Estimation of deformation temperatures, flow stresses and strain rates from an intra-continental shear zone: The Main Boundary Thrust, NW Himalaya (Uttarakhand, India)

This work generates thermal and tectonic data from the mylonitized rocks of the Main Boundary Thrust (MBT). South and north to this major thrust, the Siwalik range and the Lesser Himalaya, respectively have been receiving a renewed interest amongst the petroleum geoscientists. Understanding temperature, flow stress and strain rates related to a major thrust is crucial in thermal-mechanical modelling of the terrains adjacent to it. Strongly sheared Proterozoic Chandpur phyllites and abundant syntectonic quartz veins occur along the MBT zone (one of the major foreland verging thrusts in the Himalayan orogen) near the Sahenshahi temple at Dehradun, Uttarakhand state, India. Microstructure analyses show intense ductile top-to-S shear in terms of S-C fabric and XRD analyses indicate presence of clays e.g., illite and clinochlore. These clays not only bear the signatures of the intense fluid activity, but also enhance the fault movement by reducing the mechanical strength of the fault zone rocks. Thermometry based on qualitative quartz grain boundary mobility (similar to 300-550 degrees C) and quantitative Laser Raman Spectroscopy for carbonaceous materials (similar to 340-370 degrees C) reveal the maximum estimates of the deformation temperature and metamorphic temperature, respectively. The higher temperature microstructures can be found in lower temperature domains under the presence of fluids. Hence the quartz thermometry estimates a broader temperature range than the Raman thermometry. Previous works report that, the phyllites present at the base of a thrust sheet, experiences deformation at < 500 degrees C. The thermometric estimates in this study indicate a higher temperature at the base than that at further north known from previous studies, possibly by shear heating. A flow stress of similar to 6-49 MPa has been calculated from recrystallised quartz grain piezometry. This broad range of flow stress value indicates temporally variable tectonic stress responsible in the evolution of the MBT. Using these magnitudes, similar to 10(-15)-10-(16) S-1 of strain rate has been estimated. Such a range can also indicate seismic cycles in the tectonically active Himalaya.