New Data on the Geology of Osborn Plateau, Indian Ocean

The Osborn Plateau is a large intraplate rise in the eastern part of the Indian Ocean, which has been poorly studied by the geological and geophysical methods. In cruise SO258/1 on the R/V Sonne , new data were collected using Parasound seismic profiling and a multibeam echo-sounder survey. Faults in the sedimentary cover, which extend to the bottom surface, indicate high neotectonic activity in the Osborn Plateau area. It may continue up to the present, as well as in the adjacent segment of the Ninetyeast Ridge, where strong earthquakes have been recorded. Two reflectors in the upper part of the sedimentary cover mark the global lowering of the World Ocean level at the Miocene/Pliocene and Pliocene/Pleistocene boundaries. The reflector in the sediments at the Lower/Upper Pliocene boundary is associated with a change in the regional hydrodynamic regime that occurred at that time in the eastern Indian Ocean. The rocks dredged on Osborn Plateau are identical to some volcanic rocks of the Ninetyeast Ridge, confirming their assumed genetic link, but they are more similar to the basalts of the Kerguelen Plateau. Extremely altered vitroclastic tuffs appear to have been formed as a result of explosive volcanic eruptions of alkali basalts or foidites under subaeral or relatively shallow water conditions and represent the most recent eruptions in the region.