Geometry and kinematics of Main Frontal thrust-related fault propagation folding in the Mohand Range, northwest Himalaya

摘要

The Main Frontal thrust (MFT) shapes the similar to 2500 km-long frontal Himalayan arc between two syntaxes. Surprisingly, the structural geometry and kinematics of the MFT sheet remains poorly constrained along portions of this arc even though it is a major seismogenic plate boundary hazard that affects 53 million people living in the Himalaya and roughly 600 million living in the combined Himalayan drainage basin (Apollo, 2017). Here, we integrate methods to constrain and quantify MFT deformation and its associated topographic growth in the Mohand Range at the northwestern Himalayan front. We use (a) new structural data and trishear modeling to detail the MFT-related structural geometry across a portion of the range and confirm it to be a fault propagation fold with an exposed monocline in the hanging wall as previously hypothesized (Srivastava et al., 2016), and (b) microstructural, finite strain, and grain size analysis from 11 transport-parallel thin section samples of Middle Siwalik sandstones to propose that the fold formed by near-surface (similar to 1-5 km), frictional-sliding and block-supported cataclastic flow. New luminescence dating of fault gouge and uplifted sand bodies indicate that MFT activity is older (similar to 105 ka) in the central compared to the western (similar to 15 ka) Mohand Range, implying that recent MFT motion has been variable and segmented. We next present Boundary Element Method-based dislocation modeling results along 3 transects to confirm that the along-strike topographic growth is variable, but consistent with the existing data and our interpretations. Balanced cross sections indicate that a maximum of 23-26 km(2) area was eroded from the Mohand Range since folding began. Modern era estimates of minimum erosion from our dislocation modeling compared to existing topographic profiles suggest that similar to 1 km(2) erosion occurred in the range. Our integrated approach helps us to better constrain, resolve reported inconsistencies, and advance our understanding of MFT-related deformation, topographic growth and erosion within the Dehradun recess and across its outboard range front. Himalayan arc-segmentation into salients and recesses likely impacts along-strike earthquake rupture propagation, thus it is important to understand how each segment behaves, evolves, and perhaps interrelates as part of a larger perspective towards evaluating active collisional wedge fronts and their potential seismic hazards.