Aspects of the dynamic and thermal metamorphic history of the Vredefort cryptoexplosion structure: implications for its origin

Abstract

The Vredefort structure is the oldest and the largest known cryptoexplosion structure on earth. An approximately 36 km deep section through the Archean sialic crust and the overlying Precambrian strata of the Kaapvaal craton is exposed in the core of the structure. The geology presented in the exposed section includes all the principal metamorphic facies in the crust and records a long and complex thermo-tectonic history which dates back to at least 3.5 Ga. The petrographie and geological observations in the basement rocks indicate that there is a complex interrelationship between the Archean geology and the 2.0 Ga dynamic and thermal metamorphic overprint (some of which are postulated to be indicative of impact processes). The dynamic and thermal metamorphic effects do not increase progressively towards the centre of the structure as found at known impact structures. In particular, dynamic deformation phenomena such as pseudotachylite and planar features in quartz reach maximum intensity in the rocks close to the Vredefort discontinuity, a brittle-ductile shear zone which separates upper crustal amphibolite facies rocks from lower crustal granulites. In certain other lithological zones, deformation phenomena are noticeably absent or diminished. We suggest that changes in the physical and chemical properties of the rocks from margin to centre of the basement may account for the variation in the intensity of the 2.0 Ga thermal and dynamic metamorphic effects observed at Vredefort. In conclusion, our overall impression of the Vredefort structure is that it is a relic of an ancient meteorite impact crater, but that there were thermo-tectonic events which occurred both prior to and after the postulated impact event, which complicates the interpretation of its origin.