3D Strain Analysis and Mathematical Modelling of L-tectonites from Eastern Sikkim Himalayan Fold Thrust Belt: A Step Towards Understanding the Structural Evolution of the Local Constrictional Zone

Dwivedi, Harsh Vardhan (2015) 3D Strain Analysis and Mathematical Modelling of L-tectonites from Eastern Sikkim Himalayan Fold Thrust Belt: A Step Towards Understanding the Structural Evolution of the Local Constrictional Zone. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Harsh Vardhan Dwivedi (10MS23)) Thesis_Harsh_V_Dwivedi_10MS23.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

L-tectonites are locally exposed within the hanging wall Lingtse orthogneiss of the Pelling thrust, as linear N-S trending discontinuous klippen in eastern Sikkim Himalayan fold thrust belt (FTB). These rocks are spatially associated with L>S and S>L tectonites; they are folded along a synform with the stretching lineations and the hinge line being sub-parallel to the regional transport direction (N-S). The L-tectonites are best developed along the hinge zones of the fold. Within the klippen, the strongest constrictional strain is recorded in the hinge zone of the synform and decreases progressively towards the eastern and the western structural boundary. The constrictional strain increases from the northern klippe (k = 2.8) to south (k = 6.4), and then decreases farther south to k = 1.5. Strain partitioning between quartz and feldspar suggests that only plastically deformed quartz records constrictional strain. I generated 3-D strain ellipsoids that reveal strong transport-parallel constrictional strain and supports the field observation. Asymmetric feldspar porphyroclasts indicate a dominantly transport parallel and a subsidiary transport-perpendicular flow. These observations, along with the presence of a transport-parallel fold, suggest a possible existence of a lateral ramp beneath the PT in this region. To validate the proposed structural setting for transport-parallel constrictional strain, I generate a displacement-based mathematical model of the deforming thrust wedge. I studied three different cases. In the first case, I examined the effect of the presence of frontal ramp; in the second case, the effect of a lateral ramp. In the third case, based on the natural observations, I integrated the combined effect of a frontal and a lateral ramp. The synthetic data from the mathematical models were compared with the natural data in each of the cases. The results suggest that a presence of a lateral ramp can explain the observed natural data, and local transport-parallel constrictional strain in a FTB. We report such a structure for the first time in the Himalayan FTB.

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