Active tectonics, landscape evolution and sediment dynamics in Dehra Dun, Northwest Himalaya inferred from geomorphic indices and GIS tools

Abstract

Geomorphic evolution of the frontal parts of active mountain belts is strongly influenced by tectonic processes, usually manifested in fluvial, structural and morphotectonic parameters of the landscape. Geomorphic indices have long been used for understanding the tectonic imprints in river basins. However, their applicability and validation from field observations in the Himalayan basins have often been debated. Here we test the applicability of several geomorphic indices in the western Dehra Dun (intermontane valley) that includes well-known tectonic elements such as Main Boundary thrust (MBT), Main Frontal Thrust (MFT), Santaurgarh Thrust (ST), and other smaller faults. Based on the spatial distribution of the values of different geomorphic indices, we map four different tectonic zones to represent the relative tectonic activity from low to very high. We conclude from morphometric analysis that the MBT and the ST are active faults. However, we do not find any geomorphic evidence of recent activity along several other faults mapped in this region or any deformation in the major fan units. We argue that active tectonics has not played a significant role in shaping the landforms in this region except for providing accommodation space and sediment supply for the deposition of fan units. We have further employed DEM and limited borehole data to understand the sediment storage and excavation at Late Late Quaternary time scale. We compute the average stored volume of sediments since ~50 ka as 8.03 × 1010 m3 (using borehole data) and 12.24 × 1010 m3 (GIS approach). Total eroded volume has been computed as 0.47 × 1010 m3 (using borehole data) and 0.64 × 1010 m3 (GIS approach). These volumes give a time-averaged sediment discharge leaving the dun as 0.21–0.39 × 106 m3/yr over Late Quaternary period. We note that the estimated average eroded material during the incision phase is only 5–6% of the total accumulated sediment volume. Further, the average estimated volume (8.03–12.24 × 1010 m3) in the valley is nearly half of the volume of the sediment that can be accommodated (27.2 × 1010 m3) in the valley. Also, the present-day sediment supply to the Dun (~0.14 × 106 m3/yr) is low in comparison to estimated average erosion rate. Our results therefore suggest that the valley is only half filled, and is in excavation phase since Late Quaternary mostly driven by climatic perturbations.