PLATE-TECTONIC SETTING OF THE TASMANIAN REGION

The South Tasman Rise is a large submarine plateau of continental orig in, located south of Tasmania. In the light of satellite-derived gravi ty data and shipboard swath-bathymetry and magnetic data collected in 1994 in the South Tasman Rise region, this paper re-examines the sea-f loor spreading history of the surrounding ocean basins (northeastern A ustralian-Antarctic Basin and southwestern Tasman Sea). This informati on is synthesised in seven plate tectonic reconstructions of the South Tasman Rise region from the mid-Cretaceous (95 Ma) to the Early Oligo cene (34 Ma). Our interpretation and model confirm that the South Tasm an Rise is composed of two distinct terranes. A western domain, limite d to the west by a transform margin along the Tasman Fracture Zone and to the east by a N170 degrees E oriented boundary at 146.5 degrees E, was initially attached to Antarctica. The western terrane rifted away from Antarctica in the Late Paleocene/Early Eocene and underwent seve re wrench deformation as the Antarctic plate moved southward relative to the Australian plate. Shear motion continued to shape the Tasman Fr acture Zone transform margin until the Early Miocene (chron 6B, 23 Ma) after which the Southeast Indian Ridge axis cleared from the western edge of the South Tasman Rise. An eastern domain limited to the east b y a boundary at 146.5 degrees E, rifted off from Tasmania and the East Tasman Plateau. After an initial phase of stretching between Tasmania , the East Tasman Plateau and the Lord Howe Rise that lasted until the mid-Cretaceous, sea-floor spreading in the Tasman Sea started in the Late Cretaceous (chron 3-4y) north of the East Tasman Plateau. Seismic , magnetic and gravity profiles between the eastern South Tasman Rise and the East Tasman Plateau suggest that sea-floor spreading began bet ween the two microcontinental blocks in the Late Cretaceous (ca chron 33y) and failed shortly after (chron 30y).