GONDWANIDE OROGENESIS IN EAST AUSTRALIA: CRATONIZATION OF AN ACCRETIONARY OROGEN

The 310-230 Ma Gondwanide orogeny developed along the pan-Pacific margin of Gondwana and marked the termination of the accretionary Neoproterozoic to Paleozoic Terra Australis Orogen. Orogenesis commenced at around 310 Ma with termination of activity along the long established magmatic arc. Over the next 40 m.y. the region underwent pulses of compression, including oroclinal bending, extension and basin formation, and crustal melting and S-type granite emplacement, prior to establishment of a new magmatic arc located within the old subduction complex at around 270 Ma. This arc was active until around 230-220 Ma and was associated with further deformation and metamorphism of variable intensity. The apparent eastward displacement of the magmatic arc between its pre-310 Ma and its post-270 Ma positions probably reflects foreshortening and westward displacement during oroclinal bending of the upper crustal lithosphere rather than a dramatic shift in the position of the subduction zone within an asthenospheric reference frame.

The orocline deformed the pre-existing tectonic elements of the orogen including a western magmatic arc, an adjoining forearc basin and an eastern subduction complex. The orocline is doubly vergent with the southern, Manning-Hastings segment having undergone up to 230° of counter-clockwise rotation and the northern Texas-Coffs Harbour segment showing clockwise rotation of at least 120°. Assuming an originally linear trend for the arc system, oroclinal bending has resulted in a minimum of 50% shortening in New England from an original length of at least 1200 km. This has in part been accommodated by some 300 km of lateral displacement of the arc onto the Gondwana foreland. This model requires a detachment within the lithosphere above which oroclinal motion and foreland displacement took place. The spatial correspondence of oroclinal bending with earlier extension and crustal melting suggests that thermal weakening of the crust may have facilitated development of a crustal decollement above which the orocline formed. The westward motion of the orocline onto the foreland during buckling and vertical axis rotation may have been buttressed by the rigid oceanic lithosphere of the Pacific plate restricting eastward motion of the arc system as it thickened.