Considerations and Evidence Regarding the Size of the Vredefort Structure and its Spatial Relation to the Witwatersrand Basin

Originally, Vredefort researchers focused purely on the Vredefort Dome. Most early size estimates for the Vredefort Structure ranged from 70 (only the inner parts of the Vredefort Dome) to 140 (Dome plus surrounding Potchefstroom Synclinorium) kilometers (e.g., Bishopp 1941; Dietz 1961; Simpson 1978; Grieve and Pesonen 1992). Grieve and Masaitis (1994), however, estimated - on the basis of a map with concentric structural features published by McCarthy et al. (1990) - that the original diameter of the structure could have been as wide as 300 kilometers. Therriault et al. (1997) applied the spatial distribution information for various styles of deformation (occurrences of planar deformation features, shatter cones and impact-related brecciation) to empirically derived equations based on observations from other impact structures (Melosh 1989) to predict an original diameter for the Vredefort impact structure (Fig. 4), of similar magnitude. Henkel and Reimold (1996, 1998), who carried out integrated geophysical modeling of the whole Witwatersrand basin, derived at a

Transvaal Basin

Downfaulted Transvaal Supergroup

Transvaal Supergroup il'^'i Dominion Group il'^'i Dominion Group

Outer limil oi PDF occurence

Outer limil ol shatter cone occurence

Downfaulted Transvaal Supergroup

Transvaal Supergroup

Fig. 4. Scaling of the spatial distribution of various deformation features caused by the Vredefort impact event in the environs of the Vredefort Dome (based on work by Therriault et al. 1997).

diameter in the order of 250-300 km, and remote sensing applications by Phillips et al. (1999) are in excellent agreement with these figures. Generally, most workers have accepted that Vredefort is the remnant of an originally ca. 300-km-diameter impact structure.

Vredefort, with regard to its size, falls into the same category as two other terrestrial impact structures, namely the Sudbury and Chicxulub structures (e.g., Grieve and Therriault 2000), for which original diameters of 200-250 (or even larger) and ca. 200 km are favored, respectively (also Deutsch et al. 1995; Sharpton et al. 1996; Morgan and Warner 1999; Snyder and Hobbs 1999). However, Vredefort differs from the other two structures in that it does not contain any evidence for a significant coherent impact melt body and impact breccia fill. The Sudbury Structure still includes a ca 3.5 km thick impact melt body and even overlying fallback breccia (see below), and Chicxulub is more or less fully preserved underneath Tertiary sediments. Based on what we know about the Sudbury Structure, the currently exposed erosion level at Vredefort is considerably lower than that at Sudbury. Published estimates of the depth of erosion of the Vredefort Dome range from a few hundred meters (Martini 1991) to nearly 18 kilometers (Schreyer and Abraham 1978). Figures of 7-10 km have been favored in recent years (e.g., Henkel and Reimold 1998; Gibson et al., 1998; Gibson and Reimold 2000, 2001b), as the absence of a coherent impact melt body and impact breccia crater fill make the lower estimates highly unlikely. In addition, as the stratigraphic and intrusive units that originally lay close to the surface (namely Transvaal Supergroup and Bushveld Complex strata) are still preserved with only gentle dips in the environs of the Vredefort Dome, the upper limit is unrealistic as well. The prefered values are also generally consistent with scaling equations (Grieve and Pilkington 1996) that relate the amount of structural uplift (SU) to crater (rim) diameter D as SU = 0.086 D103. Also, if the diameter of the central uplift (Vredefort Dome to axis of Potchefstroom Synclinorium) is taken at 80-100 km, the empirical relation between diameter of central uplift (DSU) and total diameter of the impact structure (D) by Therriault et al. (1997), (Dsu = 0.31 D10), gives a 300 km diameter for the Vredefort impact structure.

In addition to the shock deformation features found in the rocks of the dome, several synclinal and anticlinal structures have been described from the region of the Witwatersrand Basin, and have been related to the Vredefort impact by McCarthy et al. (1986, 1990) and Brink et al. (1997, 1999, 2000). Beyond the Rand Anticline along the northern margin of the conventionally considered area of the Witwatersrand Basin (Pretorius et al. 1986), a series of smaller, and more periclinal, synclinal and anticlinal structures has been described (McCarthy et al 1990; Gibson et al. 1999), up to a distance of 150 km from the center of the Dome. Several workers (Spray 1998; Brink et al. 1997, 1999) have speculated that this series of morphological rings and/or alleged zones of enhanced development of pseudotachylitic breccia that generally have been associated with the Vredefort impact (Grieve and Therriault 2000) could be equivalent to the ring features of multi-ring basins (Spudis 1993). Reimold (1998) discussed the Vredefort case and emphasized that apparent enhancement of breccia development at specific distances from the center of the Vredefort dome could be a result of insufficient, discontinuous outcrop and localized underground observation that is restricted to the Witwatersrand goldfields (Fig. 5) where strata are accessible for geological analysis to a maximum depth of around 4 km.

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