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Abstract

When continents are thrown into extension a rift system evolves until eventually the continent breaks apart and a new ocean basin is opened. However, not all rift zones developed into an ocean. Some became inactive at an intermediate stage and are preserved as subsurface graben structures within continental interiors. But what controls the success or the failure of a rift system? During separation of South America from Africa about 130 Ma ago, the Equatorial segment of the South Atlantic Rift competed with the West-African Rift Zone. After 20 Ma of coeval rifting, extension in the West-African Rift stopped while the Equatorial Atlantic opened. Our platekinematic and numerical models show that the evolution of continental break-up is strongly influenced by the orientation of a rift system relative to its extension direction. The larger the angle between rift trend and extensional direction, the more force is required to maintain a rift system. But even single rift systems can show a surprising dynamic evolution - the centre of the rift, where the continental crust gets actively thinned through faulting, may not stay fixed during continental break-up, but migrate laterally. During this motion, highly stretched crustal blocks are transferred from one side of the rift centre to the other so that the resulting passive margins exhibit a strong crustal asymmetry. Such a relocation of a rift takes its time – during formation of the asymmetric Angolan and Brazilian margin pair, the rift centre migrated more than 200 km westward. This delayed continental break-up and the generation of oceanic crust by up to 20 Ma.