Decades after the recognition of theWest Antarctic Rift System, and in spite of its global importance, the location and nature of the plate boundary it formed at are unknown east of the Byrd Subglacial Basin. Alternative constructions of the circuit of South Pacific plate boundaries suggest the presence of either a transcurrent plate boundary or a continuation of the extensional rift system. We identify George VI Sound, a curved depression separating Alexander Island from Palmer Land, as the easternmost basin of a rift system that terminated at a triple junction with the Antarctic Peninsula subduction zone. The history of the triple junction's third, transform, arm suggests extension started around 33.5-30 Ma. A more speculatively identified basin further west may have formed earlier during the same episode of rifting, starting around 43Ma. Proposals of earlier Cenozoic relative motion between East and West Antarctica cannot be verified from this region. Copyright 2009 by the American Geophysical Union.