Abstract

Processed gravity and aeromagnetic data covering the Witwatersrand basin and adjacent areas have been used to delineate the boundary of the basin and to investigate other features such as the Vredefort structure, the Bethlehem gravity high, and the Colesberg magnetic anomaly trend. A model is proposed in which the regional aeromagnetic and gravity signatures of these features can be related to the disposition of a semi-continuous mid-crustal magnetite-rich layer which is exposed or has a shallow suboutcrop in three regions: in the centre of the Vredefort structure, along the Kaapvaal craton boundary, and centrally through the craton northwards from Colesberg. The Bethlehem high is thought to be due to elevation of lower levels of the crust, along a NW-trending geanticlinal arch. The apex of this broad arch is termed the Vredefort axis and plunges northwestwards beneath the Witwatersrand basin. The Vredefort structure lies at the intersection of this axis and a major NNE-trending axis of crustal downwarp which also constitutes the long axis of the Witwatersrand basin. Two contrasting scenarios for the development of these major axes are discussed. With either scenario, genesis of the Vredefort structure should be integrated with the genesis and evolution of the Vredefort axis and with the tectonic evolution of the Kaapvaal craton.