The South Tasman Rise is an area of

seafloor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth an ...

that lies 550 km south of Hobart,

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

in the

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...

where water depths are about 1,500 metres. The South Tasman Rise is also known as the Tasmania Ridge or South Tasmania Ridge. The South Tasman Rise is a sunken landbridge that used to connect Tasmania to Antarctica. Exploration of this oceanic region has taken place via sonar, gravity field, magnetic field, a few drill holes and deliberate or accidental dredging of rocks up to the surface.

Geology

The rise forms a northern extension from the Macquarie-Balleny Ridge which extends to the subantarctic Balleny Islands. To east of the rise is the Tasman Basin, while the South Australian Basin extends westwards, south of the

Great Australian Bight The Great Australian Bight is a large oceanic bight, or open bay, off the central and western portions of the southern coastline of mainland Australia. Extent Two definitions of the extent are in use – one used by the International Hydrog ...

. The rise contains a number of seamounts, some of which have flat summits, indicating exposure above the sea surface at some time. The rise most probably originates from subsided

continental crust Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial'' be ...

that fragmented as Australia and

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

separated. This area of the

is slated for exploration of offshore

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...

reserves. No oil has been recovered from the South Tasman Rise.

Rocks

A quartz syenite sample has an age of 1119 Ma which is Grenville in age. This kind of rock resembles rocks from the Grenville magmatic province in the west of the United States. It supports the idea that

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...

and eastern Australia, including the South Tasman Rise were once joined to each other during the time of the Rodinia

supercontinent In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", which leav ...

. Dolerite similar to the Jurassic dolerite in Tasmania has been recovered by dredging from the centre of the South Tasman Rise on ''RV Rig Seismic'' on its cruise 147. '' Glomar Challenger'' drilled a borehole on the southern slope of STR. Hole 280 in oceanic crust resulted in basaltic intrusive of age 64.2 Ma. The hole at site 281 resulted in Precambrian quartz mica schist. ''RV Sonne'' did a cruise titled SO-36B in 1985 which did seismic profiles.

Tertiary sediments

Bottle green coloured microtectites were found in borehole 1169 at around early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58

Exploration

Ocean Drilling Program The Ocean Drilling Program (ODP) was a multinational effort to explore and study the composition and structure of the Earth's oceanic basins. ODP, which began in 1985, was the successor to the Deep Sea Drilling Project initiated in 1968 by th ...

Leg 189 made a borehole into the sediments termed Site 1171. Site 1169 was drilled at 145°14.2′E, 47°3.9′S.

Fishery

The rise forms a sector of an international fishery managed by Australia and

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

. New Zealand ceased fishing the area in the 2000-01 fishing season. By agreement between New Zealand and Australia the zone has been closed to commercial fishing since 2007. The principal species of fish sought after are the

Orange Roughy The orange roughy (''Hoplostethus atlanticus''), also known as the red roughy, slimehead and deep sea perch, is a relatively large deep-sea fish belonging to the slimehead family (Trachichthyidae). The UK Marine Conservation Society has categori ...

and Oreo Dory using deepwater demersal trawlers. The northern part of the South Tasman Rise lies in the Australian Fishing Zone, in the exclusive economic zone of Australia, but the south end lies in international waters. Orange Roughy ( Hoplostethus atlanticus) is an important fish found between 1 and 2 km deep on the South Tasman Rise. Fishing for orange roughy in international waters ramped up in late 1997. 1998 saw the introduction of ''Arrangement Between the Government of Australia and the Government of New Zealand for the Conservation and Management of Orange Roughy on the South Tasman Rise''. This agreement set a research target of 150 tonnes per country, intended to input data to scientific research on the fish population structure. Australian fishermen were allowed to catch three times as much fish as the New Zealanders. The quota was set at 2100 and then 2400 tonnes for 1999-2000. However it was exceeded so the fishing ground was closed till the end of February 2000. In June 1999 fishing boats from South Africa and one registered in Belize came in to fish the rise outside of the bilateral agreement. Diplomatic pressure from Australia caused the Belize government to deregister its vessel and for the South Africans to withdraw their boats. A new agreement was made in 2000 about the orange roughy fishery. One feature of this new agreement was a 2:1 payback for over fishing: if the country's quota was exceeded, it had to pay back via a double reduction in its future fish allowance.

Marine reserve

The South Tasman Rise Commonwealth Marine Reserve was declared in 2007. The reserve seeks to protect unique environments for marine life which are of significant scientific interest.

References

{{Coord, 47, 30, S, 148, 00, E, display=title, region:AU_type:mountain Underwater ridges of the Southern Ocean