The Australian-Antarctic Depression

Australia: The Land Where Time Began

The Australian-Antarctic Depression

This depression (AAD), with an area of 15 million sq km, straddles the South East Indian Ocean Ridge (SEIR), extending onto the Australian continent in the north and Antarctica in the south. 'It is disposed about the A-A Discordance at the crest of the SEIR about NNE and SSE axes' (Veevers, 2000). It is traceable through the flanks of the ridge and the continent-ocean boundary for 1750 km to the north and 2000 km to the south. The deep depression of the Southeast Indian Ocean dominates the north-south profile of the seafloor at 132^oE. The depth varies from 3 km at the ridge crest to 5 km at the foot of the continental margins, modelled as due to oceanic lithospheric thermal contraction, all being anomalously deep. The ridge flank on the north is slightly wider and much deeper than on the south flank. The sediment cover of the abyssal plains is up to 1000 m in the north and 3500 m in the south, being thin over other parts of the seafloor. The spacing of the magnetic anomalies, based on the spreading half-rates, is of similar symmetry for anomalies older than anomaly 6. During the first half of seafloor spreading, between 96 and 45 Ma, that was slow, comprised 12 % of the spread. During the second, fast half, from 45 Ma to the present, the spreading comprised 88 % of the total spread. The azimuth of the spreading varied between the fast spreading of the present, when it is to the north and south. During the slow spreading period, between 96 and 45 Ma, it was to the NNE and SSW. The asymmetry was added to by early ridge jumps See Link 1.

On the east of the depression are the Eastern Highlands, Tasmania, South Tasman Rise, a high point at 150^oE on thr SEIR, the submarine ridge beneath the Balleny Islands and the Transantarctic Mountains. To the west, the boundary is less distinct, comprising the Gamburtsev Mountains, through a long spur enclosing Mt Sandow, a high point at 95^oE on SEIR, and the southwestern of the Great western Plateau. In the north it reaches central Australia and Mt Isa, and across to the Eastern Highlands at 22^oS. The axis extends through Lake Eyre and Lake Torrens, Spencer Gulf, across the SEIR flank to Adelie land (data points at 30 and 10 Ma isochrons) in a 2.2 km deep trough. On the west, a subsidiary axis runs from the Nullarbor Plain across the SEIR to the Budd Coast and the Aurora sub-ice basin. The subaxis on the east runs from the Murray-Darling Basin to George V Land and the Wilkes sub-ice basin.

According to west-east profiles of the depression it has a first-order trough shape, secondary crests and troughs flanking the median axis on the continent. Profiles are of much greater amplitude on Antarctica than elsewhere, though without the ice loading that originated in the mid-Tertiary, reaching an ice cover of 4 km, it has been estimated that the deepest parts would rise by about 1 km by isostatic rebound, that would bring the axis to or above sea level, the highlands about the axis would then probably be about 1 km higher. There is a much thinner ice covering on the crest of the Antarctic Mountains, so they would rise by a much lesser amount. Even after the distortion of the deep ice cover of Antarctica was removed it would retain greater relief than Australia, though the differences would be less. Flexure of the ice sheet edge is believed to be the main cause of the outcrop arising above sea level along parts of the coast of East Antarctica.

Late Cretaceous

There was an early manifestation of the AAD in the Late Cretaceous, the perimeter of which was similar to that of the present, with a low-lying centre in the Ceduna depocentre, with a presumed mirror image in Antarctica. Uplift and loss of section marked the eastern flank along the Australian Eastern Highlands, that continued to the south along the Antarctic Mountains. The initial breakup of Australia and Antarctica is thought to possibly be associated with a phase of uplift, known extensively in the Transantarctic Mountains of the present (Balestrieri et al., 1997). The southern flank is conjectural. Widespread reworked palynomorphs of Cretaceous age are seen as evidence of uplift during the Cretaceous.

Definitive uplift of eastern Australia-Transantarctic Mountains impounded sediments in rim basins on either side of the nascent SEIR between Antarctica and Australia, leading to the reversal of drainage towards the interior with the formation of the depression of the Mid-Cretaceous.

Permian

During the earliest Permian Stage A, the lowland in the eastern half of southern Australia, occupied by an ephemeral epeiric sea, with sediment or ice transport from Antarctica, suggests a possible earlier manifestation of the AAD. Sand was flowing to southeastern Australia from Antarctica by the Triassic, across the area occupied by the former Permian sea. The definitive inception of the AAD occurred in the Middle Cretaceous. It has been suggested that in the Neogene it may have been rejuvenated when it drifted over the sinking slab from subduction off Queensland during the Jurassic (Gurnis et al., 1998). The inception of the AAD in the Cretaceous requires either downwelling or slower upwelling.

Sources & Further reading

Veevers, J.J. (ed.), 2000, Billion-year earth history of Australia and neighbours in Gondwanaland, GEMOC Press Sydney

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