Beveling the Colorado Plateau: Early Mesozoic Rift-Related Flexure Explains Erosion and Anomalous Deposition in the Southern Cordilleran Foreland Basin

Deposition of the Late Jurassic Morrison Formation in a back-bulge depozone and formation of the overlying sub-Cretaceous unconformity above a forebulge mark the birth of the foreland basin system in the central U.S. Cordillera. In the southern U.S. Cordillera, the Morrison Formation is either anomalously thick or absent and the sub-Cretaceous unconformity cuts out progressively older stratigraphy toward the south on the Colorado Plateau. Based on results of 2D and 3D flexural modeling, we suggest that flexural uplift of the northern rift flank of the Bisbee segment of the Borderland Rift Belt can explain these observations. Structural restoration of the sub-Cretaceous unconformity indicates a minimum of 1.5 km of uplift and flexural models with an effective elastic thickness of 55 ± 5 km can reproduce the geometry of the unconformity and rift flank. This implies that effective elastic thickness has decreased between the Jurassic and the present, consistent with hypotheses for uplift and modification of the Colorado Plateau lithosphere during the Late Mesozoic to Cenozoic. Modeling results also predict the presence of a rift-related flexural trough in the Four Corners region of the Colorado Plateau, which may explain above-average thickness of the Morrison Formation. Constructive interference between a flexural back-bulge depozone and a flexural rift-flank trough may help explain anomalously high Late Jurassic subsidence. © 2021. American Geophysical Union. All Rights Reserved.6 month embargo; first published: 25 May 2021This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Coupled Rapid Erosion and Foreland Sedimentation Control Orogenic Wedge Kinematics in the Himalayan Thrust Belt of Central Nepal

Spatial and temporal coincidence among rapid Pliocene-Holocene bedrock exhumation, development of a topographic bight, abundant monsoonal precipitation, accumulation of anomalously thick proximal foreland basin deposits, and development of an opposite-polarity salient-reentrant couple on the two most frontal major thrust faults in the Himalayan orogenic wedge of central Nepal provide a basis for a model that links these diverse phenomena and could be operating in other parts of the frontal Himalaya. Rapid bedrock erosion is documented by a concentration of young (<5 Ma) low-temperature thermochronologic ages in the Narayani River catchment basin. Where the river exits the Lesser Himalayan Zone, the Main Boundary thrust has a 15-km-amplitude reentrant. Directly south of the reentrant lies the ∼50 km wide Chitwan wedge-top basin, which is confined by a large salient on the Main Frontal thrust. Rapid erosion and sediment flux out of the Narayani catchment basin, possibly due to anomalously intense monsoonal precipitation in this topographically depressed region of central Nepal, causes greater flexural subsidence and surface aggradation in the foreland, both of which increase initial wedge taper and render this region more susceptible to anomalous forward propagation of the thrust front. Analysis of the modern and post-early Miocene taper history of the thrust belt suggests that rapid erosion hindered forward propagation of the contemporaneous Main Boundary thrust, but simultaneously produced conditions in the foreland that eventually elevated initial taper to a critical/supercritical value promoting forelandward propagation of the Main Frontal thrust. This analysis has implications for large damaging earthquakes in the Himalaya. © 2021. American Geophysical Union. All Rights Reserved.6 month embargo; first published: 06 March 2021This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Coupling Between Lithosphere Removal and Mantle Flow in the Central Andes

The central Andes is part of a Cordilleran orogen formed through continent-ocean convergence. In contrast to the thickened crust, the mantle lithosphere below much of the orogen is anomalously thin. Additionally, the surface is characterized by widespread backarc magmatism and transient ∼100-km-wide basins that developed the over last 30 million years, with basins located systematically seaward of major backarc ignimbrite centers and basin formation predating the late Miocene magma/ignimbrite flare-up. Using numerical models, we propose a novel mechanism whereby lithosphere removal is coupled with mantle flow. First, a small area of high-density eclogitized lower crust initiates a gravitational instability, causing a localized basin at the surface that subsides and then uplifts. Foundering crust and adjacent lithosphere are entrained by subduction-induced mantle flow, driving regional lithosphere removal and magmatism. The models demonstrate that mantle flow can amplify a local lithosphere instability to orogen-wide lithosphere removal, rapidly eliminating accumulated mass in the orogen. © 2021. American Geophysical Union. All Rights Reserved.6 month embargo; first published: 12 August 2021This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Geologic control of Sr and major element chemistry in Himalayan Rivers, Nepal

