Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..586t&link_type=abstract
Meteoritics, vol. 30, no. 5, page 586-587
Physics
3
Craters, Impact, Vredefort, Impacts, Melting, Metamorphism, Shock
Scientific paper
Consisting mainly of uplifted crystalline basement, the 40-50 km Vredefort Dome is in a near-central position within the Witwatersrand Basin [1,2](Fig. 1). The Vredefort impact structure (27 degrees 00'S, 27 degrees 30'E) consists of the Vredefort Dome and a wide "collar" of metasediments and metavolcanic rocks. Prior to the 1970's the diameter of the Vredefort structure was estimated at 60 km, and possibly up to 200 km [3,4]. In the 1970's and 1980's, the diameter was estimated at 100-120 km, while in the early 1990's 170-180 km was preferred. In relation to the outer edge of the Dome, the radial limit of shatter cones (26-37 km), PDFs (~13 km), and downfaulted Transvaal outliers (45-55 km), yield an estimate of 192-300 km for the final crater diameter 15](Fig. 2). These spatial indicators are subject to erosional effects. At the Sudbury Structure, a 45 km thick impact melt sheet has been preserved, under which PDFs have been annealed from the target rocks for at least 1 km. At Vredefort, the Granophyre is the only remanent of this melt sheet [6], and PDFs are observed in the Dome. The Vredefort structure, therefore, has been eroded below the crater floor and, comparing to Sudbury, the area has undergone at least 6 km of erosion. This corresponds with the regional erosion estimate of 5 to 10 km [7]. To better define an original crater diameter estimate, an indicator relatively independent of erosional effects is needed. The exposed 36 km "crust-on-edge" section serves as such an indicator, constraining the structural uplift (SU) to 36 km [81 and giving an estimated final crater diameter of 335 km [9] This diameter estimate is supported by a concentric fold pattern of anticlinal and synclinal sturctures [7] (Fig. 2). Such a large diameter suggests that the final form was that of a multi-ring basin, with the inner ring corresponding to an anticlinal structure and a concentration of pseudotachylite occurences (Fig. 2), and makes Vredefort the largest known terrestrial impact structure. The preservation of the upper Witwatersrand and Transvaal Supergroups and the many gold fields in the Witwatersrand Basin during post-Transvaal erosion is due to these broad fold structures produced by the Vredefort event (Figs. 1 and 2). Thus like Sudbury, a world class mining camp, owes its origin to the effects of a major impact event [10]. Different terms have been employed for the impact structure (e.g., Vredefort Dome, Vredefort Ring, and Vredefort structure). Given the recent findings, we suggest that Witwatersrand Structure would be a more appropriate name for the highly eroded impact structure encompassing, both the Witwatersrand Basin and Vredefort Dome. References: [1] Stepto D. (1990) Tectonophys., 171, 75-103. [2] Corner B. et al. (1990) Tectonophys., 171, 49-61. [3] Bishopp D. W. (1941) Geol. Soc. S. Afr. Trans., 44, 1-18. [4] Dietz R. S. (1961) J. Geol., 69, 499-516. [5] Therriault A. M. et al. (1993) LPS XXIV, 1419-1420. [6] Therriault et al. (1995) S. Afr. J. Geol., 99, in press. [7] McCarthy T. S. et al. (1990) S. Afr. J. Geol., 93, 1-4. [8] Hart R. J. et al. (1991) Tectonophys., 192, 313-331. [9] Grieve R. A. F. et al. (1981) Multi-Ring Basins, 37-58. [10] Grieve R. A. F. and Masaitis V. L. (1994) Int. Geol. Rev., 36, 105-151.
Grieve Richard A. F.
Reimold Wolf Uwe
Therriault Ann M.
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