust belt of Bolivia. Although west vergent structures in the central Andes have been recognized previously, we describe the backthrust belt at a regional scale, emphasizing its implications for the kinematic development of the Andes and the subsequent influence of these kinematics on amounts of tectonic shortening. We use techniques such as line length balancing, restorability, and the viability of the progressive development of the structures to construct balanced cross sections across the backthrust belt and Altiplano. The cross sections are taken to a regional depth of detachment (basement) to examine the relationship between mapped surface structures and inferred subsurface structures. The relationship of the backthrust belt to the Altiplano suggests that the Altiplano basin is a crustal-scale piggyback basin created as a basement megathrust propagated up and over a half-crustal scale ramp located just west of the physiographic boundary of the Eastern Cordillera. This basement megathrust was the means by which a narrow Paleocene fold-thrust belt located to the west of the Altiplano propagated eastward and emerged in the present Eastern Cordillera. The relationship between the basement thrusts and the physiographic boundaries of the Central Andean plateau (as defined by Isacks [1988]) suggests that extensive megathrust sheets (involving crystalline basement or quartzite) may play an important role in the formation of orogenic plateaus. The kinematic development of the Andean fold-thrust belt indicates that the backthrust belt developed as a taper-building mechanism after the basement megathrust overextended the system eastward. The mechanism proposed in this study for the development of the central Andean backthrust belt requires a minimum of 200 km of shortening within the Altiplano/Eastern Cordillera alone. This increases minimum shortening estimates across the fold-thrust belt in Bolivia to as much as 300-340 km.
• Robinson, D. M., DeCelles, P. G., Patchett, P., & Garzione, C. N. (2001). The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes. Earth and Planetary Science Letters, 192(4), 507-521.
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  Abstract: Neodymium (Nd) isotopes from the Himalayan fold-thrust belt and its associated foreland basin deposits are useful for distinguishing between Himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen. Neodymium isotopic data from the Himalayan fold-thrust belt in Nepal (n = 35) reveal that the Lesser Himalayan zone consistently has a more negative εNd(0) value than the Greater and Tibetan Himalayan zones. Our data show the average εNd(0) value in the Lesser Himalayan zone is - 21.5, whereas the Greater and Tibetan Himalayan zones have an average εNd(0) value of - 16. These consistently distinct values throughout Nepal enable the use of Nd isotopes as a technique for distinguishing between Lesser Himalayan zone and Greater Himalayan zone rock. The less negative εNd(0) values of the Greater Himalayan rocks support the idea that the Greater Himalayan zone is not Indian basement, but rather a terrane that accreted onto India during Early Paleozoic time. Neodymium isotopic data from Eocene through Pliocene foreland basin deposits (n = 34) show that sediment provenance has been dominated by Greater and Tibetan Himalayan detritus across Nepal. The εNd(T) values in the synorogenic rocks in western and central Nepal generally show an up-section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones. At Ο 10 Ma in Khutia Khola and Ο 11 Ma in Surai Khola, a shift in εNd(T) values from - 16 to - 18 marks the erosional breaching of a large duplex in the northern part of the Lesser Himalayan zone. This shift is not seen in eastern Nepal, where the εNd(T) values remain close to - 16 throughout Miocene time because there has been less erosional unroofing in this region. © 2001 Elsevier Science B.V. All rights reserved.
• DeCelles, P. G., & Cavazza, W. (1999). A comparison of fluvial megafans in the Cordilleran (Upper Cretaceous) and modern Himalayan foreland basin systems. Bulletin of the Geological Society of America, 111(9), 1315-1334.
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  Abstract: The Campanian-Maastrichtian Hams Fork Conglomerate Member of the Evanston Formation in northeastern Utah and southwestern Wyoming consists of a widespread (>10 000 km2) boulder to pebble, quartzitic conglomerate that was deposited by east-southeastward-flowing, gravelly braided rivers on top of the frontal part of the Sevier fold-thrust belt and in the adjacent foredeep of the Cordilleran foreland basin. In northeastern Utah the conglomerate was deposited in a lobate fan-shaped body, up to 122 m thick, that trends southeastward away from its principal source terrane in the southern end of the Willard thrust sheet The Willard sheet contains thick Proterozoic quartzite units that produced highly durable clasts capable of surviving long-distance fluvial transport. Although the main source of sediment for the Hams Fork Conglomerate was the Willard sheet, the active front of the thrust belt lay 40-50 km to the east along the Absaroka thrust system. Displacement along the Absaroka system uplifted and topographically rejuvenated the Willard sheet, and antecedent drainages carried detritus from hinterland source terranes into the proximal foreland basin. Although topographic ridges associated with fault-propagation anticlines along frontal thrusts locally influenced transport directions, they provided relatively little sediment to the Hams Fork Conglomerate. Lithofacies, paleocurrent, and isopach data indicate that the Hams Fork Conglomerate was deposited in fluvial megafans and stream-dominated alluvial fans, similar in scale and processes to megafans and alluvial fans in southern Nepal and northern India that are forming along the proximal side of the Himalayan foreland basin system. The Himalayan fluvial megafans have areas of 103-104 km2, slopes of 0.05°-0.18°, and are deposited by large transverse rivers that are an-tecedent to frontal Himalayan structures and topography. The main fluvial channels on the upper parts of the megafans are anastomosed and braided at bankfull stage but commonly have braided thalwegs at low-flow stage. Downstream, these channels become predominantly braided and meandering and ultimately merge with the axial Ganges trunk river system. Stream-dominated alluvial fans in the Himalayan foreland basin system fringe the topographic front of the fold-thrust belt in the intermegafan areas. These fans have areas of ∼102 km2 and slopes of ∼ 0.5°. The proximal parts of both types of fans are dominated by extremely coarse (boulder-cobble) bedload that is in transit mainly during the monsoon. The prevalence of fluvial megafans in the modern and Miocene Himalayan foreland and in the Upper Cretaceous-lower Tertiary stratigraphic record of the Cordilleran foreland suggests that these types of deposits may be the volumetrically largest gravel accumulations in nonmarine foreland basin systems.
• Cavazza, W., & DeCelles, P. G. (1998). Upper Messinian siliciclastic rocks in southeastern Calabria (southern Italy): Paleotectonic and eustatic implications for the evolution of the central Mediterranean region. Tectonophysics, 298(1-3), 223-241.
