OROGEN-SCALE ALONG-STRIKE CONTINUITY IN QUARTZ RECRYSTALLIZATION MICROSTRUCTURES ADJACENT TO THE MAIN CENTRAL THRUST: IMPLICATIONS FOR DEFORMATION TEMPERATURES, STRAIN RATES AND FLOW STRESSES

Traced for ~ 1500 km along the foreland edge of the Himalaya from NW India to Bhutan published reports indicate a remarkable along-strike continuity of quartz recrystallization microstructures adjacent to the Main Central Thrust (MCT). In foreland-positioned high-strain rocks adjacent to the MCT recrystallization is dominated by subgrain rotation (SGR) suggesting that shearing occurred under a relatively restricted range of deformation temperatures and strain rates. Quartz c-axis fabric opening angles, mineral phase equilibria and Raman spectroscopy of carbonaceous material (RSCM) data indicate deformation temperatures and 'close to peak' temperatures of metamorphism of 450-550 °C for SGR-dominated rocks located at up to a few hundred meters below/above the MCT.

Plotting these temperature estimates on the quartz recrystallization map developed by Stipp et al. (2002) indicates 'ball-park' strain rates of ~ 10^-13 to 10^-10 sec^-1on the MCT. However, only strain rates slower than 10^-12 sec^-1on the MCT are likely to be compatible with know convergence rates between the Indian and Asian plates. The presence of a single recrystallization microstructure traced over a large (regional scale) distance does not necessarily mean that deformation temperature (or strain rate) remains constant but could, for example, indicate that spatial variations in deformation temperature are compensated for by changes in strain rate, with grain-scale deformation remaining within a particular recrystallization regime.

Constant stress conditions plot along a straight line in the 1/T versus log strain rate space used in the quartz recrystallization mechanism map. This suggests that the observed along-strike consistency of SGR-dominated recrystallization microstructures may indicate near to constant stress boundary conditions (albeit with varying temperatures and strain rates) prevailing along what are now the more foreland-positioned exposures of the MCT. Extrapolation of the Hirth et al. (2001) flow law suggests a flow stress of ~ 30-50 MPa based on the deformation temperatures and strain rates inferred for foreland-positioned exposures of the MCT, in agreement with flow stresses estimated from recrystallized quartz grain size data using the Stipp and Tullis (2003) piezometer.