Tomographic Image of Shear Wave Structure of NE India Based on Analysis of Rayleigh Wave Data

Abstract

The major scientific purpose of this work is to evaluate the geodynamic processes involved in the development of tectonic features of NE India and its surroundings. In this work, we have obtained tomographic images of the crust and uppermost mantle using inversion of Rayleigh waveform data to augment information about the subsurface gleaned by previous works. The images obtained reveal a very complicated tectonic regime. The Bengal Basin comprises a thick layer of sediments with the thickness increasing from west to east and a sudden steepening of the basement on the eastern side of the Eocene Hinge zone. The nature of the crust below the Bengal Basin varies from oceanic in the south to continental in the north. Indo-Gangetic and Brahmaputra River Valleys comprise ∼5–6-km-thick sediments. Crustal thickness in the higher Himalayas and southern Tibet is ∼70 km but varies between ∼30 and ∼40 km in the remaining part. Several patches of low-velocity medium present in the mid-to-lower crust of southern Tibet along and across the major rifts indicate the presence of either partially molten materials or aqueous fluid. Moho depth decreases drastically from west to east across the Yadong-Gulu rift indicating the complex effect of underthrusting of the Indian plate below the Eurasian plate. Crust and upper mantle below the Shillong Massif and Mikir Hills are at a shallow level. This observation indicates that tectonic forces contribute to the uprising of the Massif.

Along‐strike variation in the shear wave crustal structure of the NE Himalayan and Indo‐Burmese arc: Evidence based on surface wave dispersion analysis

“…For Group 2, we suggest the Moho at ~45 km depth where the V S at the uppermost mantle reaches 4.1 km/s. In earlier studies in NE India and NW Himalaya, the Moho discontinuity is marked at depths where V S obtained from inversion of Rayleigh wave group velocity is ~4.0–4.1 km/s (Acton, Priestley, Gaur, & Rai, 2010; N. Kumar et al, 2019; A. Kumar et al, 2021). In the frontal part of north‐west and central Himalayas, Moho depth varies between ~40 and ~80 km.…”

“…According to S. N. Bhattacharya (1993), the total crustal thickness of central Burma is 55 km. Low velocity in the near surface indicates the presence of sedimentary rocks (A. Kumar et al, 2021). Group 4 shows that V S increases systematically up to 80 km depth.…”

“…The anisotropy patterns demonstrate that east Asia consists of several tectonic domains including the Indian Plate, Burma, and Andaman sea region having strong anisotropy in the asthenosphere, indicating that the anisotropy is caused by convectional flow in the mantle. A. Kumar et al (2021) obtained the tomographic images of shear wave velocity structure of NE India and surroundings using Rayleigh wave group velocity and found that the depth of Moho decreases from ~70 km in the high Himalaya and southern Tibet to ~40 km in the south of NE India.…”

“…Also, for low periods up to 20 s, the velocity is lowest for this group compared to all the present data sets. The low velocity may be related to the thick sedimentary rocks of the BB in the uppermost crust (A. Kumar et al, 2021). Therefore, this group indicates a different sub‐surface structure for these paths towards the southern section of the study region.…”

“…The Shillong Plateau and Mikir Hills are part of the Indian Plate and are bounded by the Alpine‐Himalayan collision zone in the north, by the Arakan‐Yoma subduction zone in the east, and the Dauki Fault in the south (Evan, 1964). E‐W running Dauki Fault separates the Precambrian basement of Shillong Plateau to the north from the Tertiary sediments of the Bengal Basin (BB) to the south (P. M. Bhattacharya, Mukhopadhyay, Majumdar, & Kayal, 2008; Kayal, 2001; A. Kumar, Kumar, Mukhopadhyay, & Klemperer, 2021; Raoof et al, 2017). Hence the tectonics of the Shillong Plateau and Mikir Hills are very complex.…”