Structural control on the landscape evolution of son alluvial fan system in ganga foreland basin

Abstract

We have applied transform index in the bedrock-alluvium mixed and alluvial sub-basins of thick alluvial cover in the marginal plain of the Ganga River Basin, located in the Himalayan foredeep. This index has been tested to delineate zones of drainage (dis)equilibrium wherein divide reorganization is in progress. The Himalayan foreland basin (HFB) is one of the largest and most dynamic terrestrial basins on the Earth's surface. It is interspersed with a number of mega- and micro-sized alluvial fans deposited by progenitors of the three of the world's largest rivers and their tributary systems. The thick alluvial deposits record sensitivity of the controlling factors continuously engaged in the Himalayan orogenesis. The disequilibrium of the stream channels and their corresponding basins also record the interplay of control factors responsible for shaping the channel and watershed ridge geometry. Landscape (in)stability and stream network reorganization of the bedrock channel stream profiles quantified through the transform have been proved to be a promising tool to work out the behavioral changes in the stream basin and network geometry. transform maps predict the direction of movement of (dis)equilibrium in the landscape through assessment across divide anomalies. This metric has never been applied in purely alluvial basins. Here, we have applied it as an experiment for streams flowing over the Son Alluvial Fan System (SAFS) to examine whether the effects of surface, subsurface, lithologic, and climatic controls are discernible in the transform map. Geomorphological and other field photo evidences have also been used to corroborate the findings. We found that zones of some of the reported surface and subsurface faults, e.g. East Patna fault (EPF) and West Patna fault (WPF), Munger-Saharsa Ridge fault (MSRF), and many such reported tectonic features, are apparently highlighted in the map of the first order basins in the map of the SAFS and surroundings. We have also attempted to attribute the probable dominant factors which might have contributed to the differences in the values along the first-order streams in the alluvial fan setting and its surroundings at the southern margin of the HFB leading to landscape evolution of the present day SAFS. The problem of delineating the extent of SAFS has also been revisited. © 2021 John Wiley & Sons Ltd. All rights reserved.