Conductive structures in southernmost Africa: a magnetometer array study

Summary A magnetometer array study in 1971 led to the discovery of a zone of electrically conductive material in the crust or upper mantle, elongated east—west and located beneath the Cape—Karroo Basin of South Africa. In 1977 a second array study was made, with 53 magnetometers distributed over the tip of Africa south of 30° S. This array recorded magnetic storms and substorms which provided signals of good amplitude in the period range 24—158 min and of various polarizations. Magnetograms for three time sequences are presented and used in locating the edges of the conductive region. Six sets of maps of Fourier transform amplitudes and phases, at periods for which the events supply adequate energy, show anomalies related to currents flowing in three conductive structures. These are the continental edge structures off the south-east and west coasts, and an intracontinental conductive strip running approximately east—west from coast to coast. The use of several magnetic variation events, with various phase differences between X and Y components, proves highly effective in isolating the intracontinental conductor from the continental edge (including oceanic) induction effects. The former is named the Southern Cape Conductive Belt and has been well mapped from the Fourier transform maps and the magnetograms. Anomalous fields, normalized to the horizontal field at a station within the craton, are used to locate the induced currents where they cross three profiles, and to yield maximum depth estimates between 46 and 52 km. The Southern Cape Conductive Belt is thus in the crust or uppermost mantle. It correlates closely with a static magnetic anomaly, whose source must lie above the Curie isotherm and so no deeper than 38 km. The Conductive Belt is strongly related to geochronologic provinces, and lies outside the edge of the Namaqua—Natal Mobile Belt which forms the edge of the craton. Heat-flow data indicate that the Conductive Belt is probably not thermally related. It is provisionally associated with an accumulation of oceanic crustal rocks, containing hydrated minerals such as serpentine, in either a Proterozoic zone of subduction or a marginal sea of Proterozoic Gondwanaland.