Active tectonics and geomorphology in the Sone-Ganga alluvial tract in mid-Ganga Basin, India

Abstract

Active tectonics plays a major role in landscape evolution by bringing changes in topography and channel slopes. The alluvial plains in Ganga Basin are divided into a number of tectonic blocks by several active subsurface faults. The associated disturbances along these faults have given rise to a distinctive fluvial regime and geomorphology in the region. In Sone-Ganga alluvial plain in parts of South Ganga Plains in middle Ganga Basin, the Sone and Ganga rivers have responded to the ongoing tectonic deformation in the area. The study reports a classical example of tilt- induced avulsion and channel migration during Holocene from the parts of South Ganga Plains within the extensional half-graben between East Patna Fault (EPF) and West Patna Fault (WPF). The rate of lateral tilt appears to control the style of channel movement, with gradual migration occurring at low tilt rates, and avulsion at higher rates. The river Sone, lying laterally close to the line of uplift, has undergone avulsions at different times of tilting, whereas the Ganga River located at the lower parts of the tilting-block away from the line of uplift has migrated in the down-tilt direction. Various fluvial anomalies such as gradient reversals in the longitudinal profile of channels, sinuosity variations, channel incision, frequency in the braid bar distribution and variation in the cross-sectional parameters of the channels most probably created by tectonics have been studied. Satellite images and maps coupled with relative bank height measurements and other geomorphologic studies in field have helped in identifying such fluvial signatures and their relevance to tectonics. The subsurface faults in the Sone-Ganga plains cut across and also run parallel the river channels, showing the effects of transverse, and lateral tilting manifested in channel avulsions and migrations.