Neotectonics of the central Nepalese Himalaya: Constraints from geomorphology, detrital 40Ar/39Ar thermochronology, and thermal modeling

[ 1] The southern flanks of the central Nepalese Himalaya correspond to a sharp transition in landscape morphology and bedrock mineral cooling ages that suggests a change in rock uplift rate. This transition can be explained by either ( 1) accretion of footwall material to the hanging wall across a ramp in the decollement separating India from Eurasia, thereby enhancing rock uplift rates above the zone of accretion or ( 2) out-of-sequence surface thrust faulting at the physiographic transition. Here we use geomorphic data, 649 new detrital 40Ar/39Ar cooling ages, and a simple thermokinematic model to evaluate which of these tectonic configurations is most appropriate for the central Nepalese Himalaya. We first define and delineate the physiographic transition in central Nepal using maps of knickpoints, river steepness indices, local relief, and the distribution of thick alluvial fill deposits. We then report new detrital 40Ar/39Ar data from two trans-Himalayan transects, each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition. Thermokinematic modeling suggests that either of the two developmental scenarios for the transition is plausible but that an accretion model is viable only under an extremely narrow range of conditions. We contend that the physiographic and thermochronologic data in our study area are most simply explained by recent out-of-sequence surface thrusting within the Lesser Himalayan metasedimentary sequence, approximately 15 - 30 km south of the mapped surface trace of the Main Central Thrust system. An important finding of this work is that there are substantial along-strike variations in physiography and thermal history that reflect along-strike changes in the degree and location of out-of-sequence surface thrusting.