ABSTRACT
More than 1,426 mi (2,300 km) of 48-trace, 12-fold seismic reflection profiles were used to examine the nature of the continent-ocean boundary off southwest Africa. 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. Although structural details are obscured by a thick, seaward-dipping wedge of pre-All (pre-late Early Cretaceous) overburden, oceanic crust at the presumed contact is not older than magnetic anomaly M9 (126 to 121 m.y.B.P.).
North of the Orange River, a pronounced hinge in continental crust correlates with magnetic anomaly G of Rabinowitz (1976). Seaward of the hinge, a complex fault-block terrane is evident. Oceanic basement can not be traced with any certainty landward of anomaly M4.
Intracontinental stretching and associated volcanism appear to have been important in the early history of the Cape Basin. Rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea-floor spreading. Related extrusives form at least part of the pre-AII wedge just seaward of the hinge zone. Our 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.
