INHERITED STRUCTURES AND EXHUMATION OF MID-CRUSTAL ROCKS IN CONVERGENT OROGENS: INSIGHTS FROM THE HIMALAYA, NW INDIA

Convergent orogens include inherited structures that can influence the subsequent localization of deformation during extrusion and doming. In some areas of the Himalaya, rocks that experienced different pressure-temperature-time paths, degrees of partial melting, and exhumation during the Miocene can be attributed to shear zones and detachments that formed during previous stages of crustal thickening (i.e., Eocene and Oligocene). Pelitic rocks of the Haimanta Group in the Sutlej Valley, NW India record burial and Barrovian metamorphism. Pressure-temperature-time-deformation paths indicate that Barrovian metamorphism culminated at ~30 Ma, resulting in a metamorphic field gradient that increases from garnet-grade (567 ± 105°C and 6.7 ± 1.6 kbar) to kyanite-grade (650-750ºC, 6-8 kbar). Northeast from the zone of Barrovian metamorphism, a protracted period of partial melting (23-18 Ma) was injected into a localized area around the Leo Pargil dome. Partial melting in the Leo Pargil dome overlapped with near isothermal decompression at ~23 Ma. The Leo Pargil shear zone nucleated along a detachment at the base of a fold and thrust belt within the Tethyan Sedimentary Series and exhumed the dome during top-to-the-west shearing. At 23 Ma, as decompression was accommodated by the Leo Pargil shear zone, the Greater Himalayan Series was extruded southward beneath the Sangla detachment (local equivalent of the South Tibetan detachment). The Sangla detachment was localized along the contact between the Greater Himalayan Series and the Haimanta Group. These data suggest that structures inherited from early stages of crustal shortening controlled the subsequent localization of shear zones that accommodated nearly simultaneous exhumation of mid-crustal rocks during south-directed extrusion in the foreland and doming in the hinterland.