Deformation history from microstructures: an example from Ramgarh Thrust Sheet, Eastern Himalaya

The emplacement of thrust sheets in a fold and thrust belt is an integral part in the evolution of orogenic belts. The signatures of deformation present in a thrust sheet is often very useful to elucidate the history of deformation and also to understand the mechanisms of deformation that operates in stages during the emplacement of thrust sheets. The Himalayan orogenic belt has experienced multiple stages of thrusting and duplex formation. Ramgarh thrust sheet (RTS), one of the major thrust sheets bounded by the Main Central Thrust to the North and Ramgarh thrust (RT) in the south, is studied in the eastern Himalaya to understand its emplacement history and formation of microstructures in response to the deformation. The RTS consists dominantly of metapelitic sequence with intercalated quartzite bands. The present study attempts to understand the deformation history and mechanisms of deformation from microstructures developed in Daling group of rocks of the RTS, in the immediate northern side outside the Rangit window, Sikkim Himalaya. Three stages of cleavage formation in phyllite have been identified. The cleavages of the early stages are transposed to the present stage through the process of shearing adjacent to the RT fault zone as deformation progresses. The remnants of earlier cleavages are present as obliterated relict structures within the dominant last stage fabric. The evidence of shearing is present in both quartz-poor as well as in quartz-rich sericite schist. Schistosity plane is sheared to form S-C structure along with the development of pressure solution planes usually parallel to C-plane. Shear band cleavage is a common micro-scale structure in the deformed schistose rocks. Quartz grains in quartz mylonite have undergone crystal-plastic deformation and exhibit sweeping undulatory extinction, deformation bands, sub-grain formation and incipient to almost complete recrystallization of original grains. In phyllosilicate grains, undulatory extinction, bending of grains and local recrystallization are common. All these structures suggest that pressure solution, crystal-plastic dislocation creep, and recrystallization are the dominant deformation mechanisms operated in the process of deformation. The effects of crystal-plastic deformation and dynamic recrystallization near the RT fault are more pronounced while the pressure solution structure is predominantly developed further north in the sheet away from the RT fault. So, it is possible that the deformation of the rock within the RTS took place in varying deformation conditions, spatially arranged with respect to the RT fault. All the quasi-plastic deformation structures are overprinted by a later localized elastico-frictional deformation. The microstructural data suggest that the structures are formed in stages in a continuous tectonic regime related to the emplacement of RTS during the N-S shortening of the Himalayan orogen. Deformation partitioning is evident in large scale, spatially related with the RT fault as well as in micro-scale. The microstructure also suggests that relatively higher temperature and higher strain condition prevailed near the fault and are dissipated away from the RT fault.