Geological Setting

The 80-90-km-wide Vredefort Dome (Gibson and Reimold 2001a,b) is located in Archean and Paleoproterozoic rocks of the Kaapvaal craton of southern Africa (Fig. 1). The Dome comprises a central core of about 4550 km diameter that is composed of poly-deformed, pre-3.1 Ga, Archean granitoid gneisses and granites, with fragments of upper amphibolite to granulite facies, mafic and felsic ortho- and paragneisses. The core is surrounded by a circa 20-km-wide collar of greenschist to amphibolite facies, metasedimentary and metavolcanic, Late Archean to Paleoproterozoic (ca. 3.1-2.15 Ga) strata of the Dominion Group and the Witwatersrand, Ventersdorp, and Transvaal supergroups (Gibson and Reimold 2001a). The southern parts of the Dome, as the southern part of the Witwatersrand Basin, are largely covered by Phanerozoic sedimentary rocks and dolerite intrusions of the 300-180 Ma Karoo Supergroup (Fig. 1, 2).

The Dome is surrounded by a 50-70 km wide rim syncline known as the Potchefstroom Synclinorium (Fig. 3), which is characterized by shallow dips of the Transvaal Supergroup and older strata. The Transvaal Supergroup strata are affected by kilometer-scale open folds that are tangentially arranged around the Dome (Simpson 1978). Ductile shear zones with associated meter-scale folds and a cleavage that displays centrifugal vergence in relation to the Dome are found in Transvaal Supergroup rocks along the northern margin of the Witwatersrand Basin, as far from the Dome as 150-200 km (McCarthy et al. 1986, 1990; Gibson et al. 1999). Intercalated with the collar strata of the Dome are intrusions that have been related to various magmatic events, including the emplacement of the 2.06 Ga Bush veld Complex and the 2.7 Ga Ventersdorp extrusive event.

300 km cfiametei

2SCE

300 km cfiametei

Transvaal Supergroup

Pretoria Group "Tl C huni es poort Group

Fig. 3. Large-scale tectonic structure (anticlinal/synclinal structures) in the region of the Witwatersrand Basin and in the environs of the Vredefort Dome. The Rand Anticline is generally taken as the northern limit of the Witwatersrand Basin. It also represents the northernmost limit of known occurrence of pseudotachylitic breccia (personal observation, WUR). Modified after Therriault et al. (1997). The position of the Potchefstroom Synclinorium is marked by the wide occurrence of Chuniespoort strarta in the environs of the dome.

Pretoria Group "Tl C huni es poort Group

Ventersdorp Supergroup Wijwalersrand Supergroup "7" | Dominion Group Basement

Transvaal Supergroup

2ffS

Fig. 3. Large-scale tectonic structure (anticlinal/synclinal structures) in the region of the Witwatersrand Basin and in the environs of the Vredefort Dome. The Rand Anticline is generally taken as the northern limit of the Witwatersrand Basin. It also represents the northernmost limit of known occurrence of pseudotachylitic breccia (personal observation, WUR). Modified after Therriault et al. (1997). The position of the Potchefstroom Synclinorium is marked by the wide occurrence of Chuniespoort strarta in the environs of the dome.

One controversial issue has been the question of whether strata exposed in the crystalline core of the Vredefort Dome are upturned, or even overturned, as observed in the adjacent supracrustal strata of the collar. Hart et al. (1981, 1990), Tredoux et al. (1999), and others have subscribed to this idea of a "crust-on-edge" geometry of the basement rocks. The geophysical modeling of Henkel and Reimold (1998) and detailed metamorphic and structural work by, for example, Stevens et al. (1997), Gibson and Reimold (2000, 2001b), Gibson et al. (1998, 1999), Lana (2004), and Lana et al. (2003a-d) has provided much evidence against the crust-on-edge hypothesis. This hypothesis postulates that in the center of the Dome rocks of the lower crust and, possibly, even of the upper mantle (Tredoux et al. 1999) are exposed. This is, however, not supported by the modeling of basement uplift based on geophysical data and the metamorphic grades determined for rocks of the central core. Recent detailed, small-scale mapping of the exposed basement in the core of the Vredefort Dome by Lana et al. (2003a, c, and d) also did not provide any structural geological support for the so-called 'crust-on-edge' model. Rotation of the subvertical Archean fabric by impact tectonics is restricted to a zone of a few kilometer width just inward from the core-collar contact. Henkel and Reimold (1998) suggested only gentle upward flexing of the lower crust, perhaps with a maximum gradient of about 4 km.

The best estimate for the age of the Vredefort impact event of 2020 ± 5 Ma (Kamo et al. 1996; Moser 1997; Gibson et al. 1997; Spray et al. 1995) clearly distinguishes this impact event from the about 50 Ma earlier (2.06 Ga) emplacement of the Bushveld Complex - a finding that with regard to thermal and hydrothermal overprint on the Witwatersrand ores (see below) is of significance.

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