MULTIPLE PALEOSTRESS AND STRAIN-RATE GRADIENTS ABUTTING THE GREATER HIMALAYAN SERIES: INSIGHTS FROM THE SOUTH TIBETAN DETACHMENT SYSTEM, SOUTHERN TIBET, AND THE MAIN CENTRAL THRUST, NW INDIA

Microstructural and quartz fabric analyses across shear zones marking the upper (South Tibetan Detachment System - STDS) and lower (Main Central Thrust - MCT) margins of the Greater Himalayan Series (GHS) indicate significant changes in high temperature ductile deformation traced towards each fault. Recent work on several structural transects through GHS rocks in these shear zones in Sutlej Valley, NW India (STDS), and Rongbuk Valley, S Tibet (MCT), documents extreme telescoping of deformation isotherms associated with normal and reverse sense motion on the STDS and MCT, respectively, in agreement with previously well established distribution of metamorphic isograds. Traced towards both the STDS and MCT, and commencing at a structural distance of ~ 200 m, a change in quartz recrystallization mechanism from grain boundary migration to subgrain rotation occurs in the GHS rocks. The recrystallization mechanism switch leads to a decrease in quartz grain sizes. At distances of 1150 - 200 m from the STDS/MCT, differential flow stress estimates from the Stipp & Tullis (2003) recrystallized quartz grain size piezometer are relatively constant at 5-20 MPa, but increase sharply to 30-35 MPa at ~ 200 m from the STDS/MCT. Combined flow stress and deformation temperature (~ 450 - 650 °C, inferred from quartz c-axis fabric opening angles) data indicate strain rates of 9.1 x 10^-13 to 2.9 x 10^-15 sec^-1 using the Hirth et al. (2001) quartz flow law. Traced towards both faults into progressively lower deformation temperature tectonites, strain rates first decrease and then abruptly increase at structural distances of 200 m. This abrupt increase in strain rate coincides spatially with the first appearance of subgrain rotation recrystallization and may be associated with a later phase of shearing that overprints the higher temperature / lower differential stress microstructures preserved at greater structural distances from the faults.