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  Abstract: The Messinian stratigraphy of eastern Calabria (southern Italy) is characterised by a threefold subdivision: (1) a pelite section with local limestone and gypsum, deposited in a restricted-marine environment, is unconformably, or disconformably, overlain by (2) coarse-grained alluvial conglomerate, which is in turn locally overlain by (3) a thin and discontinuous ribbon-shaped sedimentary body of sandstone and pelite, commonly displaying a shallow-marine to continental progradational trend. The basal unconformity/disconformity, coarse grain-size, and abrupt compositional-sedimentological change of unit 2 with respect to unit 1 can be explained as a response to tectonic instability and out-of-sequence thrusting in the Calabrian orogenic wedge, possibly induced by isostatic back-tilting of the wedge following the desiccation of the Mediterranean Sea. This mechanism could explain widespread late Messinian thrusting and syntectonic sedimentation along the Apenninic-Maghrebian orogenic belt. The uppermost Messinian continental to shallow-marine siliciclastic deposits of unit 3 crop out today at elevations of up to 300 m. Similar, age-equivalent sedimentary deposits can be traced along the Apennines and the Sicilian Maghrebides, thus, indicating that the Mediterranean area was flooded before deposition of the Trubi Formation, the base of which is traditionally regarded as marking the reestablishment of marine conditions in the Mediterranean region.
• Coogan, J. C., & DeCelles, P. G. (1998). Extensional collapse along the Sevier Desert reflection, northern Sevier Desert basin, western United States: Reply. Geology, 26(5), 475-.
• Camilleri, P., Yonkee, A., Coogan, J., Decelles, P., McGrew, A., & Wells, M. (1997). Hinterland to foreland transect through the Sevier Orogen, Northeast Nevada to North central Utah: Structural style, metamorphism, and kinematic history of a large contractional orogenic wedge. Brigham Young University Geology Studies, 42(1), 297-380.
• Horton, B. K., & DeCelles, P. G. (1997). The modern foreland basin system adjacent to the Central Andes. Geology, 25(10), 895-898.
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  Abstract: Regional variations in sediment thickness, internal structures, average elevation, and Bouguer gravity define a four-component foreland basin system adjacent to the Central Andes. In the most proximal part of the foreland basin system, the eastern Subandean zone and western-most Chaco Plain, 1-3 km of Cenozoic deposits overlies active folds and thrusts of the frontal Andean orogenic wedge. These wedge-top deposits pass cratonward into a foredeep depozone containing a 3-4-km-thick sedimentary prism that tapers toward (and locally pinches out against) a broad-wavelength forebulge in the central-eastern Chaco Plain. The forebulge is underlain by Precambrian-Mesozoic rocks and is largely covered by a thin veneer of Quaternary alluvium. East of the forebulge, a thin (0.5 km) saucer-shaped accumulation of sediment beneath the Pantanal Wetland represents a back-bulge depozone. Ancient counterparts of these four depozones can be identified in the Central Andes, suggesting that modern basin architecture is the result of continuous, eastward migration of the coupled orogenic wedge and foreland basin system since the Late Cretaceous-Paleocene.
• Lawton, T. F., Sprinkel, D. A., Decelles, P. G., Mitra, G., Sussman, A. J., & Weiss, M. P. (1997). Stratigraphy and structure of the Sevier thrust belt and proximal foreland-basin system in central Utah: A transect from the Sevier Desert to the Wasatch Plateau. Brigham Young University Geology Studies, 42(2), 33-67.
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  Abstract: The Sevier orogenic belt in central Utah comprises four north-northwest trending thrust plates and two structural culminations that record crustal shortening and uplift in late Mesozoic and early Tertiary time. Synorogenic clastic rocks, mostly conglomerate and sandstone, exposed within the thrust belt were deposited in wedge-top and foredeep depozones within the proximal part of the foreland-basin system. The geologic relations preserved between thrust structures and synorogenic deposits demonstrate a foreland-breaking sequence of thrust deformation that was modified by minor out-of-sequence thrust displacement. Structural culminations in the interior part of the thrust belt deformed and uplifted some of the thrust sheets following their emplacement. Strata in the foreland basin indicate that the thrust sheets of central Utah were emplaced between latest Jurassic and Eocene time. The oldest strata of the foredeep depozone (Cedar Mountain Formation) are Neocomian and were derived from the hanging wall of the Canyon Range thrust. The foredeep depozone subsided most rapidly during Albian through Santonian or early Campanian time and accumulated about 2.5 km of conglomeratic strata (Indianola Group). The overlying North Horn Formation accumulated in a wedge-top basin from the Campanian to the Eocene and records propagation of the Gunnison thrust beneath the former foredeep. The Canyon Range Conglomerate of the Canyon Mountains, equivalent to the Indianola Group and the North Horn Formation, was deposited exclusively in a wedge-top setting on the Canyon Range and Pavant thrust sheets. This field trip, a three day, west-to-east traverse of the Sevier orogenic belt in central Utah, visits localities where timing of thrust structures is demonstrated by geometry of cross-cutting relations, growth strata associated with faults and folds, or deformation of foredeep deposits. Stops in the Canyon Mountains emphasize geometry of late structural culminations and relationships of the Canyon Range thrust to growth strata deposited in the wedge-top depozone. Stops in the San Pitch Mountains illustrate deposits of the foredeep depozone and younger, superjacent wedge-top depozone. Stops in the Sanpete Valley and western part of the Wasatch Plateau examine the evolution of the foreland-basin system from foredeep to wedge-top during growth of a triangle zone near the front of the Gunnison thrust.
• Yonkee, W. A., Decelles, P. G., & Coogan, J. (1997). Kinematics and synorogenic sedimentation of the eastern frontal part of the sevier orogenic wedge, northern Utah. Brigham Young University Geology Studies, 42(1), 335-380.
• Coogan, J. C., & DeCelles, P. G. (1996). Extensional collapse along the Sevier Desert reflection, northern Sevier Desert basin, western United States. Geology, 24(10), 933-936.
