Seafloor Mapping of the South-east Region and Adjacent Waters : AUSTREA-2 Cruise Report: South-east of Tasmania and Southern Macquarie Ridge

In January/February 2000, the Australian Geological Survey Organisation (AGSO) completed a 2S-day seabed swath-mapping and geophysical survey off south and south-east Tasmania and south of Macquarie Island for the National Oceans Office and Environment Australia. The survey, which is named AUSTREA-2 and designated as AGSO Cruise 223, used the 8S-m French oceanographic and geoscience research vessel N/O L 'Atalante, which departed Hobart on January IS and arrived in Bluff, New Zealand, on February 9. The survey covered about 10,200 km and mapped about 140,000 km2 of seabed. The initial impetus for the work was to map the foot-of-slope position in several areas to support definition of Australia's legal Continental Shelf under the United Nations Convention on the Law of the Sea. A significant additional but complementary aspect was to support marine zone planning and management, and assessment of seabed living and non-living (petroleum and mineral) resources, as an important step towards implementation of Australia's Oceans Policy and Australia's Marine Science and Technology Plan, and, in particular, the development of the South-east Regional Marine Plan by the National Oceans Office. Geophysical data collected included Simrad EM12D swath bathymetry and backscatter, 6-channel GI-gun seismic, 3.S kHz sub-bottom profiling, and gravity and magnetic profiles. This was augmented by a suite of oceanographic data, such as seawater temperature, and both current and salinity depth profiles. Weather and sea conditions were highly favourable, particularly in the more southern latitudes. Occasional periods of rough weather resulted in higher noise levels, but did not seriously affect acquisition, and data quality was generally excellent. The work conducted off Tasmania was mostly to fill in and extend previous swath coverage, and map the foot-of-slope along the eastern margin of the South Tasman Rise. It highlighted features such as the major development of slope canyons down the eastern Tasmanian margin, and the complex character of the Cascade Seamount and other seamounts adjacent to the South Tasman Rise. The work conducted over the southern Macquarie Ridge Complex highlighted features such as: a high-relief axial valley adjoining the deep Hjort Trench; the broadening to the south of the submerged Hjort Ridge, east of the Hjort Trench; the development of seafloor spreading tectonic fabric across the Hjort Ridge summit; and the presence of a linear trough/ridge feature that obliquely truncates the southern end of the Hjort Trench and adjoining axial valley. A full set of shipboard maps are held by the National Oceans Office and AGSO, and copies of the digital data are stored at AGSO. All data from the survey will be managed jointly by AGSO and the National Oceans Office.