Strain Partitioning in the Sub-MCT Thrust Sheets from Siang Valley, Arunachal Himalayan Fold Thrust Belt: Insights into Structural Evolution of the Wedge

Gogoi, Jonti (2015) Strain Partitioning in the Sub-MCT Thrust Sheets from Siang Valley, Arunachal Himalayan Fold Thrust Belt: Insights into Structural Evolution of the Wedge. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Jonti Gogoi (10MS52)) Jonti_10MS52.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

Grain-scale strain typically accommodates ~50% of total shortening in fold thrust belts (FTB). Therefore, the magnitude and orientation of strain ellipses provide critical insights into the kinematic and structural evolution of an orogenic wedge. We focus our study on the sub-MCT thrust sheets of the Siang window in far-eastern Arunachal Himalayan FTB to examine how strain is partitioned. We quantified strain from the Ramgarh thrust (RT) in the north followed by Main Boundary Thrust (MBT) and Geku Thrust (GT). We used quartzites, sandstones and vesicular basalts as strain markers from these thrust sheets. 2D strain from the Fry Method shows that the strain is decreasing in the thrust sheets from hinterland (R=1.908) to foreland (R=1.607); this suggests a foreland-ward progression of deformation. The strain is also varying within the same thrust sheet as a function of its structural position within the sheet. In a larger scale we could see that the strain recorded in the pebbles is slightly higher than the microspoic scale, from the same thrust sheet indicating that coarse grains record higher strain amount. We found 55% transport-perpendicular and 45% transport-parallel strain ellipses in the study region. This suggests the role of footwall thrusts of the RT in controlling the strain geometries. The rocks record an overall flattening strain. Comparison of the strain data with the Bhutan Himalaya in the west suggests that there is a lateral variation in strain partitioning in the Himalayan FTB. Vorticity analysis using rigid grain method and subgrain analysis indicates overall general deformation of the studied thrust sheets in the Arunachal Himalayan FTB.

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