Pre-transvaal wrench tectonics along the northern margin of the witwatersrand basin, South Africa

Abstract

The pre-Transvaal tectonic history of the northern margin of the Witwatersrand basin is re-evaluated in terms of wrench fault tectonics. Three major east-northeast trending left-lateral wrench fault systems are recognized: the Sugarbush, Rietfontein, and Kromdraai systems. They are characterized by pull-apart basins (e.g., Bezuidenhout Valley and Kromdraai grabens), narrow slivers of displaced stratigraphy (e.g., Langermanskop, Rietfontein block) and marginal, convex upward thrusts. Changes in fault orientation result in areas of compression passing laterally into areas of extension over short distances. Splays terminate individual fault systems. Regions of wrench fault overlap are characterized by fold domains (e.g., West Rand syncline, East Rand basin), with folds characteristically trending NW-SE. The left-lateral strike-slip system appears to be of craton-wide scale and probably extends at least as far as the Barberton greenstone belt. The wrench system appears to have come into existence during Witwatersrand times, but experienced its major movement during middle Ventersdorp times. The left-lateral strike-slip system is believed to be related to rifting along the Ventersdorp trough, and to right-lateral faulting along the southwestern margin of the Witwatersrand basin. These tectonics occurred prior to a post-Transvaal period of folding and thrusting related to the emplacement of the Vredefort dome. The latter is responsible for the penetrative regional cleavage and bedding sub-parallel faults throughout the area concerned. The structural model proposed here has implications for the existence of possible outliers of mineralized upper Witwatersrand stratigraphy.