Continental Rifting in the Western Ross Sea

The Ross Sea forms the north western end of the West Antarctic Rift system, a major continental rift that lies across the western part of Antarctica, and results from rifting during the break-up of Gondwana starting some 180 m.y. ago. In the Ross Sea region, extension comprised a regional thinning associated with the break-up of New Zealand and Australia from Antarctica, and a more focussed extension during the Cenozoic. The last episode of extension, largely from 46 Ma - 25 Ma, formed the Victoria Land Basin (VLB) in the southwest, the Northern Basin (offset from the VLB) in north western Ross Sea and the Adare Basin in the deep ocean to the north. Marine magnetic anomalies associated with the seafloor spreading that formed the Adare Basin, extend continuously onto the continental shelf of the Northern Basin, suggesting that the basin is underlain by oceanic crust, consistent with high gravity anomalies across the continental shelf edge. No seismic data exist for the deeper crust of Northern Basin. The amplitude and gradient of gravity anomalies across the basin limit the depth, density contrast and thickness of the dense body underlying it and are consistent with oceanic crust with steep margins at a depth of about 8 - 10 km. The VLB in contrast shows an extensional thinning of the continental crust. The three basins thus show seafloor spreading in the north, continental rifting at the continental margin, and continental thinning in the south. The pole of rotation for the extension lies to the south of the VLB so the rate of extension increases to the north. In addition, the azimuth of the axis of spreading changes relative to the extension direction, presumably as it followed pre-existing zones of weakness in the Antarctic lithosphere. This leads to a much larger degree of strike slip motion in the Northern Basin rifting that may be a significant factor in the development there of narrow rifting of the continental lithosphere.