Tectonics and basin development of the offshore Tasmanian area incorporating results from deep ocean drilling

Abstract

Tasmania and adjoining continental blocks, the South Tasman Rise (STR) and East Tasman Plateau (ETP), are central components of a major fragmentation of east Gondwana that began in early Late Cretaceous. The tectonic development of the Tasmanian region is key to understanding (i) the kinematics and geological evolution of adjacent plates and former neighboring continents and microcontinents that have now dispersed, (ii) formation of the extensive sedimentary basins off Tasmania, and (iii) major changes in ocean circulation, climate and sedimentation patterns associated with the opening of a deep-water seaway between Tasmania and Antarctica at the end of the Eocene. From Late Cretaceous to early Miocene the western and southern parts of the region were part of a large sinistral transform system, which until the Paleocene trended NW (Tasmanian-Antarctic Shear Zone) then changed orientation to N-S (Tasman Fracture Zone). Wrench basins up to 6 km deep developed in the Sorell Basin off west Tasmania, and narrow transtensional rifts with up to 4 km of fill were created on the STR. During the Paleocene shift in plate kinematics the western STR terrane, including the Ninene Basin, transferred to the Australian plate. Before that, the Ninene Basin had been part of the NNE-trending Rennick Graben in northern Victoria Land. In the east of the region, Tasman Basin rifting began in early Late Cretaceous, with onset of seafloor spreading in early Campanian. Small rift grabens, now filled with up to 4 km of sediment, developed on the east Tasmanian margin, eastern STR, and ETP. The Australian and Antarctic continents separated at the SW tip of the STR at 33.5 Ma, opening a deep seaway that led to circum-Antarctic circulation as the continents continued to drift apart. Significant flow-through may have begun several million years earlier across passages in the southern Ninene Basin and South Tasman Saddle, due to basin extension and increased margin subsidence. Sedimentation changed from siliciclastics-dominated to biogenic (mainly pelagic) carbonate-dominated, with post-Eocene accumulations no thicker than about 1000 m anywhere in the region. Although there is no recorded activity since the late Miocene, volcanism was extensive earlier. It is attributed to breakup volcanism (Campanian-Paleocene) close to the continent-ocean boundary, and an Eocene episode of widespread and voluminous basaltic eruptions that produced large seamounts and fields of volcanic cones.