Western margin of Australia: Evolution of a rifted arch system

The 4,000-km-long western Australian margin and adjacent ocean floor are probably unique among older (>100 m.y.) passive margins and adjacent oceans in having such a thin (<1.5 km) cover of sediments deposited since continental break-up in Late Jurassic and Early Cretaceous time. The oceanic seismic basement and the unconformity on the faulted continental surface at break-up (collectively, reflector R₄) are thus traceable in seismic-reflection profiles across the ocean-continent boundary, and in many places are a continuous surface. Drilling shows that the oldest oceanic crust adjacent to the margin is almost the same age as the oldest part of the continental break-up unconformity. Two types of margin are distinguished by the shape of R₄: stepped, in which R₄ is offset at the ocean-continent boundary by a long transform fault, and smooth, including the transition from normal ocean floor through oceanic upgrowths, called epiliths, that developed after the start of sea-floor spreading. The marginal plateaus of the western margin originated variously as epiliths or from the post-break-up subsidence of regions that originally lay between rifted arches. After 100 to 150 m.y. of rifting along a multiple rift valley arch system analogous to that of modern East Africa, with concomitant deposition in inter-arch and extra-arch basins, the northwestern margin was initiated by plate divergence 160 m.y. ago (Late Jurassic time) and the southwestern margin 125 m.y. ago (Early Cretaceous time). After break-up, a diachronous clay was deposited on the newly generated sea floor and behind the subsiding continental rim or half-arch (the former rift valley shoulder) in what is called a rim basin. The rim subsided below sea level 30 to 45 m.y. after break-up, and thereafter sediments were dispersed seaward across the entire margin.