Neotectonic influence on geomorphology in the Gandak megafan of the Indo-Gangetic foreland basin, India

English Abstract:

Geomorphic evolution of any foreland basin is mainly controlled by spatial and temporal occurrence of tectonism and sediment inflow into it. The Indo-Gangetic foreland basin, bordered by the Himalayas witnesses rapid changes in geomorphology and sedimentation processes due to frequent subsidence and upliftment of the fault bounded tectonic blocks. The fluvial geomorphology of the basin is greatly controlled by this neotectonic process.

The Gandak megafan, a witness to the neotectonism in the Indo-Gangetic foreland basin records three distinct phases of shifting of course of the Gandak River due to eastward tilting of the megafan block. Post tilting surface faulting developed four terminal fans in different times. Temporal and spatial distribution of these depositional units contributed significant geomorphological changes on the megafan surface, particularly in the Gandak depression of the Indo-Gangetic foreland basin. The Oldest exposed part of the Gandak megafan shows gentle slope with numerous SW flowing small channels. However, the overlying subsequent terminal fans show south flowing channels with greater slopes. Localized sediment accumulation as terminal fans significantly modified the geomorphology of the megafan system. Surface faulting too modified the fluvial system in the region as down-thrown blocks show high drainage density and surface ponding which are generally not observed on the up-thrown blocks. Differential upliftment and subsidence divided the whole block into two distinct geomorphic entities i.e. uplands in the north and low-lying swampy area in the south. As the terminal fans are the indicator of surface faulting in the Indo-Gangetic foreland basin, these have been dated by OSL technique to work out the temporal faulting activities and the geomorphological evolution of the region. Satellite imageries since 1985-2005 show continuous growth of the younger terminal fans and gradual cover up the older units. This study highlights the role of neotectonism in geomorphological modification in the Indo-Gangetic plain.