Fault Slip and Exhumation History of the Willard Thrust Sheet, Sevier Fold‐Thrust Belt, Utah: Relations to Wedge Propagation, Hinterland Uplift, and Foreland Basin Sedimentation

Zircon (U‐Th)/He (ZHe) and zircon fission track thermochronometric data for 47 samples spanning the areally extensive Willard thrust sheet within the western part of the Sevier fold‐thrust belt record enhanced cooling and exhumation during major thrust slip spanning approximately 125–90 Ma. ZHe and zircon fission track age‐paleodepth patterns along structural transects and age‐distance relations along stratigraphic‐parallel traverses, combined with thermo‐kinematic modeling, constrain the fault slip history, with estimated slip rates of ~1 km/Myr from 125 to 105 Ma, increasing to ~3 km/Myr from 105 to 92 Ma, and then decreasing as major slip was transferred onto eastern thrusts. Exhumation was concentrated during motion up thrust ramps with estimated erosion rates of ~0.1 to 0.3 km/Myr. Local cooling ages of approximately 160–150 Ma may record a period of regional erosion, or alternatively an early phase of limited... (see full abstract in article)

Thermochronological Constraints on the Timing and Magnitude of Miocene and Pliocene Extension in the Central Wassuk Range, Western Nevada

Apatite fission track and (U-Th)/He thermochronological data provide new constraints on the timing of faulting and exhumation of the Wassuk Range, western Nevada, where east dipping normal faults have accommodated large-magnitude ENE-WSW oriented extension. Extensional deformation has resulted in the exhumation of structurally coherent fault blocks that expose sections of preextensional mostly granitic upper crust in the Grey Hills and central Wassuk Range. These fault blocks display westward tilts of ∼60° and expose preextensional paleodepths of up to ∼8.5 km, based on the structural reconstruction of tilted preextensional Tertiary andesite flows that unconformably overlie Mesozoic basement rocks. Apatite fission track and (U-Th)/He thermochronological data from the fault blocks constrain the onset of rapid footwall exhumation at ∼15 Ma. Fission track modeling results indicate rapid fault block exhumation occurred between ∼15 and 12 Ma, which is in agreement with Miocene volcanic rocks that bracket the tilting history. In addition, fission track and (U-Th)/He data suggest reduced rates of cooling following major extension, as well as renewed cooling related to active, high-angle faulting along the present-day range front starting at ∼4 Ma. Thermochronological data from structurally restored fault blocks indicate a preextensional Miocene geothermal gradient of 27° ± 5°C/km. The thermochronological constraints on the timing of extensional faulting and the eruptive history in the Wassuk Range imply a model for extension where crustal heating and volcanism precede the onset of rapid large magnitude extension, and where synextensional magmatism is suppressed during the highest rates of extension

Regional Pliocene exhumation of the Lesser Himalaya in the Indus drainage

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Clift, P. D., Zhou, P., Stockli, D. F., & Blusztajn, J. Regional Pliocene exhumation of the Lesser Himalaya in the Indus drainage. Solid Earth, 10(3), (2019): 647-661, doi:10.5194/se-10-647-2019.New bulk sediment Sr and Nd isotope data, coupled with U–Pb dating of detrital zircon grains from sediment cored by the International Ocean Discovery Program in the Arabian Sea, allow the reconstruction of erosion in the Indus catchment since ∼17 Ma. Increasing εNd values from 17 to 9.5 Ma imply relatively more erosion from the Karakoram and Kohistan, likely linked to slip on the Karakoram Fault and compression in the southern and eastern Karakoram. After a period of relative stability from 9.5 to 5.7 Ma, there is a long-term decrease in εNd values that corresponds with increasing relative abundance of >300 Ma zircon grains that are most common in Himalayan bedrocks. The continuous presence of abundant Himalayan zircons precludes large-scale drainage capture as the cause of decreasing εNd values in the submarine fan. Although the initial increase in Lesser Himalaya-derived 1500–2300 Ma zircons after 8.3 Ma is consistent with earlier records from the foreland basin, the much greater rise after 1.9 Ma has not previously been recognized and suggests that widespread unroofing of the Crystalline Lesser Himalaya and to a lesser extent Nanga Parbat did not occur until after 1.9 Ma. Because regional erosion increased in the Pleistocene compared to the Pliocene, the relative increase in erosion from the Lesser Himalaya does not reflect slowing erosion in the Karakoram and Greater Himalaya. No simple links can be made between erosion and the development of the South Asian Monsoon, implying a largely tectonic control on Lesser Himalayan unroofing.This research has been supported by the USSSP (grant no. 355-001)

