The significance of the Vredefort Dome for the thermal and structural evolution of the Witwatersrand Basin, South Africa

The Vredefort Dome represents an area of significant (similar to 10 km) str uctural uplift within the central parts of the economically important Witwa tersrand Basin. Its rocks experienced higher grades of metamorphism than th e equivalent stratigraphic horizons exposed around the periphery of the bas in. Recent studies of this medium- to high grade metamorphism, as well as n ew evidence concerning the origin of the dome, have contributed to a metamo rphic model for the Witwatersrand Basin as a whole. This evidence shows tha t the gold-bearing strata experienced at least two metamorphic events at ca . 2 Ga. The unusually high strain rate and shock deformation features expos ed in the rocks of the dome rule out an endogenous origin by tectonic or di apiric processes. Recent work on these features has shown that the dome is best explained as the central uplift of a large, 250-300 km diameter, 2023/-4 Ma old meteorite impact structure, the extent of which closely correlat es with the present-day limits of the Witwatersrand Basin. Impact-related d eformation features in the Vredefort rocks facilitate the separation of met amorphic textures developed during a pre-impact event associated with the 2 .05-2.06 Ga Bushveld magmatism, and textures developed during a slightly lo wer-grade, post-impact, static overprint. The post-impact overprint decreas es in intensity outwards from the dome. It is attributed to the massive dis turbance of the thermal structure of the crust by impact-induced exhumation , and to shock heating of the rocks as a consequence of the impact event.