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

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• 作者：Sunipa Mandal ; ^{sunipam@gmail.com" class="auth_mail" title="E-mail the corresponding author} ; Subir Sarkar
• 关键词：Uplift of Main Frontal Thrust ; Neotectonism ; Hypsometric index ; Stream-gradient index ; Alluvial fan system evolution ; Fan attributes ; Clast size and composition
• 刊名：Journal of Palaeogeography
• 出版年：2016
• 出版时间：July 2016
• 年：2016
• 卷：5
• 期：3
• 页码：221-240
• 全文大小：6468 K

文摘

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 Darjeeling–Jalpaiguri 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.