Arc-continent collision as a key mechanism for continental growth: Examples from the Phanerozoic of eastern Australia Jonathan AITCHISON 1 , Solomon BUCKMAN 2 1 Department of Geosciences, University of Sydney, Australia, 2 School of Earth and Environmental Sciences, University of Wollongong, Australia Prevailing ‘consensus’ models for the Phanerozoic development of eastern Australia invoke a “retreating accretionary orogen” model in which slab retreat results in development of offshore island arcs in front of marginal basins. Periodically, during intervals of plate-coupling, these basins close and the arcs are retro-thrust back onto the continental margin. Implicit in this model is the notion that all elements develop in an upper plate location upon the accretionary margin and that west-dipping subduction beneath eastern Gondwana was a long- lived phenomenon. We suggest this concept is flawed and instead propose a new testable hypothesis that eastern Australia grew through a series of arc-continent collisions in which east-directed subduction beneath intra-oceanic island arcs led to their collision with eastern Australia. At least four such arc-continent collisions are posited in mid-Cambrian (Mt Stavely arc), Late Ordovician (Macquarie arc), mid-Devonian (Gamilaroi arc) and Late Permian (Gympie arc) times. This process effectively transferred new material to the continental crust resulting in net growth. When collisions were followed by subduction flip episodes of west-directed subduction beneath the continental margin gave rise to the S- and I-type granites for which eastern Australia is famous.  Tectonic reconstruction of Australia, Tasmania and Antarctica in the Middle Cambrian after Veevers et al. (1991). WSTR, western South Tasman Rise, position relative to Antarctica after Royer & Rollet (1997); ESTR, eastern South Tasman Rise; black areas, pre-Cambrian continental crust; grey areas, inferred Precambrian continental crust. From: Meffre, S., Berry, R.F., and Hall, M., 2000, Cambrian metamorphic complexes in Tasmania: tectonic implications Australian Journal of Earth Sciences, v. 47, p. 971-985. The late Cambrian of far western NSW, Victoria and Tasmania in Australia, NW Nelson in New Zealand and North Victoria Land in Antarctica contains exotic island arc elements that have been thrust over continental margin rocks. In each case it is possible that this represents the products of an arc-continent collision where oceanic lithosphere that lay to the east of the Gondwana margin has been subducted under an intra-oceanic island arc. When all the intervening oceanic lithosphere was removed by subduction the arc elements were then obducted onto the continental margin. The culmination of this process was a subduction flip akin to that seen in present-day Taiwan. Proposed tectonic model for Cambrian arc collision in Antarctica and Tasmania. Either a New Caledonian- type model (after Aitchison et al. 1995) or an Oman-type model (after Miller et al. 1998) could account for the geology of the Tasmania Cambrian metamorphic complexes. (a), Tasmania arc collision with passive margin with thin sedimentary cover; (b), North Victoria Land collision with passive margin with thick sedimentary cover. Also from Meffre, S., et al (2000) The Lachlan Fold Belt in eastern Australia also contains exotic island arc elements that have been thrust over continental margin rocks. In this case, the rocks include miogeoclinal strata of the Adaminaby Group that developed on the eastern continental margin of Gondwana. They were overthrust by intra-oceanic island arc rocks of the Macquarie arc in the Late Ordovician. Both suites of rocks are distinctive and cannot have formed in the same tectonic setting. The Lachlan Fold Belt contains a suite of rocks that includes basalt, chert and sandstone that have been incorrectly interpreted to have formed as part of an ocean plate stratigraphy. The cherts are quite different from the red ribbon-bedded radiolarian-bearing varieties that characterise open ocean settings. Moreover, that are intercalated with quartz arenites that dominate the Lachlan Fold Belt and are unlike any sediments deposited in trenches associated with convergent plate margins. Left: Contrast the typical Lachlan Fold Belt stratigraphy (note that ages of various rock packages are consistent across the terrane) with that of a typical ocean plate stratigraphy being accreted into a subduction complex. In this case the ages of chert packages young progressively trenchward and individual thrust slices young towards the arc. Note also the comparison between detrital sandstone modes for Adaminaby Group sediments compared with those that typify sediments from oceanic trenches. Below: Cartoons outlining the existing model for Lachlan Fold Belt evolution (left) vs. arc-continent collision model (right). Above left: Pillow basalts in the Lachlan Fold Belt at Narooma on the NSW, south coast. Alkali basalts such as these are not necessarily indicative of any particular plate setting. Mafic igneous rocks dominate the Macquarie arc. They host world-class Cu-Au deposits such as this one at the Cadia mine in central NSW (below). They are unlike any autochthonous elements of the Lachlan Fold Belt. The Siluro-Devonian Gamilaroi terrane is an intra-oceanic island arc within the New England orogen of eastern Australia. It formed above an east-dipping subduction zone and accreted to the Gondwana margin in the Late Devonian (Aitchison and Flood 1995). Following this event, west-dipping subduction began again along the margin of continental Australia. The figure on the right is a model of the system from Offler and Murray (2011). Aitchison, J.C., and Flood, P.G., 1995, Gamilaroi Terrane: a rifted Devonian intra-oceanic island arc assemblage, NSW, Australia, in Smellie, J., ed., Volcanism associated with extension at consuming plate margins, Geological Society of London Special Publication 81, p. 155-168. Offler, R., and Murray, C., 2011, Devonian volcanics in the New England Orogen: tectonic setting and polarity: Gondwana Research, v. 19, p. 706-715. A further arc-continent collision may have affected eastern Gondwana at some time in the Late Permian or Triassic. Intra-oceanic island arc rocks in the Gympie terrane (southern Queensland), Teremba terrane (New Caledonia) and Brook Street terrane (New Zealand have been correlated together and are widely recognised as exotic elements. However, little research has been conducted on the nature and timing of their docking with the eastern margin of Gondwana. Right: Permian volcaniclastic rocks of the Gympie terrane in southern Queensland. Right: Boninitic pillow basalts in the Gamilaroi terrane exposed on the Back River property NE NSW. Left: The pit at the Mt Morgan Cu-Au deposit which is hosted in volcanic rocks associated with the Gamilaroi terrane arc. Left and above left: Typical sedimentary rocks of the Adaminaby Group in the Lachlan Fold Belt exposed along the NSW south coast. Here thin zones of black chert are intercalated with turbidite packages that are dominated by quartz arenites. Below: Model contrasting different modes of continental growth along a convergent plate margin FOR ADDITIONAL MATERIAL SEE ALSO: Aitchison, J.C. and Buckman, S. 2012. Accordion vs. quantum tectonics: Insights into continental growth processes from the Paleozoic of eastern Gondwana. Gondwana Research DOI: 10.1016/j.gr.2012.05.013. Right: Red ribbon-bedded cherts of the variety typically found in accretionary wedges. In this case the rocks are part of the Djungati terrane in the New England orogen of NE NSW, Australia.