Thermochronologic constraints on the slip history of the South Tibetan detachment system in the Everest region, southern Tibet

North-dipping, low-angle normal faults of the South Tibetan detachment system (STDS) are tectonically important features of the Himalayan-Tibetan orogenic system. The STDS is best exposed in the N-S-trending Rongbuk Valley in southern Tibet, where the primary strand of the system - the Qomolangma detachment - can be traced down dip from the summit of Everest for a distance of over 30 km. The metamorphic discontinuity across this detachment implies a large net displacement, with previous studies suggesting >200 km of slip. Here we refine those estimates through thermal-kinematic modeling of new (U-Th)/He and 40Ar/39Ar data from deformed footwall leucogranites. While previous studies focused on the early ductile history of deformation along the detachment, our data provide new insights regarding the brittle-ductile to brittle slip history. Thermal modeling results generated with the program QTQt indicate rapid, monotonic cooling from muscovite 40Ar/39Ar closure (ca. 15.4-14.4 Ma at ca. 490 °C) to zircon (U-Th)/He closure (ca. 14.3-11.0 Ma at ca. 200 °C), followed by slower cooling to apatite (U-Th)/He closure at ca. 9-8 Ma (at ca. 70 °C). Although previous work has suggested that ductile slip on the detachment lasted only until ca. 15.6 Ma, thermal-kinematic modeling of our new data suggests that rapid (ca. 3-4 km/Ma) tectonic exhumation by brittle-ductile to brittle fault slip continued to at least ca. 13.0 Ma. Much lower modeled exhumation rates (≤0.5 km/Ma) after ca. 13 Ma are interpreted to reflect erosional denudation rather than tectonic exhumation. Projection of fault-related exhumation rates backward through time suggests total slip of ca. 61 to 289 km on the Qomolangma detachment, with slightly more than a third of that slip occurring under brittle-ductile to brittle conditions.