LAUSR

Abstract The bulk of the crustal shortening across the Himalayas is accommodated across the Main Frontal Thrust (MFT), forming the topographic ridges of the Siwalik Hills. Here, we quantify topographic metrics to document the geomorphic response to progressive shortening. We use the integral method of channel profile analysis to assess the topographic response to the development of thrust anticline ridges in the eastern and western Churia, Dundwa, and Mohand Ranges. River channel steepness (ksn), the distribution of knickpoints, and channel concavity (θ) in the Siwalik Hills indicate geomorphic evolution during ongoing shortening across the deformation front. Mean channel steepness indices increase with increasing shortening over the Quaternary amongst the eastern Churia (~3.8 km shortening), Mohand (~6.9 km shortening), and Dundwa Ranges (~9 km shortening). Higher channel steepness at the western tip of the Churia Range (~0.4 km shortening) than in the eastern Churia and Mohand Ranges may indicate that the topography is still responding to the initial stages of shortening and relative uplift. Balanced cross sections reveal that there is spatial variability in the growth of these ranges, and as shortening increases, the drainage divide migrates southward (away from the mountains) relative to the flat-ramp transition of the underlying thrust; this results in steepening of the southward-draining catchments (basinward facing), and elongation of the northward flowing catchments. Basinward facing channels also have more prominent knickpoints and greater channel concavities caused by ongoing shortening between their fluvial base level and the drainage divide; this contrasts with the mountain facing channels where there is net growth of the distance between the drainage divide and the base level. Despite the asymmetry of erosional processes, the southward displacement of the drainage divide over the underlying thrust ramp drives topographic advection into the foreland succession which eventually leads to the propagation of new thrust-cored ridges.