Identification of deformation variability and active structures using geomorphic markers in the Nahan salient, NW Himalaya, India

The morphotectonic study was carried out in the Nahan salient, located between the well known Kangra and Dehradun recesses in western Himalaya, India. We used various geomorphic indices namely stream length gradient index (steepness index), Hack profile convexities, and hypsometric integral (HI) with hypsometric curve and elevation profiles to map the spatial variability in the tectonic process across the salient. Field observations and planform mapping of river terraces in the area support the morphometric data. We found a systematic variation of the gradient in long profiles (convexity) with numerous knickpoints in main rivers as well as their tributaries. The geomorphic processes responsible for anomalous behaviour in these fluvial indices also causes variability in the HI values for watersheds confined to different thrust sheets. We also carried out analysis of incision patterns in different river basins through variability in base level change, Topographic Ruggedness Index (TRI) and basin relief across the major thrust sheets. The western part of the Nahan salient area is relatively tectonically more active in comparison to the east. Further, fault(s) to the north of Main Frontal thrust (MFT) are undergoing more deformation than the MFT. We interpret this deformation as out-of-sequence activity in the Nahan salient. Integration of the geomorphic indices with field observations also helped us find new structure(s) that are necessary for the seismotectonic evaluation of the area.