Mesozoic Rifting in SW Gondwana and Breakup of the Southern South Atlantic Ocean

Abstract

The opening of the South Atlantic Ocean in the Early Cretaceous was only the final stage of the complex rifting process of SW Gondwana. In this contribution we reassess the chronology of Mesozoic basin formation in southern South America and Africa and integrate it in the long-term rifting and breakup history of SW Gondwana. During the Triassic, after the Gondwanides orogeny, plate-scale instabilities produced intracontinental rifting in Africa, and retro-arc extension on the SW-margin of Gondwana. This process was followed and accentuated by the impingement of the Karoo plume in the Early Jurassic, which triggered rifting in East Africa and ultimately produced the breakup of Eastern from Western Gondwana in the Middle Jurassic. Retroarc extension continued affecting the paleo-Pacific margin, with emplacement of the Chon Aike magmatic province in the Patagonian retro-arc during the Early-Middle Jurassic. By the Late Jurassic retroarc rifting reached a point of oceanic crust accretion, with the establishment of the Rocas Verdes back-arc basin in southern Patagonia, together with the formation of the Weddell Sea further south, between the South American plate and Antarctica. The core of the Late Paleozoic Gondwanides orogen, between southern South America and Africa, was subjected to oblique rifting at this time and produced the Outeniqua and Rawson/Valdés basins. This area was the locus of extension and oceanization in the Early Cretaceous associated with a rotation of the stress field from NE-SW to E-W extension. The formation of the South Atlantic Ocean resulted from lithospheric extension and was accompanied by extensive intrusive magmatism and extrusive flood basalts identified as seaward dipping reflectors (SDRs), which were emplaced diachronically from south to north, along different segments along both conjugate margins. These volcanic rocks form the South Atlantic Large Igneous Province. The chronology of the South Atlantic opening and the magmatic sources and processes associated with SDR formation remain interpretative since they have only been studied on seismic data but are still undrilled, hence scientific drilling will be key to unravel many of these unknowns.