The Vredefort structure: an introduction and a guide to recent literature

Abstract

The ∼ 50 km wide Vredefort structure appears to present the eroded roots of an area affected by rapid uplift and violent rock deformation—widely regarded as shock deformation—on an immense scale. The origin of the structure is disputed: external (impact) and internal agencies have been suggested. This note introduces the Vredefort structure and its geological environment, emphasizing information published in the decade prior to the 1987 International Workshop on Cryptoexplosions and Catastrophes in the Geological Record, with a Special Focus on the Vredefort Structure. This typical cryptoexplosion structure has a central and polygonal basement uplift containing violently deformed and brecciated rocks, with the northern half surrounded by steeply-dipping and heavily faulted collar strata, and then by a rim synclinorium of less deformed strata. The gains made in research on all three sectors are noted. Traverses through the basement, from collar to centre, effectively penetrate through a ∼ 15 km thick section of crust. The Archaean history of the crustal section has been documented. The thermal and fluid inclusion evolution near the time of uplift has been studied and a deep-seated CO 2-rich gas explosion has been proposed as an alternative to the impact hypothesis for the “shock deformation”. Meticulous study of the effects of thermal metamorphism on the shock deformations in the collar strata led to a bold proposal: more than one “shock event” in the evolution of Vredefort. Analysis of the character of the faulting of the collar supports an earlier proposal regarding inward movement of blocks of collar strata. Renewed research on shatter cone orientations in the collar strata and two detailed studies of the structural history within the rim synclinorium are noted. Through the intensive mapping, geophysical exploration and drilling which have been completed within the Witwatersrand and Transvaal basins, close spatial and temporal connections between Vredefort and the Bushveld Complex have been documented. In both cases, basin centres were intruded by horizontal mafic eruptives and basin subsidence took place after these eruptives had consolidated and cooled. The Vredefort structure has long been discussed as a possible deeply eroded remnant of a major cratering event or events; the connections of this structure with the Bushveld Complex carry important implications for such discussion.