| Summary: | The regional Barrovian facies metamorphic rocks of the High Himalayan Slab in Zanskar are bounded along the base by the southwest vergent Main Central Thrust (MCT) with its characteristic zone of inverted isograds, and along the top of the slab by the northeast dipping Zanskar Shear Zone (ZSZ), part of the South Tibetan Detachment (STD) System of normal faults. Summarized here are the results of systematic mapping combined with detailed P-T-t data from the Zanskar Himalaya, and models for the thermal and mechanical evolution of the middle and deep crustal rocks of the Himalaya are discussed. Temperatures and pressures increase dramatically up-structural section across the MCT Zone from biotite through garnet, staurolite and kyanite grade to sillimanite + muscovite, and decrease along the top of the stab beneath the ZSZ normal faults. In Zanskar, peak P-T conditions of the M1 kyanite-grade rocks are 550-680°C and 9.5-10.5 kbar, and M2 sillimanite-grade gneisses were formed at 650-770°C and 4.5-7 kbar. The core of the High Himalayan Zone is a 30 km wide zone of sillimanite + K-feldspar-grade gneisses, migmatites and anatectic leucogranites with a right way-up isograd sequence above and an inverted sequence below. Thermal models of thrusting a hot slab over a cold slab and frictional heating along the MCT are not supported by the structural and thermobarometric data. Mechanical models of post-metamorphic structural disruption by folding and thrusting of a pre-existing, right way-up metamorphic sequence are compatible with structural and P-T data. Ductile shearing along the MCT Zone has structurally condensed the isograds, and final motion along the ZSZ postdated leucogranite crystallization (21.5-19.5 Ma; U-Pb monazite ages) and emplacement in the footwall. Crustal shortening and thickening, resulting in prograde metamorphism, lasted from the time of India-Asia collision at 54-50 Ma until at least 30-25 Ma. Crustal melting is constrained at 20.8-19.5 Ma along the core of the stab. Late stage out-of-sequence thrusting within the High Himalayan Slab effectively maintained the crustal thickening process enabling P-T conditions to remain high for 10-5 Ma. Rapid exhumation between 21-18.5 Ma was accompanied by removal of 18-25 km of overburden by erosion at exhumation rates of 6-10 mm yr-1. This early Miocene period of high exhumation rates, rapid erosion and exhumation of rocks buried at >25 km depth is interpreted to indicate the uplift of high mountains and erosion of deep valleys, probably with high precipitation during that time.
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