Sn wave tomography of the uppermost mantle beneath the Indian shield and its adjacent regions

Abstract

We present a high-resolution Sn velocity image beneath the Indian shield and its adjoining regions by inverting 11,243 Sn arrivals. The average apparent Sn velocity is 4.60 km/s with a velocity perturbation of ±0.2 km/s. High velocities are found under some parts of the Indian cratons, the Tarim Basin, the North-west part of the Indian plate and the Bay of Bengal, whereas the Burmese arc, Hindu-Kush, Tibetan Plateau and eastern parts of the Indian shield are dominated by low Sn velocities. The uppermost mantle shear wave velocity beneath the Indian shield is hitherto not known. Our Sn image clearly shows the heterogeneous character of the Indian lithosphere. The cratonic blocks of Singhbhum, Bastar and Aravalli show low Sn. The intracratonic Cuddapah Basin and north of Southern Granulitic Province also exhibit higher uppermost mantle shear velocity. We also observe a localized high beneath the exotic Coorg microcontinent located in the south of the western Dharwar Craton. The intriguing feature is the central Indian highs, which we interpreted as the basaltic lava pillows concomitant to the Deccan volcanism. Along the Himalayan arc, we observe higher Sn velocity which supports the idea of subduction of the Indian lithosphere into the mantle. Interestingly, the uppermost mantle beneath the Bay of Bengal shows faster and slower Sn in the western and the eastern regions respectively implying their distinct compositions. The Sn velocity distribution is discussed in conjunction with the Pn tomography results obtained previously. Low-velocity anomaly is conspicuous all along the Andaman-Nicobar island arc.