Evolution of landscape in a piedmont section of Eastern Himalayan foothills along India-Bhutan border: A tectono-geomorphic perspective

The present study area involves part of a deformed coalesced fan located along the Himalayan Frontal Thrust (HFT) on the east of river Tista near the India-Bhutan border. The area is marked by two spectacular E-W trending south-sloping scarps namely the Matiali (ca. 60 m) and Chalsa (ca. 90 m) Scarps and a north-sloping E-W trending Thaljhora (ca. 80 m) Scarp. Our work comprises of a comparative study of geomorphology and geologic history in the adjacent interfluves of Jaldhaka-Gathia and Neora-Murti rivers to understand the tectonic history of the area. We mapped the Jaldhaka-Gathia river interfluve at a 1:25,000 scale and report a hitherto unidentified northerly sloping small scarp of ca. 5m height named the Nagrakata Scarp. This scarp was identified using satellite images, DEMs, and total station survey. We interpret that the two north-sloping, E-W trending scarps (Thaljhora and Nagrakata Scarps) are manifestations of steep limbs of anticlines over blind south-dipping back thrusts. Together they form a wrinkle-ridge pair behind the north-dipping HFT, which is manifested by south-sloping Chalsa Scarp. We propose a plausible geomorphic model interpreting that deformation along the small fan in the Jaldhaka-Gathia interfluves is younger compared to fan deposition and deformation in the adjacent Mal-Murti interfluve. The most recent geomorphology of the Jaldhaka-Gathia interfluve is controlled by tectonism associated with the thrust below the Nagrakata Scarp where the youngest deformation episode is recorded to at around ∼6 ka and is likely related to motion on a splay off of the thrust beneath the Thaljhora Scarp.