- © 2005 Geological Society of South Africa
Ever since Slawson (1976) suggested that the Archaean crystalline basement rocks in the core of the Vredefort dome southwest of Johannesburg might represent “crust-on-edge”, debate has existed about the extent to which the deep crust was exhumed by the ~2.02 Ga, impact-induced doming event. In a suite of papers since 1981, Hart and co-workers (Hart et al., 1981; 1990a; b; 2004; De Wit et al., 1992; Tredoux et al., 1999; Moser et al., 2001; Flowers et al., 2003) postulated that doming (and subsequent erosion) exposed a complete section through the crust of the central Kaapvaal craton, and that even rocks of the uppermost mantle are exposed near the centre of the dome. Their argument has been compiled based on:
geochemical traverses across the Meso-Archaean trondhjemite-tonalite-granodiorite (TTG) gneisses, which Hart et al. (1981; 1990a) interpreted as indicating a more refractory character towards the centre of the dome;
the presence, and distinctive chemical character, of a small body of ultramafic rocks (harzburgite or peridotite) found in the Beta-1 borehole near the “centre” of the dome, which were inferred as the deepest-exhumed levels of the section (Hart et al., 1990b; Tredoux et al., 1999); and
the alleged existence of a major, north-northeast- to northeast-trending structural discontinuity located southeast of the ultramafic rocks, commonly referred to as the Southeast Boundary Fault (SEBF), that separates them from lower-grade rocks to the southeast (Hart et al., 1987; 1990a).
Geophysical evidence has also been used in support of this model - a positive Bouguer gravity anomaly in the central parts of the dome (Maree, 1944; Stepto, 1990) was interpreted by Hart et al. (1990b) as indicative of the existence of dense upper mantle rocks close to surface, …