THE DEVELOPMENT OF CONTINENTAL CRUST IN AN ACCRETIONARY OROGEN: EVIDENCE FROM THE PALEOZOIC MARGIN OF AUSTRALIA

Backarc and forearc basins have opened and closed in the western Pacific for the past 600 million years. A record of basin development remains for some basins while others are completely lost to subduction. Basins with extensive and thick sediment fans are more likely to be converted to continental crust. Eastern Australia evolved in Paleozoic-Mesozoic times through accretion of backarc basins, volcanic arcs, ribbon continents, and sediment fans with a complexity similar to the modern western Pacific. Eastern Australia was assembled through accretion of the Delamerian, Lachlan and New England orogens between ca. 520 and 260 Ma. The Lachlan was constructed within a >1500 km wide marginal ocean basin that formed due to rollback of the paleo-Pacific subduction zone. Lachlan accretion involved at least three subduction-accretion systems that shortened a massive turbidite fan and island arc complex. An east–verging thrust wedge in the western Lachlan formed in two stages between 455-439 Ma and 410-395 Ma. In the central Lachlan a SW-propagating thrust wedge was active between 440-395 Ma in front of a 440-410 Ma magmatic arc. The eastern Lachlan history includes formation (480-440 Ma) and shortening (400-340 Ma) of the Macquarie volcanic arc along with sedimentation, extension, and magmatism between these times. Extensional basins and inverted rifts are prominent in the central and eastern Lachlan Orogen. Syntectonic and post-tectonic magmatism throughout the Lachlan reflects subduction geometries, areas of crustal thickening, basaltic underplating, periods of extension, and stabilization of the crust. Metallogenic associations across the Lachlan Orogen are correlated with tectonic setting and the geodynamics of evolving basins, magmatic arcs, forearcs, and accretionary prisms. Maturation into continental crust in this accretionary system, therefore, involved multiple cycles of mafic magmatism, extension, sedimentation, metamorphism and partial melting of lower crust.