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

摘要

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 similar to 10% drop occurs in shear wave velocity. The seismic velocity structure indicates that the lithosphere thickness varies from similar to 60 km beneath the rift to similar to 110 km beneath rift walls. Furthermore, the upper mantle discontinuities (410 and 660 km discontinuities) are found to be thinner by similar to 2 sec (i.e. similar to 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.