Late Cenozoic growth and exhumation of the northern Lunggar extensional basin, west-central Tibetan plateau
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Despite major shortening during India-Asia convergence, the dominant late Cenozoic geological features across the southern to central Tibetan plateau are north-trending rifts and associated strike-slip faults. Fault orientations and slip directions suggest broadly distributed east-west extension, but the dominant modes of extensional deformation and basin formation are unclear. The Lunggar extensional basin in west-central Tibet is bounded by a <40° low-angle normal fault (detachment), contains active high-angle normal faults, and displays higher topography toward the basin center with outward axial drainage toward the northern and southern basin terminations. Structural and stratigraphic features of the Lunggar extensional system are variably consistent with a high-angle half-graben geometry or a low-angle fault-related supradetachment basin, possibly suggesting a change in deformational style. This study aims to constrain the depositional and exhumational history of the Lunggar extensional basin and bounding fault system by assessing the sedimentologic, structural, and thermochronological record of basin fill. Clastic sedimentary facies filling the Lunggar extensional basin include alluvial-fan debris-flow conglomerates and fluviolacustrine sandstones and siltstones. Sandstone petrographic point count data, conglomerate clast compositions, and detrital zircon U-Pb age spectra suggest systematic unroofing of the western footwall (including Upper Jurassic-Cretaceous and Miocene granitic crystalline rocks that intruded Permian to Cretaceous strata), which strongly influenced sediment transport and basin sedimentation. Paleoflow measurements are effectively opposite to the current flow regime, suggesting that the modern topographic high in the central basin modified the original basin configuration. As a proxy for cooling histories of previously eroded footwall rocks, 4 sandstones and 11 leucogranite boulder clasts from proximal hanging-wall strata were sampled for low-temperature thermochronometry. Apatite and zircon (U-Th)/He results support conglomerate clast compositional trends in suggesting rapid footwall exhumation and a pattern of hanging-wall exhumation in which the youngest exposed range-front deposits are focused along the central segment of the basin-bounding detachment fault. Collectively, the sedimentologic, structural, and thermochronological results support a model of rift evolution predicting upper crustal thinning, supradetachment basin subsidence, and subsequent isostatic rebound that intiates along the more-evolved central segments of Tibetan extensional systems.