The Himalayas are a surface manifestation of the ongoing collision of the Indian and Asian plates. As stress accumulates along the collision zone, it is released during large earthquakes that pose an enormous threat to the growing populations in the region. The destructive power of these earthquakes was demonstrated most recently by the 2015 Gorkha earthquake in central Nepal, which caused over ten thousand casualties. Most large Himalayan earthquakes occur along the Main Himalayan Thrust (MHT), which is the active plate boundary beneath the range.

In addition to generating large earthquakes, tectonic movement along the MHT results in mountain building. Recent theoretical and technical advances have led to the realization that rates of rock uplift, which reflect the structure of the faults in the subsurface, are, to a degree, encoded into the landscape form (i.e., tectonics can be read from topography). Analysis of Himalayan topography reveals that the mountain front in western Nepal is characterized by two steps, as opposed to the rest of the central Himalaya, which features a single, dramatic mountain front. This distinction is likely related to a previously unknown along-strike change in the geometry and history of tectonic structures beneath the range. Specifically, it is hypothesized that mid-crustal deformation has recently shifted from the northern topographic step to the southern, raising a broad area between them that is now preserved as a high-elevation, low-relief landscape.

In order to test the hypothesis that the study area in western Nepal is undergoing transient incision due to recent uplift of the low-relief landscape between the two mountain fronts, modern erosion rates are estimated in twelve watersheds that drain the two topographic steps and the high-elevation, low-relief landscape between them. Erosion rates vary by an order of magnitude and appear to be a function of watershed morphology, which is in turn a function of the watershed's position with respect to an upstream-migrating wave of incision that appears to be related to the hypothesized rejuvenated uplift along the southern mountain front.

In a parallel study aimed at constraining the longer-term history of deformation in the study area, low-temperature thermochronology is employed to estimate rock-uplift rates from bedrock cooling ages. Inversion of these data using a thermo-kinematic model yields best-fit rock-uplift rates, which are compared to predictions from the hypothesis presented above. Results show that the main zone of rock uplift in well-studied central Nepal makes a sharp turn to follow the northern mountain front in western Nepal, away from the trend of the rest of the range, which would otherwise follow the southern mountain front. In contrast, rock uplift has been slow since ~10 Ma along the southern mountain front, and the hypothesized recent uplift there can be no older than 4 Ma.

In summary, the recent tectonic history in western Nepal differs significantly from the trend exhibited in adjacent segments of the range. This distinction has important implications for the possible locations of future seismic activity, the paleoseismic record in western Nepal, and the ongoing evolution of the range.