ALONG-STRIKE VARIATIONS IN THE TECTONIC ARCHITECTURE OF THE CENTRAL NEPALESE HIMALAYA

The central Nepalese Himalaya are characterized by a sharp physiographic transition that does not correspond to previously mapped faults. This study integrates observations from geomorphology, ⁴⁰Ar/³⁹Ar thermochronology, and cosmogenic isotopes, to evaluate the degree to which active faulting is implied by this physiographic transition. Across-strike profiles of topography, local relief, hillslope gradients, and river channel gradients are used to characterize the position and the width of the physiographic transition at various positions along strike. ⁴⁰Ar/³⁹Ar and cosmogenic isotope data are used to delineate breaks in exhumation and erosion histories across the transition. Our results document along-strike variations in the nature of Neogene faulting in the Himalaya. In the two central transects, all of the data suggest a narrowly-defined and recently active fault zone, approximately 20 km south of the surface trace of the Main Central Thrust (MCT) system. Farther to the east and to the west, changes in physiography are both more diffuse and more nearly coincident with the surface trace of the MCT, suggesting more broadly distributed strain during out-of-sequence reactivation of this regionally significant thrust fault. The geometry of the MCT system, which varies considerably along strike, thus appears to be an important control on the nature of Neogene faulting in central Nepal. Our results underscore the utility of integrating geomorphic, thermochronologic, and cosmogenic isotopic methods for delineating Neogene faults in active orogens.