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  Abstract: Newly released and previously published seismic reflection data from the northern Sevier Desert basin provide a complete seismic transect between the tilted western margin of the basin and the eastern breakaway zone. When tied to well and surface age data, the transect delineates a continuum of extensional fault and basin fill geometries that developed between late Oligocene and Pleistocene time across the basin. A minimum of 18 km of top-to-the-west normal displacement is estimated across the Sevier Desert from only the most conspicuous growth geometries and offsets across listric normal faults that sole downward into the Sevier Desert reflection (SDR). The SDR clearly marks a normal fault zone beneath the entire basin, where stratal truncations are imaged for 50% of the 39 km length of the reflection east of the Cricket Mountains block. Restoration of extensional displacement along this entire 39 km fault length is necessary to reconstruct the pre-Oligocene configuration and erosion level of Sevier thrust sheets across the Sevier Desert area. The SDR normal fault zone underlies the former topographic crest of the Sevier orogenic belt, where it accommodated extensional collapse after cessation of regional contractile tectonism.
• DeCelles, P. G., & Currie, B. S. (1996). Long-term sediment accumulation in the Middle Jurassic-early Eocene Cordilleran retroarc foreland-basin system. Geology, 24(7), 591-594.
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  Abstract: The late Middle Jurassic-early Eocene (∼120 m.y.) sediment-accumulation history of the Cordilleran foreland basin in northern Utah exhibits a sigmoidal pattern on a rate vs. time plot, with moderate rates of accumulation during late Middle Jurassic, very low net rates during Late Jurassic-earliest Cretaceous, increasingly rapid rates during Early-middle Cretaceous, and low rates during Late Cretaceous-early Eocene time. This pattern is consistent with deposition in a prograding foreland-basin system that comprised integrated back-bulge, forebulge, foredeep, and wedge-top depozones. The upper Middle Jurassic represents the back-bulge depozone; the Upper Jurassic was deposited on the eastern flank of a flexural forebulge; the basal Cretaceous unconformity is the result of eastward migration of the forebulge; the thick, Lower-middle Cretaceous succession represents the foredeep depozone; and the Upper Cretaceous-early Eocene embodies the syndepositionally deformed wedge-top depozone. Previous models that explain Middle-Late Jurassic stratigraphic patterns in terms of foredeep subsidence (alone) and a Late Jurassic hiatus in crustal shortening in the Cordilleran orogen are shown to be neither necessary nor supported by evidence from the Cordilleran hinterland.
• DeCelles, P. G., & Giles, K. A. (1996). Foreland basin systems. Basin Research, 8(2), 105-123.
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  Abstract: A foreland basin system is denned as: (a) an elongate region of potential sediment accommodation that forms on continental crust between a contractional orogenic belt and the adjacent craton, mainly in response to geodynamic processes related to subduction and the resulting peripheral or retroarc fold-thrust belt; (b) it consists of four discrete depozones, referred to as the wedge-lop, foredeep, forebulge and back-bulge depozones - which of these depozones a sediment particle occupies depends on its location at the time of deposition, rather than its ultimate geometric relationship with the thrust belt; (c) the longitudinal dimension of the foreland basin system is roughly equal to the length of the fold-thrust belt, and does not include sediment that spills into remnant ocean basins or continental rifts (impactogens). The wedge-top depozone is the mass of sediment that accumulates on top of the frontal part of the orogenic wedge, including 'piggyback' and 'thrust top' basins. Wedge-top sediment tapers toward the hinterland and is characterized by extreme coarseness, numerous tectonic unconformities and progressive deformation. The foredeep depozone consists of the sediment deposited between the structural front of the thrust belt and the proximal flank of the forebulge. This sediment typically thickens rapidly toward the front of the thrust belt, where it joins the distal end of the wedge-top depozone. The forebulge depozone is the broad region of potential flexural uplift between the foredeep and the back-bulge depozones. The back-bulge depozone is the mass of sediment that accumulates in the shallow but broad zone of potential flexural subsidence cratonward of the forebulge. This more inclusive definition of a foreland basin system is more realistic than the popular conception of a foreland basin, which generally ignores large masses of sediment derived from the thrust belt that accumulate on top of the orogenic wedge and cratonward of the forebulge. The generally accepted definition of a foreland basin attributes sediment accommodation solely to flexural subsidence driven by the topographic load of the thrust belt and sediment loads in the foreland basin. Equally or more important in some foreland basin systems are the effects of subduction loads (in peripheral systems) and far-field subsidence in response to viscous coupling between subducted slabs and mantle-wedge material beneath the outboard part of the overlying continent (in retroarc systems). Wedge-top depozones accumulate under the competing influences of uplift due to forward propagation of the orogenic wedge and regional flexural subsidence under the load of the orogenic wedge and/or subsurface loads. Whereas most of the sediment accommodation in the foredeep depozone is a result of flexural subsidence due to topographic, sediment and subduction loads, many back-bulge depozones contain an order of magnitude thicker sediment fill than is predicted from flexure of reasonably rigid continental lithosphere. Sediment accommodation in back-bulge depozones may result mainly from aggradation up to an equilibrium drainage profile (in subaerial systems) or base level (in flooded systems). Forebulge depozones are commonly sites of unconformity development, condensation and stratal thinning, local fault-controlled depocentres, and, in marine systems, carbonate platform growth. Inclusion of the wedge-top depozone in the definition of a foreland basin system requires that stratigraphic models be geometrically parameterized as doubly tapered prisms in transverse cross-sections, rather than the typical 'doorstop' wedge shape that is used in most models. For the same reason, sequence stratigraphic models of foreland basin systems need to admit the possible development of type I unconformities on the proximal side of the system. The oft-ignored forebulge and back-bulge depozones contain abundant information about tectonic processes that occur on the scales of orogenic belt and subduction system.
• Decelles, P. G., & Cavazza, W. (1995). Upper Messinian conglomerates in Calabria, southern Italy: response to orogenic wedge adjustment following Mediterranean sea-level changes. Geology, 23(9), 775-778.
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  Abstract: Widespread uppermost Miocene conglomerate and sandstone along the Apenninic-Maghrebian orogenic belt in the central Mediterranean region cannot be explained as a result of the Messinian base-level falls. Along the Ionian coast of Calabria, southern Italy, these rocks were deposited in marine fan deltas and rest in angular unconformity or disconformity upon the internal part of the Calabrian accretionary wedge. It is proposed that the upper Messinian deposits were produced by internal shortening of the Calabrian accretionary wedge as it compensated for the decrease in upper surface slope caused by flexural rebound as the ~3.4-km-thick Ionian water mass evaporated. Latest Miocene-Pliocene marine inundation reloaded the basin, restored the wedge to a critical state, and caused the rear part of the wedge again to become tectonically stable. -from Authors
• Decelles, P. G., & Mitra, G. (1995). History of the Sevier orogenic wedge in terms of critical taper models, northeast Utah and southwest Wyoming. Geological Society of America Bulletin, 107(4), 454-462.
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  Abstract: Problems with applying critical taper models to ancient orogenic wedges are overcome in the Late Cretaceous-late Paleocene Sevier orogenic wedge in Utah and Wyoming by a symptomatic approach in which wedge behavior is inferred from the timespace distribution of thrust faulting, erosion, and synorogenic sediment accumulation in association with the orogenic wedge. In turn, the causes of wedge behavior are interpreted in terms of features that are known about the Sevier wedge, such as the locations of major decollements and the lithologic constituents of major thrust sheets. From Coniacian through late Paleocene time (~35 m.y.), basement and cover rocks in the Sevier wedge were shortened by ~100 km in three major and one minor events. An overall eastward progression of thrusting was punctuated by several episodes of out-of-sequence and hinterland vergent thrusting. -from Authors
• Decelles, P. G., Lawton, T. F., & Mitra, G. (1995). Thrust timing, growth of structural culminations, and synorogenic sedimentation in the type Sevier orogenic belt, western United States. Geology, 23(8), 699-702.
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  Abstract: Most of the regional shortening in the type area of the Sevier orogenic belt in central Utah was accommodated by displacement on the Canyon Range (Neocomian-Aptian), Pavant (Aptian-Albian), Paxton (Cenomanian-Campanian), and Gunnison (late Campanian-Paleocene) thrust systems. Inception of each thrust system generated synorogenic sediment associated with frontal thrust-tip anticlines or triangle zones and older thrust sheets that were elevated above major ramps farther toward the hinterland. The Sevier culmination, a large antiformal duplex cored by crystalline basement rocks, developed during Paxton aand Gunnison thrusting west of and structurally beneath the Canyon Range and Pavant thrusts. Growth of the Sevier culmination was coeval with reactivation of the Canyon Range and Pavant thrust systems and produced a second culmination in Proterozoic-Lower Cambrian rocks in the Canyon Range. -from Authors
• Decelles, P. G. (1994). Late Cretaceous-Paleocene synorogenic sedimentation and kinematic history of the Sevier thrust belt, northeast Utah and southwest Wyoming. Geological Society of America Bulletin, 106(1), 32-56.
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  Abstract: Integration of cross-cutting structural relationships, overlapping sedimentary units, new conglomerate provenance data, and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of Late Cretaceous through Paleocene thrust faulting in the northeast Utah-southwest Wyoming part of the Sevier thrust belt. These data indicate a general eastward progression of deformation that was punctuated by local out-of-sequence and hinterlandward-verging events. Provenance data delimit a sequential restoration of a regional cross section. Comparison of the sequential restoration with the Late Cretaceous subsidence history and isopach patterns in the distal foreland basin of western Wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the Wasatch culmination. Growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge. -from Author
• Cavazza, W., & Decelles, P. G. (1993). Geometry of a Miocene submarine canyon and asociated sedimentary facies in southeastern Calabria, southern Italy. Geological Society of America Bulletin, 105(10), 1297-1309.
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  Abstract: Major erosion surfaces cutting into basement rocks define several submarine paleocanyons. Paleocanyon fills consist of large, lenticular conglomerate bodies that are 200-580 m thick and 3-6 km wide. The best example of these paleocanyons is located near the town of Stilo. The conglomeratic canyon fill and the adjacent muddy slope deposits are both overlain by a laterally continuous sequence, 160 m thick, composed of two units of fine-grained, thin-bedded turbidites alternating with two units of thicker sandstone and minor pebble-conglomerate beds. The paleocanyons probably originated as subaerial valleys in response to a major fall in relative sea level at 30 Ma and were later submerged by a combination of relative sea-level rise and concomitant tectonic activity. -from Authors
• Decelles, P. G., Pile, H. T., & Coogan, J. C. (1993). Kinematic history of the Meade Thrust based on provenance of the Bechler conglomerate at Red Mountain, Idaho, Sevier thrust belt. Tectonics, 12(6), 1436-1450.
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  Abstract: The Meade thrust fault is one of the major thrusts in the Sevier thrust belt, yet its age of displacement has remained enigmatic because of the lack of a derivative synorogenic deposit. We proposed that the synorogenic conglomerate produced by initial Meade thrusting crops out on Red Mountain, in southeastern Idaho, ~1.5 km east of the present trace of the Meade thrust. This conglomerate historically has been considered part of the Ephraim Formation of the Lower Cretaceous Gannett Group, but is is suggested that it is a conglomeratic facies of the Bechler Formation localized to the area of Red Mountain. Regional stratigraphic considerations and paleontological dates from underlying and overlying strata indicate that the Bechler conglomerate facies is Aptian in age. -from Authors
• Ridgway, K. D., & DeCelles, P. G. (1993). Petrology of mid-Cenozoic strike-slip basins in an accretionary orogeny St. Elias Mountains, Yukon Territory, Canada. Special Paper of the Geological Society of America, 284, 67-89.
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  Abstract: Eocene-Oligocene strike-slip faulting along the Denali fault system in Yukon Territory produced several local strike-slip basins that were filled by coarse-grained alluvial, fluvial, and lacustrine sediment referred to as the Amphitheatre Formation. The Amphitheatre Formation provides an excellent opportunity to study in detail the composition of siliciclastic sediment in a major, accretionary strike-slip orogen. Petrographic analyses of sandstones and clast counts of conglomerates reveal a lithologically diverse provenance. On standard QFL and QmFLt diagrams, sandstones from the Burwash and Bates Lake basins are arkosic, overlapping the uplifted basement and dissected magmatic arc provenance fields of Dickinson and Suczek (1979). Conglomerates were derived from volcanic, plutonic, and medium- to low-grade metasedimentary rocks that currently are found in several accreted terranes associated with the Denali fault system. Wrangellia and the Alexander terrane provided voluminous volcanic, metavolcanic (greenstone), sedimentary, and low-grade metasedimentary material. The Yukon Crystalline terrane provided medium-grade (gneissic and schistose) material and plutonic material, and the Gravina-Nutzotin belt provided pelitic and metasedimentary material. These data support the contention that the Amphitheatre Formation was derived mainly from local, high-relief sources associated with major uplift in the eastern St. Elias Mountains, as well as the southern Yukon Crystalline terrane. The Eocene-Oligocene climatic event apparently had no significant effect on Amphitheatre sandstone composition. This is attributed to the proximal character of the sandstones studied; steep local relief kept soil residence times to a minimum in spite of climatic change. In addition, the persistence of coal- and stream-dominated alluvial fan facies throughout the entire stratigraphic section indicates that climate remained humid in southwestern Yukon Territory, possibly owing to local orographic effects. It is proposed that, in general, climate should not have a major impact on detrital sand composition in large strike-slip orogens, where sediment accumulates within the orogen itself and is subject neither to long distances of transport nor to long-term residence in lowland soil profiles where most compositional modification is expected.
