Analyses of marine geophysical data from the Adare Trough and Basin in the deep sea adjacent to the Ross Sea, Antarctica suggest that there has been 170 km of extension between East and West Antarctica during 46 to 21 Ma with 95 km of this extension in the Northern and Victoria Land Basins in the western Ross Sea adjacent to the Transantarctic Mountains. There are several kilometers of Oligocene sediments 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 southern Ross Sea extension was comparable to extension in the Adare system. The early Tertiary geometry was of one oceanic rift that branched into at least three rifts in the continental lithosphere.