An integrated tectonothermal model for the evolution of the High Himalaya in western Zanskar with constraints from thermobarometry and metamorphic modeling

We present an integrated model for the tectonothermal evolution of the High Himalaya in NW Zanskar based on detailed field mapping, petrographic and microstructural analysis, thermobarometric techniques, and metamorphic modeling. Metasedimentary lithologies in the Suru valley can be correlated with the Palaeozoic-Mesozoic Tethyan shelf sediments along the north Indian continental margin in Kashmir and Ladakh, and metaigneous amphibolites correlate with Permian rift-related igneous units. Subsequent to India-Asia collision at ca. 54 Ma, crustal thickening of Indian plate rocks resulted in a polyphase deformational and metamorphic history. The large-scale structure of the area is that of kilometer-scale, SW vergent recumbent folds that have been folded by structurally lower, later domes such as the Suru Dome. Prograde M1 metamorphism reached a maximum of kyanite grade and is believed to be synkinematic to postkinematic with respect to the formation of the large folds. Thermobarometric analysis indicates that peak conditions relating to this Harrovian event between 33 and 28 Ma were 9.5-10.5 kbar and 620°–650°C. A later metamorphic event (M2) associated with doming throughout the Zanskar Himalaya and crustal anatexis in the sillimanite + K-feldspar-grade core of the High Himalaya caused reequilibration of deeper Suru Dome rocks to slightly lower pressures (4.5–7 kbar). Metamorphic modeling, involving phase diagram construction and pressure-temperature (P-T) path determination, suggests that metamorphic garnets grew under conditions of heating and burial along moderate slopes in P-T space. Rapid exhumation of the High Himalayan Crystallines between the Main Central Thrust and the Zanskar Shear Zone occurred during or immediately after peak M2 metamorphism (21.5–19.5 Ma).