Neotectonic Fault in the Middle Part of Lesser Himalaya, Arunachal Pradesh: A Study Based on Structural and Morphotectonic Analyses

The landscape morphology, and structural and geomorphometric analyses in the middle part of the Lesser Himalaya suggest the neotectonic activity along the ENE-WSW trending course of the Kamla River, Arunachal Pradesh. In the upper reaches, the Kamla River flows transverse to the regional strike of rocks, but becomes sub- parallel to parallel along the Kamla River Fault (KRF), which passes through the river. Along the KRF the rocks are characterized by brittle extensional fractures with relative displacements across them. The foliation-parallel shear fabrics also show extension in between. Locally, the river valley is straight for a distance of>l km and forms a deep and narrow gorge with convex and steep valley-slopes. A series of landslides with fresh-looking slip-circles exposing triangular steep facets are aligned along the Kamla River valley and its nearby region. On the hill-slope (dipping >50° towards SSW and S directions) of ENE-WSW trending sharp ridge, the recurrence of old and active landslides took place parallel to the trace of the KRF. The high stream length-gradient index (SL) values of 3 230 of Pa and Paja tributaries of the Kamla River suggest young vertical uplift in the region combined with deep incision by the river. The SL of the Kamla River has low value (1 0) as it passes through the KRF zone. The out-of-sequence neotectonic activity along the KRF is attributed to the locking of the detachment surface below the Lesser Himalaya.