Evidence for thick continental roots beneath south Indian shield

Abstract

Teleseismic P-wave arrivals recorded at South Indian seismological stations have been used to construct a 3-D velocity image of the upper mantle beneath this region. Analysis of a suite of models lead us to conclude that an anomalously high velocity region (1-6% contrast) exists in the upper mantle beneath the whole of South Indian shield including the Deccan Traps in the depth range of 60-300 km. In contrast, the westernmost part of the Deccan Traps and its subjacent mantle in the northwest is characterised by a relatively low velocity. This observed low velocity zone may be a still warm remnant of the spreading centre offset eastwards from the Carlsberg ridge, which might have been the dominant source of the flood basalts that flowed over the Deccan Plateau 65 Ma ago. The high velocity upper mantle of the entire South Indian shield is viewed as a continental root formed by a process of geochemical reordering which happens to be more stable.