Crustal underplating and effective elastic plate thickness of the Laxmi Ridge, northern Arabian Sea

Abstract

Laxmi Ridge, a prominent basement high feature with negative gravity anomaly in the northern Arabian Sea, is generally considered as stretched continental crust. However, recently it is proposed that the ridge could be an extinct spreading ridge. Ship-borne and satellite-altimetry derived bathymetry, gravity and geoid data over the Laxmi Ridge are analysed and modelled to determine effective elastic plate thickness (Te) and crustal structure beneath the ridge. The estimation of Te and compensation mechanism of the ridge are examined based on 3D coherence and admittance analysis considering surface and subsurface loads which involve computation of coherence and admittance between Mantle Bouguer Anomaly (MBA) and sediment corrected bathymetry. The best fit model reveals that Te is about 3–4 km and amplitude of subsurface load at Moho is approximately equal to the surface load. Low value of Te implies elastically weak lithospheric plate which can be attributed to either weakening of lithosphere by re-heating and underplating caused by the then nearby Réunion Hotspot or volcanic emplacement on the young oceanic crust. Further, all along the length of the Laxmi Ridge, the MBA is negative and its magnitude reaches up to 115 mGal, which indicates either presence of thickened crust or less dense mantle beneath the ridge. 2D crustal models suggest that underplating material is prevalent all along the Laxmi Ridge. High amplitude (>400 nT) magnetic anomaly over the southern part of the ridge reveals several lineations in WNW-ESE direction suggesting magma outpouring along week zones. Geoid to topographic ratio over the ridge is negative (−1.98 m/km), possibly indicating shallow subsurface load (<20 km) with small wavelength (<200 km). Based on present study and available geophysical results, we infer that the Laxmi Ridge is a stretched continental sliver which is underplated, intruded by volcanics and isostatically compensated by Airy Model of Compensation.