College of Letters & Science
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The College of Letters and Science, the largest center for learning, teaching and research at Montana State University, offers students an excellent liberal arts and sciences education in nearly 50 majors, 25 minors and over 25 graduate degrees within the four areas of the humanities, natural sciences, mathematics and social sciences.
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Item Regional Exhumation and Tectonic History of the Shanxi Rift and Taihangshan, North China(American Geophysical Union, 2021-03) Clinkscales, Christopher; Kapp, Paul; Thomson, Stuart; Wang, Houqi; Laskowski, Andrew; Orme, Devon A.; Pullen, AlexThis study presents a comprehensive low-temperature thermochronometric data set from the Shanxi Rift, Taihangshan, and eastern Ordos block in North China, including new apatite fission track and apatite (U-Th-Sm)/He data and published apatite and zircon fission track and (U-Th-Sm)/He data. We use these data and new thermal history inversion models to reveal that the Shanxi Rift and Taihangshan experienced an increase in cooling rates between ca. 110–70 Ma and ca. 50–30 Ma. A preceding ca. 160–135 Ma cooling event is generally restricted to the western rift margin in the Lüliangshan and Hengshan. In contrast, the ca. 50–30 Ma cooling event was widespread and occurred coevally with the opening of the Bohai Basin and slip across the NNE-striking Eastern Taihangshan fault. In the southern rift zone, however, exhumation beginning ca. 50 Ma was likely associated with fault block uplift across the ESE–striking Qinling and Huashan faults, which accompanied the extensional opening of the Weihe Graben. Coeval fault slip along the NNE–striking Eastern Taihangshan faults and ESE–striking Qinling and Huashan faults was associated with NW-SE extension in North China related to oblique subduction of the Pacific plate under Eastern Asia and slow convergence rates. The Shanxi Rift is commonly attributed to Late Miocene and younger extension, but our new thermochronologic data do not precisely record the onset of rifting. However, our inversion models do suggest ≤∼50°C of Neogene–Quaternary cooling, consistent with ≤∼2 km of footwall uplift across most range-bounding faults.Item Sedimentology and provenance of newly identified Upper Cretaceous trench basin strata, Dênggar, southern Tibet: Implications for development of the Eurasian margin prior to India–Asia collision(Wiley, 2021-04) Orme, Devon A.; Laskowski, Andrew K.; Zilinsky, Misia F.; Chao, Wang; Guo, Xudong; Cai, Fulong; Lin, DingTrench basins preserved along the southern margin of the Lhasa Terrane, Tibet, are sedimentologic records of convergent margin processes preceding Cenozoic India–Asia collision. We present new sedimentologic, petrographic and geochronologic data from the Rongmawa Formation and surrounding strata near Dênggar, Tibet, to determine depositional environment, provenance and age. Depositional ages range from ca. 92 to 87 Ma and lithofacies are consistent with deposition by low- and high-density turbidity currents and suspension settling of pelagic detritus in a deep-marine, trench basin setting. Sandstone modal analyses and U–Pb geochronology indicate that trench basin detritus in this region was derived from the Lhasa Terrane. We interpret that the Cretaceous subduction trench received detritus from an axial sediment dispersal system that transported sediment from headwaters in the central-southern Lhasa terrane near Lhasa City directly to the trench and then flowed westwards parallel to the trench. The preservation of trench basin strata deposited during Late Cretaceous time compared with the lack of trench deposits prior to ca. 92 Ma and after ca. 80 Ma suggests the margin experienced a period of significant accretion during this interval. In addition, deposition of trench basin strata occurred during Late Cretaceous adakitic magmatism and high-temperature metamorphism, which are hypothesized to be explained by subduction of an oceanic ridge or subduction zone retreat and related upper plate extension along the southern margin of the Lhasa terrane. Subduction of an oceanic ridge may provide a mechanism to potentially erode forearc basin strata and promote increased sediment delivery directly to the trench.Item Record of Crustal Thickening and Synconvergent Extension from the Dajiamang Tso Rift, Southern Tibet(MDPI AG, 2021-05) Burke, William B.; Laskowski, Andrew K.; Orme, Devon A.; Sundell, Kurt E.; Taylor, Michael H.; Guo, Xudong; Ding, LinNorth-trending rifts throughout south-central Tibet provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts. In this study, we present new data from the Dajiamang Tso rift, including quantitative crustal thickness estimates calculated from trace/rare earth element zircon data, U-Pb geochronology, and zircon-He thermochronology. These data constrain the timing and rates of exhumation in the Dajiamang Tso rift and provide a basis for evaluating dynamic models of synconvergent extension. Our results also provide a semi-continuous record of Mid-Cretaceous to Miocene evolution of the Himalayan-Tibetan orogenic belt along the India-Asia suture zone. We report igneous zircon U-Pb ages of ~103 Ma and 70–42 Ma for samples collected from the Xigaze forearc basin and Gangdese Batholith/Linzizong