The tectonothermal evolution of the high himalayas in the Suru valley, NW Zanskar, with constraints from metamorphic modelling
This thesis attempts to construct an integrated model for the tectonothermal evolution of the Suru valley region of the High Himalayan Crystalline unit of north-west India, and combines detailed field mapping, petrographic and microstructural analysis, thermobarometric techniques and metamorphic modelling. This work confirms that the metasedimentary lithologies of the Suru valley correspond to the Palaeozoic-Mesozoic Tethyan shelf sediments found in Kashmir and Zanskar and that the meta-igneous bodies correlate with Permian rift-related igneous units. This demonstrates that all the metamorphism in the Suru valley is Himalayan in age. Subsequent to India-Asia collision at ~54 Ma the units of the Suru valley underwent a polyphase deformational and metamorphic history. The large scale structure of the area is that of kilometre-scale, SWvergent recumbent folds that are domed by a later structure, the Suru Dome. Barrovian metamorphism resulting from collision and burial reached a maximum of kyanite grade, and is believed to be syn- to post-kinematic with respect to the formation of the large folds. Thermobarometric analysis indicates that peak conditions relating to this Barrovian event were of the order of 9.5-10.5 kbar and 620-650 0C. A later metamorphic event associated with doming throughout the Zanskar Himalaya and crustal anatexis in the core of the High Himalaya caused re-equilibration of deeper Suru Dome rocks to conditions of 8.7-9.7 kbar and 630-640 0C. Metamorphic modelling, involving phase diagram construction and P-T path determination using Gibbs method calculations, suggests that metamorphic garnets grew under conditions of burial and heating. Rapid exhumation of the High Himalayan Crystallines on the Main Central Thrust and the Zanskar Shear Zone occurred shortly after peak metamorphism. The results suggest that phase diagram construction and P-T path calculation should be used in conjunction in order to confidently model metamorphic rocks.