Dynamic recrystallization mechanisms and their transition in the Daling Thrust (DT) zone, Darjeeling–Sikkim Himalaya

Abstract

The Daling Thrust (DT) delineates a zone of intense shear localization in the Lesser Himalayan Sequence (LHS) of the Darjeeling–Sikkim Himalaya. From microstructural studies of deformed quartzite samples, we show a transition in the dynamic recrystallization mechanism with increasing distance from the DT, dominated by grain boundary bulging (BLG) recrystallization closest to the DT, and progressively replaced by sub-grain rotation (SGR) recrystallization away from the thrust. The transition is marked by a characteristic variation in the fractal dimension (D) of grain boundaries, estimated from the area–perimeter method. For the BLG regime, D≈1.046, which decreases significantly to a value as low as 1.025 for the SGR regime. Using the available thermal data for BLG and SGR recrystallization, we infer increasing deformation temperatures away from the DT in the hanging wall. Based on the quartz piezometer our estimates reveal strong variations in the flow stress (59.00MPa to 16.00MPa) over a distance of ~1.2km from the DT. Deformation mechanism maps constructed for different temperatures indicate that the strain rates (10−12S−1 to 10−14S−1) comply with the geologically possible range. Finally, we present a mechanical model to provide a possible explanation for the cause of stress intensification along the DT.