Crustal structure of the Darjeeling-Sikkim Himalaya and southern Tibet

Abstract

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.