Estimates of Effective Elastic Thickness along the Southwest Continental Margin of India Using Coherence Analysis of Gravity and Bathymetry Data - Geodynamic Implications

The southwest continental margin of India has evolved due to rifting between India and Madagascar at 88 Ma and subsequent seafloor spreading. During this period, several major ridges and horst-graben structures were formed at the margin. In the southern most part, the Comorin Ridge aligned along the margin, is another topographic feature believed to be related to the earliest phase of margin evolution. In the present study, Coherence analysis of gravity and bathymetry data of the region based on Maximum Entropy Spectral Estimation (MESE) method is used to understand the regional geodynamic processes and lithospheric strength that controls the flexure and basin evolution. The study brings out effective elastic thickness (Te) values of 5-10 km along the Konkan and Kerala basins, and the Comorin Ridge at the margin and 5-8 km along the Chagos Laccadive Ridge north of the equator. The elastic thickness values increase southward and maximum Te value of 10 km is observed below the Comorin Ridge. These results, combined with the earlier estimates of Te indicate low strength < 15 km for the lithosphere below the western margin, but not as low as that observed below plume affected margins. The observed low elastic strength at the margin will have implications on long term topographic evolution of the Western Ghats.

The Marion Plume seems to have played limited role in the rifting and evolution process of the western margin. In the absence of such an active plume role, the substantial partial melt zone observed in the Comorin Ridge region based on geomagnetic induction models can be alternatively explained by invoking the convective partia1 melting model. The Comorin Ridge might have been emplaced as an oceanic basement high due to large-scale volcanism during the onset of seafloor spreading. Absence of diagnostic magnetic signature associated with the isostatic gradient over the Comorin Ridge that marks the Ocean Continent Boundary indicates that active volcanism due to strong convection might have blurred the crustal transition in the Comorin Ridge region.