Diversity in the Indian lithosphere revealed from ambient noise and earthquake tomography

We present evidence of significant diversity in the Indian cratonic lithosphere mantle based on the analysis of 3-D shear wave velocity maps. These images are obtained through the inversion of 21600 fundamental mode Rayleigh wave group velocity dispersion data retrieved from ambient noise and from earthquake waveforms. The velocity model is constructed using two step approach-firstly generating group velocity maps at 1^° square grid at time periods from 10s to 100s; and subsequently inversion of dispersion data at each grid node to a depth of 200 km in terms of velocity-depth model. Analysis of velocity images suggest a bipolar characteristics of lithospheric mantle. We observe a two layer-lithospheric mantle correlated with the Eastern Peninsular India comprising of Archean cratons like east Dharwar, Bastar, Singhbhum, Chotanagpur, Bundelkhand and Proterozoic Vindhyan Basin. The intra lithospheric mantle boundary is at a depth of ~90 km where Vs increases from 4.5 km/s to over 4.7 km/s. The positive velocity gradient continues to a depth of 140-180 km beyond which it reverses the trend and mapped as layer with lower velocity Vs of 4.3-4.4 km/s, as which could be possibly defined as the lithosphere-asthenosphere boundary. Geologically, the region correlates with the kimberlite fields with the xenoliths showing presence of eclogite in them. The other group of Precambrian terrains like 3.36 Ga western Dharwar, eastern Deccan Volcanics, southern Granulite terrane and the Marwar block in western India are characterized by an almost uniform mantle with shear wave velocity of 4.4-4.5 km/s, also supported by other seismological studies. We do not observe any low-velocity layer underlying these terrains. Presence of such a uniform lower than expected mantle velocity could be due to its fertilization through an early geodynamic process. The velocity imprint of Deccan volcanism is best preserved in term of the thinned lithosphere (100-120 km) restricted to the westernmost part of Deccan Volcanic Province (DVP). This suggests that the plume-Indian lithosphere interaction was primarily confined to the western most Deccan volcanic province and possibly extending into the Indian ocean.