Architecture and evolution of the Australian continental margin

Brief description

Interpretation of key seismic profiles around the Australian margin clearly show that the three principal rift-drift segments (north-western/western, southern, and eastern) have distinct and different architecture. The normally rifted northwestern and oblique-slip western margins have a polyphase rift/drift history that saw the progressive separation of continental blocks from the Permo-Carboniferous until the Neocomian. These multiple tectonic episodes produced a margin that is geologically complex and also shows a strong imprint of volcanism. The continental shelf and marginal plateaux of this margin are generally underlain by thick Phanerozoic sediments of the Westralian Superbasin, while areas of shallow crystalline basement are rare. The Phanerozoic section is generally thick and flat-lying, and extension of the upper crust is observed only adjacent to the inboard confined deep rifts and on the outermost margin. Even in these regions, the amount of upper crustal extension is rarely more than 20 per cent. Profiles through the basins, together with limited velocity information, suggest strongly that they formed largely as the result of lower crustal extension; i.e. the northwestern and western margins are probably upper plate margins. The southern margin, from the Naturaliste Plateau in the west to the South Tasman Rise in the southeast, formed during a single rift/drift episode that culminated in the separation of Australia and Antarctica in the Cretaceous. The age of onset of rifting is inferred to be in the Jurassic; however, the azimuth of extension is open to conjecture, with interpretations ranging from NW-SE to NNE-SSW. The preserved rift ranges in width from about 350 km in the normally extended crust in the west to no more than 100 km on the strike-slip margin west of Tasmania. In contrast to the northwest margin, the main basins of the southern margin lie beneath the continental slope and rise, while the shelf is largely underlain by shallow crystalline basement. Marginal plateau development is rare, with the major plateau (the Ceduna Plateau) interpreted to be largely sedimentary in origin, rather than structural. The primary basin-forming mechanism is interpreted to be extension of the upper crust. Also in contrast to the northwest margin, volcanism is apparently more restricted, except at the eastern and western ends of the rift. The eastern margin has the least understood architecture on the Australian margin, largely owing to the dearth of seismic and drill information. The margin of the Tasman Basin is narrow and, with the exception of the deep and areally restricted Gippsland Basin, rift basin development is generally limited to the conjugate Lord Howe Rise. In contrast, the margins adjacent to the Coral Sea Basin are broad, particularly where the Queensland and Marion Plateaux have developed on the continental margin. These plateaux are distinct from those on the northwestern and southern margins in that they are structurally controlled, but consist largely of shallow, pre-rift basement.

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