Geotectonic evolution of prominent rift structures beneath Deccan Volcanic Province: Inferences from satellite gravity studies

The satellite gravity data over 65 Ma Deccan Volcanic Province (DVP) and the adjacent Dharwar Craton (WDC) in the south, has been analyzed to understand the regional geological and tectonic framework and possible mechanism related to the intraplate earthquake nucleation. The continued seismicity in Koyna–Warna region and its relationship with subsurface structures, concealed below thick volcanic sequences, are studied with the help of Finite Element deduced residual gravity field anomaly map. A NW–SE trending high-order residual gravity low on the western margin of Indian shield, that runs for about 700 km from Pune in the north to Coimbatore in the south, is attributed to the presence of a Precambrian rift structure, designated as ‘Koyna–Coimbatore rift’. The study suggests that the Koyna–Warna seismogenic region may be a part of Western Dharwar Craton. We have further delineated another gravity low trending WNW–ESE near Kaladgi, which could also be a probable rift zone, similar to Kurduvadi rift. The 2½D gravity models across Koyna–Kurduvadi, Udipi–Magalore and Coimbatore gravity lows, delineate the presence of low-density zones, which corroborate with the low velocity layers as inferred in previous studies. These zones are likely to contain mantle metasomatised and fractionated underplated magmatic material. The Koyna and Killari earthquakes nucleated within the upper crustal low-velocity/low-density zone. Another interesting finding of the present study has been that all the major earthquakes which occurred over the studied region fall in the Koyna–Coimbatore, Kurduvadi and Kaladgi rift zones or alternatively, over the areas of the prominent gravity lows. Further, the occurrence of earthquakes appears to be related to metasomatic alteration caused by mantle fluids influx during crust-mantle thermal interaction and magma underplating. Research highlights Tectonic framework of DVP and adjacent Dharwar Craton analysed using satellite gravity data. High order negative residual gravity field over a newly delineated Koyna–Coimbatore rift structure. Mantle metasomatism may be related to the earthquake nucleation. All the major earthquake epicentres fall over the major rift structures or conspicuous gravity low.