Our study of the Seti River in far western Nepal shows that the solute chemistry of the river and its tributaries is strongly controlled by geology. The Seti flows through four distinct terranes, starting with the Tethyan sedimentary series (TSS) and Greater Himalayan series (GHS). TSS/GHS waters display ⁸⁷Sr/⁸⁶Sr ratios of <0.73 and high Sr and Ca, consistent with the composition of limestone and marble common in these terranes. The ⁸⁷Sr/⁸⁶Sr ratio and Mg increase markedly as the river passes into the Lesser Himalayan series (LHS), where tributaries have ⁸⁷Sr/⁸⁶Sr ratios from 0.75 to 1.02 and high Sr, Ca, and Mg. The high Mg in LHS waters correlate with high ⁸⁷ Sr/ ⁸⁶ Sr ratios, which we attribute to weathering of highly radiogenic (0.71– 0.82) dolostones. Tributaries to the Seti River draining the largely carbonate-free Dadeldhura thrust sheet (DTS) have ratios near 0.74, but low Sr, Ca, and Mg and therefore have little impact on Seti mainstem chemistry. Mass balance calculations and CaMg-weathering indices show that carbonate weathering accounts for >70% of total dissolved solids to the Seti River. Sr/Ca ratios of river waters provide a minimum estimate of the %-carbonate weathering contribution to Sr, due to partitioning of Sr and Ca during incongruent dissolution and reprecipitation of calcite. Overall, we attribute high ⁸⁷Sr/⁸⁶ Sr ratios in the Seti River and its tributaries to the weathering of metacarbonates (especially dolostones in the upper Nawakhot Group) which have exchanged Sr with silicates during metamorphism. Our modeling of Sr fluxes in the Seti River indicates that the TSS/GHS accounts for 36 –39% of the Sr, the LHS for 40 –53%, and 8 –23% for the DTS. Prior to exposure of LHS rocks at ~12 Ma, TSS and GHS carbonates with low ⁸⁷Sr/⁸⁶ Sr ratios dominated Himalayan rivers. We attribute the elevated ⁸⁷Sr/⁸⁶ Sr ratios of Himalayan paleorivers during the late Miocene and Pliocene to exposure and weathering of LHS metacarbonates

40Ar–39Ar laser dating of ductile shear zones from central Corsica (France): Evidence of Alpine (middle to late Eocene) syn-burial shearing in Variscan granitoids

The island of Corsica (France) plays a central role in any reconstruction of Western Mediterranean geodynamics and paleogeography but several key aspects of its geological evolution are still uncertain. The most debated topics include the interpretation of the Corsican orogen as the result of an east- or west-directed subduction, and the actual involvement of the Variscan basement of Corsica in the Alpine orogenic cycle. This study integrates 40Ar–39Ar laserprobe, mesostructural, microtextural, and microchemical analyses and places relevant constraints on the style, P-T conditions, and timing of Alpine-age, pervasive ductile shear zones which affected the Variscan basement complex of central Corsica, a few kilometers to the west of the present-day front of the Alpine nappes. Shear zones strike ~ NNE-SSW, dip at a high angle, and are characterized by a dominant sinistral strike-slip component. Two of the three investigated shear zones contain two texturally and chemically resolvable generations of white mica, recording a prograde (burial) evolution: (1) deformed celadonite-poor relicts are finely overgrown by (2) a celadonite-rich white mica aligned along the main foliation. White mica from a third sample of another shear zone, characterized by a significantly lower porphyroclast/matrix ratio, exhibits a nearly uniform high-celadonite content, compositionally matching the texturally younger phengite from the nearby shear zones. Mineral-textural analysis, electron microprobe data, and pseudosection modeling constrain P-T conditions attained during shearing at ~ 300 °C and minimum pressures of ~ 0.6 GPa. In-situ 40Ar–39Ar analyses of coexisting low- and high-celadonite white micas from both shear zones yielded a relatively wide range of ages, ~ 45–36 Ma. Laser step-heating experiments gave sigmoidal-shaped age profiles, with step ages in line with in-situ spot dates. By contrast, the apparently chemically homogenous high-celadonite white mica yielded concordant in-situ ages at ~ 34 Ma, but a hump-shaped age spectrum, with maximum ages of ~ 35 Ma and intermediate- to high-temperature steps as young as ~ 33–32 Ma. Results indicate that the studied samples consist of an earlier celadonite-poor white mica with a minimum age of ~ 46 Ma, overgrown by a synshear high-celadonite white mica, developed at greater depth between ~ 37 and 35 Ma; faint late increments in shearing occurred at ≤ 33–32 Ma, when white mica incipiently re-equilibrated during exhumation. Results suggest that ductile shearing with a dominant strike-slip component pervasively deformed the Corsican basement complex during the emplacement and progressive thickening of the Alpine orogenic wedge and broaden the extent of the domain affected by the Alpine tectonometamorphic events. Integration of petrological modeling and geochronological data shows that the Variscan basement of central Corsica, close to the Alpine nappes, was buried during the late Eocene by ≥ 18 km of Alpine orogenic wedge and foreland deposits. Our results, combined with previously published apatite fission-track data, imply an overburden removal ≥ 15 km from the late Eocene (Priabonian) to the early Miocene (Aquitanian), pointing to a minimum average exhumation rate of 1.3–1.5 mm/a

