Australia: The Land Where Time Began

A biography of the Australian continent 

Together with Antarctica and India, Australia made up East Gondwana, and possibly with Laurasia, until other continents fused about 720 Ma. At 700 Ma Australia underwent extension. According to the SWEAT hypothesis (Kearey et al., 2009, section 11.5.3) Gondwana began forming immediately after the break up of Rodinia during the Late Palaeozoic. As the proto-Pacific opened  forming the Pan-African orogens, small ocean basins surrounding the African and South American cratons closed. The collision and amalgamation of West Gondwana with the blocks comprising East Gondwana resulted from the closure of the Mozambique Ocean. Some believe that there may have been a short-lived supercontinent called Pannotia In the Early Cambrian, that resulted from this amalgamation (Kearey et al., 2009). Whether or not Pannotia actually existed depends on the timing of rifting between Gondwana and Laurentia (Cawood et al., 2001).

See Neoproterozoic Australia,

Pan African orogenies

A number of continental blocks and probably many smaller fragments of continental crust and oceanic arcs were sutured together to form the supercontinent of Gondwana during what has been very loosely termed the "Pan-African" Orogenies (Fitzsimons, 2000; Boger et al., 200; Boger & Miller, 2004; Collins & Pisarevsky, 2005). In the period from about 850 Ma to 520 Ma it has been suggested the accretion occurred (Dalziel, 1991; Pinna et al., 1993;  Powell et al., 1994; Grunow et al., 1996) though the main accretion from 650-520 Ma (Meert, 2003; Collins & Pisarevsky, 2005). Peaking at about 620 Ma, closure and convergence in the Mozambique Suture resulted in western Gondwana being assembled by the East African Orogeny. The Mozambique Suture extended south to into Antarctica (Jacobs et al., 1998), and more recent interpretations have suggested there was a second suture that radiated from Dronning Maud Land, that continued northeast through the South Prince Charles Mountains to Prydz Bay (Boger et al., 2001, Meert, 2003) separating a fragment constituting the East Antarctic-Australian continental fragment from a block that was comprised only of India. It has been suggested (Boger et al., 2001) that associated with the final stages of the assembly of Gondwana, tectonic activity, followed by post-collisional cooling (Mezger & Cosca, 1999; Rickers et al., 2000), continued in this belt from 550 Ma to 490 Ma, in the Kuunga Orogeny proposed by Meert (2003). Recent results from the Leeuwin Complex in the southwest of Western Australia, where a mean age has been found of 537 ± 4 Ma for granulite facies metamorphism and syntectonic magmatic suites, that extend from about 600 Ma to 522 ± 5 Ma (Collins, 2203), support this conclusion.

The peak of the Pan-African events occurred between 630 Ma and 540 Ma in Antarctica-Africa and India, though they had begun about 900 Ma (Pinna et al., 1993). Static metamorphism and post-tectonic felsic magmatism dating to 510-480 Ma (Jung & Mezger, 2001; Jung et al., 2001; Meert, 2003), the ages being similar to those of the late Ross and Delamerian Orogens, have been found in the Damara Orogen, Namibia, in southern Africa, by recent geochronological studies. The  mobile belt, the main synkinematic activity of which occurred prior to 570-550 Ma (Yung & Mezger, 2001), associated with active plate convergence, subduction and accretion had these imposed on it. As with the model proposed earlier by the authors³ for the Delamerian Orogen, the presence of A-type granite in the Damara Orogen has also been suggested to result from the influx of mantle following delamination of the lithosphere (Jung et al., 1998, 2001).

The Pan-African ages mostly correspond with the time of passive margin activity in eastern Australia, in the Neoproterozoic to Early Cambrian. A shift in the locus of strain to what had previously been the trailing edge of the Australian-Antarctic (East Gondwana) continent as it merged with the new supercontinent of greater Gondwana is indicated by the age cluster marking the Delamerian Orogen that corresponds to a time of rapid decline in the Pan-African events.

See Delamerian Orogeny

Sources & Further reading

1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.
2. Veevers, J.J. (ed.), 2000, Billion-year earth history of Australia and neighbours in Gondwanaland, GEMOC Press Sydney