Geophysical study of the continental margin of southern Africa

Abstract

Marine gravity and magnetic anomalies determine a model for the boundary between oceanic and continental basement off southern Africa. A nearly linear high-amplitude positive magnetic anomaly (about 300 to 800 γ) is coincident with an isostatic gravity anomaly where the Agulhas fracture zone is bounded to the north by the African continent. The magnetic anomaly is interpreted as a magnetic “edge effect” separating oceanic basement in the south from continental basement in the north. Where deep ocean lies on either side of the fracture zone, negative magnetic anomalies occur that are modeled by assuming a broad zone of zero magnetization within the fracture zone. Mesozoic magnetic lineations M0 (∼108 m.y. B.P.) through M12 (∼128 m.y. B.P.) are adjacent to the western margin of southern Africa (Cape Sequence). West of the Orange River, the magnetic anomaly amplitudes are attenuated, which is attributed to a partial demagnetization of basement rocks due to an increase in sediment overburden. A prominent magnetic anomaly (anomaly G) borders the Cape Sequence on its landward side and is coincident with an isostatic gravity anomaly. Anomaly G is interpreted as a magnetic edge-effect anomaly separating oceanic from continental basement, similar to the magnetic anomaly associated with the Agulhas fracture zone. Southwest of Capetown, anomaly G is located on the continental slope, implying that continental basement has subsided to form the slope. Farther north, anomaly G lies nearly 125 km landward of the shelf break; this implies that the shelf break in the north was formed by a pro-grading of sediment over an oceanic basement, in general agreement with available seismic (sonobuoy) measurements.