Abstract:
The Johannesburg Dome is a structural unit comprising a basement inlier of Archaean granitoids with greenstone remnants and surrounding radially outwards dipping supracrustals. In this study, the tectonic fabrics and macrostructures of the basement inlier and immediately surrounding supracrustals were documented in order to constrain the structural evolution of the Johannesburg Dome. Following generation of the basement granitoids in the Mid Archaean (involving at least three tectono-metamorphic events) and development of the Witwatersrand and Ventersdorp Supergroup depositories, a Late Archaean contractional deformation occurred. This event produced a series of northward verging thrusts and has been recognised throughout the study area. Lithostratigraphic constraints place the timing of this thrusting between 2709 and 2600 Ma allowing correlation with the Limpopo Orogeny. Deformation features related to the thrusting dominate the rocks of the basement inlier and the West Rand Group rocks along its' southern margin. The Northcliff Promontory (a ridge West Rand Group rocks along the southern margin of the basement inlier) was mapped in detail to serve as a microcosm for the deformation of the West Rand Group in the Johannesburg area. The anomalous strike of the promontory is a result of northward directed thrusting over the granitoids followed by 2 Km translation along an ENE striking, dextral sense, strike slip shear zone. A conjugate shear zone set (NNE sinistral strike slip, NNW dextral strike slip) developed in response to north-south orientated regional compression. The shear zones postdate the northward verging thrusts but pre-date the deposition of the Black Reef Quartzite Formation. Following deposition of the Late Archaean\Early Proterozoic Transvaal Sequence, a second northward vergent thrust event occurred. This event was accommodated by bedding parallel slip in the Black Reef Quartzite Formation and is attributed to reactivation of basement thrust structures. Arching of the crust to form the dome structure was probably due to thermal activity associated with the development and emplacement of the Bushveld Igneous Complex. This arching event caused gentle tilting of the rocks above the BRQF unconformity but does not appear to have significantly influence the attitude of the older units.
