Australian-Antarctic Discordance Research Articles

Spreading of Antarctic Bottom Water and its effects on the floor of the Indian Ocean inferred from bottom-water potential temperature, turbidity, and sea-floor photography

Data on bottom-water potential temperature, turbidity and current indications show that in the Southern Ocean west of the Kerguelen Plateau, Antarctic Bottom Water (AABW) of Weddell Sea origin spreads northwards from the Atlantic—Indian Basin in two directions: (1) AABW enters the Agulhas Basin through relatively deep areas in the Mid-Indian Ridge at 20–25°E and possibly at 35°E, and flows northwards into the Mozambique Basin as far as its northern limits; (2) a more easterly spreading path extends from the Atlantic—Indian Basin through the Crozet into the Madagascar, Mascarene, Somali and Arabian Basins. The passage in the western branch of the Indian Ridge for the AABW spreading from the Crozet into the Madagascar Basin appears to be at 29-26°S and 60–64°E. East of the Kerguelen Plateau in the South Indian Basin, the bottom water formed mainly along the Adélie Coast and Ross Sea travels west towards the Kerguelen Plateau and then parallel to it. This water finally flows eastwards hugging the Southeast Indian Ridge. Significant deviations from this general circulation pattern occur due to local topographic effects. Some AABW in the South Indian Basin exits through a passage at 120–125°E in the region of the Australian—Antarctic discordance in the Southeast Indian Ridge and enters the South Australian Basin and subsequently the Wharton Basin. This passage is clearly indicated by the northward extension of a cold, bottom-water tongue as shown by the temperature distribution in the region; the bottom-water effects in the passage are reflected in the high turbidity and current lineations on the sea floor. In the Southern Ocean basins, bottom-water turbidity is generally high, reflecting in part the strong bottom-water activity. The effects of AABW circulation on the sea floor—in the form of well-developed small- or large-scale current ripples and erosional/depositional features, manganese-nodule formations, and unconformities and reworking of sediments observed in cores — are also marked in these basins. Even though the AABW in the Wharton Basin is cold, its spreading effects on the sea floor are minimal in this basin in contrast to the basins west of the Mid-Indian Ridge at comparable latitudes.

The Australian-Antarctic Discordance: New results and implications

By using additional geophysical data obtained from the Australian-Antarctic discordance the history of this complex part of the southeast Indian Ocean for the past 10 m.y. is now ascertainable. Magnetic lineation patterns observed over limited areas on both ridge flanks indicate that asymmetric spreading took place between 10 and 4.7 m.y. B.P. at a rate of 4.0 ± 0.2 cm/yr on the north flank and 3.0 ± 0.2 cm/yr on the south. Although anomalies formed less than 4.7 m.y. ago within the discordance cannot be identified unequivocally, the amount of crust between inferred anomaly 3 lineations on both flanks is consistent with an average total opening rate of about 7.4 cm/yr for the past 10 m.y. A survey of the north flank delineated a northerly trending morphologic feature that is probably a fracture zone. The trend of the observed feature agrees with the azimuth of a hypothetical fracture zone calculated from a previously determined pole of opening. Geophysical results from the southeast Indian Ocean provide observations with which conceptual models of plate tectonics may be compared. Notably, within the framework of thermal contraction models for the lithosphere a 2000-km-wide depression in the regional morphology occurs across the Indian and Antarctic plates. Depths up to 800 m below those expected from the empirical global depth-age relation are found in the regional depression. Further, for crust ≤30 m.y. old, depths are systematically more than 100 m shallower on the south than on the north flank of the Southeast Indian ridge. The unusual ‘depth anomalies,’ satellite gravimetry, episodes of asymmetric sea floor spreading, and the creation during the Neocene of the complex morphologic and magnetic fabrics that characterize the discordance imply to us that during the evolution of the southeast Indian Ocean, separation of the Indian and Antarctic plates has occurred over a relatively stable region of predominantly downward convective flow in the asthenosphere.