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

Abstract

AbstractThe 2.02 billion year old Vredefort impact structure in South Africa offers a unique opportunity to study large‐scale impact processes on Earth. Vredefort's large size (∼250 km in diameter) and eroded topography provides the opportunity to study the effects of shock physics at depth and post‐formation hydrothermal alteration. In this work, we simulate the formation of the Vredefort structure building upon recent shock physics (iSALE) simulations. We expand those simulations to cover a wider range of input conditions, and compute impact‐driven porosity and permeability. The latter quantities are used to perform fluid mobility simulations (HYDROTHERM). We find that the Vredefort event produced significant impact‐generated porosity (up to 30%) in an annulus from about 50 to 100 km from the center and up to several kilometers in depth. The corresponding estimated permeability (up to 10−12 m2) would have allowed for large scale subsurface fluid flows. Our hydrothermal calculations show that the Vredefort impact event could have generated a complex crustal fluid pattern within the crater rim that lasted for hundreds of thousand years, with localized flow concentration regions, opening a new interpretation for the mobilization and location of the ore deposits in the Witwatersrand basin. The combined approach utilizing impact and hydrothermal simulations constitute a powerful tool to understand geochemical processes at Vredefort, as well as to assess the ability of large impacts to drive crustal chemistry with far‐reaching consequences for the prebiotic evolution of the early Earth.