Arc-continent collision and orogenesis in western Tasmanides: insights from reactivated basement structures and formation of an ocean-continent transform boundary off western Tasmania

Crustal architecture in formerly contiguous basement terranes in SE Australia, Tasmania and northern Victoria Land is a legacy of late Neoproterozoic-Cambrian subduction-related processes, culminating in formation of the Delamerian-Ross orogen. Structures of Delamerian-Ross age were subsequently reactivated during late Mesozoic-Cenozoic Gondwana breakup, strongly influencing the geometry of continental rifting and providing clues about the origins and configuration of the pre-existing basement structures. An ocean-continent transform boundary developed off western Tasmania follows the trace of an older Paleozoic strike-slip structure (Avoca-Sorell fault) optimally oriented for reactivation during final separation of Australia from Antarctica. This boundary cuts across rocks preserving an earlier record of arc-continent collision during the course of which continental crust was subducted to mantle depths and Cambrian mafic-ultramafic island arc rocks were emplaced westwards over late Neoproterozoic passive margin sequences deposited during Rodinia breakup. Following a change to transcurrent motion along the Gondwana margin, Tasmania migrated northward along the Avoca fault system. This process arrested with entry of continental crust (Selwyn block) into a subduction zone located along the Heathcote-Governor fault systems, precipitating a second collision event accompanied by south-vergent thrusting, blueschist facies metamorphism and tectonic reworking of the already accreted Cambrian arc-forearc complex and underlying passive margin sequences.