Geological evolution of the East Tasman Plateau, a continental fragment southeast of Tasmania

The 50 000 km(2) East Tasman Plateau has been regarded as a continental block on the basis of geophysical data, but this study is the first to prove its continental nature directly by sampling of basement rocks. Continental rocks recovered from four stations on its bounding scarps include Neoproterozoic orthogneiss, rhyolite, metasediments and metabasite, quartzite, quartz, quartzose sandstone and ferricrete. Geophysical and petrological evidence suggest that the plateau was adjacent to the South Tasman Rise until it was transported about 130 km east-northeast (relative to Tasmania) during early formation of the Tasman Basin in the Late Cretaceous (95-83 Ma), as part of the breakup of East Gondwana. Stretching and sea-floor spreading formed the L'Atalante Depression between the plateau and the rise. Lord Howe Rise separated from the East Tasman Plateau at 75 Ma, as part of Tasman Basin spreading. The plateau supports Cascade Seamount, a Late Eocene guyot that is part of the trace of the Balleny mantle plume and consists of volcanic breccia, hyaloclastite and alkali olivine basalt. Seismic profiles suggest that the weight of the seamount caused subsidence of the central plateau, and up to 1500 m of sediment fill the resultant depression. Seismic profiles and some sedimentary evidence indicate that the depression is filled by 500-1000 m of Late Eocene and Early Oligocene volcaniclastic sediments, and 200-500 m of Late Oligocene and younger calcareous ooze and chalk.