The opening of Southern Ocean gateways was critical to the formation ofthe Antarctic Circumpolar Current and may have led to Cenozoic globalcooling and Antarctic glaciation. Drake Passage was probably the finalbarrier to deep circumpolar ocean currents, but the timing of opening isunclear, because the Shackleton Fracture Zone could have blocked thegateway until the early Miocene. Geophysical and geochemical evidencepresented here suggests that the Shackleton Fracture Zone is in fact anoceanic transverse ridge, formed by uplift related to compression across thefracture zone since ca. 8 Ma. Hence, there was formerly (i.e. in theMiocene) no barrier to deep circulation through Drake Passage, and a deep-water connection between the Pacific and Atlantic oceans was probablyestablished soon after spreading began in Drake Passage during the EarlyOligocene.