Estimation of Flow Stress and Strain Rate from the veins of an internal shear zone: Insights from Pelling-Munsiari thrust, Sikkim Himalaya

Ranjan, Rajiv (2022) Estimation of Flow Stress and Strain Rate from the veins of an internal shear zone: Insights from Pelling-Munsiari thrust, Sikkim Himalaya. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Rajiv Ranjan (16MS018)) 16MS018_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (17MB)

Abstract

Estimating deformation conditions such as flow stress, strain rate and deformation temperature from shear zone rocks is critical in understanding its complex deformation history. However, often mylonite deformation conditions provide information on finite state deformation conditions. On the other hand, veins may record incremental strain stages during the progressive deformation. As a result, veins were used as incremental strain markers to assess the spatial and temporal variation in deformation conditions along the transport direction of a shear zone. In this study, we estimated vein attributes at the microscopic scale, flow stress, strain rate and deformation temperature from the Pelling-Munsiari thrust in the Sikkim Himalaya. It is a regionally folded thrust which acts as the roof thrust of a complex duplex of Lesser Himalayan. At the hinterland-most (Mangan) and frontal exposures (Suntaley) in eastern Sikkim, the PT zone is exposed as a ~938 m and ~188 m thick quartz-mica mylonite zone, respectively. It is subdivided into three domains where the protomylonite domain is surrounded on both sides by mylonite domains. Based on the angular relationship to the mylonitic foliation, we can distinguish three vein-sets. At both hinterland and foreland locations of the PT zone, the low-angle (0-30°) is the dominant veinset followed by moderate-angle (30-60°) and high-angle (60-90°). Based on the overprinting and cross-cutting relationship, the high-angle vein set is the youngest. There are three generations of veins present in the hinterland and frontal part of the PT shear zone. In both of these locations, the low-angle veins are dominant. Multiple crack-and-sealed events were observed in Mangan PT zone, indicating repeated failure and mineral precipitation. On the contrary, crack-and-sealed events were restricted to low-angle veins in the frontal exposure of the PT zone. At both locations, higher distribution of veins is present near the footwall. Low-angle veins recorded coarser quartz grain size in the hinterland. However, high-angle veins record coarser grain size in the foreland. Following Stipp et al. (2003) and Twiss (1977), we estimate flow stress from recrystallized quartz grain-size piezometer. In the hinterland, the high-angle (~25MPa) vein-set records the highest flow stress. On the contrary, moderate-angle (~30MPa) vein-set records the highest flow stress in the foreland exposure. Flow stress variation in the veins of different orientations at a single location indicates that veins are tracking different stages of progressive deformation. Following Hirth et al., (2001), we estimated similar strain rates (~10⁻¹⁵ sec⁻¹) from both the PT zones. Interestingly, the high-angle veins record the fastest strain rate (~6*10⁻¹⁵ sec⁻¹) in the hinterland most exposure, whereas, in the frontal part, the moderate-angle veins record the fastest strain rate (~9*10⁻¹⁵ sec⁻¹). Variations in strain rate along the transport direction has been observed in veins. In general, strain rate increases in veins along the transport direction.

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