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Marine geophysical data from the deep sea adjacent to the Ross Sea, Antarctica suggest that 170 km of extension occurred between East and West Antarctica from 46 to 21 Ma. The Northern and Victoria Land Basins in the western Ross Sea adjacent to the Transantarctic Mountains accommodated 95 km of this extension. Several kilometers of Oligocene sediments are found in the Central Trough and Eastern Basin in the eastern Ross Sea. Subsidence modeling accounts for these accumulations with about 40 km of extension in each basin centered on 35 Ma; therefore Ross Sea-wide Tertiary extension was comparable to extension in the deep-sea system. The early Tertiary geometry was of one oceanic rift that branched into at least three rifts in the continental lithosphere. This pattern is likely due to the contrast of physical properties and thermal state between the two different lithospheres at the continent-ocean boundary.