Oceanic Spreading and Continental Rifting in Ross Sea, West Antarctica

The initial process of rifting of continental lithosphere is not well understood and has been the focus of research for many years. Analysis of marine magnetic anomalies indicates relatively slow extension between East and West Antarctica from about 43 to 26 Ma that, in the Ross Sea, is focussed along the its western margin. Magnetic anomalies of this age associated with new ocean crust formation in the Adare Basin off north-western Ross Sea, can be traced directly into the Northern Basin that underlies the adjacent morphological continental shelf, implying a continuity in the emplacement of oceanic crust. Modelling of the steep gravity gradients along the margins of the Northern Basin, particularly in the east, suggests that little extension and thinning of continental crust occurred before it ruptured. Immediately south of this and offset to the west, the Victoria Land Basin has formed by stretching and thinning of continental crust, similar to that found elsewhere and in modelling studies. Under the Ross Ice Shelf, extension is small and may be of a distributed nature. These changes in style of continental rifting are associated with changes in strike slip components of extension and in the rate of extension in the basins. The changes in strike slip motion result from changes in orientation of the rift axis relative to the direction of spreading, probably caused by rupturing along pre-existing sutures that change orientation along strike, with a higher degree of strike slip motion associated with early continental rupture. Minor Cenozoic extension and right lateral transcurrent faulting in Northern Victoria Land may be a second order feature of the main 43 - 26 Ma extension across the western Ross Sea. The sediments in the basins were probably derived mainly from the adjacent Transantarctic Mountains.