Exploring the roles of climate and tectonics in the geomorphic evolution of the Chitwan Intermontane valley, Central Himalaya

Abstract

The Himalayan front exhibits varying convergence rates along its length. This study examines the morphotectonic evolution of the landforms in an intermontane valley (viz., Chitwan dun) present in a relatively rapidly converging segment of the Himalaya. Using field investigations, geomorphic analysis, and geochronology, we demonstrate that five geomorphic levels have developed in the past >112 ka. Four major phases of aggradation occurred in the Chitwan, i.e., >112 ka, ~112–>70 ka, ~70–25 ka, and ~18–11 ka. Tectonic activity on the Jharahi (JHT) and Belani thrusts (BT) at >112 ka and >70 ka respectively resulted in the uplift of the geomorphic units to Level 5 and Level 4. Continuous aggradation in the valley occurred between 70 and 25 ka (Level 3); the tectonic activity during this period is marked by the reactivation of the West Chitwan thrust (WCT) and the Central Churia Thrust (CCT). The Level 3 surfaces were abandoned during the LGM (25–18 ka) and subsequently incised. Enhanced monsoon in the post-LGM period favored aggradation of the Level 2 and 1 geomorphic units. The Level 2 surfaces were abandoned due to activity on the Danda (DT) and Shaktikhor Thrusts (ST) whereas the Level 1 landforms were incised due to climatic changes in the Holocene. Comparing the results of this study with other parts of the Himalaya, we infer that despite variable tectonic convergence rates, major aggradation and incision cycles are uniform. This suggests that Late Quaternary climatic perturbations have strongly influenced the fluvial systems across the Himalaya. However, the role of tectonics is also manifested in the form of multiple levels of surfaces and terraces.