Constraining the deformation history of the South Tibetan detachment system in the Marsyandi Valley, central Nepal

Abstract Outcropping over a distance of roughly 2000 km along the Himalayan arc, the South Tibetan detachment system (STDS) is one of the most important structural features in the Cenozoic evolution of the orogen, but its temporal and spatial evolution remains poorly constrained in all but a few transects. We present new data from the Marsyandi valley and base of the Nar valley, central Nepal, in an effort to constrain the timing of normal-sense slip on two strands of the STDS in this area—the Chame and Bhratang detachments. These detachments represent two different structural levels of the STDS with the lower (deeper) level being the Chame detachment. We present new geochronologic dates of variably deformed leucogranites to constrain the timing of mylonitic fabric formation associated with ductile slip on the basal Chame detachment, which formed prior to 16.0 ± 1.1 Ma. Thermal-kinematic models of new thermochronologic data from a transect across the footwall of the Chame detachment and ending in the footwall of the overlying Bhratang detachment suggest that normal-sense slip on both detachments was broadly contemporaneous. Major normal-sense slip on both detachments had mostly ceased by ca. 15 Ma, with only minor, lower temperature slip continuing until ca. 12 Ma. Thermal-kinematic models suggest temporal and spatial variations in exhumation rates (from 0.3 to 2.5 km/m.y.) across the transect over the past 10 m.y.