Shallow lithosphere-asthenosphere boundary beneath Cambay Rift Zone of India: Inferred presence of carbonated partial melt

Abstract

Cambay Rift Zone (CRZ) is an ancient rift that had last witnessed volcanism and rifting activity 68-65 million years ago, when the Indian plate migrated over the Deccan plume, potentially signifying the presence of a shallow LAB. Here we apply converted wave techniques to a teleseismic earthquake dataset to image the mantle below the CRZ. We place the Lithosphere-Asthenosphere Boundary (LAB) beneath this rift at a depth where a ∼10% drop occurs in shear wave velocity. The seismic velocity structure indicates that the lithosphere thickness varies from ∼60 km beneath the rift to ∼110 km beneath rift walls. Furthermore, the upper mantle discontinuities (410 and 660 km discontinuities) are found to be thinner by ∼2 sec (i.e. ∼20km) with delayed 410-km discontinuity and normal 660-km discontinuities. We infer this to be caused by a thermal anomaly in the upper mantle. The surface heat flow at CRZ is high. A geothermal gradient extrapolated from the surface heat flow intercepts CO2-bearing mantle peridotite solidus at 60 km, and thus could signal the presence of small amounts of carbonatite-type magma. We suggest that the CRZ might become reactivated by the generation of small pockets of melt at the LAB, which is being triggered by a thermal anomaly in the upper mantle beneath this currently inactive rift.