Geology and evolution of the Vredefort impact structure, South Africa

  • Reimold W
  • Gibson R
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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 geological community only in the past 15 years. More than 150 impact structures are now known on the Earth's surface. The origin of the Vredefort dome, a ca 70 kilometre wide structural uplift terrane in the centre of the economically important Witwatersrand basin, by either endogenic or impact processes, has long been controversial. Detailed microdeformation studies have recently proven that the dome represents the central uplift of one of the largest (original diameter: ca 300 kilometres) and oldest (2023 ± 4 Ma) known terrestrial impact structures. The Vredefort structure is not only of importance because of its controversial origin and particular setting in the centre of the economically important Witwatersrand basin. This structure is also the type locality for pseudotachylitic breccia and well-known for its abundant shatter cones and the enigmatic Vredefort granophyre. A wealth of new data on the Vredefort structure have become available in recent years: detailed 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 information regarding the time of impact and have permitted temporal separation of the Vredefort impact event from the emplacement of the 2050 Ma Bushveld complex. This achievement has major implications for the understanding of the impact-related thermal and hydrothermal effects in the Witwatersrand basin as a whole and the regional metamorphic evolution. As continued study of the Vredefort structure has the potential to contribute much more to the understanding of large impact cratering events and their potential geological and economic effects, besides contributing to the general geological knowledge of the evolution of the Kaapvaal craton, this paper reviews the current knowledge on the Vredefort structure and to emphasise those areas in need of future investigation. Copyright © 1996 Elsevier Science Ltd.

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