Fluid-fault Interaction during Progressive Deformation in Pelling-Munsiari Thrust Fault Zone: A Case Study from Darjeeling-Sikkim Himalayan Fold Thrust Belt

Singh, Akanksha (2017) Fluid-fault Interaction during Progressive Deformation in Pelling-Munsiari Thrust Fault Zone: A Case Study from Darjeeling-Sikkim Himalayan Fold Thrust Belt. Masters thesis, Indian Institute of Science, Education and Research Kolkata.

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Abstract

Fault rocks associated with thrust faults with large displacement and prolonged activity in a fold thrust belt (FTB) generally records rotation, reorientation and reactivation of early-formed fractures and generation of new sets of fractures with progressive deformation. Thus, within the same fault zone fracture population evolves temporarily and spatially along its transport direction. Fluids are an integral component during evolution of fault zones where they can create hydraulic fractures, seal existing fractures by forming veins, or cause hydrolytic weakening. In Sikkim Himalayan FTB, we examine the fault-fluid interaction along such a fault, i.e., the Pelling-Munsiari thrust (PT) zone, that acts as the roof thrust of the Lesser Himalayan duplex. The hinterlandmost exposure of the PT zone at Mangan records, three dominant fracture orientations at ~0°–30° (~46%), ~30°-60° (~37%), and~60°-90° (~17%) with respect to the mylonitic foliation. High-angle fractures offset the early formed lower angle fractures. Vein population study reveals that early-formed, mostly shear fractures are fluid-filled forming silicate veins, while the later ones are unfilled. The forelandmost exposure records higher fracture population with orientations of~0°-30° (~16%), ~30°-60° (~47%) and ~60°-90° (~37%) with respect to the mylonitic foliation. Fluids are present in both low- and high-angle fractures indicating greater dominance of fluids in the foreland part than in the hinterland. Proportion of extensional fractures increases toward the foreland. The Pelling PT zone in the western segment records higher fracture population than eastern segment with orientations of 0°-30° (~23%), ~30°-60° (~43%), and ~60°-90° (~34%) with respect to the mylonitic foliation Veins form along both low- and high-angle fractures. Although the location records the deepest initial overburden conditions, extensional fractures increase toward the western segment. We explain this discrepancy due to higher structural culmination associated with the exposed Rangit duplex that lies in the footwall of the Pelling PT zone, while it is blind below the Mangan PT zone. Recrystallized quartz grain-size analysis shows similar temperature distribution (~555°C-548°C±50°C) for differently oriented veins in Mangan PT zone. The forelandmost PT zone records progressive lowering of temperature from early-stage, low-angle veins (515°C±50°C) to late-stage high-angle veins (415°C ±50°C). Preliminary results from isotopic analysis indicate that the source of fluids was intraformational in all the studied locations.

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