THE INITIATION OF THE SOUTH ATLANTIC OCEAN BASIN

The initial stage of opening of the South Atlantic Ocean basin was at its southern extremity. There the process commenced with stretching of the continental crust between Africa, South America and Antarctica in the area that was to become the Weddell Sea, Natal embayment and Falkland Plateau. This was associated with the long-controversial, but now well documented, counterclockwise rotation of the Ellsworth-Whitmore mountains crustal block of Antarctica and the clockwise rotation of the Falkland/Malvinas Islands crustal block (‘Lafonian microplate’) of South America. All this took place prior to the formation of any oceanic lithosphere, as in the case of the rotation of the Danakil horst during stretching in the Afar triangle. We have suggested (Dalziel et al., Earth and Planetary Science Letters, 2000) that the late Permian-Triassic formation of the Gondwanide fold belt, the initial stretching and fragmentation of Gondwana in that region, and the Jurassic (ca. 180 Ma) Karoo-Ferrar LIP were all the result of a plume impinging on a downgoing Panthalassic margin slab, thermo-mechanically breaking through that oceanic slab, updoming and fragmenting the continental lithosphere, and finally manifesting itself as the Karoo-Ferrar LIP. The best candidate for a present day hotspot related to such a plume is Bouvet Island. The interaction between a plume and a downgoing slab of oceanic lithosphere has now been imaged in the Pacific Northwest of the United States by high resolution tomographic images obtained using shear and and compressional data from the ongoing USArray deployment (Obrebski et al., Geophysical Research Letters, 2010). This presents an opportunity to reassess the potential involvement of a single plume head in both the formation and the dismemberment of the Gondwanide fold thrust belt as well as the stretching and block rotation associated with the initiation of the South Atlantic Ocean basin.