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

Abstract

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 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.

Tectonometamorphic evolution of the tip of the Himalayan metamorphic core in the Jajarkot klippe, west Nepal

“…The Jajarkot klippe is located in the Himalayan foreland, ~75 km south of the main hinterland‐positioned exposures of the HMC, and as such may provide important insights on the early mid‐crustal evolution of the Himalayan orogen (e.g., Soucy La Roche, Godin, Cottle, et al., ). However, the extent of a pre‐Himalayan metamorphic event, as recognized in other klippen in the central Himalaya (e.g., Gehrels et al., 2006a, 2006b; Joshi & Tiwari, ), must first be evaluated.…”

“…The GHS records a cryptic phase of early Palaeozoic metamorphism that is rarely preserved in the hinterland and is better documented in the foreland klippen (e.g., Argles et al., ; Cawood et al., ; DeCelles et al., ; Gehrels et al., , 2006a; Godin et al., ). During the Himalayan orogeny, the GHS was metamorphosed at greenschist to granulite facies conditions and was extensively deformed from the Eocene to the Miocene (Carosi et al., ; Godin et al., ; Grujic, Hollister, & Parrish, ; Grujic, Warren, & Wooden, ; Iaccarino, Montomoli, Carosi, Massone, et al., ; Iaccarino et al., ; Inger & Harris, ; Larson & Cottle, ; Larson et al., ; Pêcher, ; Soucy La Roche, Godin, Cottle, et al., ; Streule, Searle, Waters, & Horstwood, ; Vannay & Hodges, ). The base of the GHS is marked by the Main Central thrust (MCT) zone, a several km‐thick top‐to‐the‐SW shear zone that propagated down‐section from the early Oligocene to the late Miocene as slices of footwall rocks were successively accreted to the hangingwall (Gansser, ; Hunter, Weinberg, Wilson, Luzin, & Misra, ; Larson, Ambrose, Webb, Cottle, & Shrestha, ; Martin, 2017b; Mottram et al., ; Searle et al., ).…”

“…In the Almora klippe in northwest India, greenschist to upper amphibolite facies metamorphism is inferred to be dominantly Precambrian based on folded regional metamorphic isograds that are cut by the Himalayan‐age Almora thrust at the base of the klippe, contact metamorphism that overprints regional metamorphism around the Neoproterozoic Almora granite, and evidence for two generations of garnet growth (Joshi, Singh, & Goel, ; Joshi & Tiwari, , ). In contrast, kyanite‐grade suprasolidus metamorphism in the Karnali klippe in west Nepal is entirely constrained to the Eocene–Oligocene based on monazite petrochronology (Soucy La Roche, Godin, Cottle, et al., ). Due to the foreland position and early exhumation of the Karnali klippe, these metamorphic conditions are interpreted to represent the state of the middle crust during the early evolution of the Himalaya (Soucy La Roche, Godin, Cottle, et al., ).…”

“…In contrast, kyanite‐grade suprasolidus metamorphism in the Karnali klippe in west Nepal is entirely constrained to the Eocene–Oligocene based on monazite petrochronology (Soucy La Roche, Godin, Cottle, et al., ). Due to the foreland position and early exhumation of the Karnali klippe, these metamorphic conditions are interpreted to represent the state of the middle crust during the early evolution of the Himalaya (Soucy La Roche, Godin, Cottle, et al., ).…”

“…(), McQuarrie et al. (), Murphy and Copeland (), Soucy La Roche, Godin, Cottle al., (), and Yin (). Black rectangle in the Jajarkot klippe indicates the map area of Figure .…”