Comparative Study of two Aseismic Ridges in the Northwest India Ocean Using Geopotential Data

Abstract:

The prominent aseismic ridges in the western offshore of India are the Laccadive Ridge and the Laxmi Ridge. The satellite derived Free Air Gravity(FAG) anomaly map over the western Indian offshore region depicts a high over the Laccadive ridge and a low over the Laxmi ridge in the Arabian Sea. In addition to the evolution of these ridges, it is also debated if these ridges are of continental or oceanic nature. We studied the two ridges individually.

We undertook analysis of ship-borne gravity-magnetic and the re-tracked satellite derived FAG data to have a relook into the crustal architecture of the Laxmi ridge and adjoining areas using different techniques. 3D models using Energy Spectral Analysis and 2D crustal modelling suggests the ridge is continental and the gravity low associated with the NW-SE segment of the Ridge is due to under-plating. Magnetic sources from EMAG2 data, various filtered maps and absence of under-plating in the EW section suggest that the EW and NW-SE segment of the Laxmi ridge are structurally and characteristically different and probably associated with different stages of evolution. The region to the north of Laxmi ridge, between Laxmi & Gop basins, is composed of volcanic / basaltic flows having Deccan affinity which may have been emplaced in an already existing crust. The calculated inclination parameters derived from the best fit 2D model suggests that the rifting in the Gop basin preceded the emplacement of the volcanics in the region between Laxmi & Gop basins. The emplacement of volcanic / basaltic flows may be associated with the passage of India over the Reunion hotspot.

The tectonic evolution of the Laccadive Ridge has been deduced from 2D satellite derived free air gravity data by utilizing wavelength filtering to resolve different depth structures representing different epochs. From such an analysis it has been possible to conclusively identify the extension of the older onshore NE-SW lineaments/fault into the Laccadive ridge which is later intruded by volcanic material as India moved over the Reunion hotspot.The crustal structure derived from the 2D models depicted a thick crust underneath the Laccdaive ridge suggesting continental nature of the ridge. From the combined analysis of the two Ridges an evolutionary model of Western Continental Margin of India will be presented.