Australia: The Land Where Time Began

A biography of the Australian continent 

Australian Western Margin – Evolution During the Supercontinent of Rodinia and Gondwana

According to Wilde it appears the proto-Darling Fault Zone and the Darling Fault, its successor that extend 1,000 km along the western margin of the continent, have been active at several periods in the geological past. At ~ 2,570 Ma deformation began which affected granitoids of Late Archaean age along the western margin of the Yilgarn Craton. Events related to the accretion and breakup that was associated with the supercontinent cycles of Gondwana and Rodinia are reflected in much of the later activity.

Northampton and Mullingarra Complexes in the north are part of a high-grade Grenvillian orogenic belt that is located to the west of the Darling Fault, which is referred to as the Pinjarra Orogen. They form part of a global collisional event that resulted in the amalgamation of Rodinia, having undergone granulite facies metamorphism ~1,080 Ma. These rocks extend to the south beneath the Phanerozoic sedimentary cover of the Perth Basin, where they are constrained by the Darling Fault and to the west by the Dunsborough Fault, the latter marking the boundary of the Leeuwin Complex.

The Leeuwin Complex, which is traditionally considered to be part of the Pinjarra Orogen, is a fragment of the pan African crust. Its composition is predominantly of upper amphibolite to granulite facies felsic orthogneisses that are derived from A-type anorogenic granitoids. It has been established by conventional and SHRIMP U-Pb zircon geochronology that the granitoids evolved between ~780 Ma and ~520 Ma and were metamorphosed ~615 Ma. These events have been equated with the breakup of Rodinia. The juvenile nature of the crust and the lack of evidence for the involvement of Archaean continental material that was pre-existing are supported by Sm-Nd whole rock data.

The Dunsborough and Darling Faults were reactivated during the Phanerozoic as normal faults that define the inner arm of a major rift system within eastern Gondwana and controlling sedimentation in the Perth Basin that at present overlies the Grenvillian terrane. By the Late Jurassic major normal movement of the Darling Fault ceased and in the Early Cretaceous continental breakup occurred along fractures that were closely related to the western boundary of the Leeuwin Complex that defined the eastern margin of the outer arm of the rift system. At ~132 Ma breakup began between Australia and Greater India and was followed at 130 Ma and 123 Ma by the eruption of the Bunbury Basalt. Wilde suggests this was probably the result of hotspot activity beneath Eastern Gondwana and may have been a reflection of the Kerguelen Plume, though the evidence is equivocal.

Based on the petrographic, geochemical and isotopic characteristics, as well as with the likely contiguity of the Eastern Gondwana continents since the assembly of Rodinia, Wilde says it is argued that the Leeuwin Complex evolved within an intracrustal rift and is not an exotic terrane. It should be considered a separate terrane as it is distinct from the portions of the Pinjarra Orogen that is adjacent. Wilde recommends that the term ‘Pinjarra Orogen’ be confined to rocks that record the Grenvillian events, which thereby excludes those rocks (the Leeuwin Complex) that evolved during the Pan-African Orogeny that occurred later.

Sources & Further reading

1. Wilde, S. A. (1999). "Evolution of the Western Margin of Australia during the Rodinian and Gondwanan Supercontinent Cycles." Gondwana Research 2(3): 481-499.