Tectonic control on the drainage system in a piedmont region in tectonically active eastern Himalayas

Abstract

The impact of neotectonic activity on drainage system has been studied in a large alluvial fan in the eastern Himalayan piedmont area between the Mal River and the Murti River. Two distinct E–W lineaments passing through this area had been identified by Nakata (1972, 1989) as active faults. The northern lineament manifested as Matiali scarp and the southern one manifested as Chalsa scarp represent the ramp anticlines over two blind faults, probably the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT), respectively. The fan surface is folded into two antiforms with a synform in between. These folds are interpreted as fault propagation folds over the two north dipping blind thrusts. Two lineaments trending NNE–SSW and nearly N–S, respectively, are identified, and parts of present day courses of the Murti and Neora Rivers follow them. These lineaments are named as Murti and Neora lineaments and are interpreted to represent a conjugate set of normal faults. The rivers have changed their courses by the influence of these normal faults along the Murti and Neora lineaments and their profiles show knick points where they cross E–W thrusts. The overall drainage pattern is changed from radial pattern in north of the Matiali scarp to a subparallel one in south due to these conjugate normal faults. The interfluve area between these two rivers is uplifted as a result of vertical movements on the above mentioned faults. Four major terraces and some minor terraces are present along the major river valleys and these are formed due to episodic upliftment of the ground and subsequent downcutting of the rivers. The uppermost terrace shows a northerly slope north of the Chalsa scarp as a result of folding mentioned above. But rivers on this terrace form incised channels keeping their flow southerly suggesting that they are antecedent to the folding and their downcutting kept pace with the tectonism.

Keywords

alluvial fan / drainage pattern / thrust / normal fault / active tectonic

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Acknowledgements

We thank Dr. Mallika Mallick, Woman Scientist, Raman Center for Applied and Interdisciplinary Sciences, for providing the geographic location obtained from Geodetic survey of a point used as primary coordinate in Total Station Survey. A part of the present study was carried out in a project funded by Department of Science and Technology, Govt. of India and executed in Centre for studies on Man and Environment, Kolkata. C G. thanks UGC for funding a minor research project, Principal, Asutosh College for necessary permissions and help, and Head of the Department of Geology, University of Calcutta for infrastructural facilities for research. C G. also thanks Mr. A Ball and Mrs. B Ball for their hospitality during field work, Dr. P Jana, Dr. B Purkait, Dr. T Chakraborty and Dr. B Goswami for discussions and constant encouragement. Cooperation of colleagues in Asutosh College is also acknowledged. The manuscript is considerably improved by the constructive comments of the two anonymous reviewers and the pain staking corrections by the managing editor.

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