Preservation of the Early Evolution of the Himalayan Middle Crust in Foreland Klippen: Insights from the Karnali Klippe, West Nepal

Although the India-Asia collision has been ongoing since the Eocene, the exposed hinterland of the Himalayan orogen was pervasively deformed and metamorphosed at high temperature during the Miocene and hence reveals little information about the Eocene-Oligocene period of collision. New pressure-temperature-time-deformation data from the Karnali klippe in west Nepal foreland demonstrate that Greater Himalayan sequence (GHS) rocks there escaped the Miocene overprint and consequently unveil the early tectonometamorphic evolution of the middle crust. Prograde metamorphism in the GHS occurred at 40Ma and peak suprasolidus conditions in the kyanite stability field (i.e., >650-700 degrees C and >0.7-1.0GPa) were attained between 35 and 30Ma. Peak metamorphism was followed by cooling, decompression, and melt crystallization at circa 30Ma during tectonic exhumation below the South Tibetan detachment. As the middle crust was exhumed, strain propagated up section within the South Tibetan detachment high-strain zone, which remained active through circa 16Ma. The GHS cooled below similar to 450-475 degrees C at 20-17Ma on the southwest flank and 17-14Ma on the northeast flank of the Karnali klippe. In marked contrast, GHS rocks now exposed in the hinterland were still buried, hot and actively deforming, while the foreland was cooled and exhumed. Oligocene cooling of the frontal tip of the GHS is compatible with the southward extrusion of partially molten midcrustal rocks followed by renewed shortening along out-of-sequence shear zones in the hinterland.