Detailed analysis of magnetic and bathymetric data over shelf and slope regions of Cauvery basin demarcated three major offshore lineaments. The N-S lineation has been interpreted as due to dyke intrusions. The NE-SW lineament reflects the offshore extension of a major basement depression, viz., the Pondicherry depression. The E-W lineation, south of Porto Novo reveals a basement high suggesting the seaward extension of Kumbakonam ridge within the Pondicherry depression. The present analysis reveals a deep seated graben block flanked by rift related volcanic intrusives.