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• W1527760338 abstract "The resistivity structure of the South African crust and upper mantle is being studied by deep and ultradeep electrical soundings (Schlumberger array). Deep electrical soundings are carried out by laying out cable for emission lines (maximum current electrode spacing AB of ∼40 km), while ultradeep soundings are made by using overhead telephone or power lines (maximum AB of ∼1000 km). Sounding results have been obtained on a variety of tectonic provinces that are given here with the age of the main metamorphic event shown in parentheses: the Rhodesian and Kaapvaal cratons (>2600 m.y.), the Limpopo Mobile Belt (>2700 m.y.), the Namaqua Mobile Belt (1000 m.y.), and the Damara Orogen (500 m.y.). The results show that the cratons are characterized by a highly resistive zone extending downward from the surface to a depth of 5–10 km depending on the locality. The average resistivity of this zone is about 100,000 ohm m with values varying between 30,000 and 400,000 ohm m. In contrast, the mobile belts invariably indicate the presence of a thick but only moderately resistive zone with a resistivity ranging from 2000 to 10,000 ohm m (average, 5000 ohm m) extending from the surface to a depth of 25–30 km. An important feature, deduced from the electrical results, is that this zone continues laterally into the cratons, where it underlies the highly resistive zone. Correlation of these electrical zones with geological information implies that first, the highly resistive zone in cratonic type terrain is associated with vast amounts of massive, strongly consolidated unfractured granitoid material in the form of granitic intrusions, gneisses, and migmatites, second, the moderate resistivities in mobile belts are due to the presence of intensely deformed, fractured, and highly metamorphosed rocks, and third, the lower crust under both cratons and mobile belts is metamorphic and fractured, consisting mainly of a refractory residuum which has been depleted of lighter granitic components. A surprising feature of the electrical sounding results is that a major portion of the upper crust (in mobile belts) and the entire lower crust to a depth of 25–30 km is only moderately resistive, and this provides strong evidence for the presence of free water in these zones. Calculations based on measurements of subsurface water resistivities and the electrical results have provided some crude constraints on the distribution of porosity in the crust. The highly resistive zone of the cratonic terrains has a porosity of less than 0.5%, but there is a severalfold increase in porosity at the transition to the moderately resistive zone which occurs at a depth ranging from 5–10 km. In the mobile belts the porosity varies from about 3% near the surface to 1% at a depth of 25 km. If water is present in the lower crust as is suggested by the electrical results, it may provide a means of explaining the origin of the conductive zone of variable conductance that has been recognized in the depth range 25–40 km. The preferred explanation at this stage is that the conductive zone forms by hydration, and it is shown that serpentinite (hydrated mantle rock) could account for its electrical properties. It is also tentatively suggested that water, by preferential movement, may determine the heat flow pattern observed in cratons and mobile belts. Finally, it is shown that the uppermost mantle is characterized by a thick, highly resistive zone which becomes more conductive as the depth and temperature increase. Its electrical resistivity suggests that it is dry." @default.
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• W1527760338 date "2013-03-19" @default.
• W1527760338 modified "2023-09-26" @default.
• W1527760338 title "Electrical Studies of the Deep Crust in Various Tectonic Provinces of Southern Africa" @default.
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• W1527760338 doi "https://doi.org/10.1029/gm020p0470" @default.
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