Estimation of vein attributes, stress ratio (Φ), driving pressure ratio (Rʹ) and local stress axes orientations from veins: Insights from Pelling-Munsiari Thrust zone of Sikkim Himalayan FTB

Ray, Deblina (2021) Estimation of vein attributes, stress ratio (Φ), driving pressure ratio (Rʹ) and local stress axes orientations from veins: Insights from Pelling-Munsiari Thrust zone of Sikkim Himalayan FTB. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Deblina Ray (16MS067)) 16MS067_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (6MB)

Abstract

To estimate vein attributes, stress ratio (Φ), driving pressure ratio (Rʹ), and their spatial and temporal variation, I analyze extension veins from the deepest and shallowest exposures of the regionally folded Pelling-Musiari thrust (PT), the roof thrust of the Lesser Himalayan duplex, in the Sikkim Himalaya, along its transport direction. Based on the angular relationship with respect to the mylonitic foliation, we recognize three different fracture- and associated vein-sets at the outcrop scale. These are low-angle (<30°), , moderate-angle (30°-60°) and high-angle set (>60°). The high-angle fracture set overprints the mylonitic foliation and is the youngest set. These are also the most dominant fracture set (~58 %), followed by the moderate-angle (~32%) and low-angle (~10%) sets. Interestingly, the low-angle vein set (mean orientation ~ 29°, 054°) is the most dominant set (~59%), followed by the moderate-angle set (~28%; mean orientation ~ 19°, 055°), and the high-angle set (~13% ; mean ~23°, 340°).Field analysis indicates that ~95% of low-angle, ~71% of moderate-angle and ~ 40% of high-angle fracture-sets form veins. The co-efficient of variation (Cv) of spacing of both the fracture and vein sets are less than 1, indicating that these follow anti-clustered distribution. Some of the low- and moderate-angle veins are locally folded along with the mylonitic foliation, while the high-angle veins overprint the mylonitic foliation. The poles to the veins indicate two distinct patterns. The low-, moderate- and high-angle veins from the hinterland exposure define a girdle distribution, implying pore fluid pressure (Pf) exceeded intermediate principal stress axis (σ2), whereas the veins from the foreland exposure show a clustered distribution indicating σ2 exceeded Pf. A preliminary microstructural study reveals presence of blocky texture in the low- and moderate-angle veins with quartz growing at high angles with respect to the vein walls. Based on stress-inversion method (Yamaji, 2016), there are 4 generations of veins in hinterland exposure and 3 generations in the foreland exposure. Based on Rʹ, Φ and stress axes orientations in finite deformed state, the different generations of veins possibly formed under different deformation conditions. The stress axes orientations and mylonitic foliation, along with Rʹ and Φ, have contributed to vein development. Results from stress analysis (Jolly and Sanderson, 1997; Yamaji, 2016) and field observations are internally consistent. Pore fluid pressure, vein orientations, vein generations and local paleostress orientations varied spatially and temporally along the PT shear zone along the transport direction. Deformation conditions have thus varied spatially and temporally along the transport direction of the PT shear zone

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