The Antarctic Ross and the Australian Delamerian orogenies are the consequence of stress transfer to the outboard trailing edge of the newly assembled Gondwana supercontinent. This tectonic reorganization occurred in the Early to Middle Cambrian on completion of Pan-African deformation and subduction along the sutures between eastern and western Gondwanan continental fragments. Before this, Neoproterozoic to Early Cambrian rocks in eastern Australia were formed in a passive margin and record dispersion of Rodinia with consequent opening of the proto-Pacific. Our new U-Pb and Rb-Sr geochronology shows that in the South Australian (Adelaide Fold Belt) domain of the Delamerian Orogen, contractional orogenesis commenced at 514 ± 3 Ma and persisted for ∼24 m.yr. until 490 ± 3 Ma, terminated by rapid uplift, cooling, and extension in association with posttectonic magmatism. Integration of new and published U-Pb and 40Ar-39Ar geochronology from the entire Ross-Delamerian belt shows that although both the Delamerian and Ross have a synchronous late magmatic and terminal cooling history, the Ross commenced its convergent orogenic history at ∼540 Ma. This was 25 m.yr. before Delamerian deformation began. During the Early Cambrian, eastern Australia was still in a state of extension (or transtension), with opening of the Kanmantoo Basin and associated anorogenic, largely mafic magmatism. This basin received sediment from the already exposed Ross Orogen to the south. The simultaneous first occurrence of strain fabrics and subduction-related magmatism (including boninite, granite, and andesite lavas) at ∼514 Ma in New Zealand, Victoria, South Australia, New South Wales, and Tasmania implies that the Delamerian Orogeny was driven by ridge-push forces transmitted on the initiation of westward-dipping subduction. Subsequent eastward slab rollback at 490 Ma may have occurred when the new slab had reached the transition zone at 650-km depth, resulting in upper plate extension and anorogenic Basin and Range–style magmatism in South Australia and Tasmania (Mount Read belt). The delayed onset of subduction in the Australian sector of the margin implies that westward motion of the Australian portion of eastern Gondwana continued to be accommodated during the late Early Cambrian by subduction or deformation along either the Mozambique Suture or at the northern end of the South Prince Charles Mountains–Prydz Bay suture. ; John Foden, Marlina A. Elburg, Jon Dougherty-Page, and Andrew Burtt ; Copyright © 2006 by the University of Chicago Press