THERMAL STRUCTURE OF THE MAIN CENTRAL THRUST, EVEREST TRANSECT, HIMALAYAS

Inversion of metamorphic isograds is a common phenomena in large scale mid-crustal thrust zones, with metamorphic grade increasing up structural section. Structural models for inversion involve motion on discreet thrusts and/or pervasive shearing within thrust sheets that may be either synchronous with peak metamorphic temperatures (T_max) or postdate T_max with late-stage shear fabrics forming at lower T. The Main Central Thrust (MCT) of the Himalayas is a high strain shear zone with well documented telescoping of inverted metamorphic isograds (Hubbard, 1996; Searle et al., 2003). Thermal data of the MCT is limited to the Sutlej region (Law et al., 2011), showing an increase in deformation temperature up structural section from 535˚C to 615˚C. This thermal data also corresponds to the quartz microstructures in the high strain samples.

The Kruhl (1998) c-axis fabric opening angle geothermometer was implemented to expand the thermal data set for the MCT using samples from the Everest Region (collected by Hubbard, 1988, and Struele, 2010). Deformation in both the Lesser Himalayan Series (LHS) in the footwall and the Greater Himalayan Series (GHS) in the hanging wall to the MCT was emphasized. Comparing quartz c-axis fabrics to the microstructures preserved in the samples yields similar results to that of the Sutlej region, with a general increase in deformation temperatures from 490˚C in the LHS hanging wall. Increase in temperature is also visible with a change in quartz recrystallization regimes, from subgrain rotation to grain boundary migration. . This is in agreement with the ductile shearing model for inverted metamorphism.