PALEOZOIC CONTINENTAL ACCRETION AND OROGENY IN THE TASMANIDES OF EASTERN AUSTRALIA

Orogeny in the Tasmanides (Lachlan, Delamerian, and New England Orogens) added 30% to the surface area of Australia between 520 and 260 Ma. Accretion involved inversion of a continental margin sedimentary sequence, collision with continental fragments and island arcs, accretion of a thick submarine turbidite fan, thickening/imbrication of oceanic crust, and partial melting. Continental crust was constructed from recycled continental detritus and juvenile material in an oceanic setting as subduction alternated from phases of roll-back to phases of intense coupling. Recent structural syntheses and detailed geochronology has redefined the timing and nature of the Delamerian and Lachlan orogenies. The inversion of the Neoproterozoic-Cambrian passive margin of East Gondwana occurred during the Delamerian (Ross) Orogeny, which was initiated by subduction that started near the end of Pan-African suturing of east and west Gondwana. The Delamerian started with highly oblique convergence between Gondwana and the Pacific between 550 and 520 Ma during which a Jura-style fold-thrust belt and basement reactivation occurred followed by opening deep rift basins (Kanmantoo Rifts) that rapidly filled with clastic sediment. At ~520 Ma subduction became margin orthogonal leading to the obduction of an island arc complex, high-grade metamorphism and major shortening. By 508-500 Ma the high-grade metamorphic rocks were exhumed beneath extensional shear zones. Extension was caused by steep west-dipping subduction after the collisional phase of the Delamerian Orogeny. Subduction roll-back between 508 and 460 Ma produced a back-arc-basin >1000 km wide, that became the basement for the 5-10 km thick continental detritus deposited in the Lachlan turbidite fan. The Lachlan back-arc-basin closed by double-divergent subduction between 450 and 340 Ma. Subduction formed three oceanic accretionary thrust-systems with opposing structural vergence. Episodes of magmatism were syntectonic and post-tectonic throughout the Tasmanides and reflect changing subduction geometries, areas of intense crustal thickening, basaltic underplating, and extension. The partial melting and magmatism within the thickened Lachlan back-arc-basin chemically and thermally maturated and stabilized the new continental crustal.