• Ridgway, K. D., & Decelles, P. G. (1993). Stream-dominated alluvial fan and lacustrine depositional systems in Cenozoic strike-slip basins, Denali fault system, Yukon Territory, Canada. Sedimentology, 40(4), 645-666.
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  Abstract: The Amphitheatre Formation filled several strike-slip basins in Yukon Territory and consists of up to 1200 m of coarse siliciclastic rocks and coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent analysis of the Amphitheatre Formation in two of these strike-slip basins document the transition from proximal, to middle, to distal and fringing environments within ancient stream-dominated alluvial-fan systems. Coal distribution in the Amphitheatre Formation is closely coupled with predominant depositional processes on stream-dominated alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. -from Authors
• DeCelles, P. G., & Burden, E. T. (1992). Non-marine sedimentation in the overfilled part of the jurassic-cretaceous Cordilleran foreland basin: Morrison and Cloverly Formations, central Wyoming, USA. Basin Research, 4(3-4), 291-313.
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  Abstract: Lithostratigraphic, chronostratigraphic, sedimentological and petrological data from the Upper Jurassic-Lower Cretaceous Morrison and Cloverly Formations in central Wyoming allow detailed characterization of the early history of the central part of the Cordilleran foreland basin. The Morrison is divisible into three informal members: (1) a lower sandstone, deposited by a complex coastal dune-foreshore-fluvial system during retreat of the Sundance sea; (2) a middle mudstone, deposited by muddy fluvial and ephemeral lacustrine systems during a period of regional, seasonal aridity; and (3) an upper sandstone, deposited by a sandy fluvial system of variable sinuosity. The overlying Cloverly Formation is divisible into two informal members: (1) a lower mudstone (previously considered as part of the Morrison Formation), deposited by muddy fluvial and lacustrine systems; and (2) an upper chert-pebble conglomerate and sandstone, deposited primarily by gravel-dominant braided rivers. Palynological data and a single fission-track date indicate that the lower part of the middle Morrison mudstone is early to middle Oxfordian and the upper part of the lower Cloverly mudstone is Valanginian. Morrison sandstones are subarkosic, with average %QFL = 91,6, 3 and %QmFLt = 83, 6, 11. Cloverly sandstones are cherty litharenites and sublitharenites, with average %QFL = 99.6, 0,0.4 and %QmFLt = 82,0,18 (Gazzi-Dickinson point-counting method). Palaeocurrent data and sandstone compositions indicate a complex provenance including exirrabasinal sources in lower Mesozoic and upper Palaeozoic sedimentary and volcanic rocks of the Cordillera and intrabasinal sources of Proterozoic clasts in south-central Wyoming. Cloverly sandstone compositions in the eastern part of the study area were influenced by short-term fluvial reworking within the basin. The thickness of the composite Morrison-Cloverly succession is practically constant over a distance of several hundred km east of the Idaho-Wyoming thrust belt, and its internal chronostratigraphic zones are subparallel. On the other hand, equivalent strata in the Gannett Group of the thrust belt are at least three times thicker. This indicates that the Morrison and Cloverly in central Wyoming were deposited within the overfilled part of the foreland basin. Preliminary regional correlation indicates that coarse-grained lithofacies in these rocks are significantly time-transgressive, generally becoming younger toward the E and NE. Overfilling of the early Cordilleran foreland basin in central Wyoming was accomplished by progradation from the W and S. In spite of their three-dimensional (3D) complexity, the Morrison and Cloverly Formations generally confirm theoretical model predictions for overfilled foreland basins.
• Decelles, P. G., & Cavazza, W. (1992). Constraints on the formation of Pliocene hummocky cross- stratification in Calabria (southern Italy) from consideration of hydraulic and dispersive equivalence, grain-flow theory, and suspended-load fallout rate. Journal of Sedimentary Petrology, 62(4), 555-568.
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  Abstract: Hummocky cross-stratification (HCS) occurs in two Pliocene progradational nearshore-marine to fluvial sequences. The HCS is notable because of its shallow depth of deposition (~2-5m) and its coarse grain size (up to coarse sand). Individual units of HCS consist of, in ascending order, three subfacies which are characterized by distinctive internal structure: a massive subfacies; a centimeter-scale laminated (meso-laminated) subfacies; and a millimeter-scale laminated (micro-laminated) subfacies. Preservation of the Guardavalle HCS in a very shallow-marine environment was promoted by the microtidal, storm-wave-dominated character of the Pliocene Ionian coast of Calabria. -from Authors
• Fraser, G. S., & DeCelles, P. G. (1992). Geomorphic controls on sediment accumulation at margins of foreland basins. Basin Research, 4(3-4), 233-252.
