Control of structural position on the deformation behaviour of fault zone rocks: Insights from Pelling-Munisiari thrust

Ammu, J. K. (2018) Control of structural position on the deformation behaviour of fault zone rocks: Insights from Pelling-Munisiari thrust. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Ammu J. K. (13MS044)) 13MS044.pdf - Submitted Version Restricted to Repository staff only Download (4MB)

Abstract

In fold-thrust belts (FTB), shallow crustal rocks accommodate strain at various scales. Penetrative strain in hinterland thrust sheets can be envisioned as two end-member cases of Layer parallel shortening (LPS) strain and Layer Normal Flattening strain (LNF). In the Himalayan FTB, at the orogenic scale, the convergence-related shortening is accommodated by a folded thrust fault system. The Lesser Himalayan Duplex (LHD) has folded the overlying roof thrust, the Pelling-Munsiari thrust (PT), and has exposed it at various structural positions. Due to its longer connectivity to LHD, the PT zone is expected to record a prolonged deformation history. Additionally, in Sikkim-Himalaya, the LHD is exposed in the western transect but is partially blind in the eastern transect exposing the PT zone at different structural elevations with respect to the basal decollement. Therefore, in this study, I integrate finite strain and kinematic analyses from the PT fault zones at two locations along its transport direction to examine how variation in structural position affects the deformation and kinematic evolutionary history of these fault rocks. Penetrative strain is partitioned at all studied locations of the PT zone. The angle that the long axis of the strain ellipse makes with the mylonitic foliation (Ɵ’) is very low (<16º). The hinterlandmost exposed Pelling PT fault zone records the highest finite strain magnitude (Rxzmax = ~2.14) and it decreases towards the foreland at Soreng (Rxzmax = 2.09) and eastward at Mangan (Rxzmax = 2.01), where the lower LHD is blind. The overburden, at the time of initiation of thrusting on the PT, is estimated to be highest in Pelling (~16 km), followed by Mangan (~14 km) and Soreng (~7.1 km). Ɵ’ estimated from the PT sheet rocks from Pelling and Soreng is low (Ɵ’< 17º). Additionally, Ry/z >1 indicates stretching along thrust perpendicular Y-direction. Thus, the PT sheet dominantly records LNF strain. Pelling records lowest Ɵ’ indicating maximum parallelism (Ɵ’<10º). The PT sheet rocks record comparable strain magnitude (Rxz~1.7-1.8) as PT fault zone rocks (Rxz ~1.7-2.1) suggesting a penetrative strain that pre-dated thrusting. In Pelling and Soreng which lie in the western transect, the strain ellipses are oriented towards west and in Mangan which lies in the eastern transect, the XZ plane dips eastward indicating that the penetrative strain is pre-folding. Vorticity analysis using different proxies show a progressive simple-shear dominated to pure-shear dominated deformation. Soreng records the highest simple-shear component (~ 67% - 90%) which is explained by the proximity of Soreng to the footwall duplex structure and hence an effective slip transfer. The microfracture intensity is higher in Pelling (0.016) than in Soreng (0.010). In Pelling the fractures are anti-clustered (Cv<1) and clustering increases towards foreland.

Actions (login required)

View Item