Two-Phase Westward Encroachment of Basin and Range Extension into the Northern Sierra Nevada

Structural, geophysical, and thermochronological data from the transition zone between the Sierra Nevada and the Basin and Range province at latitude ~39°N suggest ~100 km westward encroachment of Basin and Range extensional deformation since the middle Miocene. Extension, accommodated primarily by cast dipping normal faults that bound west tilted, range-forming fault blocks, varies in magnitude from150% in the Wassuk and Singatse Ranges to the east. Geological and apatite fission track data from exhumed upper crustal sections in the Wassuk and Singatse Ranges point to rapid footwall cooling related to large magnitude extension starting at ~14-15 Ma. Farther to the west, geological and thermochronological data indicate a younger period of extension in the previously unextended Pine Nut Mountains, the Carson Range, and the Tahoe-Truckee depression initiated between 10 Ma and 3 Ma, and incipient post-0.5 Ma faulting to the west of the Tahoe-Truckee area. These data imply the presence of an extensional breakaway zone between the Singatse Range and the Pine Nut Mountains at ~14-15 Ma, forming the boundary between the Sierra Nevada and Basin and Range at that time. In addition, fission track data imply a Miocene preextensional geothermal gradient of 27 ± 5°C km -1 in the central Wassuk Range and 20 ± 5°C km -1 in the Singatse Range, much higher than the estimated early Tertiary gradient of 10 ± 5°C km -1 for the Sierra Nevada batholith. This might point to a significant increase in geothermal gradients coupled with a likely decrease in crustal strength enabling the initiation of extensional faulting. Apatite fission track, geophysical, and geological constraints across the Sierra Nevada-Basin and Range transition zone indicate a two-stage, coupled structural and thermal westward encroachment of the Basin and Range province into the Sierra Nevada since the middle Miocene

Zircon U-Pb age constraints on NW Himalayan exhumation from the Laxmi Basin, Arabian Sea

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Zhou, P., Stockli, D. F., Ireland, T., Murray, R. W., & Clift, P. D. Zircon U-Pb age constraints on NW Himalayan exhumation from the Laxmi Basin, Arabian Sea. Geochemistry Geophysics Geosystems, 23(1), (2022): e2021GC010158, https://doi.org/10.1029/2021GC010158.The Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program recovered a discontinuous erosional record for the Indus River drainage dating back to at least 9.8 Ma, and with a single sample from 15.6 Ma. We dated detrital zircon grains by U-Pb geochronology to reconstruct how erosion patterns changed through time. Long-term increases in detrital zircon U-Pb components of 750–1,200 and 1,500–2,300 Ma record increasing preferential erosion of the Himalaya relative to the Karakoram between 8.3–7.0 and 5.9–5.7 Ma. The average contribution of Karakoram-derived sediment to the Indus Fan fell from 70% of the total at 8.3–7.0 Ma to 35% between 5.9 and 5.7 Ma. An increase in the contribution of 1,500–2,300 Ma zircons starting between 2.5 and 1.6 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon age spectra is consistent with bulk sediment Nd isotope data. The initial change in spatial erosion patterns at 7.0–5.9 Ma occurred during a time of drying climate in the foreland. The increase in ILH erosion postdated the onset of dry-wet glacial-interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the ILH thrust duplex, especially during the Pliocene, also played an important role, while the influence of the Nanga Parbat Massif to the total sediment flux was modest.This work was partially funded by a grant from the USSSP, as well as additional funding from the Charles T. McCord Chair in petroleum geology at LSU, and the Chevron (Gulf) Centennial professorship and the UTChron Laboratory at the University of Texas

Geochronology of the middle Eocene Purple Bench locality (Devil’s Graveyard Formation), Trans-Pecos Texas, USA