40Ar\u201339Ar laser dating of ductile shear zones from central Corsica (France): evidence of Alpine (middle to late Eocene) syn-burial shearing in Variscan granitoids

The island of Corsica (France) plays a central role in any reconstruction of Western Mediterranean geodynamics and paleogeography but several key aspects of its geological evolution are still uncertain. Themost debated topics include the interpretation of the Corsican orogen as the result of an east- or west-directed subduction, and the actual involvement of the Variscan basement of Corsica in the Alpine orogenic cycle. This study integrates 40Ar\u201339Ar laserprobe, mesostructural, microtextural, and microchemical analyses and places relevant constraints on the style, P\u2013T conditions, and timing of Alpine-age, pervasive ductile shear zones which affected the Variscan basement complex of central Corsica, a few kilometers to the west of the present-day front of the Alpine nappes. Shear zones strike ~NNE\u2013SSW, dip at a high angle, and are characterized by a dominant sinistral strike-slip component. Two of the three investigated shear zones contain two texturally and chemically resolvable generations of white mica, recording a prograde (burial) evolution: (1) deformed celadonite-poor relicts are finely overgrown by (2) a celadonite-rich white mica aligned along the main foliation. White mica from a third sample of another shear zone, characterized by a significantly lower porphyroclast/matrix ratio, exhibits a nearly uniform high-celadonite content, compositionally matching the texturally younger phengite from the nearby shear zones. Mineral-textural analysis, electron microprobe data, and pseudosection modeling constrain P\u2013T conditions attained during shearing at ~300 \ub0C and minimum pressures of ~0.6 GPa. In-situ 40Ar\u201339Ar analyses of coexisting low- and high-celadonite white micas from both shear zones yielded a relatively wide range of ages, ~45\u201336 Ma. Laser step-heating experiments gave sigmoidal-shaped age profiles, with step ages in line with in-situ spot dates. By contrast, the apparently chemically homogenous high-celadonite white mica yielded concordant in-situ ages at ~34 Ma, but a hump-shaped age spectrum, with maximum ages of ~35 Ma and intermediate- to high-temperature steps as young as ~33\u201332 Ma. Results indicate that the studied samples consist of an earlier celadonite-poor white mica with a minimum age of ~46 Ma, overgrown by a synshear highceladonite white mica, developed at greater depth between ~37 and 35 Ma; faint late increments in shearing occurred at 6433\u201332 Ma, when white mica incipiently re-equilibrated during exhumation. Results suggest that ductile shearing with a dominant strike-slip component pervasively deformed the Corsican basement complex during the emplacement and progressive thickening of the Alpine orogenic wedge and broaden the extent of the domain affected by the Alpine tectonometamorphic events. Integration of petrological modeling and geochronological data shows that the Variscan basement of central Corsica, close to the Alpine nappes, was buried during the late Eocene by 6518 km of Alpine orogenic wedge and foreland deposits. Our results, combined with previously published apatite fission-track data, imply an overburden removal 6515 km from the late Eocene (Priabonian) to the early Miocene (Aquitanian), pointing to a minimum average exhumation rate of 1.3\u20131.5 mm/a

Asymmetric exhumation of the Mount Everest region: Implications for the tectono-topographic evolution of the Himalaya

International audienceThe tectonic and topographic history of the Himalaya-Tibet orogenic system remains controversial, with several competing models that predict different exhumation histories. Here, we present new low-temperature thermochronological data from the Mount Everest region, which, combined with thermal-kinematic landscape evolution modeling, indicate asymmetric exhumation of Mount Everest consistent with a scenario in which the southern edge of the Tibetan Plateau was located >100 km farther south during the mid-Miocene. Northward plateau retreat was caused by erosional incision during the Pliocene. Our results suggest that the South Tibetan Detachment was a localized structure and that no coupling between precipitation and erosion is required for Miocene exhumation of Greater Himalayan Sequence rocks on Mount Everest

Restoration of Cenozoic deformation in Asia and the size of Greater India

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

Climate as the Great Equalizer of Continental-Scale Erosion

Central Asia hosts the most extensive and highest topography on Earth, which is the result of the feedbacks among rock uplift, atmospheric circulation and moisture transport, and erosion. Here, we analyze 2,511 published low-temperature thermochronometric ages as a proxy of the regional-scale erosion of Central Asia. We compare these ages to tectonic and climate proxies, and state-of-the-art paleoclimate simulations to constrain the influences of climate and tectonics on the topographic architecture of Central Asia. We observe a first-order relationship between younger cooling ages in areas of high precipitation and older ages (Mesozoic) in areas that have been sheltered from precipitation, despite high strain rates. Thus, we suggest that climate enhances erosion in areas where rock uplift produces significant orographic gradients, whereas in the continental interior, areas which are tectonically active but have been sheltered from significant precipitation record older ages and a longer erosional history. © 2021. American Geophysical Union. All Rights Reserved.6 month embargo; first published: 10 October 2021This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]