Characterising the metamorphic discontinuity across the Main Central Thrust Zone of eastern-central Nepal

Abstract

The Main Central Thrust Zone (MCTZ) is a top-to-south shear zone that has exhumed the high-grade Himalayan metamorphic core during the orogeny. Identifying the location of the MCTZ is a major challenge and the characteristics of the metamorphic discontinuity remain under debate. To clarify this issue, petrologic and thermobarometric studies were carried out on metapelites and metapsammites that were collected from the basal Nyalam transect in eastern-central Nepal. Results reveal that the metamorphic discontinuity across the MCTZ is characterised by a continuous increase in peak P–T conditions toward higher structural levels, a relatively high field temperature gradient (25–50°Ckm−1) and different types of P–T paths. Specifically, representative rocks in the MCTZ record sub-solidus peak conditions (637±16°C and 9.2±1.0kbar) and a hairpin-type P–T path. The lower GHC rocks record supra-solidus peak conditions (690±32°C and 10.3+1.1/−1.4kbar) and a prograde loading path with a small segment of decompression. The presence of a high field pressure gradient across the MCTZ is debatable in the Nyalam transect due to the large uncertainties in pressure estimates. Comparison between obtained P–T results and model predictions indicates that a multiple thrusting process dominated exhumation of the MCTZ and lower GHC rocks, while crustal flow contributed partly to exhumation of the lower GHC rocks.