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  Abstract: The occurrence of cyclic patterns of sedimentation on a large scale, or abrupt changes in lithology or facies patterns in foreland basins, are most commonly attributed to tectonism. Climatic controls are invoked much less often, and geomorphic controls are rarely considered except for small-scale features. Tectonism is the first-order control on sedimentation at mountain fronts by providing accommodation space for sediment accumulation, and the requisite energy for the system to operate. However, geomorphic controls on sediment yield from source areas, transformation of sediment yield in transfer systems, and feedback mechanisms between source areas and depositional basins may be the secondary controls on sediment dispersal and accumulation near mountain fronts. Drainage basins pass through a series of stages during their evolution. Sediment flux is large during initial stages of basin evolution, allowing fans to prograde rapidly. But as drainage nets in the source area expand, and valleys increase their capacity to store sediment eroded from interfluves, the quantity and caliber of the sediment load at the outlet diminishes, and fan sequences begin to fine upward. Eventually the source area drainage network will expand to its maximum size. Relief is greatest in the source area at this time, and the quantity and calibre of the sediment eroded from valley walls reaches a maximum. Dynamic equilibrium between fan and source area is attained and a period during which spontaneous incision of source area valleys, fanhead entrenchment, and depositional lobe progradation occurs The amount and size of sediment supplied to the fan reaches a maximum at this time, and fan deposits coarsen upward through this period. However, feedback relationships between fan and depositional basin limit the ability of fans to prograde basinward. Fans begin to retrograde as relief in the source area declines and storage capacity increases. Fining-upward sequences arc deposited and basinal facies encroach on the fan during this final phase. Ancient alluvial fans commonly consist of coarsening-then fining-upward stratigraphic sequences consistent with this evolutionary model, and cross-sections of alluvial fan deposits normally show that fan facies stack vertically near their upland sources. A typical first-order fan deposit ranges in thickness between 100 m and 250 m, but those in foreland basins near thrust margins are considerably thicker, possibly in response to increased accommodation space provided by basin subsidence, extended periods of downcutting caused by continued movement on the thrust, and to basinward translation of the source area.
• Cole, R. B., & Decelles, P. G. (1991). Subaerial to submarine transitions in early Miocene pyroclastic flow deposits, southern San Joaquin basin, California. Geological Society of America Bulletin, 103(2), 221-235.
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  Abstract: The Tecuya volcanic member is an important ancient example in which the depositional environments of subaerial to submarine pyroclastic deposits are well constrained by bounding units. The characteristics of the subaerial and submarine pyroclastic flow deposits thus may be useful in other ancient volcaniclastic successions which lack bounding units of obvious paleoenvironmental origin. -from Authors
• Decelles, P. G. (1991). Controls on synorogenic alluvial-fan architecture, Beartooth Conglomerate (Palaeocene), Wyoming and Montana. Sedimentology, 38(4), 567-590.
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  Abstract: Systematic unroofing sequences and intraformational unconformities, folds, and faults in the conglomerate attest to deposition during uplift. Along the eastern flank, at least three ancient alluvial-fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies. A seven-fold hierarchy of bounding surfaces and enclosed lithosomes exists in the Beartooth Conglomerate. -from Authors
• Decelles, P. G. (1991). Kinematic history of a foreland uplift from Paleocene synorogenic conglomerate, Beartooth Range, Wyoming and Montana. Geological Society of America Bulletin, 103(11), 1458-1475.
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  Abstract: An integrated structural, sedimentological, and provenance study of the upper Paleocene Beartooth Conglomerate along the eastern flank of the Beartooth Range yields new information about Laramide tectonics in the northern Rocky Mountain foreland and has implications for models of fault-propagation folding and the development of synorogenic basins in thrust-faulted terranes. By utilizing the results of provenance modelling and the orientations of the basal and intraformational angular unconformities, we have been able to retrodeform stepwise an area balanced cross section through the eastern Beartooth uplift. Retrodeformation of the cross section indicates that 3.8km of uplift occurred before deposition of the oldest part of the proximal Beartooth Conglomerate. Further uplift of ~1.7km and total horizontal shortening of ~3.7km accompanied ~1.3km of displacement on the Beartooth fault. Our data indicate that during the early stages of uplift by fault-propagation folding, sediment derived from the uplift bypassed the proximal realm and was deposited in distal settings. Bypassing occurred because the eventual footwall of the Beartooth fault was uplifted during fault-propagation folding. Viewed from this perspective, the early fine-grained fills of many Laramide basins may owe their existence to a combination of fine-grained source material and sediment bypassing during the early stages of Laramide uplifts. -from Authors
• Sharma, M., Basu, A. R., Cole, R. B., & DeCelles, P. G. (1991). Basalt-rhyolite volcanism by MORB-continental crust interaction: Nd, Sr-isotopic and geochemical evidence from southern San Joaquin Basin, California. Contributions to Mineralogy and Petrology, 109(2), 159-172.
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  Abstract: The early miocene Tecuya volcanic center in the southern San Joaquin basin of California consists of flows and tuffs of basalt and rhyolite that erupted, closely spaced in time, in both submarine and subaerial conditions. The rhyolites are overlain by the basalts and constitute approximately 45% of a total of at least 180 km3 of the Tecuya volcanic rocks. The basalts have εNd(t) values of +2 to +6 and (87Sr/86Sr)i values between 0.7035 and 0.7052. These rocks show LREE enrichment [(La/Yb)N =2.4-5.5; La=28-150 times chondrite] and higher Th/U, Th/Ta, Rb/Ta, Ba/Ta, Cs/Rb but lower K/Rb ratios than MORB. Combined major- and trace-element, and Sr-Nd isotopic data suggest the involvement of subcontinental lithosphere, depleted upper mantle source (MORB), and local continental crust in the basalt petrogenesis. εNd(t) values in rhyolites vary from +1.5 to +3.7 while (87Sr/86Sr)i ratios range from 0.7051 to 0.7064. The rhyolites display LREE enrichment [(La/Yb)N=10; La=100 times chondrite] along with a distinct negative Eu anomaly (Eu/Eu*=0.75) and depletion of Ti and P. Mixing relations in (87/86Sr)i - εNd(t) space among basalts, rhyolites, and local continental crust indicate that the Tecuya rhyolites were produced by assimilation of variable amounts of continental crust by MORB-related magmas and subcontinental lithosphere-derived melts. This conclusion is supported by the synchroneity of Tecuya volcanism at 22 Ma with interaction of a segment of the East Pacific Rise along the southern California margin. The Tecuya volcanic rocks thus provide an example for the generation of rhyolitic melts owing to crustal assimilation by basaltic melts during mid-oceanic ridge-induced magmatism along a continental margin. © 1991 Springer-Verlag.
• Basu, A. R., Sharma, M., & DeCelles, P. G. (1990). Nd, Sr-isotopic provenance and trace element geochemistry of Amazonian foreland basin fluvial sands, Bolivia and Peru: implications for ensialic Andean orogeny. Earth and Planetary Science Letters, 100(1-3), 1-17.
