Along-Strike Variation in Structural Architecture and Penetrative Strain: Insights from Sikkim, Eastern Himalayan Fold-Thrust Belt

Parui, Chirantan (2022) Along-Strike Variation in Structural Architecture and Penetrative Strain: Insights from Sikkim, Eastern Himalayan Fold-Thrust Belt. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Chirantan Parui (13IP039)) 13IP039.pdf - Submitted Version Restricted to Repository staff only Download (36MB)

Abstract

The Sikkim Lesser Himalayan fold-thrust belt (FTB) records along-strike structural variations over ~15 km. The lower Lesser Himalayan (Rangit) duplex (LHD) is blind, and the overlying Pelling and Ramgarh thrusts are more intensely folded and preserved in eastern Sikkim than in the west. A regional balanced cross-section reveals a minimum shortening of ~542 km (~80%) with an average long-term shortening rate of ~18 mm/yr in eastern Sikkim. In western Sikkim, an ~2 km high footwall Main Himalayan thrust (MHT) ramp below the Rangit duplex caused higher structural culmination exposing the duplex. In contrast, an ~0.35 km high ramp below the Rangit duplex in eastern Sikkim was insufficient to expose the duplex. The partially blind LHD with 11 gently dipping (~40°) horses transferred more displacement to the roof thrust in eastern Sikkim than the 12 steeply dipping (~60°) horses in western Sikkim, illustrating the importance of constraining blind structures in FTBs. Lateral variations in location and height of footwall MHT ramp, lithological variation along the MHT, initial width of the Lesser Himalayan basin, and presence of a lateral ramp explain the structural variation in Sikkim. Integrating quartz plastic strain results from 102 quartz-rich rocks with two balanced cross-sections reveal variations in shortening partitioning across scales during orogenesis. The LHD in the middle of the orogen records the highest thrust-sheet scale shortening, while its roof thrust toward the hinterland records the highest grain-scale shortening. The Lesser- and the Sub- Himalayan thrust sheets record ~78-91 km of shortening as penetrative strain. The layer-normal shortening (LNS) strain dominates in the internal thrusts, while the external thrust sheets record a combination of LNS and layer-parallel shortening (LPS) strain. Lateral structural variations in the thrust sheets are reflected in the thrust sheet shortening and the strain ellipsoid orientations but not in the strain magnitude. Incorporating grain-scale shortening into the large-scale shortening will decrease the total minimum shortening of Sikkim Himalaya by ~13% due to the dominance of LNS in thrust sheets. Estimating grain-scale strain from low-magnitude penetratively deformed rocks are typically fraught with uncertainties. Therefore, to find the most suitable strain analysis method(s) for low strained rocks (Rs <1.5), we examined the accuracy and the precision of eight commonly used methods by varying the strain magnitude (Rs) and deformation types (pure shear, simple shear, and general shear). We generated 5097 strain measurements by deforming synthetic rock images and analyzing 102 quartz-rich rocks from the Sikkim Himalaya. Results indicate that the shape matrix eigenvector (SME) and stereographic projection (SP) methods provide the most accurate and precise strain estimates. A minimum of 100 markers for low strain (Rs <1.5) and 75 for moderate to high strain (Rs >1.5) are required for generating accurate strain results. Results from this study have implications for estimating grain-scale strain from similar rocks deformed in any structural setting.

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