Deformation Kinematics of Main Central Thrust Zone (MCTZ) in the Western Himalayas

Abstract

The main central thrust (MCT) is one of the major thrusts in Himalayas. In central Himalaya, MCT was defined as a contact between underlying Lesser Himalayan Sequence (LHS) and overlying higher Himalayan crystallines (HHC). However, in the Kashmir Himalayas, the main central thrust zone (MCTZ), shear zone associated with MCT, is overlain by Kashmir Tethyan Sequence suggesting that the MCTZ has been deformed through a mechanism different than the mechanism responsible for MCTZ evolution in other parts of the Himalayas. In the present study we used structural, microfabric and kinematic analyses to investigate the deformation kinematics of MCTZ. Microstructural investigation revealed that the quartz in orthogneiss mylonites of MCTZ was dynamically recrystallized by grain boundary migration (GBM) and sub-grain rotation recrystallisation (SGR) with top-to-SW sense of shear. The mean kinematic vorticity number (W _m) just above the thrust ranges from 0.72 to 0.84 (40%–52% pure shear component) decreasing upwards to 0.65–0.71 (35%–50% pure shear component). Deformation in the MCTZ is characterized by R _xz strain ratio varying from 2.7 to 8. The present study suggested that the MCTZ suffered 3%–40% vertical shortening and 3%–66% transport-parallel elongation. The results suggested that the HHC’s were not completely exhumed to the topographic surfaces in the Kashmir Himalayas. Along the basal decollement, i. e., the main Himalayan thrust (MHT), the deformation continued until MCTZ reached the brittle-ductile transition where deformation mechanism changed to the brittle and the MCTZ rocks were transported to the surface through slip on brittle MCT.

Keywords

kinematics / mylonites / finite strain / mean kinematic vorticity number (W _m) / general shear / tectonics

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