Himalayan Geology, Vol. 43 (1B), 2022, pp. 151-163, Printed in India
Channel Flow: Structural and Thermal evolution of the Greater Himalayan Sequence
MICHAEL P. SEARLE1,2,3*
1Dept. Earth Sciences, University of Oxford, Oxford, OX1 3AN
2Oxford University Museum of Natural History, Oxford, OX1 3PW
3Camborne School of Mines, University of Exeter (Cornwall campus), Penryn, Cornwall. TR10 9EZ
*Email : mike.searle@earth.ox.ac.uk
Abstract: Crustal thickening and shortening along the northern continental margin of India started soon after obduction of ophiolites during the Late Cretaceous and India-Asia collision during the Early Eocene. Increasing pressure and temperature led to regional metamorphism which resulted in peak staurolite-kyanite grade metamorphism between ca 44-30 Ma. Minor kyanite-bearing partial melts are the first indication of partial melting at pressures up to 1 GPa. The initiation of the Main Central Thrust (MCT) zone along the base of the Greater Himalayan Sequence (GHS), and the South Tibetan Detachment (STD), a north-dipping, low-angle normal fault along the top of the GHS at this time, led to S- or SW-extrusion of a hot, ductile deforming mid-crustal layer. Decompression melting of sillimanite + muscovite and sillimanite + K-feldspar migmatites led to widespread partial melting and formation of migmatites and garnet + tourmaline + muscovite ± cordierite leucogranite melts between ca 28-15 Ma, facilitating southward extrusion of the mid-crustal GHS, and large-scale folding of metamorphic isograds (Channel Flow). Inverted metamorphic isograds along the MCT zone at the base can be linked to right way-up isograds beneath the STD along the top around a large-scale S- or SW-verging giant fold of the metamorphic isograds. Ductile shearing at mid-crustal depths progressed downwards to brittle faulting along the base of the MCT zone, forming the base of the extruding channel, and upwards to brittle faulting along the top of the STD zone during exhumation. Mid-crustal channel flow was operating during the Late Eocene to Mid-Miocene, and ended with the final leucogranite melts as thrusting propagated down-structural section to the Main Boundary thrust.
Keywords: Himalaya, Channel Flow, Main Central Thrust, South Tibet Detachment, leucogranites, structure, metamorphism
