Virtual field guide for the Darjeeling-Sikkim Himalaya, India

Dawn Kellett , Djordje Grujic, Catherine Mottram, and Malay Mukul

The Darjeeling-Sikkim Himalaya marks the beginning of the Eastern Himalaya. It is located between Bhutan to the east and Nepal to the west. In the west Sikkim shares Kanchenjunga, Earth’s third highest mountain, with Nepal, while the ridge line forming Sikkim’s eastern border flanks the Yadong-Gulu rift graben. The South Tibet detachment (STD) system with a small portion of the Tibetan plateau defines the northern boundary of the Darjeeling-Sikkim Himalaya while the Main Frontal fault system defines its southern boundary. The Main Boundary thrust (MBT), the Ramgarh thrust (RT) and the Main Central thrust (MCT) are other thrusts that are well-exposed in the area along a south to north, transport-parallel traverse. The Lesser Himalayan Duplex (LHD) with horses involving rocks from the RT and MBT thrust sheets stacked in a complex geometry dominates the Lesser Himalaya in the region. The puzzling MCT Zone, with two strands of the MCT, is a classical region of inverted metamorphic isograds, and new ideas about its tectonics will be introduced. Along-strike variations in the geometry and kinematics of the Darjeeling-Sikkim Himalaya is evident from the presence of a salient-recess pair separated by a transverse, sinistral strike-slip fault that cuts across the frontal thrust systems and extends into the Yadong-Gulu rift system. The seismicity in the region is dominated by transverse strike-slip tectonics as evident from moderate strike-slip earthquakes in the region since 1980 and 2011 in contrast with the Himalaya further west that is dominated by thrust seismicity. Out-of sequence neotectonic deformation is also seen in the frontal Darjeeling-Sikkim Himalaya. This virtual guide tours key outcrops at which field expressions of first-order Himalayan structures such as the MBT, MCT, STD system and the Gish fault can be observed. Sites of neo-tectonic activity and horses within the LHD are also visited.

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