Heterogeneous Deformation of MCT Zone from Rakdong-Dikchu, Eastern Sikkim: Implications for Structural Evolution of the Sikkim Himalayan Fold Thrust Belt

Raj, Nikhil Kumar (2013) Heterogeneous Deformation of MCT Zone from Rakdong-Dikchu, Eastern Sikkim: Implications for Structural Evolution of the Sikkim Himalayan Fold Thrust Belt. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS Dissertation of Nikhil Kumar Raj (08MS58)) Nikhil_Kumar_Raj-08MS58-MSThesis.pdf - Submitted Version Restricted to Repository staff only Download (5MB)

Abstract

The thrust systems in the Himalayan fold thrust belt play a critical role in accommodating the large shortening resulting from ongoing collision between the Indian and the Eurasian plates. In the Darjeeling-Sikkim Himalaya, the Main Central thrust system (MCT) is composed of a structurally higher MCT1, and a lower MCT2 (Bhattacharyya and Mitra, 2011). Each of these faults have translations greater than 100 km that helped carry the overlying MCT sheets within ~7 km of the mountain front; this is greater than anywhere else in the western Himalaya. The sub-MCT Lesser Himalayan duplex has folded the overlying MCT sheets in a regional antiform-synform pair exposing the fault zone rocks at many structural positions in this region. To evaluate the mechanics of large translations along the fault zone, I studied the MCT2 fault zone rocks from the northern limb of the antiform that is exposed near Dikchu-Rakdong in east Sikkim. In this area, the MCT2 is exposed at two strands at Rakdong as an east dipping (~35°,118°) with true thickness 19-840m and at Dikchu (~41º,108°) with true thickness 284-1015m, where the amphibolite grade, quartzo-feldspathic Paro gneiss of the hanging wall has undergone grain size reduction and change in composition to form a quartz-mica mylonite. The footwall rocks are the greenschist grade Daling phyllites of the Lesser Himalayan sequence. The eastward dip is related to the plunge of the structurally underlying Rangit anticline of the Lesser Himalayan duplex (Bhattacharyya & Mitra, 2009). Within the mylonite zone, there is evidence of a stretching lineation along the N-S direction (8°→010°) that is consistent with the regional transport direction. My preliminary study suggests that quartz have undergone grain size reduction dominantly by dislocation creep, while feldspar grains by microfracturing. In addition, sericitization of feldspars is prominent within the mylonite zone. Reaction softening of feldspars to micas in the presence of fluids has been a dominant mechanism which has resulted in strain softening along the MCT and has allowed a large translation within a thin fault zone. The study shows there is a effect of Rangit duplex in the deformation mechanism of grains.

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