Describe the features of the Darjeeling and Sikkim Himalayas
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Answer:
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
Answer:
Analysis of data from nine, temporary broadband seismic stations operated across West Bengal and Sikkim, along with publicly available data from seismographs in the surrounding region, provides the first image of the descending Indian Plate beneath the Darjeeling—Sikkim Himalaya. The down-going Indian crust is imaged by receiver function common conversion point stacking using data from 32 sites in combination with more detailed analyses from simultaneous modelling of receiver function data and Rayleigh wave group velocity dispersion at 13 stations. Compared to locations farther south on the Indian Shield our southernmost station shows evidence for thickened crust beneath the Mahanadi Rift basin with a possible mafic basal layer. North of the Mahanadi Rift the Indian Moho is ∼38 km deep below the Archean terranes of the northeast part of the Indian Shield. The Moho then dips gently northward beneath the Himalayan foreland basin reaching a depth of 44–48 km below the Himalayan foothills. Below Sikkim the Moho continues to deepen but there are indications of secondary structures in the receiver function image and modelling results suggesting some imbrication of the crust as it flexes downward. The crust thickens further beneath the Greater Himalaya and southern Tibet reaching depths of ∼65–70 km below the Southern Tibet Detachment (STD). Below the Lhasa Terrane north of the STD a double discontinuity exists with interfaces at 55–60 km and ∼80 km depth. There is a significant reduction in the average shear wave velocity of the crystalline crust between sites to the south of and on the Himalayan foreland basin and sites in the Himalaya and to the north. Below the Himalaya and southern Tibet the P-to-S conversion (Ps) has a lower amplitude compared to that observed at sites on the undeformed Indian Shield. This decrease in amplitude of the Moho Ps phase could arise from a lower impedance contrast across the crust—mantle boundary or from scattering due to deformation of the crust and Moho. A coherent negative arrival beneath the Darjeeling—Sikkim Himalaya indicate the presence of a low velocity zone (LVZ), possibly associated with the Main Himalayan Thrust. This LVZ can be traced beneath the Darjeeling—Sikkim Himalaya but disappears beneath the Greater Himalaya.
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