rifted in the Jurassic, and the South Atlantic, Tasman Sea, and the East African margin broke up in the Cretaceous. By the end of the Mesozoic, India and Africa had begun to separate from Antarctica (Bois et al. 1982). The diachronous initiation of seafloor spreading along the present-day margin of the central North Atlantic Ocean is part of a larger trend that reflects the progressive dismemberment of Pangea (Withjack et al. 1998). During the Jurassic, Pangea broke up into Laurasia to the north and Gondwana to the south, and this peak in rifting (Muller et al. 2019) is reflected in an abrupt increase in paleocoastline (Kocsis and Scotese 2021) (Fig. 11). The breakup of Gondwana initiated in the Early Jurassic with initial rifting between East and West Gondwana being preceded by the emplacement of extensive plume-related flood basalts in southern Africa and the Transantarctic Mountains. East Gondwanaland (Australia, Antarctica, India, Madagascar, and the Kalahari craton of southern Africa) separated from West Gondwanaland (Africa, Arabia and South America) (Baillie et al. 1994; Gibbons et al. 2013). The second major phase of Gondwana breakup commenced in the earliest Cretaceous, when the South Atlantic Ocean began to open and Greater India commenced its northwestward rotation from Australia and Antarctica. This Early Cretaceous phase of seafloor spreading affected the Western Australian margin (Baillie et al. 1994). An example of a Cretaceous breakup unconformity is shown in Fig. 12.There was widespread basic volcanism along the southwestern margin of Australia related to breakup and the formation of oceanic crust (Norvick 2004). The Bunbury Basalt was extruded in the onshore Bunbury Trough of the southernmost Perth Basin. Aeromagnetic data over the Bunbury Trough shows basalt flows confined to paleovalleys on the breakup unconformity (Olierook et al. 2015) (Fig. 13). The basalt is columnar rather than pillow basalt, consistent with subaerial extrusion in paleovalleys. Paleodrainage appears to have been from south to north, which would have been similar to the south-to-north drainage system inferred to have existed between Antarctica and Australia during the Permian and Early Triassic (Olierook et al. 2015). By the Mid-Cretaceous, a series of rift valleys formed a broad rift-divergence zone between Australia and Antarctica in which thick accumulations of terrestrial and lacustrine sediments accumulated. Australia became isolated from Antarctica by the Oligocene (Baillie et al. 1994) (Fig. 10). Passive margins are found around many continents of the world. Examples include margins of the Atlantic Ocean, the Gulf of Mexico, and around Australia and Antarctica. They represent the transition between oceanic crust and continental crust which is not an active plate margin. They were constructed by sedimentation above continental rifts which created new ocean basins. Because there was no collision or subduction taking place in these particular zones, deformation and tectonic activity was minimal.2. Post-Paleozoic sedimentation a. Presence of non-marine sequences In this paper reference has already been made to non-marine sedimentation. This includes Late Paleozoic lacustrine and fluvial deposition, Carboniferous karst, Permian fires, numerous indicators of drying in Triassic strata, floral trends, and Cretaceous subaerial Figure 12. Cross-section extending from the crystalline basement of the Yilgarn Craton in the east to Mesozoic sedimentary rocks of the offshore Perth Basin in the west (after Crostella and Backhouse 2000). This cross-section shows an example of a Cretaceous continental breakup unconformity which truncates fault blocks of the underlying half-graben. From the Late Cretaceous, the southwestern margin of Australia became tectonically quiescent. The post-breakup passive margin strata show little deformation. DICKENS Flood Waters Lead to Seafloor Spreading 2023 ICC 458
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