More than 1,426 mi (2,300 km) of 48-trace, 12-fold seismic reflection profiles. South of the Orange River, faulted blocks, which we interpret as rifted continental crust, can be traced seaward to the 3 km isobath. The transition to oceanic basement occurs in a zone 19 to 25 mi (30 to 40 km) wide beneath the continental rise. Oceanic crust at the presumed contact is not older than magnetic anomaly M9 (126 to 121 m.y.B.P.). Intracontinental stretching and associated volcanism appear to have been important in the early history of the Cape Basin. Interpretation of the seismic data suggests that the initiation of normal spreading in the Cape Basin postdates by 4 to 9 m. y. the Valanginian age derived from prevailing plate tectonic reconstructions of the South Atlantic.--Modified journal abstract.

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