Late Pleistocene - Holocene tectonic activities in the frontal part of NE Himalaya between Siang and Dibang river valleys, Arunachal Pradesh, India

摘要

From the geomorphic and geological perspectives, Pasighat and its environs serve as a transitional zone between the low-lying Brahmaputra alluvial plain and the steep SSE facing hill slopes of the Neogene Sub-Himalaya and Pre-Tertiary Lesser Himalaya, NE Himalaya, Arunachal Pradesh. This region, drained by major river system, represents an ideal environment for fluvial fan and terrace formation. On the basis of analysis of Quaternary landforms the faults that were active during the Late Pleistocene-Holocene were identified. Imprints of active tectonics on the landforms are discernible in the form of tilted fans, colluvial deposits and terraces, truncation of toe of fan, linear fault scarps and thrusting of Siwalik rocks over the colluvial deposits. OSL dates for fans and terrace deposits indicate tectonic activities between 8 and 56 ka. Dextral strike-slip faulting has deflected the Siang River course resulting in the formation of a shutter ridge and change in river gradient across this fault. Strath terraces are observed in the hanging wall of the Himalayan Frontal Thrust (HFT) and Main Boundary Thrust (MBT). Abandoned channels, incised valley profiles, strath terraces and linear fault scarps are found along the mountain front. Soft-sediment deformation structures are developed at the footwall of the HFT. Geomorphic evidence, chronology of depositional landforms, deformation of Quaternary landforms and soft-sediment deformation structures provide baseline information about neotectonic activity in the region.