A Revision of the Formation Conditions of the Vredefort Crater

Abstract

The Vredefort impact structure, located in South Africa and formed 2.02 Ga, is the largest confirmed remnant impact crater on Earth. The widely accepted impactor diameter and velocity to form this crater are 15 km and 15 km/s, respectively, which produce a crater diameter of 172 km. This is much smaller than the most commonly cited estimates (250–280 km), and while previous results were able to match the geologic evidence known at that time, these impact parameters are not consistent with more recent geological constraints. Here, we conduct impact simulations to model the Vredefort crater formation with the shock physics code impact Simplified Arbitrary Lagrangian Eulerian (iSALE). Our numerical simulations show that combinations of the impactor diameter and impact velocity of 25 km and 15 km/s or 20 km and 25 km/s are able to recreate the larger crater size of ∼250 km. Moreover, these configurations can reproduce shock‐metamorphic features present in the impact structure today, including the distributions of breccia, shatter cones, planar deformation features in quartz and zircon, and melt. Our model also predicts that Vredefort and Karelia, Russia, where an ejecta layer from the impact was found, were approximately 2,000–2,500 km apart based on the layer thickness. Additionally, we use this model to predict the potential global effects of such a large impact by estimating the amount of climatically important gases released to the atmosphere at the time. Our work demonstrates the need to revisit previously estimated impactor parameters for large terrestrial craters in order to better characterize impact events on Earth and elsewhere.

Genesis of the mafic impact melt rock in the northwest sector of the Vredefort Dome, South Africa

“…The Vredefort Dome experienced post‐impact temperatures that increase from the collar toward the center of the structure, with metamorphic conditions progressing inward from greenschist to granulite facies. They are interpreted to reflect a combination of the centripetally increasing effects of shock heating and uplift of progressively deeper crustal levels as a consequence of impact (Allen et al., 2022; Gibson, 2019; Gibson & Reimold, 2005; Ivanov, 2005). Numerical modeling (Allen et al., 2022; Ivanov, 2005) showed that the rocks increase in post‐shock temperature overprint radially inward from ~300°C in the collar rocks to >1000°C in the lithologies at the center of the dome, in good agreement with the shock petrographic findings of Gibson and Reimold (2005).…”

“…They are interpreted to reflect a combination of the centripetally increasing effects of shock heating and uplift of progressively deeper crustal levels as a consequence of impact (Allen et al, 2022;Gibson, 2019;Gibson & Reimold, 2005;Ivanov, 2005). Numerical modeling (Allen et al, 2022;Ivanov, 2005) showed that the rocks increase in post-shock temperature overprint radially inward from ~300°C in the collar rocks to >1000°C in the lithologies at the center of the dome, in good agreement with the shock petrographic findings of Gibson and Reimold (2005). Beaton et al (2022) recently provided a detailed analysis of metamorphic overprints in the collar of the Vredefort Dome and concluded that the regional Ventersdorp volcanism on the central Kaapvaal craton at about 2.7 Ga was likely responsible for lower amphibolite to upper greenschist facies metamorphism that affected the collar strata prior to the impact event.…”

“…Large-scale impact melt generation requires that a very large volume of target rock is subjected to shock pressures of 60-100 GPa (e.g., Allen et al, 2022;French, 1998;Ivanov, 2005), as illustrated by the Sudbury Igneous Complex impact melt body (Lightfoot, 2016) and as also to be assumed in the case of the Vredefort impact, where remnants of impact melt rock are exposed still today over an area of 2000 km 2 , the entire terrain of the Vredefort Dome (e.g., Reimold & Koeberl, 2014). The resulting post-shock temperatures are in the order of 1500-2500°C, clearly sufficient to melt quartzite (as obvious from the numerous completely annealed quartzite clasts in Granophyre).…”

“…The Vredefort impact structure (VIS) in north‐central South Africa is centered roughly 130 km southwest of Johannesburg on the Vredefort Dome, an extensive window into Archean and Paleoproterozoic crust of the central Kaapvaal Craton. The VIS represents the remnant of the largest known impact structure on Earth, with an estimated original diameter of 250–300 km (e.g., Allen et al., 2022; Gibson, 2019; Gibson & Reimold, 2008; Henkel & Reimold, 1998). This large structure encompasses the entire region of the erosional remnant of the Witwatersrand Basin between Johannesburg in the northeast and the Welkom gold mining district in the southwest.…”

Impact‐Generated Permeability and Hydrothermal Circulation at the Vredefort Impact Structure, South Africa

“…The formation of Vredefort has been modeled with shock physics code SALEB by Ivanov (2005), and iSALE by Allen et al. (2022). The latter work reports the most updated simulations for the Vredefort impact and have been shown to match a number of observational constraints, particularly the spatial distribution of shock features.…”