A CRUSTAL GEOELECTRICAL MODEL FOR SOUTH AFRICAN PRECAMBRIAN GRANITIC TERRAINS BASED ON DEEP SCHLUMBERGER SOUNDINGS

Abstract

Three crustal Schlumberger soundings on widely separated granitic basement areas were recently completed. One sounding was carried out on the 1000 my old metamorphic complex of Namaqualand, a second on the 2600 my old Rhodesian Craton, and the third on the Cape‐Vaal Craton, also of 2600 my age. The fact that the sounding curves are very similar in shape suggests that the sounding data reflect vertical changes in resistivity at crustal depths. This contention is substantiated by an analysis of the results from several measuring stations for one of the soundings. Interpretation of the field measurements leads to a model which has the following characteristics: The crust has a maximum resistivity not exceeding [Formula: see text] ohm‐m at a depth of less than 10 km. Its transverse resistance varies from 5 to [Formula: see text]. The resistivity then decreases progressively to reach a value of 700–1300 ohm‐m at a depth of 16–20 km, after which the resistivity does not decrease appreciably until a depth of at least 200 km is reached. The finding that resistivities as low as 700 ohm‐m exist at a depth of 16–20 km is at variance with Brace’s 1971 laboratory model for the stable crustal area of the eastern U.S. However, this zone of low resistivities coincides with low velocity layers reported by Bloch et al. (1969), and it is suggested by the authors that hydrated rocks near the base of the sialic layer are the cause of the anomalous zone.