Late Cenozoic Tectonic Evolution of the Western Nepal Himalaya: Insights from Low-Temperature Thermochronology

In the central Himalaya, an abrupt physiographic transition at the foot of the Greater Himalaya (PT2) marks the southern edge of a zone of rapid rock uplift along a ramp in the Main Himalayan Thrust (MHT). Despite being traceable along ~1500 km of the central Himalaya, PT2 is less distinct in western Nepal, reflecting along-strike changes in MHT geometry and/or a migrating locus of midcrustal deformation, the details of which have important implications for seismic hazard in western Nepal. New mineral cooling ages (apatite and zircon U-Th/He and muscovite Ar-Ar) from a series of relief transects provide constraints on exhumation rates and histories in western Nepal. Inversion of these data using Pecube and QTQt models yields results that require rapid (~1.4–2.7 mm/yr) exhumation in the rocks near the along-strike projection of PT2 until around 9–11 Ma, followed by much slower (~0.1–0.4 mm/yr) exhumation until at least the late Pliocene. In contrast, transects from ~75 km hinterlandward are best fit by rapid exhumation rates (~1.5–2.1 mm/yr) over at least the past ~4 Myr. Midcrustal deformation in western Nepal is occurring well north of the position expected from along-strike structures in central Nepal, and a growing dataset suggests that rapid exhumation has been sustained there since the late Miocene. These new constraints on the late Cenozoic exhumation history of the western Nepal Himalaya provide key insight on the active structures behind the complex seismic hazards in the region.