Lithospheric Structure of the South India Precambrian Terrains From Surface Wave Tomography

We have developed a 3-D shear velocity (Vsv) model of Precambrian terrains of South India with a lateral resolution of 55 km to a depth of 250 km by inversion of fundamental mode Rayleigh wave phase velocity dispersion data in the period of 30-140s, combined with a well-constrained crustal velocity model from an earlier study. The dispersion data were computed from 748 earthquakes recorded at 85 homogeneously distributed seismographs. Our velocity model shows 150-200 km thick lithosphere in most of the Archean Dharwar craton where Vsv is >4.7 km/s in the depth of 50-100 km and progressively decreases to 4.6 km/s at 120-150 km followed by a constant velocity of 4.5 km/s beyond 150-200 km depth. It correlates well with the results of the petrological studies of kimberlite xenoliths. An extraordinary high shear velocity (up to 4.8 km/s) and thick lithosphere (150 km) are observed beneath the Proterozoic Carbonatite complex, located at the south-eastern edge of the Dharwar craton. We infer compositional modification of lower lithosphere at the south-western margin of the Dharwar craton and lithospheric erosion at the Granulite terrain both possibly due to interaction with the Marion mantle plume at similar to 90 Ma. The region is underlain by a 2%-3% lower velocity channel at similar to 180-220 km depth in the asthenosphere, uncorrelated with the overlying lithosphere, possibly due to relative motion between them. It may be attributed to deep Earth-processes such as asthenospheric upwelling.