Seismic landscape from Sarpang re-entrant, Bhutan Himalaya foredeep, Assam, India: Constraints from geomorphology and geology

Abstract

Geomorphic landscape and late Quaternary geological attributes from the Raidak–Manas interfluve in the Bhutan–Himalayan foothills, Kokrajhar District, Assam led towards documenting the east–west trending, south dipping, 30km long active Frontal Back Thrust (FBT), well within the foredeep south of the Main Frontal Thrust (MFT). Spectacular north facing 6–50m high tectonic-scarp generated by the north-propagating emerging thrust front along with a complementary subdued south-facing scarp defines the terrain as a pop-up structure. The entire belt is made up of 5 to 8km wide six distinct blocks, separated by antecedent rivers/streams. Scarp parallel east–west drainage along with linear lakes characterises the emerging thrust front. Field evidence for a major fault-propagation fold structure along with thrust faulting within the late-Quaternary fluvial sediments is ubiquitous. Clay beds deposited in lakes along the footwall of FBT have formed due to blockade of south flowing rivers by episodic upliftment of the hanging wall block; three such episodes of uplift since 16k years correspond to three morphogenic earthquakes of magnitude ~6.9 rupturing the FBT during late Pleistocene–Holocene. In light of geomorphological and geological studies, neotectonic activity has been modelled as an active south dipping backthrust that originates at shallow crustal depth from south vergent basal Himalayan Decollement in response to the advancing Himalayan wedge.