Structure and evolution of the Afanasy Nikitin seamount, buried hills and 85 degrees E Ridge in the northeastern Indian Ocean
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Geophysical data of the Afanasy Nikitin seamount (ANS), partly buried hills and 85 degrees E Ridge in the northeastern Indian Ocean were studied together with published seismic refraction results to understand genesis and evolution of the structures. The ANS joins the 85 degrees E Ridge through isolated buried hills and intervening subsurface structures and together they form a linear ridge system. The gravity signature of the ridge system changes from negative to positive towards south of 5 degrees N, which seems to coincide with termination/thinning of pre-collision continental sediments in the Bay of Bengal. Thick pile of Bengal Fan sediments had great impact on underneath pre-collision sediments as well as on basement rocks; the process resulted in attaining higher velocities, up to 6.6 and 7.1 km/s, respectively. Gravity model studies suggest that structures of the ridge system are compensated in different modes. The ANS is underlain by an 8 km thick, deep crustal body of magmatic rocks, while beneath other structures oceanic crust is down-flexed up to 2.5 km. The presence of metasediments, more dense than volcanic rocks, and flexure of the lithosphere would explain the negative gravity anomaly over the 85 degrees E Ridge, whereas lack of metasediments and magmatic rocks at depth would explain the compensated positive anomaly over the ANS. The width of the 85 degrees E Ridge, the wavelength (approx. 190 km) and amplitude (approx. 2.5 km) of the flexed oceanic crust and the intersection of the ridge with the Mesozoic fracture zones suggest that the ridge was formed in intraplate position when the lithosphere underneath was approximately 35 Myr old. Initial emplacement of the ANS was coeval with the formation of oceanic lithosphere at 80-73 Ma. The hotspot forming the 85 degrees E Ridge had reactivated the ANS during the Paleocene and brought it to the sea surface. Then it underwent erosion and subsidence processes. The deformation activity had converged the north and south parts of the ANS at late Miocene and again at late Pleistocene and northward buried hills at early Pliocene.
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