RAPID GENERATION OF CONTINENTAL CRUST IN PLATE-CONVECTION MODE

Current plate- and plume-convection models for the mantle do not adequately explain how continental crust is rapidly formed. Thirty percent of the Australian continent formed in 300 Ma as the Paleozoic Tasmanides of eastern Australia. To explain this rapid growth, a plate model is developed involving creation and collapse of a vast Early Paleozoic oceanic backarc substrate. Critical to the continent creation process was swamping of the substrate by Cambrian/Ordovician quartzose turbidites and segmentation into microplates. Each plate then progressively deformed as an independent (behind-the-arc) orogenic belt during microplate subduction, characterised by separate granite- dominated, magmatic belts.

Pene-contemporaneously with collapse of some backarc segments in the Silurian-Devonian, the leading-edge, outer arc continued splitting during ongoing slab retreat to produce a series of outboard-migrating silicic magmatic belts and volcaniclastic rift basins atop the older backarc turbidites. Intermittent interruption by transient shortening events (tectonic switching), probably associated with episodic arrival and subduction of buoyant oceanic crustal fragments (plateaus), progressively converted the extending crust into a series of outboard migrating orogenic belts until the Late Carboniferous. In the Early Permian, the arc finally migrated onto the long-lived (Cambrian- Carboniferous), leading-edge, accretionary prism, which also underwent shortening, followed by voluminous granite plutonism in the Permo-Triassic, representing terminal orogeny activity in the Tasmanides. Throughout the orogenic history, minor primitive basalts from each extensional stage retained their intraoceanic arc/backarc chemical signature. This reflects protracted and pronounced lithospheric stretching, which was the cause of semi-continuous mafic underplating and ultimately widespread granite generation in the Tasmanides.

Based on the above, the critical elements of rapid continental creation in this example of plate-convection mode are: growth of a vast oceanic substrate; flooding with detritus; segmentation and formation of independent subduction zones; leading-edge protracted extension, and repeated "tectonic switching" between extension and compression to build outboard migrating arcs.