Fluvial response to active tectonics: analysis of DEM-derived longitudinal profiles in the Rangit River Basin, Eastern Himalayas, India

The Himalayan Mountain Range has originated from the ongoing collision of the Eurasian and the Indo-Australian plates since the Paleogene. It is widely accepted that this tectonically-driven uplift is still continuing, as reflected by a large number of earthquakes in the area. Apart from the sub-surface and geophysical signatures, the surficial geomorphic markers deserve due attention. Fluvial systems, which preserve the evidence of the past and present tectonic perturbations on the surface, have often been investigated to assess the imprints of uplift in a region. Sensitive to long-term tectonic, structural and climatic regimes, the general forms of the longitudinal profile and its derivatives have been analysed across the globe for determining the varying roles of tectonics, litho-structure and climate. This article assesses the degree of tectonic and lithological control on the drainage network of the Rangit Basin in the Eastern Himalayas. One of the important characteristics of the studied basin is that the Main Central Thrust (MCT), which is located between the Greater Himalayas and the Lesser Himalayas, divides the basin into two distinct domains. Longitudinal profiles and their derivatives of 16 major tributaries of the Rangit River were extracted from the ALOS–PALSAR DEM and analysed. The controls on this Himalayan river were evaluated based on investigations of longitudinal profile shapes, stream gradient (SL) indices, longitudinal profile concavities and steepness. Prominent drainage anomalies such as above-grade conditions, exponential and linear fitting of longitudinal profiles, elevated values of SL indices, barbed drainage, over-steepened stream segments and fluvial hanging valleys imply rapid erosion rates in the basin. This is noticeable particularly in the lower domain of the Rangit Basin, especially in the areas located downstream of the MCT. A comparison of steep segments with the geological and lineament maps reveals that many of these anomalies are lineament-controlled. Furthermore, a large number of such features do not conform to lithological intersections, suggesting a possible tectonic factor behind the occurrence of such anomalies.