Deformation mechanisms in the frontal Lesser Himalayan Duplex in Sikkim Himalaya, India

Abstract

Understanding deformation mechanisms in Himalayan rocks is a challenging proposition due to the complex nature of the deformed rocks and their genesis. Crustal deformation in the Himalayan thrust belt typically occurs in elastico-frictional (EF) or quasi-plastic (QP) regimes at depths controlled mainly by regional strain-rate and geothermal gradient. However, material property, grain-size and their progressive changes during deformation are also important controlling factors. We present evidence of EF deformation from Gondwana rocks developed during the emplacement of one of the frontal horses (Jorthang horse) in the Lesser Himalayan Duplex (LHD) structure associated with Lesser Himalayan rocks in the footwall of the Ramgarh thrust in the Rangit window near Jorthang in the Sikkim Himalaya. The rocks in the horse exhibit systematic changes in microand meso-structures from an undeformed protolith to cataclasite suggesting that it was emplaced under elastico-frictional conditions. Meso-to micro-scale shear fractures are seen developed in Gondwana sandstone and slate while intercalated fine-grained shale-coal-carbonates are deformed by cataclastic flow suggesting that material property and grain-size have played an important role in the deformation of the Jorthang horse. In contrast, the hanging wall schists and quartzites of the Ramgarh thrust exhibit quasi-plastic deformation structures. This suggests that the Jorthang horse was emplaced under shallower crustal conditions than the antiformally folded Ramgarh thrust sheet even though the Ramgarh sheet presently overlies the Jorthang horse.

A review of thermal history and timescales of tectonometamorphic processes in Sikkim Himalaya (NEIndia) and implications for rates of metamorphic processes

“…Since then, it has been a topic of continuous study by the Geological Survey of India as well as academic researchers (see for example, Ray, ; Ghosh, ; Acharya, ; Acharya & Ray, ; Ghosh & Singh, ; Gangopadhyay, ; Sinha‐Roy, ) which have attempted to clarify the stratigraphic and structural relationships in the area. Interest in the structural, petrological, geophysical and geochronological evolution of the Sikkim Himalaya continues to the present day with an increase in the number of publications in the last few years ( Structure and regional patterns : Mukul, ; Searle & Szulc, ; Bhattacharyya & Mitra, , ; Matin & Mazumdar, ; Mitra et al ., ; Bose et al ., ; Bhattacharyya et al ., ,b; Bhattacharyya & Ahmed, ; Geophysics: Kayal, ; De & Kayal, ; Nath, ; Tiwari et al ., ; Singh et al ., ; Patro & Harinarayana, ; Hazarika et al ., ; Acton et al ., ; Mukhopadhyay, ; Hazarika et al ., ; Sharma et al ., ; Petrology and Geochemistry, including isotopic dating and thermochronometry: Banerjee & Bhattacharya, ; Lal et al ., ; Mohan et al ., ; Neogi et al ., ; Ganguly et al ., ; Chakraborty et al ., ; Catlos et al ., ; Dasgupta et al ., , ; Harris et al ., ; Dubey et al ., ; Chakraborty et al ., ; Gupta et al ., ; Faak et al ., ; Kellett et al ., ; Rubatto et al ., ; Anczkiewicz et al ., ; Mottram et al ., ,b, ,b; Neogi et al ., ; Sorcar et al ., ; Gaidies et al ., ; Landry et al ., ). We have used the references cited above, data from the reports of the Geological Survey of India (Chakrabarti et al ., ; Chakrabarti & Banerjee,…”

Petrographic and Raman spectroscopic characterization of coal from Himalayan fold-thrust belts of Sikkim, India

“…The eastern edge of the Rangit window is demarcated by a steep fault, which may be considered as the surface expression of an N-S trending lateral ramp (Bhattacharyya and Mitra, 2009). The Jorethang horse is underlain by the Jorethang thrust on its north side (Matin and Mazumdar, 2009) and is exposed along Jorethang ridge (Bhattacharyya and Mitra, 2009). The Gondwana coaly sandstone of thickness about 430 m is exposed by this…”