Constraints on the geometry and recent evolution of the Main Himalayan Thrust in western Nepal from U-Th/He thermochronology

Recent work has revealed an along-strike topographic discontinuity in the Himalaya of western Nepal. We interpret this discontinuity as reflecting a change in geometry and architecture of the Main Himalayan Thrust (MHT). Specifically, the ~4-km high Greater Himalayan mountainfront that characterizes much of the central Himalaya diffuses into a gentler, two-step mountainfront in western Nepal. This observation suggests that the belt of rapid rock uplift that has been identified along much of the range front (commonly interpreted as a result of of transport of the orogenic wedge over a mid-crustal ramp in the MHT) widens or bifurcates into two bands in western Nepal. This tectonic discontinuity could perhaps be explained by recent southward migration of the Main Himalayan Thrust ramp in western Nepal, a hypothesis supported by the presence of several high-elevation, low-relief landscape remnants between the two topographic steps. In an effort to test the above hypothesis, we will present a suite of apatite and zircon U-Th/He cooling ages collected along several relief transects in western Nepal. Transects were situated such that they will capture exhumation histories above the southern and northern topographic steps as well as the high elevation, low-relief plateau between them. Exhumation rates and histories will be interpreted with the aid of thermo-kinematic modeling. This work will: 1) provide important constraints on possible tectonic models explaining the anomalous topography of western Nepal; 2) allow comparison with cooling ages collected along strike to the NW and E; and 3) ultimately provide insight into the mechanisms of orogen growth and evolution. If this work confirms the presence of a bifurcation or recent forward propagation of the MHT in western Nepal, it will have key implications for the seismic hazard presented by the plate boundary as well as for our understanding of the longevity of mid-crustal structures within the Himalaya.