Tectonics of the Himalayas

Abstract

The Himalayan range was formed and uplifted in association with the southward migration of plate boundary thrusts that separate the Himalaya into four belts. Initially, during 50-35 Ma, the Tibetan marginal mountain range was uplifted after slab break-off of the Tethyan oceanic plate, which was subducted under the Asian continent to depths of up to 100 km. During the second stage at ~35-25 Ma, the Mesoproterozoic sediments deposited on the northern passive margin of the Greater India were subducted and underwent moderate-pressure metamorphism at depths of ~40 km. Subsequent to metamorphism, metamorphosed continental crust was separated from the underlying mantle by delamination, and its rapid exhumation and associated amphibolite facies metamorphism occurred during 22-16 Ma. Partially melted metamorphic rocks generated granitic melt that intruded both metamorphic rocks and Tibetan Tethys sediments during the Miocene. Exhumation of the metamorphic belt continued after its exposure at ~15 Ma, forming an extensive metamorphic nappe covering the Lesser Himalayan sediments. After ceasing movement at 11-10 Ma, the Indian plate started to subduct along the Main Boundary Thrust (MBT), which was newly formed in front of the southern margin of the metamorphic nappe. At the same time, the nappe and weakly metamorphosed underlying Lesser Himalayan sediments started to cool laterally from the southern front to the root zone at a rate of ~10 km/Myr. At 3-2.5 Ma, the plate boundary fault shifted to the Main Frontal Thrust (MFT) to the south of the MBT, causing rapid uplift of the marginal range of the Lesser Himalaya and the Siwalik Hills. Today, the Indian Plate is converging with the Asian Continent at the rate of 58 mm/yr, and half of this convergence is consumed by uplift of the Siwalik Hills along the MFT.