Exhumation of the Trans-Himalayan Ama Drime Massif: Implications for Orogen-Parallel Mid-Crustal Flow and Exhumation on the Southern Margin of the Tibetan Plateau

The Himalayan orogen can be roughly divided into regions of crustal extension within the Tibetan Plateau and crustal shortening across the range front. In between lies a transition zone where exhumation rates are high and the potential relationships between tectonically- and climatically-driven processes can be tested. In the Mount Everest area, the east-west striking South Tibetan detachment system accommodated south-directed mid-crustal flow until the middle Miocene. Since then, deformation has been partitioned into north-south striking extensional faults and shear zones. The trans-Himalayan Ama Drime Massif (ADM) is bounded by oppositely dipping normal-sense shear zones that are kinematically linked to brittle faults that offset Quaternary deposits. Consistency in shear sense indicators in rocks that deformed at a range in temperatures suggest that normal-sense displacement was maintained during exhumation. U(-Th-)Pb geochronology conducted on rocks within the core of the ADM demonstrate that they reached granulite facies at ~12 Ma. Geochemical data from hot springs along the western shear zone indicate devolatilization of crustal rocks and potentially active metamorphism and/or melting beneath the ADM. The ADM coincides with the Xainza-Dinggye rifts system that extends into the Tibetan Plateau. Previous geomorphological investigations defined the Arun River gorge, located within the southern end of the ADM, as a candidate for climate-tectonic coupling. New (U-Th)/He apatite ages from the footwall block of the ADM yield a minimum exhumation rate of ~1 mm/yr between ~1.5 and 3.0 Ma. We propose that the deep crustal level exposed within the ADM was exhumed during orogen-parallel extension along the southern margin of the Tibetan Plateau. Focused surface denudation in the Arun River gorge interacted with tectonically-driven extension resulting in enhanced exhumation rates. These data begin to quantify the timing of mid-crustal flow and exhumation and provide insights into the dynamic interactions between climate and tectonic processes during orogen-parallel extension.