Wed 2nd March @ 12 pm, Rm 312.222

Abstract

There is probably no such thing as a “typical” passive margin, although the southern margin of Australia comes close.  On this margin an Upper Jurassic rift propagated through an ancient cratonic block, deep within a continent, culminating in the generation of oceanic crust and the eventual separation of Australia from Antarctica – albeit with a significant delay between early slow spreading and later rapid spreading.

 By contrast the western margin of Australia contains suture zones across which the final stages of continental accretion occurred relatively shortly before the onset of sedimentary basin formation, while the north west margin of Australia appears to have been located close to a continental margin throughout much of its history.  Rift basins have developed intermittently over a period of about 300 Ma on the western margin, culminating in the separation of Grater India from Australia in the Early Cretaceous, while a series of continental ribbons appear to have detached from the north west margin of Australia over a shorter time period.  The tectonic setting in which ribbons can separate from a continental margin remains enigmatic.

 The Carboniferous and Permian was probably the time at which the fundamental architecture of the rift system was established, but understanding the tectonics of this period is made difficult by poor imaging of these sections on seismic data – a result at of the depth at which they occur.  However, integrating observations from onshore basins and the margins of offshore basins is beginning to shed light on events in this period and may help to explain the origin of the large depocentre on the Exmouth Plateau in which significant thicknesses of Triassic sediment accumulated.

 Lower to Middle Jurassic rifting is clearly (and beautifully) imaged on several seismic data sets.  Although generally associated with the separation of Argoland, and formation of the Argo Abyssal Plain, it is much less uniform both in timing and location than is often assumed.  The orientation of extension across the margin is also variable, hinting at a much greater influence of the early stages of India-Australia rifting at this time.

 Upper Jurassic and Lower Cretaceous deformation is widely attributed to India-Australia separation, although deformation appears to be confined to two widely separated areas of the NW Shelf – the Exmouth sub-basin (close to the site of separation) and the Vulcan sub-basin (far removed from it).  Deformation in the Exmouth sub-basin is unusually short lived, associated with uplift and erosion, and extensive igneous activity.  Uplift and erosion is also marked on the western Australian margin, although extensional faulting is probably more pronounced.  A recently proposed mantle plume may explain a number of these observations, and has implications for heat flow and maturity of petroleum source rocks.

 The final enigma is the presence of a number of inversion structures of Pliocene to Recent age, mainly confined to the western margin, which suggests that it is not such a passive margin after all.