ORIGINAL SIZE OF THE VREDEFORT STRUCTURE - IMPLICATIONS FOR THE GEOLOGICAL EVOLUTION OF THE WITWATERSRAND BASIN

Historically, there have been a range of diameter estimates for the la rge, deeply eroded Vredefort impact structure within the Witwatersrand Basin, South Africa. Here, we estimate the diameter of the transient cavity at the present level of erosion as similar to 124-140 km, based on the spatial distribution of shock metamorphic features in the floo r of the structure and downfaulted Transvaal outliers. Taking erosion into account (greater than or equal to 6 km) and scaling to original f inal rim diameter, an estimate of close to 300 km for the rim diameter is obtained. Independent estimates of the final rim diameter, based o n an empirical relation of central uplift diameter to rim diameter, sp atial distribution of pseudotachylites, and concentric large scale str uctural patterns, give a similar estimate of close to 300 km for the o riginal final rim diameter. An impact structure of this size is expect ed to have had an original multi-ring form. At this size, the Vredefor t impact structure encompasses the hulk of the Witwatersrand Basin, wh ich appears to owe its preservation to the Vredefort impact. In additi on, the Vredefort impact event may have been the thermal driver for so me of the widespread hydrothermal activity in the area, which, in rece nt interpretations, is believed to be a component in the creation of t he world-class gold deposits of the Witwatersrand Basin.