3D Crustal Imaging of North-East India and Indo-Burmese Subduction Zone

Abstract

We present a high-resolution 3D shear wave velocity (Vsv) model for the North-East India Region (NER) and Indo-Burmese Subduction Zone (IBSZ) down to an 80 km depth by inverting a new and extensive Rayleigh wave dispersion dataset. We retrieved vertical component dataset of more than 900 seismic events recorded by 26 global and regional seismic broadband networks. We picked and analyzed ~20,000 paths of Rayleigh wave fundamental mode group velocity dispersion curves across a wide period range of 4-70s. The methodology involved a two-step inversion approach: a 2D continuous regionalization incorporating azimuthal anisotropy was utilized to produce tomographic images from the local velocity dispersion curves. Subsequently, a Markov Chain Monte Carlo scheme within a trans-dimensional Bayesian framework was employed for the inversion process. The resulting 2D tomograms of the regionalized dispersion data at different periods and the subsequent inverted 3D model are consistent with the velocity values associated with the known geologic features, accurately outlining the primary tectonic boundaries in the study area. The region under study encompasses the Bengal basin (BB), Shillong plateau (SP), Mikir Hills, Assam-Brahmaputra-valley (BV), North-Eastern Himalayas, Indo-Burma ranges (IBR) and western Myanmar. Sediment thickness is highest in the southern delta region (18-22km), increasing from west to east towards the northern part of the BB and thinnest at the Dauki fault zone (8-10km). Crustal thickness under BB varies widely, from 33-46km. The velocity model reveals an undulating mantle and higher upper-crustal shear wave velocity (Vsv~3.3-3.6km/s) under the SP than its surrounding regions. Moho thickness varies across the region: ~33km in Garo Hills, ~38km in Assam valley, and ~42km beneath the Lesser Himalayan foredeep. There is a clear eastward dipping subduction geometry along ~22°N from under BB towards the Central Myanmar Basin (CMB) (at ~95°E). Sediment thickness in the CMB varies from 10-12km. The Main Himalayan Thrust (MHT) depth is ~18km under the Arunachal Himalaya, which moving northward dips down to ~28km under the Greater Himalaya. The crustal thickness at the syntaxial corner and BV is significantly greater than its surficial topography. The maximum crustal thickness of ~52km is on the southern IBR, along its eastern side.     Keywords: Crustal Structure, North-East India, Indo-Burma subduction zone, Rayleigh wave, group velocity, 3D shear wave velocity model, Bayesian inversion.