Deeply exhumed impact structures: A case study of the Vredefort structure, South Africa

Abstract

The main evidence in the Vredefort Dome and surrounding parts of the Witwatersrand Basin is presented, which indicates that this terrain represents the deeply exhumed root zone of a large, 2023 _+ 4 Ma, complex impact structure.Shock metamorphic features such as impact melt, shatter cones, high-pressure quartz polymorphs and shock microdeformation features are restricted to the Vredefort Dome, which constitutes the central uplift of the impact structure. High strain rate brittle deformation features, including pseudotachylites and clastic breccias, are found over a larger radial distance of at least 100 km from the center of the structure. Low-temperature (~300 °C) hydrothermal effects occur up to a similar radial distance but, in the central uplift, metamorphic mineral parageneses overprinting the shock and brittle deformation features in pelites indicate a strong radially-inward increase in temperature to between 700 and 900 °C. Pressure estimates of 0.2-0.3 GPa obtained from these mineral parageneses indicate that the Vredefort structure has been eroded by between 7 and 10 kin. The original depth of burial of these rocks after the impact and their elevated temperatures explain the anomalous appearance and restricted distribution of many of the shock features compared to other, less eroded, structures. The metamorphism and hydrothermal activity are attributed to the combined effects of exhumation of deep (hot) crustal levels by the cratering event and formation of the central uplift, and shock heating, possibly with a minor component of heating caused by an overlying impact melt volume. The structural, shock and thermal impact-related features in the Vredefort Dome and Witwatersrand Basin provide a case study that should assist in the identification of other deeply exhumed impact structures. 250 Gibson and Reimold

Shock pressure distribution in the Vredefort impact structure, South Africa

“…The study of these parageneses has enabled quantifi cation of the impact-related thermal overprint across the structure. This overprint has been shown to vary strongly from a background value of ~300 °C, in rocks at radial distances of >25 km from the center of the structure, to >700 °C within 10 km of the center (Gibson et al, 1998;Gibson and Reimold, 2000). Recently, Gibson (2002) attributed ultra-high temperature pelitic metamorphic parageneses and granitic anatectic melts from near the center of the dome to the attainment of post-impact temperatures in excess of 1000 °C.…”

The Modification Stage of Crater Formation

“…Owing to the immense stratigraphic uplift, very large complex impact structures on Earth, such as the Vredefort Dome, South Africa, show an increase in metamorphic grade toward the centre of the structure. In such cases, an eroded central uplift provides a section through the upper part of the crust (Gibson and Reimold, 2000).…”

Impact‐induced melting of Archean granulites in the Vredefort Dome, South Africa. I: anatexis of metapelitic granulites

“…The Vredefort Dome in South Africa (Fig. 1) has long been regarded as the eroded remnant of the central region of a large impact structure, and evidence of a static metamorphic overprint of the impact‐related shock features is widespread in its rocks (see reviews in Reimold & Gibson, 1996; Gibson & Reimold, 2000). However, it is only recently that efforts have been directed towards quantifying the magnitude and distribution of the post‐shock thermal effects (Gibson et al ., 1998).…”