CURIOUS CASE OF MAIN FRONTAL THRUST DEFORMATION IN NORTHWEST HIMALAYA

The Main Frontal thrust (MFT) defines the front of the Himalayan arc and is the surface expression of the Himalayan décollement or the Main Himalayan thrust. Curiously, the kinematics of the MFT sheet remains poorly constrained in most places although major seismic hazard is associated with the MFT. This is true even in regions such as the Dehradun recess in the northwest Himalaya after decades of study. All models in the recess, however, accept the asymmetric “Mohand antiform” defined by a dominant NE-dipping limb and a short SW-dipping limb as the first-order structure in the MFT sheet. Curiously, our field data refute the SW-dipping limb and we mapped a monocline with a dominant north-dipping limb and a short, frontal, sub-horizontal limb cross-cut by an emergent MFT. Field observations and deformation microstructures suggest that the monocline was formed by near-surface (~1-5 km) fault propagation folding. However, finite strain measurements from 11 transport-parallel thin sections of Middle Siwalik sandstones using the Bootstrapped Modified Normalized Fry method indicate a systematic decrease in axial ratios (R_f) from 1.7 to 1.4 and mean grain-size increase from ~8 to 50 x 10^-5cm² in the MFT sheet from south to north. We found the presence of strain in the MFT sheet very curious as it was emplaced under near-surface elastico-frictional conditions. However, strain and the northward decrease in R_f in the sheet occurred probably due to cataclastic flow induced by increase in finer fraction during MFT related grain-size reduction. Simulation of along-strike topographic growth using Boundary Element Method based dislocation model in the MFT sheet suggests differential and segmented growth along 3 SRTM 1-arc topographic profiles. Comparison of 3 modeled and SRTM profiles as well as two balanced cross-sections across the MFT sheet suggest that the western part of the MFT sheet was eroded more that the eastern part. This was probably due to proximity of the MFT sheet to Yamuna tear fault defining the western boundary of the Dehradun recess and additional deformation along it. Our approach can be used to study lateral variation in MFT-related deformation, consequent topographic growth and subsequent climate-induced erosion in all salients and recesses along the Himalayan arc and other active compressional settings worldwide.