Structural Evolution of Arunachal Lesser Himalayan Fold Thrust Belt: Insights from Fracture Analysis and Preliminary Stable Isotope Analysis

Basa, Abhisek (2017) Structural Evolution of Arunachal Lesser Himalayan Fold Thrust Belt: Insights from Fracture Analysis and Preliminary Stable Isotope Analysis. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Abhisek Basa (14IP046)) Abhisek_thesis.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

Fractures of different orientations form as deformation progresses in an evolving fold thrust belt (FTB). These fractures act as strong conduits for fluid migration within the wedge. In Siang Valley of Arunachal Lesser Himalayan FTB, the Buxa dolomites of the Lesser Himalayan sequence (LHS) are exposed in Geku thrust sheet in the hinterland and in the Main Boundary thrust (MBT) sheet in the foreland. Fractures are the most discernible deformation features in these carbonate rocks across different scales. Fracture population analysis and temporal evolution of fracture sets at the outcrop scale suggests that late stage, high-angle fractures have the highest population while the early formed low-angle fractures record the least population in the foreland MBT sheet. Fracture population increases towards the fault zone in both outcrop and microscopic scales suggesting dominance of frictional deformation mechanisms closer to the fault zone. Structural analysis indicates MBT sheet is folded in a fault-bed fold. Based on unfolding the fracture sets, early formed, low-angle fractures are prefolding structures while the late stage, high-angle fractures formed synchronous to folding. Microscopic analysis shows that most of the high angle fractures (~83%) are extensional fractures. Microfracture analysis from five samples each from the hinterland and the foreland MBT sheet suggests that the Geku sheet records ~36% of microfractures that increases to ~64% in the foreland. This increase is due to forelandward dominance of frictional deformation mechanisms as overburden decreases towards the foreland, coupled with shallower depth of fault rocks due to fault cutting up-section. Additionally, there is a spatial variation in the orientation of fractures in the carbonate samples. Buxa carbonates from the hinterland thrust sheet record almost similar percentages of parallel to sub-parallel (~32%) fractures and high angle fractures (~40%), all measured with respect to the bedding. In contrast, the forelandmost thrust sheet records a dominance of late, high angle fractures (~54%), while abundance of parallel to sub-parallel and oblique fractures are ~24% and ~22%, respectively. ~69 % of all fractures form calcite, dolomite and quartz veins. Vein population and aperture studies reveal that the low-angle fractures act as preferential conduits for fluid migration in the hinterland Geku thrust sheet, while the high-angle fractures record maximum fluid activity in the foreland. Preliminary results from stable isotope analysis of calcite veins suggests that early formed, low-angle veins have less depleted δ¹⁸O (per mil VPDB) values (~2 ‰) relative to the host rock. In contrast, the high-angle veins record maximum difference of ~4%. This indicates that the low-angle veins had intra-formational fluid source, while the high-angle veins possibly had an external source. δ¹⁸O vs δ¹³C plot of the high-angle veins shows a linear trend indicating meteoric influence.

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