Crustal Structure at the Continental Margin South of South Africa

Abstract

Four gravity profiles over the continental margin south of South Africa are interpreted in the light of existing seismic, gravity and magnetic results. There is a marked difference in shallow and deep crustal structure between the rifted segment of margin facing the Atlantic Ocean and the margin offset facing the Indian Ocean. Both margin segments have a mass excess beneath the continental rise and a mass deficit beneath the outer continental shelf and continental slope, but this distribution is much more pronounced at the margin offset. It can be explained by a rapid transition from continental to oceanic crust which, in the Indian Ocean, probably results from the mode of formation of the margin (strike slip faulting). In the Atlantic Ocean the cause of the rapid transition is less obvious. The shallow structure at the continent‐ocean boundary changes around the margin. At the rifted segment, the sedimentary sequence of the continental rise north of 36° S gives way southwards to a basement high thought to be of volcanic origin. At the margin offset the Agulhas fracture zone appears to contain slivers of both oceanic and continental crust. The latter is in the form of a marginal fracture ridge. Beneath the continental shelf a high density lower crust may compensate for the low density sedimentary rocks in the Outeniqua Basin. Copyright © 1976, Wiley Blackwell. All rights reserved