Crustal thickening, Barrovian metamorphism, and exhumation of midcrustal rocks during doming and extrusion: Insights from the Himalaya, NW India

Abstract

Rocks exposed in NW India constrain the burial, partial melting, and exhumation history of the Himalayan crust. New microscale and mesoscale structural analysis, combined with pressure-temperature estimates from the Haimanta Group exposed in the Sutlej Valley, indicate that the rocks were folded by two generations of NW trending folds. Barrovian metamorphism culminated at ~30 Ma with staurolite and kyanite overgrowths of F2 folds. Crustal thickening created a metamorphic field gradient that increases from the garnet zone (499 ± 99°C and 4.5 ± 1.4 kbar to 571 ± 92°C and 7.8 ± 1.4 kbar) to the staurolite-kyanite zone (567 ± 105°C and 6.7 ± 1.6 kbar). These data are combined with previous studies to modify a two-stage conceptual model for the thermal and deformation conditions of the middle and upper crust during the Eocene-Miocene, excluding the late Miocene to recent. In the Miocene (~23 Ma), Barrovian metamorphism was overprinted by decompression during coeval south directed extrusion of the Greater Himalayan Series beneath the Sangla detachment in the foreland and doming during top-down-to-the-west displacement along the Leo Pargil shear zone in the hinterland. These data demonstrate that shear zones and detachments, such as the South Tibetan detachment that initially formed during crustal thickening (e.g., Eocene-Oligocene), contributed to the subsequent distribution of rocks that experienced different pressure-temperature-time paths, degrees of partial melting, and exhumation histories during the Miocene.