Quaternary deformation in the Gorubathan recess: Insights on the structural and landscape evolution in the frontal Darjiling Himalaya

The Gorubathan recess in the Darjiling frontal Himalaya is an unusual and unique region in the Himalaya as the Ramgarh thrust (RT) rather than the MFT defines the mountain front and the width of the Himalayan arc or the N-S aerial distance between the foreland (27.0947 degrees N, 88.8744 degrees E) and the Tibet Plateau (27.6781 degrees N, 89.1175 degrees E) is merely similar to 70 km here. South of the mountain front, blind faults have deformed several Quaternary alluvial fans and formed scarps over a similar to 35 km N-S zone. We first used field observations and fold-thrust belt concepts to re-interpret thrust nomenclature and geometry in the recess by correlating structures and lithology with the adjacent Dharan salient across the salient-recess separating Gish Transverse fault (GTF) and renamed the locally recognized faults to their regional equivalents. We then used Real Time Kinematic Global Navigation Satellite Systems (RTKGNSS) corrected SRTM 30 m C-Band data to compute geomorphic indices along with their uncertainties. Our results indicated that neotectonic deformation has generated an active but mature landscape in the Gorubathan recess. Quantitative topographic profiles based on RTKGNSS measurements reveal neotectonic warping and formation of geomorphic scarps on the fan surfaces. Geomorphic scarps form due to motion along thrusts but are not actual fault scarps. These profiles were used to model topographic growth and perturbation of fan slopes by active thrusting and fault-propagation folding by Boundary Element Method based dislocation modelling. Results indicate activity through much of the Quaternary on the Ramgarh thrust, a hanging wall imbricate and a rejoining splay in its footwall as well as the Main Boundary thrust. Comparison of modelled and measured topographic profiles in the recess points to erosion of the fault-related scarp-tops in the deformed alluvial fans on the windward side of the SW monsoon in both the Gorubathan and the Samsing-Matiali fans. Our integrated methodology improves the understanding of coupled fault-generated deformation and topographic growth and may be applied across the entire Himalayan front. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.