International Geologiical Congress - Oslo 2008

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OCE-01 Oceania and the 34th IGC in Brisbane

 

Geodynamic evolution of proterozoic Australia

 

George Gibson, Geoscience Australia (Australia)
David Champion, Geoscience Australia (Australia)
David Huston, Geoscience Australia (Australia)
David Maidment, Geoscience Australia (Australia)
Geoff Fraser, Geoscience Australia (Australia)
Steve Sheppard, Geological Survey of Western Australia (Australia)
 

 

Geodynamic models for crustal growth in Proterozoic Australia have long emphasised the importance of magmatic underplating and vertical accretion in an ensialic extensional setting. Recently, the emphasis has shifted to models based on collision and lateral accretion of crustal blocks linked to subduction and plate tectonic processes. In these models, cratonic elements once regarded as being relatively immobile are no longer believed to be in their original position with respect to each other. Final assembly of the Australian continent may not have occurred until the West (WAC) and South Australian (SAC) cratons collided to form the 1300-1140 Ma Albany-Fraser Orogen. Prior to that time a succession of collisional events interspersed with periods of extension and continental breakup along cratonic margins continually reshaped the Australian continent, reworking previously accreted terranes and producing orogenic belts in which conditions favoured mainly low P - high T metamorphism and anticlockwise P-T-time paths.

Such processes are best documented for the eastern margin of the Kimberley Craton where arc magmatism and subduction of oceanic crust ceased at 1830 Ma following collision with the trailing edge (passive margin) of the North Australian Craton (NAC). Plate convergence and subduction-related arc magmatism have also been postulated for the Capricorn orogen as well as along the southern margin of the NAC and northern margin of the SAC. The Capricorn orogen separates two Archaean cratons which were probably juxtaposed either side of a Palaeoproterozoic terrane during two collisional events before ∼1800 Ma. As for the suture between the NAC and SAC, opinion differs as to the longevity of subduction, whether it was north- or south-directed, and whether collision between these two cratons occurred at ∼1730 Ma or ∼1590-1550 Ma. Most researchers favour northward-directed subduction with arc magmatism commencing along the southern NAC margin no later than 1810 Ma and continuing in some models long after 1780 Ma, and possibly as late as 1635 Ma or 1550 Ma.

Other researchers have questioned the need for ongoing subduction along this margin after 1780 Ma, arguing instead that the locus of subduction had shifted to the east by this time and that much of Proterozoic Australia now occupied a back-arc extensional environment in which basin formation was accompanied by increased heat flow, bimodal magmatism and the emplacement of metamorphic core complexes. Whether the NAC and SAC were ever separated by a major ocean basin during the period 1800-1640 Ma or lay in close proximity to each other is debatable. In either event, Proterozoic Australia probably did not exist as a single entity until ∼1100 Ma by which time the WAC, NAC and SAC had all amalgamated, forming part of the Rodinia supercontinent. Australia remained tectonically stable until the onset of continent-wide extension at 830 Ma which ultimately led to the fragmentation of Rodinia.

 

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