Strain, Vorticity and Kinematic Analyses of L-tectonites from Eastern Sikkim Himalaya: Explaining the Local Transport-Parallel Constrictional Strain in the Himalayan Fold thrust Belt

Das, Jyoti Prasad (2015) Strain, Vorticity and Kinematic Analyses of L-tectonites from Eastern Sikkim Himalaya: Explaining the Local Transport-Parallel Constrictional Strain in the Himalayan Fold thrust Belt. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Jyoti Prasad Das (10MS43)) JPD_MS_thesis.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

In the Sikkim Himalayan fold-thrust belt (FTB), Pelling thrust (PT) fault rocks are locally characterized by an L tectonite mylonite zone along a linear, N-S trending, discontinuous klippe, unlike anywhere else in the Himalayan orogen. This zone lies in close spatial association with L>S, and S>L mylonites in this region. This is a study to understand the kinematic evolution of this unique deformation structure. Shear-sense analysis on asymmetric feldspar porphyroclasts record a dominant transport parallel flow with a subsidiary transport perpendicular flow. We analyzed various strain markers, for example, porphyroclasts to track finite strain, subgrain and oblique grain shape fabric to track incremental strain in order to conduct vorticity analysis. The estimates of kinematic vorticity numbers using these markers range 0.44<Wk<0.99, which suggest a general deformation with L fabric from the hinge zone of the fold recording the highest Wk. Wm (from porphyroclasts) and Wn (from oblique fabric) show a small variation from N-S in comparison to Wn from subgrains indicating the latter to be the best tracker of vorticity variation. Based on Wk, we concluded that the relative contribution of simple shear decreases from Phodong mylonite zone (30-75%) to Rumtek mylonite zone (37-47%). Comparing Wk from the L tectonites with the PT fault rocks lying immediately north of the klippe, suggest that the latter records a dominant simple shear than in the L tectonites. On comparing these results with a few published data from western Himalaya suggests that the studied area records greater amount of pure shear (48-63%) in comparison to other parts of Himalaya. The vorticity data supports the lateral ramp model proposed in order to explain the mechanism of the L tectonites in the eastern Sikkim Himalayan FTB.

Actions (login required)

View Item