GEOLOGY AND EVOLUTION OF THE VREDEFORT IMPACT STRUCTURE, SOUTH-AFRICA

The importance of large impact events for the evolution of all bodies in the Solar System has been recognised by a large proportion of the g eological community only in the past 15 years. More than 150 impact st ructures are now known on the Earth's surface. The origin of the Vrede fort dome, a ca 70 kilometre wide structural uplift terrane in the cen tre oi the economically important Witwatersrand basin, by either endog enic or impact processes, has long been controversial. Detailed microd eformation studies have recently proven that the dome represents the c entral uplift of one of the largest (original diameter: ca 300 kilomet res) and oldest (2023 +/- 4 Ma) known terrestrial impact structures. T he Vredefort structure is not only of importance because of its contro versial origin and particular setting in the centre of the economicall y important Witwatersrand basin. This structure is also the type local ity for pseudotachylitic breccia and well-known for its abundant shatt er cones and the enigmatic Vredefort granophyre. A wealth of new data on the Vredefort structure have become available in recent years: deta iled microdeformation studies have revealed the presence of bona fide shock metamorphic effects in quartz and zircon; U-Pb dating of single zircons from pseudotachylitic breccia have provided reliable age infor mation regarding the time of impact and have permitted temporal separa tion of the Vredefort impact event from the emplacement of the 2050 Ma Bushveld complex. This achievement has major implications for the und erstanding of the impact-related thermal and hydrothermal effects in t he Witwatersrand basin as a whole and the regional metamorphic evoluti on. As continued study of the Vredefort structure has the potential to contribute much more to the understanding of large impact cratering e vents and their potential geological and economic effects, besides con tributing to the general geological knowledge of the evolution of the Kaapvaal craton, this paper reviews the current knowledge on the Vrede fort structure and to emphasise those areas in need of future investig ation. Copyright (C) 1996 Elsevier Science Ltd.