Magnetometer array studies and deep Schlumberger soundings in the Damara orogenic belt, South West Africa

Abstract

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.

Making and Breaking of a Continent: Following the Scent of Geodynamic Imprints on the African Continent Using Electromagnetics

“…A decade later, Losecke et al (1988) detected some highly conductive layers in the Lower Zambezi Valley and in the Zambezi Mobile Belt, for which the authors suggested a link to the conductive structures of the Damara Belt (e.g. de Beer et al, 1982bBeer et al, , 1976Beer et al, , 1975. These early MT measurements were supplemented by a first audio-magnetotelluric profile farther west through the Mana Pools Basin (Whaler and Zengeni, 1993).…”

“…In addition to deep geo-electric experiments, magnetometer array studies were conducted over extensive regions (de Beer et al, 1982b). de Beer et al (1976) revealed a zone of concentrated induced currents at lithospheric depth crossing Namibia, Botswana and Zimbabwe based on a reversal in the vertical magnetic field at periods of ∼ 1, 000 − 10, 000 s. The inferred east-west trending zone of high electrical conductivity was found to be approximately 100 km wide in Namibia and widening towards western Zimbabwe to 250 km.…”

“…Within two different experiments the regional structure of the Damara Belt with its high conductivity belt (de Beer et al, 1982b;van Zijl and de Beer, 1983) was investigated and a high resolution study of one of the major fault zones, the Waterberg Fault / Omaruru Lineament (WF/OL) was conducted. MT data were collected on both regional (200 km) and local (20 km) scales, with the majority of the 110 sites focused on the local profiles, where site spacings ranged from 500 to 2000 m. The regional-scale MT model provides insight into the deep structure of the Damara Belt .…”

An audiofrequency magnetotelluric traverse across the Mana Pools Basin, northern Zimbabwe

“…found that their deepest, most conductive layer, had >1 S m p l conductivity. This is even larger than the typically high values inferred in other mobile belts of southern Africa (de Beer, Gough & van Zijl 1975;de Beer et al 1976de Beer et al , 1982ade Beer, van Zijl & Gough 1982b), although only the longitudinal conductance (thickness-conductivity product) of these good conductors is constrained. The sediments in the Mana Pools basin are thought to be several kilometres thinner (Orpen et af.…”

“…On the escarpment, the resistivity is higher at all depths. The Mana Pools basin thus appears to be a continuation of the good conductor running from Namibia into western Zimbabwe identified by de Beer (1982a), and provides a link to another beneath the Lower Zambezi basin to the east of Mana Pools, discovered magnetotellurically by Losecke, Knodel & Muller (1988). Comparisons between the geoelectrical model and other available results for the region are presented.…”