Microscopic anatomy of a "hot-on-cold" shear zone: Insights from quartzites of the Main Central Thrust in the Alaknanda region (Garhwal Himalaya)

Microstructures inside exhumed crustal shear zones provide a window into the mechanics of Earth's interior. However, difficulties remain in quantifying their physical properties due to the numerous macro-, meso-, and grain-scale factors controlling their development and modification. The complex interplay between these factors has meant that inconsistencies remain between the knowledge derived from controlled laboratory deformation experiments and that from empirical observations in nature. We present a quantitative toolbox for identifying variations in the deformation characteristics of shear zones, using the microstructure and crystallographic preferred orientation (CPO) of quartz. The Main Central Thrust in the Alaknanda Valley (NW Himalaya) is an archetypal major ductile thrust zone that contains abundant quartzites and quartz-mica schists that permit microstructural characterization across gradients in strain and temperature. Using a combination of detailed microstructural analysis and neutron diffraction, we show that quartzites across the Main Central Thrust sequence are characterized by: (1) systematic increases in grain lobateness and coarseness, indicating the transition from subgrain rotation (SGR) to dominant grain boundary migration (GBM) dynamic recrystallization; (2) a concomitant shift in CPO from oblique (top-to-the-SSW) girdles to a strong single y maximum, indicating increasing activity of prismatic <a> slip; and (3) high CPO intensities coinciding with the switch to GBM and prismatic <a> slip activity. Our analyses suggest the Main Central Thrust may be characterized as an ~3-km-wide zone of dominantly non-coaxial deformation coinciding with a sharp change in peak metamorphic temperatures, from ~450 °C in the footwall to 750-800 °C in the hanging wall. We suggest that the observed systematic microstructural and CPO transitions may typify the deformational behavior of quartz in the Main Central Thrust across the wider Himalaya.