Formation, respectively. Zircon-He cooling ages of forearc rocks in the hanging wall of the Great Counter thrust are ~28 Ma, while Gangdese arc samples in the footwalls of the Dajiamang Tso rift are 16–8 Ma. These data reveal the approximate timing of the switch from contraction to extension along the India-Asia suture zone (minimum 16 Ma). Crustal-thickness trends from zircon geochemistry reveal possible crustal thinning (to ~40 km) immediately prior to India-Eurasia collision onset (58 Ma). Following initial collision, crustal thickness increases to 50 km by 40 Ma with continued thickening until the early Miocene supported by regional data from the Tibetan Magmatism Database. Current crustal thickness estimates based on geophysical observations show no evidence for crustal thinning following the onset of E–W extension (~16 Ma), suggesting that modern crustal thickness is likely facilitated by an underthrusting Indian lithosphere balanced by upper plate extension.Item Mesoproterozoic–Early Cretaceous provenance and paleogeographic evolution of the Northern Rocky Mountains: Insights from the detrital zircon record of the Bridger Range, Montana, USA(2020-08) Ronemus, Chance B.; Orme, Devon A.; Campbell, Sare; Black, Sophie R.; Cook, JohnThe Bridger Range of southwest Montana, USA, preserves one of the most temporally extensive sedimentary sections in North America, with strata ranging from Mesoproterozoic to Cretaceous in age. This study presents new detrital zircon geochronologic data from eight samples collected across this mountain range. Multidimensional scaling and non-negative matrix factorization statistical analyses are used to quantitatively unmix potential sediment sources from these and 54 samples compiled from previous studies on regional correlative strata. We interpret these sources based on reference data from preserved strata with detrital zircon signatures likely representative of ancient sediment sources. We link these sources to their sinks along sediment dispersal pathways interpreted using available paleogeographic constraints. Our results show that Mesoproterozoic strata in southwest Montana contain detritus derived from the nearby craton exposed along the southern margin of the fault-bounded Helena Embayment. Middle Cambrian strata were dominated by the recycling of local sources eroded during the development of the Great Unconformity. In Devonian–Pennsylvanian time, provenance in southwest Montana shifted to more distal sources along the northeastern to southeastern margins of Laurentia, but more western basins received detritus from outboard sources along a tectonically complicated margin. By the Late Jurassic, provenance in the developing retroarc foreland basin system was dominated by Cordilleran magmatic arcs and fold-thrust belt sources to the west. Eastward propagation of the fold-thrust belt caused recycling of Paleozoic and Jurassic detritus into the foreland basin to dominate by the Early Cretaceous.Item Resetting Southern Tibet: The serious challenge of obtaining primary records of Paleoaltimetry(2020-08) Quade, J.; Leary, R.; Dettinger, M. P.; Orme, Devon A.Over the last several decades a number of studies have attempted to reconstruct the rise of the Himalaya and Tibet since India-Asia collision began in the early Cenozoic, with rather contradictory results. Here, we evaluate the efforts at reconstructing the history of this major mountain-building event as archived in oxygen, carbon, and clumped isotope records from carbonates exposed on the southern Tibetan Plateau. We find that a number of potential isotopic records of paleoaltimetry from Tibet – using both conventional oxygen and clumped isotope systems–may have been reset during burial and heating. Without exception, the marine δ18O values of Cretaceous and Paleogene marine carbonates across the orogen have been reset from their primary values of 0 ± 4‰ (VPDB) to lower values between −5 and −20‰, most conspicuously in the Indo-Asian suture zone. For this and other reasons, we view isotopic records of paleoaltimetry from the suture zone and adjacent Gangdese arc with great caution, especially early Cenozoic basin sediments that have experienced similar burial and heating as the underlying marine limestones. Outside the suture zone/arc, marine carbonates retain the isotopic imprint of low-elevation, meteoric diagenesis, a result that supports paleoaltimetric reconstruction from these areas using younger non-marine carbonates. We are even more cautious about temperature reconstructions using clumped isotope analyses, which previous studies suggest are sensitive to resetting at modest burial temperatures. With a few possible exceptions, primary clumped isotope values have probably not withstood the elevated temperatures of the suture zone and magmatic arc, nor burial depths of >3 km in basins outside the suture. In light of susceptibility to alteration of oxygen isotopes in carbonates in orogenic belts, future paleoaltimetric reconstruction prior to the Miocene on the Tibetan Plateau should couple δ18O, Δ47, and Δ'17O analysis of carbonates with analysis of noncarbonate archives—such as silicates and organic matter— that are less susceptible to resetting. Carbon, unlike oxygen, isotopic values from paleosol carbonate are well preserved from all periods, due to the very low C/O ratio of most