Geomorphic evidences of lateral propagation and differential uplift of the two segments of frontal Siwalik range, central Himalaya, India

Abstract:

The Siwalik range forms the southernmost terrane of the Himalaya. The frontal mountains of this range rise abruptly against the vast Indo-Gangetic plains along the range-bounding Himalayan Frontal Thrust (HFT), which marks the present day principal displacement zone between Indian and Eurasian plates. With an aim of understanding the morphotectonics of different mountain segments in such a zone of intense tectonic activities, we have analyzed geomorphic indices like ‘basin asymmetry factor’ (AF), ‘transverse topography asymmetry factor’ (T), ‘basin elongation ratio’ (BR) and ‘hypsometric integral’ (HI) of fifty six drainage basins of 3^rd streams in conjunction with ‘mountain-front sinuosity’ (S_mf), ‘valley-floor width to height ratio’ (V_f) and drainage characteristics of the two large, neighboring segments of the frontal Siwalik mountains. Referred to here as the ‘eastern segment’ and ‘western segment’, these two mountain-segments are separated from each other and adjoining mountain-segments by antecedent streams of fourth and fifth order. In both the mountain-segments, the S_mf values are mostly between 1.0–1.6 and, likewise, the V_f of most of the streams, calculated at a fix distance of 800m from the mountain-front, is invariably <1.0. The AFs of 64% of the western-segment’s drainage basins and 65% of the eastern-segment’s drainage basins indicate their down-tilting towards the west. The azimuths of the Ts of theses basins are not always in conformity with AF suggested down-tilting, but the azimuths of the larger magnitude Ts are by and large in conformity with the AF suggested down-tilting. However, the BR and HI values of all the drainage basins of a mountain-segment do not show any spatial variation. These results suggest that both the studied mountain-segments are actively uplifting, but with faster rate in their eastern part. Moreover, the drainage pattern and deflections in and around both the segments reveal their lateral progression from east to west.