Geometry & Kinematics of the Lesser Himalayan Fold Thrust Belt, Siang Valley, Arunachal Pradesh: Insights into Himalayan Orogenic Evolution

Ahmed, Farzan (2024) Geometry & Kinematics of the Lesser Himalayan Fold Thrust Belt, Siang Valley, Arunachal Pradesh: Insights into Himalayan Orogenic Evolution. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Farzan Ahmed (13IP036)) 13IP036.pdf - Submitted Version Restricted to Repository staff only Download (7MB)

Abstract

Published, regional, balanced cross-sections across various transects along the Himalayan fold-thrust belt (FTB) show significant lateral variations in the magnitude and partitioning of total minimum shortening among various litho-tectonic units. The Lesser Himalayan sequence (LHS) accommodates ~50%-70% of the total minimum shortening and exhibits the maximum lateral variation. Existing hypotheses have not been able to completely explain the lateral variation in total minimum shortening. Based on this study, we demonstrate a direct correlation between the initial width of the LHS and the total minimum shortening distribution along the Himalayan FTB. Initial width of the LHS also controlled the lateral variation in structural architecture of the LHS as manifested by number of horses and imbricates. Lateral variation in structural geometry of the LHS also resulted in lateral variation in initial and current LHS outcrop widths. To have a comprehensive understanding of the Himalayan orogenic evolution, we also require structural and kinematic insights from the relatively less studied, far-eastern Himalaya. The Siang window in the Arunachal Himalaya exposes the LHS rocks that is repeated by a series of imbricates qualifying it as a suitable transect to evaluate the LHS geometry, kinematics and shortening. I construct a regional balanced cross-section along an inferred regional transport direction (~342°-162°) through the Siang window. The undeformed footwall template is ~16 km thick. The Main Himalayan thrust (MHT) lies at a depth of ~15 km below the Lesser Himalayan Duplex (LHD) and ramps up to ~7 km depth in the frontal part. The most prominent structure is the LHD that has 13 horses, 8 upper and 5 lower LHS horses. The estimated shortening without incorporating the slip on the Main Central thrust (MCT) and the Pelling-Munsiari-Bomdila thrust (PT) is ~361 km (~72%). The minimum LHS shortening estimates from the Siang window continues to remain high and supports the role of initial LHS width in accommodating shortening. This is reflected in the structural geometry with a greater number of horses and imbricates. The frontal Main Boundary thrust (MBT) sheet forms a fault bend fold. Outcrop scale folds in the frontal MBT Buxa dolomites are associated with extensive fracturing. The fractures occur in 3 sets: low-(~0°-20°), moderate-(~21°-60°) and high-angle (~61°-90°) with respect to the bedding. Based on crosscutting and abutting relationship in the field, relative abundance and fold test, we interpret the high-angle fractures have formed later than the low- and moderate-angle fractures. Based on a Discrete Fracture Network (DFN) model, the volumetric properties of volume fracture intensity(P32), porosity and hydraulic property of permeability show a significant increase in the post folding model as compared to the pre-folding model due to the formation of late-stage, high-angle extensional fractures. The volumetric and hydraulic properties are affected by (a) proximity to the MBT zone and (b) position on the fold. However, the latter plays a more efficient role. A part of this study may serve as an outcrop analogue for exploration of fractured reservoirs in the Himalayan FTB, and potentially for similar settings in other FTBs.

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