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

Abstract

AbstractAlthough 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 40 Ma and peak suprasolidus conditions in the kyanite stability field (i.e., >650–700°C and >0.7–1.0 GPa) were attained between 35 and 30 Ma. Peak metamorphism was followed by cooling, decompression, and melt crystallization at circa 30 Ma 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 16 Ma. The GHS cooled below ~450–475°C at 20–17 Ma on the southwest flank and 17–14 Ma 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.