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  Abstract: Nd and Sr isotopes and the trace element contents, including the rare earths, were determined for fluvial sands of lithic arenite composition from the Madre de Dios foreland basin of Bolivia and Peru. On standard petrologic ternary diagrams, the sands fall in the recycled orogen provenance field and thus are similar to typical ancient foreland basin composition. The average rare earth elemental pattern of the sands is identical to the upper continental crustal average, as estimated from post-Archean composite shales of different continents. Ratios of Th U, Co Th, La Sc and Th Sc of the fluvial sands are intermediate between an average magmatic arc and an upper crustal average compositions. The dispersion of some trace elemental patterns in the sands can be attributed to fractionation of dense minerals, including zircon, during the sedimentation process. The variations of Nd isotopes in conjunction with the petrographic parameters of lithic metamorphic (Lm) and volcanic (Lv) fragments allow a two-fold classification of the sands. These two sand types can be interpreted in terms of mixing among three different provenances: one volcanic rock-suite with less negative ε{lunate}Nd(O) parameter than the other volcanic suite, and a third metasedimentary source with ε{lunate}Nd(O) value of around -12, which is considered to be similar to the average western Brazilian shield composition. Thus the overall compositions of the sands has been modeled as mechanical mixtures of two components, an Andean magmatic arc and the Brazilian shield-derived metasediments. The model is strongly supported by a plot of ε{lunate}Nd(O) versus ε{lunate}Sr(O) of the sands. In this plot, the Type 1 and 2 sands define two coherent hyperbolic trends contiguous with two different portions of the Andean magmatic trend. This relationship has been interpreted to indicate that the observed Andean magmatic trend in an ε{lunate}Sr(O)-ε{lunate}Nd(O) diagram is the result of varying degrees of contamination of a "primitive arc-type" magma by the Precambrian continental crust of the western Brazilian Shield. The depleted mantle average Nd model age of 1.46 Ga for the fluvial sands reflects the average age of the Brazilian continental crustal source. The development of the Andean orogenic belt has been discussed schematically with the isotopic data of the sands. The model describes a trailing edge prism of sediments, derived from the Brazilian Shield during the late Paleozoic-early Mesozoic. The prism becomes part of the fold-thrust belt during the Andean orogeny in the Neogene, when the foreland basin develops with the basin fill partly derived from the fold-thrust belt. The sedimentary rocks in the fold-thrust belt are also a major source of contaminants for the Andean magmas. The contiguous nature of the Andean magmatic trend and the fluvial sand data in the ε{lunate}Sr(O)-ε{lunate}Nd(O) diagram suggests that the ensialic Andean magmatic arc has remained connected to its parent continent, the western Brazilian Shield, throughout the development of the Andean orogeny. © 1990.
• DeCelles, P. G., & Hertel, F. (1990). Petrology of fluvial sands from the Amazonian foreland basin, Peru and Bolivia: Reply. Bulletin of the Geological Society of America, 102(12), 1729-1730.
• Decelles, P. G., & Hertel, F. (1989). Petrology of fluvial sands from the Amazonian foreland basin, Peru and Bolivia. Geological Society of America Bulletin, 101(12), 1552-1562.
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  Abstract: Modal sand compositions in nine rivers draining the proximal part of the Amazonian retroarc foreland basin in Peru and Bolivia provide support for the Recycled Orogen provenance model of Dickinson and Suczek. The overriding control on sand composition is the relative area of bedrock type exposed in the fold-thrust belt. Sands reworked from Tertiary and Quaternary foreland-basin fill are relatively enriched in quartz, possibly owing to intense weathering of sediments in the soil profile. -from Authors
• Decelles, P. G. (1988). Lithologic provenance modeling applied to the Late Cretaceous synorogenic Echo Canyon Conglomerate, Utah: a case of multiple source areas. Geology, 16(11), 1039-1043.
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  Abstract: Indicates that the Echo Canyon was derived from three separate stratigraphic sections exposed on the inactive Willard thrust sheet and the eastern flank of the Wasatch basement culmination. -from Author
• Decelles, P. G. (1988). Middle Cenozoic depositional, tectonic, and sea level history of southern San Joaquin basin, California. American Association of Petroleum Geologists Bulletin, 72(11), 1297-1322.
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  Abstract: The Eocene to lower Miocene fill of the southern San Joaquin basin contains three complete depositional sequences. The Tejon sequences (lower to middle Eocene) is marine and incorporates nearshore, shelf, slope, and basinal deposits. The San Emigdio (upper Eocene and lower Oligocene) and Pleito (Upper Oligocene) sequences intertongue eastward with alluvial-fan deposits of the Tecuya Formation. The lower part of the San Emigdio sequence was deposited by a westward-prograding Gilbert-type delta. The upper part of the San Emigdio sequence and lower part of the Pleito sequence were deposited by a system of shelf fan-deltas that prograded at least 10 km to the west. The middle and upper parts of the Pleito sequence were deposited by a slope fan-delta in relatively deep (hundreds of meters) water. Regional transgression during the early Eocene initiated deposition in the southern San Joaquin basin. The lower San Emigdio Gilbert-type delta prograded from the shelf edge during a lowstand in eustatic sea level at approximately 40 Ma. Relative highstand deposits in the San Emigdio and Pleito Formations consist of widespread progradational shallow-marine and nonmarine facies. The Eocene to early Miocene tectonic history of the southern San Joaquin basin included three distinct periods of increasingly intense activity. In general, local tectonic events, rather than eustatic sea level events, seem to have exerted the predominant control on middle Cenozoic sedimentation in the southern San Joaquin basin. -from Author
• Schwartz, R. K., & DeCelles, P. G. (1988). Cordilleran foreland basin evolution in response to interactive Cretaceous thrusting and foreland partitioning, southwestern Montana. Memoir of the Geological Society of America, 171(1), 489-513.
