Laccadive Ridge as a Continental Fragment: Pre-rift Geometry, Rifting style and Volcanism based on Multi-channel Seismic and Gravity Interpretation

Laccadive Ridge located off the southwest continental margin of India, is identified as part of highly extended continental crust that is heavily intruded by volcanics or as an aseismic ridge formed by the Reunion hotspot trace. Although there is a growing body of evidence suggesting it as a continental fragment, there has not been a clear identification of rift related structures at the margin. In this study, we use multichannel seismic and gravity data to decipher the nature of the Laccadive Ridge. The multichannel seismic reflection data reveal fault structures in the Laccadive Basin which separates the Laccadive Ridge from the western continental margin of India indicating that the basin is underlain by extended continental crust. Two rifting directions are evident from the seismic data that are aligned with the Precambrian NW-SE to NNW-SSE Dharwar trend and the ENE-WSE Satpura trend of the Indian shield. These trends are conformable with the trends in the gravity anomaly map which matches very well with the identified graben structures on the Ridge. We suggest that the magma travelled through the faults in the highly extended crust and gave rise to the numerous intrusions which are present all along the ridge. To restore the pre- India Madagascar geometry of the Laccadive Ridge, the gravity anomalies have been inverted to estimate the depth to Moho beneath the ridge. The volcanic addition to the crust due to magmatism and possible underplating was calculated using the adiabatic decompression melt generation models, and used to estimate the final crustal thickness. Stretching factors were calculated from these crustal thickness values and used to understand the pre-rift extent of the continental fragment. The results altogether give important information about the rift-related structures along the ridge and insights into the importance of this continental fragment in the evolution of India and Madagascar.