SPATIOTEMPORAL VARIABILITY OF EXHUMATION AND EROSION RATES IN THE WESTERN NEPAL HIMALAYA

Much of the central Himalaya features a sharp rise in mean elevation from ~1 km in the Lesser Himalaya to ~4-5 km in the Greater Himalaya. This physiographic boundary (PT2) is commonly interpreted as the surface expression of a belt of active mid-crustal deformation along a steeper segment of the Main Himalayan Thrust (MHT). In western Nepal, however, the mountain front is decidedly more diffuse. Harvey et al (2015) hypothesized that this along-strike change in mountain front morphology is the result of a Plio-Pleistocene basin-ward shift in the locus of midcrustal deformation in western Nepal.

To test the aforementioned hypothesis, we perform [U-Th]/He dating on 39 apatite and 47 zircon samples collected along seven relief transects throughout western Nepal. We constrain exhumation histories by inverting these new cooling ages with a 3-D thermo-kinematic model. The two transects collected from the along-strike projection of PT2 require rapid (~1-2 km/Myr) exhumation until ~8-11 Ma, followed by much slower (~0.1-0.2 km/Myr) exhumation until at least the late Pliocene. In contrast, five transects collected 50-100 km north of PT2 are best fit by relatively rapid exhumation rates (~1-2 km/Myr) since at least ~5 Ma. Although these results do not directly confirm Harvey et al. (2015)'s hypothesis, they imply that the locus of active deformation lies well north of where one would expect based on published data from anywhere within 500 km along strike -- a result with important and surprising implications for the geometry of the Main Himalayan Thrust and the seismic hazard in the region.

These longer-term exhumation rates are distinct from a new suite of catchment-averaged erosion rates derived from detrital cosmogenic radionuclides. The latter range from ~0.03 to 0.5 mm/yr, and appear to be controlled more by watershed morphology and transient incision as the landscape adjusts to shifting loci of uplift. Further, our exhumation and erosion rates are notably slower than rates measured along strike, suggesting that the classic belt of rapid uplift and erosion is distributed over a broader area in western Nepal. Finally, our study reinforces the notion that rates yielded by thermochronometry and cosmogenic radionuclides respond to distinct forcing factors over distinct timescales in landscapes that have not reached a steady state.