An array of 25 three-component magnetometers was operated in South-West Africa, Botswana and north-western Rhodesia from December 1971 to February 1972. Results from three periods of geomagnetic disturbance are presented. Magnetograms and maps of Fourier transform amplitudes and phases show a large anomaly in the vertical and northward horizontal variation fields over the period range 21-171 min. A well-defined reversal in the vertical field is observed between Outjo (OUT) and Sukses (SUK) on the western edge of the array. Transfer functions between mean horizontal field components and station vertical components show that a conductor that trends east-west in the western part of the array and curves north-eastward in the east underlies the array. A study of the half-widths of normalized anomalous profiles and the induction vectors indicates that the conductor is a crustal structure. It is most probably connected to the African rift system. A separation of the in-phase and quadrature-phase induction vectors indicates a structure with a high inductive reactance off the western edge of the array.
A zone of concentrated induced electric currents crossing parts of Zimbabwe, Botswana and South West Africa was discovered during a magnetovariational study conducted in 1972. In 1977, a second study was made with 27 recording magnetometers distributed across the width of South West Africa between latitudes 19 and 22's. Several geomagnetic disturbances were recorded with high recording efficiencies. Three of these time sequences were digitized for analysis. Magnetograms and Fourier transform amplitude and phase maps in the period range 22-128 min were used to delineate the westward continuation of the conductive structure revealed by the earlier investigation. The conductive zone runs approximately east-west from the Botswana border (21OE) to 17"E longitude. From here to the Atlantic coast it trends in a NE-SW direction. Anomalous fields, normalized to the horizontal field at a station recording the normal field, were used to obtain maximum depth estimates of around 45 km for the induced currents. Several deep Schlumberger soundings were done over the anomalous zone and the results showed that the conductive structure is, in places, only 3 km from the surface and that it has a resistivity of less than 20 Qm. The resistivity of the upper crust outside the structure ranges from 5000 to more than 20000Slm. Some 14 post-Karoo alkaline igneous complexes occur along the course of the resistivity anomaly. These intrusive complexes represent the youngest igneous activity in the Damara Orogenic Belt and were most probably emplaced along a line of weakness in the lithosphere. The resistivity anomaly would seem to delineate this line of weakness.
An array of 24 three-component magnetometers were operated in central South Africa during September and October, 1971. Resultsfrom three substorm sequences are presented. Magnetograms and maps of Fourier transform amplitudes and phases show a large anomaly in the vertical and northward horizontal variation fields over the period range 24-293 min. The vertical field has a maximum near Beaufort West and the horizontal field increases southward to a presumed maximum just south of the array. The Karroo sedimentary basin can contribute only a minor part of the anomaly and the principal result is the discovery of a major conductive structure under the Cape Folded Belt and southernmost Karroo. Transfer functions between mean horizontal field components and the vertical component show that induction is principally in phase with the normal, field. The effect of the Karroo sediments is seen in the quadrature phase induction arrows and is small compared with the in-phase induction anomaly. For this and other reasons the conductive body under the Cape Folded Belt and southernmost Karroo is provisionally placed in the upper mantle. Quantitative interpretation will follow further observations south of the 1971 array.
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