The Foch Thrust-Potchefstroom Fault structural system, Vredefort, South Africa: a model for impact-related tectonic movement over a pre-existing barrier

Immediately after the Vredefort impact, crustal material was accelerated ou tward. At least two concentric anticlinal folds, separated by an annular sy ncline, were formed around the centre of the Vredefort Astrobleme structure during the outward acceleration of crustal material. Over the limbs of the anticlines, facing towards the point of impact, imbricate thrust fault zon es formed. The outcrop positions of the sole surfaces of the two thrust fau lt zones defined the original outlines of the two inner rings of the system , which are now obliterated by erosion. Beyond Ring 1, centrifugal tectonic movement took place over a major fault surface, termed the Black Reef Deco llement Zone (BRDZ). The shape of the northwestern sector of the second rin g is determined not only by the Foch Thrust sole, but also by a fault known as the Potchefstroom-Master Bedding Plane Fault (PMBF). Following the outc rop trace of the PMBF, an extensional fault with a craterward displacement, the western periphery of Ring 2, is distorted into a bulge towards the cen tre of the structure. The PMBF was formed during the stage of crater modifi cation by detachment along the eastern flank of the Blaauwbank Anticline. T he development of the anticline commenced during the period of deposition o f the Central Rand Group (Upper Witwatersrand Supergroup), and extended int o post-Transvaal times. The anticlinal axis crosses the remnant depositiona l basin of the Central Rand Group in a north-south direction. Episodes of u plift, caused by fold movement related to the formation of the anticline, c ontrolled the sedimentation of the auriferous conglomerate beds, to form th e 'reefs' of the goldfield known as the West Wits Line. The history of the fold and its interference with the shape of the second ring is explained an d it is shown how a barrier such as this can be employed to determine the s equence of ring formation in a complex multi-ring basin of the Vredefort ty pe. (C) 2000 Elsevier Science Limited. All rights reserved.