Search

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 >4000km 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. [ABSTRACT FROM AUTHOR]

Neogene collision between the Australian, Eurasia and Pacific plates was coeval with the growth of major depocentres in the Timor Sea. Seismic cross-sections across these troughs identify a characteristic architecture: all three lie inboard of diffuse horst-like structural highs and all mimic older depressions in the top-Permian and upper-Jurassic surfaces. Distortion of pre-tectonic (Aptian-Oligo-Miocene) sequences indicates apparent trough subsidence was coupled to uplift of outboard highs. Rudimentary cross-section restoration identifies negligible strike-normal length change. We interpret apparent Neogene subsidence to reflect amplification of basement topography. This was not accommodated by discrete structural inversion. Rather, landward propagation of contractional strain caused continuous vertical amplification of basement topography. At shallow levels, normal faulting accommodated flexure and thin-skinned collapse of detached sedimentary cover, permitting it to shed away flora structural highs and pond in adjacent troughs. We infer that shortening of the North West Shelf accommodated oblique convergence between Australia and the Eurasia-Pacific arc and speculate that the transcurrent component of this deformation was partitioned well outboard. Neogene modification is interpreted to reflect the earliest stages of collisional orogenesis. [ABSTRACT FROM AUTHOR]