HISTORY OF OROGENY AND ACCRETION ALONG THE EASTERN AUSTRALIAN MARGIN OF GONDWANA

Assembly of eastern Australia involved inversion of continental margin sequences and intracratonic rifts, collision with Proterozoic continental fragments and island arcs, and accretion of submarine fans. This occurred along the Pacific margin of Gondwana from early Paleozoic to Mesozoic times, and produced crust of continental thickness and character. Recent geochronology, particularly Ar-Ar dating of white mica, redefines the timing and nature of the Delamerian Orogeny (DO) and Lachlan Orogeny (LO) on mainland Australia and Tasmania. During and after Rodinia-breakup (750-700 Ma) the Australian cratonic margin underwent extension and continental margin sedimentation followed by: 1) Jura-style folding and basement reactivation (proto-DO, 550-530 Ma) in the southern Flinders Ranges; 2) partial rifting and deposition of the Kanmantoo Group (532-526 Ma); 3) peak Delamerian Orogeny (520-490 Ma) including high-T metamorphism in southern Adelaide Fold Belt and Mt Painter Inlier, inversion of the Kanmantoo basin, ophiolite obduction and deformation/metamorphism in Tasmania, and shear zone reactivation in Proterozoic basement blocks (Broken Hill and Olary); 4) island arc volcanism, subduction roll-back, back-arc spreading, and turbidite deposition in western Lachlan (505-490 Ma); 5) post-orogenic (DO) magmatism and erosional exhumation (490-460 Ma), along with extensive turbidite fan deposition in the proto-LO; 6) peak LO (450-380 Ma) comprising convergence and oceanic micro-plate interaction with E- (central Lachlan) and W- (western Lachlan) directed subduction, coupled with local high-T metamorphism, W-directed collisional shortening in NE Tasmania, transpressional reactivation of Delamerian and Proterozoic shear zones, magmatism (435-400 Ma), strike-slip translation of the central Lachlan and rifting of eastern Lachlan (420-400 Ma), and deep-level thrusting in eastern Lachlan (385-380 Ma); 7) post-orogenic magmatism in western Lachlan (365-360 Ma) and E-directed deformation in eastern Lachlan (360-340 Ma); 8) post-orogenic magmatism in eastern Lachlan (340 Ma). A detailed study in the western LO combining palinspastic restorations and Ar-Ar dates of faults suggests a decollement propagation rate of ~8 mm/yr.