Lithospheric stretching and the long wavelength free-air gravity anomaly of the Eastern Continental Margin of India and the 85^oE Ridge, Bay of Bengal

Abstract

Among the submarine ridge systems in the northern Indian Ocean, the 85^oE Ridge in the Bay of Bengal is more enigmatic owing to its peculiar anomalous negative free-air gravity. In general, this has been attributed to the isostatic overcompensation of the ridge due to overburden sediment load. In this work we attempted to know; whether, the present day oceanic lithosphere, juxtaposed to the Eastern Continental Margin of India (ECMI), holds the vestiges of lithospheric stretching due to erstwhile India-Antarctica rifting? If so, how much is the gravity effect of post rift lithospheric subsidence contributed to the excess negative gravity field of later emplaced 85^oE Ridge. Secondly, does the 85^oE Ridge is a crustal mass anomaly? Or as envisaged, was it originated from the Crozet hotspot? We address these issues by using satellite altimeter-derived gravity anomaly and its analytical upward continuation anomalies with forward modeling of ship-borne data. Results on analytical continuation of anomalies at H = 150 to 200 km show that about 40% of existing negative gravity anomaly of the 85^oE Ridge has contribution in long wavelength by the subsidence of mantle lithosphere; that plausibly arise as a consequence of rifting of the ECMI from its conjugate margin. Moreover, the absence of ridge anomaly at the same continuation heights also shows that its source is confined within the crust and the overcompensation processes caused crustal up-warping at the ridge flanks. Forward modeling studies show that the ridge crust has sagged into the mantle lithosphere. Unlike the hotspot originated Ninetyeast Ridge; the gravitational support of mantle lithosphere as sub-crustal low density mantle melt is absent for the case of the 85^oE Ridge and hence negates its hotspot origin. However, the ridge anomalies are strongly present at lower continuation heights from H = 20 to 100 km and hence suggests its crustal genesis.