Purple Bench is a middle Eocene fossil locality in the Devil’s Graveyard Formation of the Trans-Pecos region of West Texas. In addition to yielding a range of taxa characteristic of the Uintan North American Land Mammal Age, the Purple Bench locality is noteworthy in documenting a number of endemic species that are known only from the site. Despite the Uintan character of the mammalian fauna, the absolute age of Purple Bench is a matter of debate. This uncertainty stems from the wide interval of time encompassed by current radiometric dates bracketing the Purple Bench locality and from conflicting magnetostratigraphic correlations in the Devil’s Graveyard Formation. This study constrains the absolute age of the Purple Bench locality through detrital zircon U-Pb geochronological analyses. For these analyses, 147 new detrital zircon U-Pb ages were collected from five tuffaceous sandstones and reworked tuff horizons and analyzed via Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). These new detrital zircon U-Pb geochronological analyses suggest a maximum depositional age of 43.7 +0.8 / -0.2 Ma for the Purple Bench tuff, a significant marker horizon immediately below the Purple Bench locality. These new maximum depositional age dates presented here provide constraints on the true depositional age of the lower and middle members of the Devil’s Graveyard Formation, bringing clarity to the previously ambiguous age of the fossil-bearing Purple Bench locality. The age constraints presented here also aid the characterization of the temporally and spatially variable Uintan North American Land Mammal Age

Regional Pliocene exhumation of the Lesser Himalaya in the Indus drainage

New bulk sediment Sr and Nd isotope data, coupled with U–Pb dating of detrital zircon grains from sediment cored by the International Ocean Discovery Program in the Arabian Sea, allow the reconstruction of erosion in the Indus catchment since ∼17&thinsp;Ma. Increasing εNd values from 17 to 9.5&thinsp;Ma imply relatively more erosion from the Karakoram and Kohistan, likely linked to slip on the Karakoram Fault and compression in the southern and eastern Karakoram. After a period of relative stability from 9.5 to 5.7&thinsp;Ma, there is a long-term decrease in εNd values that corresponds with increasing relative abundance of &gt;300&thinsp;Ma zircon grains that are most common in Himalayan bedrocks. The continuous presence of abundant Himalayan zircons precludes large-scale drainage capture as the cause of decreasing εNd values in the submarine fan. Although the initial increase in Lesser Himalaya-derived 1500–2300&thinsp;Ma zircons after 8.3&thinsp;Ma is consistent with earlier records from the foreland basin, the much greater rise after 1.9&thinsp;Ma has not previously been recognized and suggests that widespread unroofing of the Crystalline Lesser Himalaya and to a lesser extent Nanga Parbat did not occur until after 1.9&thinsp;Ma. Because regional erosion increased in the Pleistocene compared to the Pliocene, the relative increase in erosion from the Lesser Himalaya does not reflect slowing erosion in the Karakoram and Greater Himalaya. No simple links can be made between erosion and the development of the South Asian Monsoon, implying a largely tectonic control on Lesser Himalayan unroofing.</p

Constraining Deformation in the North Pamir and the Westernmost Tarim Basin

Abstract HKT-ISTP 2013 A

Dielectronic Recombination in He+ Ions

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Use of FLUXNET in the Community Land Model development

The Community Land Model version 3 (CLM3.0) simulates land-atmosphere exchanges in response to climatic forcings. CLM3.0 has known biases in the surface energy partitioning as a result of deficiencies in its hydrological and biophysical parameterizations. Such models, however, need to be robust for multidecadal global climate simulations. FLUXNET now provides an extensive data source of carbon, water and energy exchanges for investigating land processes, and it encompasses a global range of ecosystem-climate interactions. Data from 15 FLUXNET sites are used to identify and improve model deficiencies. Including a prognostic aquifer, a bare soil evaporation resistance formulation and numerous other changes in the model result in a significantly improved soil hydrology and energy partitioning. Terrestrial water storage increased by up to 300 mm in warm climates and decreased in cold climates. Nitrogen control of photosynthesis is revealed as another missing process in the model. These improvements increase the correlation coefficient of hourly and monthly latent heat fluxes from a range of 0.5–0.6 to the range of 0.7–0.9. RMSE of the simulated sensible heat fluxes decrease by 20–50%. Primary production is overestimated during the wet season in mediterranean and tropical ecosystems. This might be related to missing carbon-nitrogen dynamics as well as to site-specific parameters. The new model (CLM3.5) with an improved terrestrial water cycle should lead to more realistic land-atmosphere exchanges in coupled simulations. FLUXNET is found to be a valuable tool to develop and validate land surface models prior to their application in computationally expensive global simulations