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

Abstract

Audiofrequency magnetotelluric soundings in the frequency range 0.016–128 Hz have been conducted to investigate the structure of the Mana Pools basin in northern Zimbabwe, a Karoo sedimentary basin in the Zambezi valley. Data from nine tensorial soundings along a 40 km N‐S traverse from the River Zambezi to the Zimbabwe craton, and a further sounding on the escarpment, have been processed giving the sounding curves required for preliminary modelling studies. A variety of l‐D inversion algorithms have subsequently been applied to these processed data sets to yield interpretive models. An approximate 2‐D geoelectrical section has been derived by collating the 1‐D models. A marked and rapid contrast in subsurface resistivity distribution between the escarpment and the basin is apparent. Within the basin, the results show the presence of a top sequence of variable thickness (up to 10 km) overlying a more resistive (0.1 S m−1) unit that may represent the geoelectrical basement. These top sequences, which have tentatively been interpreted as sedimentary formations with different stratigraphic units, consist of highly conducting (0.3 S m−1 to >1 S m−1) materials overlain by a slightly more resistive (typically 0.2 S m−1) unit, in some cases with another good conductor adjacent to the surface. The data also suggest a rapid thinning of the top sequences close to the escarpment, in contrast to the more gradual decrease of the sediments in previous geophysical models for the area. 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. Copyright © 1993, Wiley Blackwell. All rights reserved