- © 2006 Geological Society of South Africa
The Broodkop shear zone is an approximately 1 km-wide, vertical, northeasterly-trending, mylonitic shear zone that separates upper amphibolite- to granulite-facies gneisses (≥670°C, 4.5 to 5.5 kbar) from mid-greenschist-facies (~400°C) greenstones in the southeastern part of the Vredefort dome. The mylonitic fabric transposes 3.09 to 3.08 Ga peak-metamorphic migmatitic structures but shows progressive overprinting of shear-related amphibolite-facies assemblages by greenschist-facies assemblages; indicating initiation of deformation close to the metamorphic peak and continued shearing during cooling. Shear-sense indicators are consistent with southeastern-side-up dip-slip movement. Removal of Late Archaean and Proterozoic rotation effects, particularly those associated with formation of the Vredefort dome, suggests that the shear zone formed as a shallowly northwesterly-dipping extensional feature associated with exhumation of the mid-crustal amphibolite-to-granulite-facies footwall rocks and transport of the greenschist-facies hanging wall rocks to the northwest. Several kilometres of throw along the shear zone is suggested by the juxtaposition of rocks with markedly different metamorphic grades. Assuming an initial shallow dip for the shear zone, this suggests in excess of 10 km of dip-slip movement, making the Broodkop shear zone a crustal-scale feature. The timing and geometry of the shear zone suggest that it may represent a mid-crustal manifestation of the rifting event that led to the deposition of bimodal volcanics and associated sediments of the 3074 ± 6 Ma Dominion Group. The Broodkop shear zone provides an appropriate mechanism to explain how these volcanics could be deposited unconformably on mid-crustal gneisses within only a few million years of the high-grade metamorphism. It provides a link between the final major pulse of greenstone-TTG formation in the core of the Vredefort dome and the initiation of a series of large sedimentary and volcanic basins that dominated the Late Archaean history of the Kaapvaal craton.