SYNCONVERGENT EXTENSION IN NW HIMALAYA: NEW GEOCHRONOLOGICAL CONSTRAINTS ON THE EXTRUSION HISTORY OF THE HIGH HIMALAYAN CRYSTALLINE OF SE ZANSKAR

High-grade metamorphic rocks of the High Himalayan Crystalline Zone (HHCZ) of Zanskar are exposed as a 50 km long dome along the Miyar and Gianbul valleys in the NW Himalaya of India. This Gianbul dome is cored by migmatitic paragneisses formed at peak conditions of 800°C and 8 kbar. This migmatitic core is symmetrically surrounded by rocks of the sillimanite, kyanite ± staurolite, garnet, biotite, and chlorite mineral zones. The structural and metamorphic data from the Miyar-Gianbul Valley section reveal that the tectono-metamorphic evolution of the HHCZ in SE Zanskar is associated with a polyphase tectonic history involving, converging nappe structures and superimposed opposite-directed extensional structures. As a consequence of two initial converging crustal thickening phases, the HHC high-grade rocks were subducted down to 30 km depth where temperatures of up to 800° C triggered partial melting. Following these crustal thickening phases, exhumation and doming of the high-grade rocks of the HHCZ were controlled by extension along the NE-dipping Zanskar Shear Zone (ZSZ) on the northern limb of the dome. This is accompanied by coeval extension along the SW-dipping Khanjar Shear Zone on the southern limb of the dome. Hence, rapid, synconvergent extension along both of these detachments induced an nearly isothermal decompression resulting in an HT/LP metamorphic overprint. New geochronological dating of migmatites and leucogranitic dykes in the core of the dome reveals that the main ductile shearing along these detachements ended by 19.8 Ma. It likely initiated shortly before 26.6 Ma. Several recent models interpret that ductile extrusion of the high-grade, low-viscosity migmatites of HHCZ reflects combined extension along the ZSZ and thrusting along the Main Central Thrust (MCT). Hence, our new data constrain the onset of the thrusting along the MCT to start shortly before 26.6 Ma, an age significantly older than the commonly assumed age of 23 Ma.