Abstract
Abstract : Structural and geochronological data are used to assess the formation mechanisms of the Johannesburg Dome, a well-known but poorly understood regional structure central to the Kaapvaal Craton, South Africa. The Johannesburg Dome is nearly elliptically in map view and composed of Paleoarchean greenstone and Mesoarchean granitoid basement core surrounded by radial, outwardly-dipping rocks of the Meso- to Neoarchean Witwatersrand, Ventersdorp and Neoarchean to Paleoproterozoic Transvaal supergroups. It is generally agreed that the Johannesburg Dome formed by uplift of the core and tilting of the overlying sequences to their present-day geometry. However, the doming mechanism(s) and timing(s) of exhumation are debated and call for a reinvestigation of the kinematic and dynamic development of Johannesburg Dome deformation fabrics as well as the timing of their formation. Two key areas along the periphery of the Johannesburg Dome were investigated to address these issues: the Zwartkops outlier, a dome marginal klippe of Witwatersrand Supergroup rocks where the Mesoarchean basement-superstructure contact is well exposed, and the Krugersdorp Game Reserve. The Krugersdorp Game Reserve acts as a point of comparison with the geology at the Zwartkops outlier because it is located ~ 15 km away from the Mesoarchean basement-superstructure contact. Three distinct deformation events are recognised at both areas; D1, occurred prior to deposition of the Transvaal Supergroup and is expressed as subvertical to steeply E-W dipping bedding planes and contacts that juxtaposed lower Witwatersrand and middle Ventersdorp supergroups rocks. D1 possibly formed during E-W shortening. These steep N-S striking bedding planes were subsequently transposed into gently, W-plunging F2 folds with associated gently S-SW-dipping S2 axial planar cleavage. This D2 event is related to top-to-the-south or SW extensional shearing that also predated deposition of the Transvaal Supergroup and is exemplified by a newly discovered 100 m-scale overturned recumbent F2 fold at Zwartkops. D3 post-dated deposition of the Black Reef Formation and involved N-S shortening that formed open, upright, gently W-plunging folds that deform basal units of the Transvaal Supergroup, and subvertical shear zone fabrics in the underlying Mesoarchaean basement. Type 0 fold interference is therefore recorded between D2 and D3 structures and Type 1 fold interference between D1 and D2 or D3 structures if D1 represents a period of folding...
M.Sc. (Geology)