MAIN FRONTAL THRUST DEFORMATION AND RELATED TOPOGRAPHIC GROWTH IN THE NORTHWEST HIMALAYA

The Main Frontal Thrust (MFT) typically defines the Himalayan topographic front across the ~2500 km boundary between the Eurasian and the Indian plates. Deciphering MFT deformation kinematics is crucial for understanding how the orogen accommodates continuing continental collision and assessing associated hazards. We detail newly discovered fault-zone exposures along the MFT at the Mohand Range front in northwestern India and apply contemporary fault zone theory to show that the MFT is an emergent fault with a well-developed, fault zone overlain by uplifted Quaternary gravels. Northward from the front, the fault zone grades from a central, gouge-dominated core to a hanging-wall, rock-dominated damage zone. We observed incohesive, non-foliated breccia, fault gouge, and brittle deformation microstructures within the fractured country rocks (Middle Siwaliks) and outcrop scale, non-plunging folds in the proximal hanging wall. We interpret these observations to suggest that (1) elastico-frictional (brittle) deformation processes operated in the fault zone at near surface (~1-5 km depth) conditions, and (2) the folds formed first at the propagating MFT fault tip, then were subsequently dismembered by the fault itself. Thus, we interpret the Mohand Range as a fault-propagation fold driven by an emergent MFT in contrast to the consensus view that it is a fault-bend fold. A fault-propagation fold model is more consistent with these observations, the modern range-scale topography, and existing erosion estimates. To further evaluate our proposed structural model, we used a Boundary Element Method-based dislocation model to simulate topographic growth from excess slip at a propagating fault tip. Comparing modelled vs. measured high resolution (~16 cm) topographic profiles for each case provides permissible end-member scenarios of a dynamically-evolving, high erosion, northward-migrating fold or a static, low, and symmetric, MHT-related fold, respectively. Our integrated approach delivers a novel perspective for improved understanding of coupled fault-generated deformation and topographic growth that may be applied more broadly across the entire Himalayan front.