Geomorphology in relation to tectonics: A case study from the eastern Himalayan foothills of West Bengal, India

The effects of neotectonic activity on geomorphic features have been studied in a large alluvial fan in the foothills area of the eastern Himalayas. The interfluve area between the rivers Mal and Murti is an alluvial fan composed of Quaternary sediments characterized by clay, sand, pebble, and boulder beds. Most of the river valleys in this area show well developed terraces. There are four major terrace surfaces, named as T1, T2, T3 and T4 according to increasing height from the river bed. Two E–W scarps named as Matiali and Chalsa scarps that cut across the fan represent traces of the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT) respectively. There are two other NNW–SSE and NNE–SSW lineaments which have partially guided the courses of the Neora and Murti rivers. These are interpreted as conjugate sets of normal faults transverse to the orogenic trend. The E–W scarps are the manifestation of the frontal limbs of the ramp anticlines over two blind thrusts. Fault propagation folding has affected the fan surface. Recurrent movements on the thrusts and consequent downcutting of the rivers have led to the formation of the raised terraces on the banks of these rivers. The terraces are formed by cut-and-fill process. Later transverse normal faulting has given rise to a horst of the Neora–Mal interfluve block.