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  Abstract: Cretaceous strata of southwestern Montana indicate that the large, first-order Cordilleran Foreland Basin was tectonically partitioned by a complex of second-order intraforeland uplifts that occupied sites of eventual classic Laramide intraforeland uplifts. The intraforeland uplifts and intervening basins formed a block-like surficial pattern throughout the evolving Cretaceous foreland. Preliminary stratigraphic and sedimentary petrologic evidence suggests that some of the same structures were active during Jurassic time. Thus, the Laramide orogeny in southwestern Montana can be viewed as the structural culmination of a long (at least 100 m.y.) history of basementinvolved deformation. The developmental history of the Cordilleran Foreland in southwestern Montana is divisible into three stages: (1) an early stage of embryonic foreland development in Late Permian-Late Jurassic time, characterized by carbonate shallow-marine sedimentation with local coarse siliciclastic input from intraforeland structural elements; (2) a middle stage of fully developed, first-order foreland-basin subsidence in Early-early Late Cretaceous time, characterized by siliciclastic-dominated nonmarine and shallow-marine sedimentation; major reactivated intraforeland uplift (and subsidence); and eastward encroachment of the Cordilleran (Sevier) thrust belt; and (3) a late stage, during latest Cretaceous-early Eocene time, characterized by impingement of the frontal Cordilleran thrust belt upon major intraforeland uplifts (Laramide foreland uplifts) and nonmarinedominated synorogenic sedimentation. The early stage (Late Permian-Late Jurassic) of foreland development was temporally associated with terrane accretion and other tectonic events in the Cordillera. The onset of first-order basin subsidence and a coeval increase in intraforeland tectonism were temporally associated with a second phase of Cordilleran tectonism and eastward migration of the Cordilleran thrust belt. Deposition of the Cretaceous Kootenai, Blackleaf, and lower Frontier Formations in southwestern Montana occurred during this middle stage of foreland-basin evolution. Source areas consisted of the Cordilleran fold-thrust upland, intraforeland uplifts, and the unstable craton. Thus, the Cretaceous stratigraphic sequence in southwestern Montana was produced by the interaction of Cordilleran and intraforeland tectonic events. The entire Kootenai-Blackleaf-lower Frontier sequence developed during a single cycle bounded by tectonic maxima (Cordilleran and intraforeland) with an intervening period of tectonic quiescence and continued foreland-basin subsidence. However, this longer term (∼20 m.y.) sequence was punctuated by higher frequency (∼3 to 4 m.y.) events, most clearly signalled by intraforeland reactivation. The first-order behavior of the Cordilleran Foreland Basin in southwestern Montana is partly explained as the flexural response to thrust-sheet loading of a viscoelastic lithosphere. However, explanation of foreland-basin evolution in Montana requires a model that incorporates an additional, interactive mechanism for broad-scale reactivation of second-order (i.e., intraforeland) elements.
• Decelles, P. G. (1987). Variable preservation of middle Tertiary, coarse-grained, nearshore to outer-shelf storm deposits in southern California.. Journal of Sedimentary Petrology, 57(2), 250-264.
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  Abstract: The strata of the San Emigdio Range in southern California were deposited along a shoreline adjacent to a tectonically active highland that formed the southern margin of the San Joaquin Basin. Abundant storm deposits (sandstone and conglomerate) are preserved in the San Emigdio, Pleito, and lower Temblor Formations. Extensive lateral exposures allow recognition of fair-weather and storm-dominated facies in contemporaneous nearshore, inner-shelf, and outer-shelf deposits. -from Author
• Decelles, P. G., Tolson, R. B., Graham, S. A., Smith, G. A., Ingersoll, R. V., White, J., Schmidt, C. J., Rice, R., Moxon, I., Lemke, L., Handschy, J. W., Follo, M. F., Edwards, D. P., Cavazza, W., & Caldwell, M. (1987). LARAMIDE THRUST-GENERATED ALLUVIAL-FAN SEDIMENTATION, SPHINX CONGLOMERATE, SOUTHWESTERN MONTANA.. American Association of Petroleum Geologists Bulletin, 71(2), 135-155.
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  Abstract: The Sphinx Conglomerate consists of a thick overall coarsening-upward megasequence that was shed in front of the eastward migrating Madison-Gravelly thrust plate during the Laramide orogeny. Lithofacies and paleocurrent data indicate that it was deposited primarily by east-northeastward flowing, boulder- and cobble-rich streams, augmented by debris flows, on a system of at least two vertically and laterally coalesced alluvial fans. Trends in clast size, lithofacies distributions, and clast compositions indicate that both alluvial fan progradation in response to thrusting and influx of Paleozoic carbonate clasts controlled development.
• Decelles, P. G. (1986). Sedimentation in a tectonically partitioned, non-marine foreland basin: the Lower Cretaceous Kootenai Formation, southwestern Montana ( USA).. Geological Society of America Bulletin, 97(8), 911-931.
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  Abstract: The Lower Cretaceous Kootenai Formation in southwestern Montana was deposited in the nonmarine, Cordilleran foreland basin during a period of intensified uplift in the westward adjacent Sevier fold-thrust belt. Concurrently, the foreland basin was partitioned by uplift of intra-foreland structural elements and incipient plutonism. Kootenai fluvial and fluviolacustrine depositional systems developed and evolved in response to changing basin architecture. The Kootenai thus provides a case study of nonmarine sedimentary responses to tectonic partitioning in a foreland basin. Criteria for recognition of tectonic partitioning of nonmarine foreland basin are described. -from Author
• Decelles, P. G., & Gutschick, R. C. (1983). Mississippian wood-grained chert and its significance in the western interior United States ( Deseret limestone).. Journal of Sedimentary Petrology, 53(4), 1175-1191.
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  Abstract: Wood-grained chert is nodular chert which has internal light- and dark-colored, concentric, 3-D, compositional banding resembling grain in wood. The light-colored bands are dolomitic and the dark-colored bands are quartzose with carbonaceous matter. It is recognized in the Mississippian Lodgepole, Deseret, and Great Blue Limestones and some correlatives of these formations in Montana, Utah, and Idaho. The Deseret starved basin model provides a paleogeographic framework that restricts the wood-grained cherts in this basin to the foreslope between the oxygenated carbonate platform margin and the anaerobic-dysaerobic deep water basin. Wood-grained chert may mark the approximate position of the lower limit of the pycnocline in the Deseret starved basin.-from Authors
• Decelles, P. G., Langford, R. P., & Schwartz, R. K. (1983). Two new methods of paleocurrent determination from trough cross- stratification.. Journal of Sedimentary Petrology, 53(2), 629-642.
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  Abstract: Two new methods are presented for the determination of paleocurrent directions from three-dimensional exposures of oblique cuts through individual trough limbs and two-dimensional exposures of sets of trough cross-strata. Three-dimensional exposures of trough limbs can be measured and treated statistically on a stereographic plot to determine the average trough-axis orientation. In two-dimensional exposures, characteristic asymmetry of the basal scour surface and truncation of foreset laminae can aid in estimation, within about 25o, of paleocurrent direction. - from Authors