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Active tectonics along the mountain front around the easternmost part of the Himalayan Range: Implications for geomorphic evolution

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The frontal part of the easternmost zone of the Himalayas is tectonically active, which can be ascertained from different quaternary deformation features like tilted terraces, differential upliftment of terraces, and steep scarps. Along the mountain front, four and three levels of terraces are present on the right and left bank of the Siang River respectively with a maximum elevation of 303 m. Different levels of Quaternary terraces indicate episodic upliftment of the area. On the left bank of the Siang River, 300 m upstream from Dhobighat, the terrace deposits are present at an elevation of 150 m above the Siwalik Group, and 100 m downstream, tilted quaternary deposits are observed. Steep scarps near Mairam, indicate the presence of Himalayan Frontal Thrust (HFT), and based on convincing field evidence, HFT can be categorized into HFT-1 and HFT-2. Total eighteen basins are mapped which are elongated and aligned along the NW-SE direction, parallel to the trend of cross faults present in the area along the Siang River course. The mountain front sinuosity index, SL value, and elongation ratio reflect further that the area is tectonically active. Furthermore, the asymmetry factor and transverse topographic symmetry factor illustrate that all the basins are asymmetric and tilted. Most of the valley profiles are V-shaped, and the valley floor width-to-height ratio is low, indicating rapid upliftment of the area. Lineament maps show a similar trend of the orientation with the major structural features of the area and further, confirm that the area is tectonically active.