Structural and Kinematic Evolution of Shear Zones Associated with Internal Thrusts: Implications on Orogenic Wedge Evolution

Ghosh, Pritam (2022) Structural and Kinematic Evolution of Shear Zones Associated with Internal Thrusts: Implications on Orogenic Wedge Evolution. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Pritam Ghosh (14RS039)) 14RS039.pdf - Submitted Version Restricted to Repository staff only Download (17MB)

Abstract

The Himalaya is one of the best examples of an active mountain belt with well-exposed shear zones associated with internal thrusts that record minimum overprinting of past orogenic events. Hence, such thrust zones can provide the best platform to address their kinematic evolutionary paths, in terms of progressive deformation associated with growth of footwall structures, that are critical to understand orogenesis. We compare and contrast the structural evolution of two successive internal thrusts, the Main Central thrust (MCT) and its immediate footwall structure, the Pelling-Munsiari thrust (PT), from the crystalline core of the Sikkim Himalaya in the context of progressive deformation involving a footwall Lesser Himalayan duplex (LHD); the PT is its roof thrust. The LHD has folded the overlying MCT and the PT into antiform-synform pairs that are exposed at several structural positions. The mylonitic foliations in both the shear zones are overprinted by successive cleavages. The youngest cleavage from the PT zone continues within the hanging wall MCT sheet that records one additional cleavage, possibly indicating time-transgressive cleavage development. The cleavages become steeper while their intensity decreases structurally higher up within individual thrust sheets. The Rs-values and angular shear strain decrease from the mylonite zones to structurally higher up within the sheets. Both the shear zones are Type II with decelerating strain paths. Rs-θ′ relationships and microstructures indicate thrust-parallel stretch is greater than thrust-perpendicular component. Both the shear zones record strain partitioning with lesser competent mylonite domains recording higher Rxz, and a greater proportion of simple-shear than the protomylonite domains. The shear zones recorded the growth of the duplex during progressive deformation that contributed to pure-shear dominated general-shear, higher flattening strain and greater translation on its roof thrust, the PT, than the overlying MCT. Extending the study on a tectonic scale, we investigate kinematic evolutionary paths of seventeen major shear zones associated with internal thrust faults from six orogenic belts, by analyzing published minimum strain, kinematic vorticity number, and minimum translation. We estimate the pure shear component from the recorded vorticity and reconstruct a first-order kinematic path based on multiple strain markers, wherever possible. The studied shear zones follow decelerating strain paths. In general, sub-simple shear more effectively accumulates translation than pure/simple shear deformation. Shear zones with relatively high pure shear component record higher strain. Incremental strain markers from these shear zones record a progressive evolution from an earlier simple shear dominated to a pure shear dominated flow. These results are in agreement with earlier theoretical studies. Internal shear zones that act as roof thrusts of duplexes or have stacked imbricate structures in their immediate footwall, generally record relatively higher strain, greater translation and greater pure shear component toward the later stage than similar shear zones without such footwall structures. We interpret that slip-transfer and structural culmination formed during the growth of immediate footwall structures contribute to the kinematic evolution of internal shear zones. The same shear zone records along-strike variation in its kinematic path due to its varying immediate footwall geometry. Thus, deciphering comprehensive kinematic evolutionary paths of internal shear zones also requires an understanding of immediate footwall structures. Additionally, studying kinematic paths of internal shear zones may provide insights into the geometry of immediate footwall structures when they are not exposed. To address how kinematic evolutionary paths of two successive internal shear zones vary along their transport direction as a result of varying structural position, I have also studied the MCT and the PT exposures from the leading-edge. The rock-parcel at the leading-edge exposures lie foreward of the LHD, and therefore record the deformation signatures from trailing-edge locations as well during its progressive deformation. Hence, these rocks record the most complex and complete progressive deformation history, as compared to the other zones. Based on deformation profiles, the frontal-most MCT and PT zones record at least one additional cleavage and crenulation lineation as compared to the trailing-edge locations. We attribute this observation to the more complete capturing of the footwall imbrication related to the progressive deformation. Irrespective of their structural position within the orogenic wedge, exposures of internal thrusts record a non-steady, decelerating strain path. In the leading-edge exposures, the PT with greater connectivity with the immediate footwall duplex records a higher strain, higher translation, and higher pure shear component than the overlying MCT that does not have a direct connectivity with the duplex. Hence, the internal thrusts record a consistent strain, pure shear component, and translation pattern, in terms of their connectivity to the footwall structures, irrespective of their structural positions within the orogenic wedge. The leading-edge exposures of the MCT and the PT zones, with a more complete deformation history, record a comparatively higher simple shear component and a higher strain than the trailing-edge. We explain this as a result of dominance of less competent mylonite zone (~91-94% of the total shear zone thickness) over more competent protomylonite zone in the leading-edge exposures. The weaker mylonite zone accommodates a higher simple shear component and higher strain in the leading-edge than in the more competent trailing-edge that has a lesser proportion of mylonite zone (~85-86% of the total shear zone thickness). Strain partitioning due to internal structural geometry, a manifestation of the varying deformation conditions along major shear zones, also play a critical role in their kinematic evolution. This work demonstrates that the kinematics of shear zones associated with internal thrusts are affected by footwall structural geometry that can provide insights into deciphering the kinematic evolution of the orogenic wedge. This work is the first of its kind from any orogenic belts.

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