Himalayan Geology, Vol. 43 (1B), 2022, pp. 241-252, Printed in India

Exhumation processes and mechanisms in the Himalayan-Tibetan Orogen: A Review

VIKAS ADLAKHA, KALACHAND SAIN*, KUNAL MUKHERJEE

Wadia Institute of Himalayan Geology, Dehradun - 248001, India

*Email (Corresponding author): kalachandsain7@gmail.com

Abstract: Exhumation of orogenic belts is one of the essential factors that exert a first-order control on their evolution. Different processes and mechanisms act in exhuming deep-seated rocks to the surface across these orogenic belts depending upon the nature of orogeny. Here we present a review of such processes and mechanisms for one of the world's largest and youngest continent-continent collisional orogenic belts, the Himalayan-Tibetan orogen. We assess the exhumation processes within the three main domains of the orogen: (a) the Himalayan Wedge, (b) the Trans-Himalaya, and (c) the Tibetan Plateau. The exhumation rates have been highest within the southernmost wedge of the orogen, while the interior of the Tibetan Plateau is characterized as the slowest exhuming domain. Fault kinematics, i.e., thrusting and folding and the geometry of the subsurface in combination with intense orographic precipitation are the main factors that control the exhumation pattern of the southernmost Himalayan wedge. The exhumation of the ultra-high pressure rocks within the Trans-Himalaya may be explained through various models of the subducting slab, while the primary process for exhumation is governed by buoyancy effect followed by erosion. Two end-membered models, i.e., Rigid body and Soft Tibet models, may explain the exhumation mechanism of Tibet. The rigid body model considers Tibet as a rigid block that extrudes eastward in between the major strike-slip domains. In the Soft Tibet model, the topographic uplift enhances exhumation through the process of delamination of the Tibetan mantle lithosphere followed by late-stage underthrusting of the Indian slab. Morphometry of Tibet is also a prominent factor that controls the regional exhumation pattern through the formation of internally drained basins within the Tibetan interior and thus reducing the reliefs and hence exhumation. Therefore, the interplay of tectonics, climate, and topography controls the exhumation pattern of the Himalayan-Tibetan orogenic belts, while the nature and intensity of these factors remain variable depending upon the location across the orogen.

Keywords: Exhumation, Tectonics, Climate, Himalaya, Tibet