Processed and enhanced gravity and magnetic images over the Vredefort structure and their interpretation

Abstract

Cryptoexplosion structures are polygonal features which have experienced violent disruption during their formation. The Vredefort structure is one of the largest known cryptoexplosion structures and the geology is well exposed over its northern half. The origin of the violent disruption experienced by the Vredefort structure is controversial and many aspects of its history are under intensive study. Since most of the southern half of the Vredefort structure is blanketed by a cover of Karoo (Permian) strata, geophysical data are essential in any attempt to map the pre-Karoo geology. Data sets of the regional gravity and magnetic fields over the Vredefort structure have been processed and enhanced with the aid of an image processor. The results are presented as a series of images, each one enhancing particular attributes of the data sets. An interpretation of the geophysical images is presented, focusing on the regional geometry and setting of the structure. Striking features of the geophysical images include a simple concentric geometry and polygonal shape which reflect the distribution of dense and magnetic lithologies within and surrounding the structure. The kinematics of the development of a polygonal uplift are discussed. Inward movement of about seven collar sectors separated by radial faults and uplift of the central sections of each of the sectors may account for the gross polygonal shape. Pull-apart voids formed at kinks in the radial faults might account for the positions of several alkali stocks within the stratified collar of the Vredefort structure. The enhanced image of the gravity field highlights a SE-trending gravity low which extends beyond the southeastern quadrant of the Vredefort structure. This feature forms part of a fundamental NW-SE trending intracratonic lineament-the Vredefort axis. © 1990.