CONSTRAINING CENTRAL HIMALAYAN (NEPAL) FAULT GEOMETRY THROUGH INTEGRATED THERMOCHRONOLOGY AND THERMOKINEMATIC MODELING

Constraining the subsurface structural geometry of the central Himalaya continues to prove difficult, even after the 2015 Gorkha earthquake and the resulting insights into the trajectory of the Main Himalayan thrust (MHT). To this end, we apply a thermokinematic model to evaluate four possible balanced cross section geometries based on three estimates of the MHT in central Nepal. We compare the effect of different decollement and duplex geometries on predicted cooling ages and compare these to new and published ages. We find that the best-fit geometry able to reproduce the cooling ages at the surface is a hinterland-dipping duplex, which has been translated over a mid-crustal ramp located similar to 110 km north of the Main Frontal thrust. We find that the temporal evolution of the duplex and MHT mid-crustal ramp both play an integral role in producing the observed cooling ages, implying that the common assumption that the active decollement and ramp geometry solely control the distribution of cooling ages is incorrect. Furthermore, results indicate that the Ramgarh-Munsiari thrust was emplaced between 17 and similar to 10 Ma, followed by the Trishuli thrust. Duplex growth occurs between 6.5 and 0.75 Ma, with its constituent thrust sheets moving at variable rates between 10 and 42 mm/yr. Young out-of-sequence thrusting (5 km of displacement) in the hinterland produces a slightly improved fit to the cooling ages. Finally, the resulting thermal field modeled from our best-fit geometry suggests a possible basis for the nucleation and rupture characteristics of the Gorkha earthquake.