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

摘要

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 ~(40)Ar/~(39)Ar 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 ~(40)Ar/~(39)Ar data from two trans-Himalayan transects, each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition.