New Maps of Crust off Tasmania Expand Research Possibilities

Eos Vol. 76, No. 20, May 16, 1995, pp.201, 206-7. © 1995 American Geophysical Union. Permission is hereby granted to journalists to use this material so long as credit is given, and to teachers to use this material in classrooms.
N. F. Exon and P. J. Hill,
Australian Geological Survey Organisation (AGSO), GPO Box 378, Canberra 2601, Australia;
and J.- Y. Royer,
Laboratoire de Gèodynamique Sous-Marine, BP 48, 06230 Villefranche-sur-Mer, France.

A mapping exercise southwest of Tasmania in waters beyond the continental shelf (Figure 1) has provided the offshore equivalent of the topographic maps and satellite images that we all take for granted. Although this joint Australian-French program was conducted mainly to map offshore Australia for strategic reasons, it is proving to be the key to a treasure trove of information about Tasmania's geological history as it gradually separated from land masses to the west, east, and south. The swath-mapping system employed by the program produced high-quality maps that can be used to discern fault patterns on the ocean floor. These maps should help geophysicists answer long-standing questions about the split between Australia and Antarctica and its effects on regional geology. They will also help document climatic change over the last 50 m.y. and define Australia's Legal Continental Shelf in compliance with the United Nation's Law of the Sea Convention, which came into force in late 1994.

Fig. 1. The area surveyed by RV L'Atalante off Tasmania using swath-mapping and reflection seismic techniques, related to the pre-existing bathymetric contours. Total area is about 200,000 km². Smaller box shows area of Figure 3.

The work was carried out in February and March 1994 by the French research vessel L'Atalante for the Australian Geological Survey Organisation (AGSO).

Tasmanian Region

The region's structural pattern and geological history are controlled by the separation of Australia and Antarctica, which began about 130 m.y.a. This separation bears on the history of the entire southern margin of Australia, which is a major producer of petroleum from the Gippsland Basin. The Sorell Basin west of Tasmania is a southward prolongation of the gas-producing Otway Basin. The region is bounded on three sides by oceanic basalt crust of Late Cretaceous and Palaeogene age (Figure 2) that formed during seafloor spreading between 80 and 20 m.y.a., as the Lord Howe Rise broke away to the east and Antarctica moved relatively to the south.

Fig. 2. The tectonic elements of the offshore Tasmanian region as presently known. The position and ages (in millions of years) of magnetic anomalies are taken with some modifications from the Tectonic Map of the Circum-Pacific Region, Southwest Quadrant, published in 1991. The arrows point to younger oceanic crust. COB = continent-ocean boundary. Further work by our group will greatly refine this picture.

Shipboard Systems

L'Atalante's multibeam sonar system was used to map almost 200,000 km² of the continental margin and the adjacent abyssal plain, an area which is 3 times the size of Tasmania. The cruise employed the Norwegian SIMRAD EM12D system to map an area along each ship track, up to 22 km wide, at a speed of 10 knots or nearly 20 km/hour. The water depth was generally in the range 2000-4500 m but occasionally less than 1000 m. The pixel size is of the order of 60 x 60 m.

The EM12D system provides bathymetric maps and acoustic imagery in real time. It uses 162 narrow sonar beams, fanning out normal to the ship's axis, with a maximum beam angle of 150° and three frequencies around 13 kHz. The acoustic velocity profile, necessary to correct for refraction in the water column, is determined by continuous measurement of surface water temperature and occasional expendable bathythermograph casts. Despite being in the infamous latitudes of the Roaring Forties - the stormy area between 40° and 50°S - with average winds of 20-30 knots and fronts with 50 knot gales, we collected acceptable data at all times although swath width was reduced during gales. In most areas 100% coverage of the sea bed was obtained and 25-m contours were produced.

Six-channel seismic, magnetic, and gravity data were also recorded along 13,600 km of track. The seismic reflection profiles gave penetration of up to 2.5 s (two-way time) below the sea bed, thus showing the structure several kilometers deep.

The South Tasman Rise and the Sorell Basin

The South Tasman Rise, a submerged continental block larger than Tasmania, extends from 44° to 50°S. About three-quarters of it, or 150,000 km², was mapped (Figure 1). The rise only sank completely below the ocean in the last 40 m.y., and parts of it are less than 1000 m deep. Spectacular faults and giant fault blocks were seen in water depths of 2500-4500 m on its western and eastern sides. The submarine cliffs dwarf anything on Australia, reaching 2300 m high in one place, and have slopes averaging 20° (Figure 3). The rise is current-swept, so Neogene sediment cover is thin or absent. It consists of about 20% outcropping rock and 80% sedimentary cover and has outcrops of ancient basement rocks like schist, gneiss, granite, and Paleozoic sediments, as well as younger Mesozoic sediments and Tertiary basalts.

Fig. 3. Block diagram of part of the western margin of the South Tasman Rise, showing a continental cliff rising 2000 m above the abyssal plain. Water depths are in the range 1000 - 4500 m. The seafloor spreading fabric is visible at the sea bed as east-west trending ridges. Location is shown by the box in Figure 1.

The complexity of the geology of the South Tasman Rise surely rivals that of Tasmania. The basement rocks are sliced apart by faults trending 345° and 320° that form deep, narrow sedimentary basins. One 345°-trending anticline in presumed Palaeozoic sediments is of the order of 100 km long and 50 km long and is bounded by cuestas hundreds of meters high. The presence of deep sedimentary basins and geochemical evidence from surface samples that thermogenic hydrocarbons are being generated suggest that the South Tasman Rise does have some potential for petroleum.

The Sorell Basin off west Tasmania contrasts remarkably with the South Tasman Rise. It was heavily sedimented in Tertiary times, so deep structure is less apparent at the surface. Nonetheless, an area of about 50,000 km² was mapped. On the continental shelf, basement blocks separate four subbasins containing more than 3000 m of Cretaceous and Tertiary strata much like those in the Otway Basin. On the continental slope, the Sorell Basin is generally more than 3000 m thick.

Some basin-bounding faults were imaged on the lower slope, including a 2500-m-high fault scarp trending 320° from which pre-Mesozoic basement rocks and Late Cretaceous shallow marine sediments were previously dredged. The existence of thick sedimentary deposits, oil and gas shows, and some structuring suggests that the basin has considerable petroleum potential. Recent studies of cores from west of Tasmania by Vicky Kosslow of Australian National University suggest that in the last 100,000 years, the upper slope was dominated by mud flow deposition to about 2300 m. The lower slope, on the other hand, experienced the deposition of sediment that settled through the water column as well as turbidites and debris flows. This sedimentological pattern is supported by swath-mapping data that show submarine canyons running from the upper slope to the abyssal plain. The data suggest that the canyons are eroding the 4° - 5° upper slope but allow deposition below 2300 m, where the slope decreases appreciably. The mud flows are not channeled, but the turbidites and debris flows are.

Australia-Antarctic Breakup

The oceanic basalts on the abyssal plain - 4500 - 5000 m deep west of Tasmania and around the South Tasman Rise - and the magnetic anomalies associated with them record much of the history of the breaking apart of Australia and Antarctica. Antarctica ground its way slowly past Tasmania and finally cleared the South Tasman Rise about 40 m.y.a. Thereafter, what had been dry land or a shallow marine embayment west of Tasmania subsided thousands of meters below the sea, as did the margins of the South Tasman Rise.

The plate tectonic history of the region, as presently understood, indicates that the breaking apart of Antarctica and Australia started 130 m.y.a. and formed the major faults trending 320°. Then the direction of the movement changed, producing faults trending 345° at about 45 m.y.a. and forming the western and southern margins of Tasmania and the South Tasman Rise. Movement was slow until about 45 m.y.a. and much faster thereafter.

The eastern margin formed differently, when the Lord Howe Rise broke away to the east-northeast about 80 m.y.a. This motion continued for about 30 m.y. The southern and northern margins of the South Tasman Rise formed by stretching, with both shear faults and normal faults involved. There is a resultant saddle at least 3000 m deep between Tasmania and the rise, where the crust moved along faults trending 345°. Associated volcanoes are 600 m high, and a field of 70 volcanic cones about 100 km south of Tasmania forms the fishing grounds for an important deep-trawl fishery.

Early this year, AGSO's R.V. Rig Seismic performed seabed sampling to resolve questions about the nature of the continental crust and where and how the continent ends and the oceanic crust begins.

Once the thinned crust of the South Tasman Rise sank below the sea (40 m.y.a.), the easterly flowing Circum-Antarctic Current - which had flowed north of Australia - broke through in the south, leading to major oceanographic changes. The oceanic abyssal plain west of the South Tasman Rise is very lightly sedimented, and the swath-mapping shows that the surface topography mimics that of the oceanic basement. East and south of the rise lies a pile of sediment more than 1000 m thick, which was probably swept from the rise by currents.

Postcruise Work

The geophysical data gathered on the cruise are being integrated with other remotely sensed data to better define the history of the separation of Australia from Antarctica to the south and the Lord Howe Rise to the east. This will greatly refine the tectonic picture sketched out in Figure 2.

The mapping of rocky outcrops and sedimentary patterns was invaluable in planning the 1995 AGSO seafloor sampling cruise. Dredged older rocks are providing information on the history of the area before it subsided. The data were used to plan several Rig Seismic reflection seismic profiles across the region, which should show the crustal structure to its base 20 - 30 km below the seabed and help reveal how this complex region formed. Our French colleagues hope to use the deep-diving submersible Nautile to map the strata exposed in the mighty cliffs flanking the abyssal plains to gain a better understanding of the geology of the continental blocks.

The mapping of sedimentary patterns on the continental slope will help document the changes as the slope subsided following the departure of Antarctica. Cores of marine sediments will be used to study changes in oceanic circulation and climate as Australia moved steadily north away from Antarctica.

Acknowledgments

We acknowledge with gratitude the support of the rest of the shipboard party: scientific, technical and maritime.

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