DUCTILE EXTENSION AND EXHUMATION OF THE LEO PARGIL DOME, NW INDIA: IMPLICATIONS FOR THE TECTONICS IN THE WESTERN HIMALAYA

Crustal deformation in the Himalaya and Tibetan Plateau initiated due to convergence that began in the Eocene. Since the Miocene, convergence has been accommodated by south-directed thrusting at the Himalayan front and by east-west extension within the Himalaya and Tibetan Plateau along normal and strike-slip faults. The northwest Indian Himalaya contains approximately north-south oriented normal faults and northeast oriented normal-sense shear zones (i.e. shear zones that bound the Leo Pargil dome), southwest of the dextral Karakoram strike-slip fault system. West-directed extension on the normal-displacement Leo Pargil shear zone, bounding the southwest flank of the Leo Pargil dome began by 23 Ma. Deformation temperatures and mean kinematic vorticity estimates (W_m) during ductile shearing are integrated with pressure-temperature estimates to evaluate the kinematic evolution of the Leo Pargil shear zone. Pressure-temperature data suggests that the rocks within the shear zone record prograde metamorphism followed by near-isothermal decompression. Kinematic vorticity and deformation temperature analysis of quartz fabrics within the Leo Pargil shear zone suggest that these rocks were thinned during top-down-to-the-west shearing at temperatures ranging from 400-500° C at the uppermost structural levels to >650° C at the deepest levels. These data suggest that the rocks within the Leo Pargil shear zone were exhumed from mid-crustal depths during west-directed shearing. Extension resulted in near-isothermal decompression of the rocks within the Leo Pargil dome and decompression-driven melting that fed an injection complex within the dome. Data from this study combined with existing age constraints on the Leo Pargil shear zone and the dextral Karakoram fault support a model where extension along the Leo Pargil shear zone occurred during an early ductile extensional regime in a zone of transtension related to dextral movement on the Karakoram fault.