Characterisation of the active tectonics in the outer Arunachal Himalaya, India: Insights from tectono-geomorphic analysis

The Himalayan fold-thrust belt, formed due to the collision between the Indian and Eurasian plates, during ~50 Ma, shows the long-term convergence with crustal shortening, duplex development, out-of-sequence thrusting and deformations of the foreland basins. The outer Arunachal Himalaya, in the southern part of the Eastern Himalayan Syntaxis, is one of the most tectonically active areas in the Himalayan arc. Frequent earthquakes of several magnitudes, accompanied by associated ground failures, liquefactions, and subsidence in the foreland basins, as well as significant changes in river courses, indicate prominent surface manifestations that reveal ongoing deformations. Documentation of uplifted and truncated terraces, unpaired terraces, soft-sediment deformation structures, warped and tilted Quaternary layers, strath terraces and fault scarps collectively suggest active deformation along the frontal fold-thrust belt. This study integrates morphometric analysis, river terrace mapping and characterisation of paleoseismic evidences to assess active tectonics in the area. Key morphometric indices derived from remotely sensed datasets, including mountain-front sinuosity (Smf), drainage basin asymmetry (Af), transverse topographic symmetry factor (T), valley floor width-to-height ratio (Vf), stream length-gradient (SL), hypsometric integral (HI), and elongation ratio (El), consistently shows strong tectonic influence in the area. The narrow, V-shaped valleys and steep channel gradients further support ongoing upliftment in the region. Lineament mapping reveals structural trends parallel to the Himalayan Frontal Thrust (HFT) that align with the regional stress field. It is worth mentioning that the upliftment along the HFT is not uniform, leading to the development of unpaired terraces. Additionally, NW-SE and NE-SW transverse faults have segmented the mountain front, that triggered channel offsets and changes in the river widths, and also contributed to the formation of minor pull-apart basins. These transverse structures, along with the south-verging thrust system, are crucial for distribution of strain across the frontal Arunachal Himalayas. Documentation of active scarps, deformed terraces, and related landscape features are crucial for understanding the relation between surface deformation, fault activity, and seismic risk in this highly active part of the orogenic belt.

Keywords: Active tectonics, HFT, Geomorphic evidences, Frontal Arunachal Himalaya.