LINKING GEOMETRY AND LANDSCAPE EVOLUTION IN CENTRAL NEPAL OVER GEOLOGIC TIME THROUGH INTEGRATED THERMO-KINEMATIC AND SURFACE PROCESSES MODELING

Tying the subsurface structural geometry of a mountain range to thermochronologic data and topography remains a challenge. The evolution of the Main Himalayan Thrust (MHT) in central Nepal over the past ~20 Myr produced a characteristic rock cooling signal and topography reflecting both modern and past uplift. This makes the region ideal for evaluating new approaches that integrate fold-thrust belt kinematics with both thermal (PECUBE) and landscape evolution models (CASCADE) to predict low-temperature cooling ages and topography. Displacement trajectories and rates used in PECUBE, based on reconstructions of balanced cross-sections, predict cooling ages that can be compared to measured ages to test cross-section geometries and kinematics. The same input can be used in CASCADE to compare predicted to modern topography. We utilize thermo-kinematic models of proposed MHT geometries and evolution to model resulting topographies over the past ~10 Myr. Quantitative comparison of model output with the natural landscape is performed through geomorphic metrics such as river channel steepness and local relief. Highest elevations in the models develop above active mid-crustal ramps after ~1-2 Myr. Legacy landscapes emerge, characterized by convergence parallel interfluves and elevated river channel steepness beyond the region of active uplift, due to continued southward advection. Main drainage divides feature an across-strike asymmetry reflecting the horizontal displacement component. Variations in rock erodibility and/or precipitation modify elevations and the appearance of legacy landscapes, with implications on the preservation of klippen such as the Kathmandu Klippe. Modeling of orographic precipitation, adjusted to rates comparable to those observed across central Nepal, modifies the magnitude of legacy landscape preserved. 2D model landscapes, based on viable thermo-kinematic solutions, can be used in PECUBE to evaluate how higher resolution, higher relief landscapes modify predicted cooling ages. Our study emphasizes that subsurface geometry and both vertical and horizontal displacement fields strongly control both the thermal and topographic evolution of mountain ranges providing a method to quantitatively assess the modern fault geometry and how it evolved through time.