Overprint of neotectonism along the course of River Chel, North Bengal, India

Abstract

This paper aims to unveil neotectonic imprints in topography, drainage and sediments in the 46.25 km long course of the River Chel from its source down to its alluvial fan at the base of the Himalayan Mountain Front in the DarjeelingeJalpaiguri districts of India. A semi-circular ridge delimits its primary catchment. Within confinement of this watershed basin the drainage pattern is composite being convergent along the periphery and divergent on a butte inside. All these geomorphic neotectonic imprints are accompanied by ramp and flat structures and spectacular mylonitization of rocks.High hypsometric index and convex shape of the hypsometric curve derived from the central near-straight course of the river between the primary catchment and the Main Frontal Thrust (MFT) also reflects tectonic youthfulness of the river course. It is well manifested also in widely variable stream index and stream gradient index ratios (SL/K) often exceeding 2. In response to neotectonism, this river course as a whole shifted westward between 1962 and 2007. Maximum reduction of the stream gradient on top of the MFT is eloquent enough about recent uplift of the thrust ridge. The high average slope gradient of canyon wall about 45.68 is well consistent with this uplift. Very low channel-width/valley-height ratio along the river further corroborates the uplift. The alluvial fan system of the River Chel is comprised of five morphogenetic fans stacked one above another with a tendency to shrink and shift progressively upslope. They differ from each other in terms of tilt, axial orientation, primary depositional surface gradient and convexity in transverse section and thus present a writ of ongoing tectonism. Progressive upward increase in the share of distal crystalline rocks in clast composition within alluvial fan package is a clear proxy for southerly advancement of the MFT. Concomitant increase in maximum clast size is in good agreement with sediment source uplift. All the five fans are, however, dormant now. Present-day River Chel deeply incises through all of them and suggests further basement uplift in the context of frequent evidences of neotectonism all around, although the role of climate remains uncertain in absence of adequate data.

Longitudinal profiles and geomorphic indices analysis on tectonic evidence of fluvial form, process and landform deformation of Eastern Himalayan Rivers, India

“…Rivers Lethi, Gish, Ramthi Khola, Chel, (Figure 6, Table 2). Rivers Lethi, Gish, Ramthi Khola, Chel, Neora, Mal, and Murti are very active according to Vf values because low valley-floor width to height ratio indicated active tectonic evidence (Bull, 1977;Sarkar, 1999Sarkar, , 1999Mandal and Sarkar, 2016;Ayaz et al, 2018).…”

“…The present morphology of the region is that of terraced landscape with secondary alluvial fans at the debouching point of these rivers. These rivers have formed their own micro fans which are being modified and their morphology is being changed due to neotectonic activities in the region (Ayaz et al, 2018;Mandal and Sarkar, 2016). All this change in the channel pattern and river diversions has been clearly observed in the Gish and Lethi channels which are the feeder channels of the Gish alluvial fan.…”

“…All this change in the channel pattern and river diversions has been clearly observed in the Gish and Lethi channels which are the feeder channels of the Gish alluvial fan. The presence of the terraces along the river banks is an indicator of the active tectonism and the deformation of the fan surface (Mandal and Sarkar, 2016). Evidence of tectonic activities could be understood from the modifications in the drainage patterns, formation of the east-west scarps and also folding of the fan surface and the sediment strata making up the fan.…”

Avulsion potential modeling using integrated Fuzzy-AHP in lower catchment of Chel river of North Bengal, India

“…The annual rainfall varied between 3600 and 6900 mm, with an average of 4600 mm, in both the Upper Fagu and Samsing tea plantations during 1986-2015 (Prokop et al 2020). The foreland of the sub-Himalaya is built up of quaternary sediment, which shows a distinct fractional differentiation starting from boulders and gravels in the root part of piedmont fans and terraces (Nakata 1989;Mandal and Sarkar 2016). Both the high rainfall and geomorphological configuration of the foreland basin aggravate local-scale avulsion in monsoon.…”