3D shear-wave velocity structure of the Eastern Indian Shield from ambient noise tomography: Ultramafic lower crust underneath the Singhbhum Craton

Singhbhum Craton (SC) in the Eastern Indian Shield (EIS) is known to have preserved Archean crust which might provide insights into the Precambrian tectonics. In this study, ambient noise cross-correlation of about 1 year of continuous data recorded in 15 broadband seismic stations across EIS is performed. Rayleigh wave group velocities at 4-22 s periods obtained by frequency-time analysis of the cross-correlograms were subjected to tomographic inversion to produce group velocity maps. The spatial resolution of these maps was found to be -100 km. Inverted 1D shear-wave velocity (VS) models at regular grid points of these tomographic maps were combined to construct the first 3D VS model of the SC and adjoining regions within the EIS. The 3D model highlights a high VS (3.6-3.8 km/s) structure in the 4-20 km depth range and an ultra-high VS (-4.0-4.2 km/s) structure at depths >= 20 km within the SC only. The VS of the upper layer and its regionalisation match with the well-studied granite batholith within the SC. The ultra-high velocity layer in the lower crust, spatially spanning -100 km x 150 km, is distinctly imaged for the first time and its VS indicates probable ultramafic composition. The existence of ultramafic composition at -800 MPa lithostatic pressure requires ultra-high temperature metamorphism, possible through mantle upwelling/melting. The previous studies of upwarped crust-lithosphere beneath the SC and the lower-crustal ultramafic composition inferred in this study probably confirm the delamination of the lithosphere beneath the SC at some point during the Precambrian eon.