altering fluids. Samples from within the suture zone yield a record of paleo-vegetation change covering much of the Cenozoic. Carbon isotopic values from paleosols have no analog among modern Tibetan soils and most resemble in appearance and chemical composition the vegetated soils in the lowlands of northern India today. Carbon isotopes from paleosols depict a major reduction in vegetation cover in the suture zone since 20 Ma, probably due to a combination of uplift of the suture zone and global cooling. Northward on the Lhasa and Qiantang terranes, the landscape was less vegetated in the Oligocene compared to the contemporaneous suture zone, but more vegetated and less arid than it is today.Item The Ancestral Lhasa River: A Late Cretaceous trans-arc river that drained the proto-Tibetan Plateau(2019-09-19) Laskowski, Andrew K.; Orme, Devon A.; Cai, Fulong; Ding, LinLate Cretaceous trench basin strata were deposited in the subduction zone that consumed Neo-Tethyan oceanic lithosphere along the southern margin of the proto–Tibetan Plateau. We conducted detrital zircon (DZ) U-Pb geochronology on six trench basin samples (n = 1716) collected near Dênggar, Tibet (∼500 km west of Lhasa), to assess the provenance of these rocks and reconstruct Late Cretaceous sediment transport pathways. They contained DZ ages that point to a unique source around Lhasa city, north of the Late Cretaceous Gangdese magmatic arc. The modern Lhasa River catchment contains the requisite sources, and its main trunk transects the Gangdese magmatic arc, joining with the Yarlung River at a barbed junction at the India-Asia suture. We infer that the Lhasa River is an ancient feature that transported sediment to the subduction zone in Late Cretaceous time and persisted during India-Asia collision.Item Four-dimensional model of Cretaceous depositional geometry and sediment flux in the northern Great Valley forearc, California(2018-12) Orme, Devon A.; Graham, Stephan A.The Great Valley forearc basin records Jurassic(?)-Eocene sedimentation along the western margin of North America during eastward subduction of the Farallon plate and development of the Sierra Nevada magmatic arc. The four-dimensional (4-D) basin model of the northern Great Valley forearc presented here was designed to reconstruct its depositional history from Tithonian through Maastrichtian time. Based on >1200 boreholes, the tops of 13 formations produce isopach maps and cross sections that highlight the spatial and temporal variability of sediment accumulation along and across the basin. The model shows the southward migration of depocenters within the basin during the Cretaceous and eastward lapping of basin strata onto Sierra Nevada basement. In addition, the model presents the first basement map of the entire Sacramento subbasin, highlighting its topography at the onset of deposition of the Great Valley Group. Minimum volume estimates for sedimentary basin fill reveal variable periods of flux, with peak sedimentation corresponding to deposition of the Sites Sandstone during Turonian to Coniacian time. Comparison of these results with flux estimates from magmatic source regions shows a slight lag in the timing of peak sedimentation, likely reflecting the residence time from pluton emplacement to erosion. This model provides the foundation for the first three-dimensional subsidence analysis on an ancient forearc basin, which will yield insight into the mechanisms driving development of accommodation along convergent margins.Item Burial and exhumation history of the Xigaze Forearc, Yarlung Suture Zone, Tibet(2017-12) Orme, Devon A.The Cretaceous–Eocene Xigaze forearc basin is a crucial data archive for understanding the tectonic history of the Asian continental margin prior to and following collision with India during the early Cenozoic Era. This study reports apatite and zircon (U–Th)/He thermochronologic data from fourteen samples from Albian-Ypresian Xigaze forearc strata to determine the degree and timing of heating (burial) and subsequent cooling (exhumation) of two localities along the Yarlung suture zone (YSZ) near the towns of Saga and Lazi. Thirty-seven individual zircon He ages range from 31.5 ± 0.8 Ma to 6.06 ± 0.18 Ma, with the majority of grains yielding ages between 30 Ma and 10 Ma. Twenty apatite He ages range from 12.7 ± 0.5 Ma to 3.9 ± 0.3 Ma, with the majority of grains yielding ages between 9 Ma and 4 Ma. These ages suggest that the Xigaze forearc basin was heated to 140–200 °C prior to cooling in Oligocene–Miocene time. Thermal modeling supports this interpretation and shows that the samples were buried to maximum temperatures of ∼140–200 °C by 35–21 Ma, immediately followed by the onset of exhumation. The zircon He and apatite He dataset and thermal modeling results indicate rapid exhumation from ∼21 Ma to 15 Ma, and at ∼4 Ma. The 21–15 Ma thermochronometric signal appears to be regionally extensive, affecting all the lithotectonic units of the YSZ, and coincides with movement along the north-vergent Great Counter Thrust system. Thrusting, coupled with enhanced erosion possibly related to the paleo-Yarlung River, likely drove Early Miocene cooling of the Xigaze forearc basin. In contrast, the younger phase of rapid exhumation at ∼4 Ma was likely driven by enhanced rock uplift in the footwall of north-striking rifts that cross-cut the YSZ.