Abstract

Difficulties in correlating the Cambrian magmatic, depositional, structural and metamorphic events along the proto-Pacific margin of East Gondwana have led to subdivision of the region into the Delamerian, Lachlan and Tyennan Orogens in Australia, the Ross Orogen in Antarctica and the Takaka Terrane in New Zealand. As a result, the Cambrian tectonic evolution of the region is poorly understood. We present here a revised lithostratigraphic section from the late Early to Mid-Cambrian rocks exposed near Stawell in western Victoria, which is used as the basis for correlating geological events in East Gondwana. These data show that the Cambrian tectonic evolution of East Gondwana's >4000 km long proto-Pacific margin involved predominantly compressional orogenesis separated by major short-lived extensional events at c. 516–514 and 504–500 Ma. The most significant extensional event, at c. 516–514 Ma, involved extensive slab rollback along the proto-Pacific margin in response to major changes in global plate motions and plate-boundary stresses following the termination of East–West Gondwana collision. Partial subduction of a ribbon of buoyant continental crust led to localized subduction-zone failure and obduction of the young hot forearc lithosphere in Tasmania at c. 510 Ma. Collision of the continental ribbon also significantly modified the architecture of the proto-Pacific margin and ultimately controlled the extent of the second major extensional event associated with slab rollback at c. 504–500 Ma. Tectonic evolution of the proto-Pacific margin of East Gondwana thus involved the complex interaction between convergent-margin processes and collisional orogenesis.