A low seismic wavespeed anomaly beneath northwestern India

Abstract

Seismic tomography of the Indian region using P wave arrival times reveals an approximately cylindrical region of lowered seismic velocities in the upper mantle to the north of the Gulf of Cambay and the present exposure of the Deccan flood basalts. This anomaly with low velocities interrupts the characteristically high seismic velocities in the lithosphere beneath peninsular India and extends from shallow depth down to a more extensive low velocity zone beneath 200 km. The centre of the low velocity anomaly lies just below the earliest alkaline magmatism which has been directly associated with the Deccan plume, in the northern portion of the Cambay graben. These magmatic events precede the main Deccan flood eruptions by at least 3 Myr. The lowered seismic wavespeeds link into the ancient Narmada lineament and to some extent to the south, but here unfortunately the tomographic image runs out of resolution. Synthetic tests indicate that such a zone of lowered wavespeeds can be recovered with some loss of amplitude towards the south. The association of the current seismic anomaly with the Deccan traps is intriguing, since the Cambay graben remains a place of raised heat flow. One explanation would be that the seismic anomaly represents the signature of the initial conduit through which plume material forced its way through the lithosphere beneath the Cambay rift. The zone of lowered seismic velocities beneath the Indian lithosphere might then represent the remains of the initial plume head which would need to have moved with India over the 65 Myr since the Deccan eruptions. Show more