Geodetic, geomorphic, and seismic studies have converged on a model for the active tectonics of the central Himalaya wherein ~2 cm/yr of Indo-Asian plate convergence is accommodated as slip along the Main Himalayan Thrust (MHT). Stress releases along this plate boundary fault are capable of devastating earthquakes like the (April 2015) Mw 7.8 Gorkha Earthquake, which has killed nearly 9,000. Prevailing models contend that over several earthquake cycles, rapid rock uplift and mountain building occur where the orogenic wedge passes over steeper sections, or 'ramps', along the MHT. The most prominent of these ramps is thought to underlie the high peaks of the Greater Himalaya, which appear to be undergoing rapid rock uplift and exhumation in a ~50-km-wide, range-parallel belt north of the physiographic boundary between the Lesser and Greater Himalaya. This physiographic transition, known as PT2 in the literature, is traceable for ~1500 km in the central Himalaya, with the exception of a ~100-km-long segment in western Nepal where it seems to bifurcate into two, smaller topographic steps (PT2-N and PT2-S) bounding a broad, high-elevation, low-relief landscape. Given the close association of topography and zones of rock uplift, this anomalous segment of the range requires revision of the prevailing tectonic model.
In this dissertation I use a multi-pronged approach to clarify the along-strike tectonic changes driving development of the anomalous topography in western Nepal, including: 1) Detailed topographic analysis; 2) millennial-scale erosion rates from cosmogenic radionuclide chemistry; and 3) longer-term exhumation rates from apatite and zircon [U-Th]/He thermochronology. Results of these analyses suggest that the main MHT ramp and associated rapid rock uplift must bend to the north along PT2-N in western Nepal. It is likely that this lateral deviation of the MHT ramp is related to a recently-discovered oblique dextral fault that represents the southeastern extension of the Karakoram fault cutting across the Himalaya. Although the elevated low-relief landscape between PT2-N and PT2-S hints at a recent pulse of uplift in this domain, old cooling ages along PT2-S require that exhumation here has been insufficient to bring up recently-cooled apatites. This point is supported by surprisingly slow erosion rates (as low as 30 m/Myr) on this subdued surface. These results have important implications for the geometry of the MHT in western Nepal, shifting the locus of deformation and possible earthquake ruptures up to 75 km farther into the orogen than would otherwise have been expected.