Geophysical characterization of the Daskop granophyre dyke and surrounding host rocks, Vredefort impact structure, South Africa

Abstract

AbstractGranophyre dykes in the central part of the Vredefort impact structure are believed to be the remnants of the impact melt sheet, which intruded downwards along the fractures in the crater floor. Little is known about their original penetration depth, dip, structural relationships with the host rocks, and their general geophysical characteristics. This information is critical to understand the emplacement history of the granophyre dykes, as it relates to the formation and modification of large impact structures. We conducted magnetic and resistivity surveys across the Daskop granophyre dyke (DGD), one of the impact melt dykes in the structure's core. The magnetic survey revealed that the DGD gives a strong magnetic response at positions where the dyke outcrop exceeds the surface topography, but a very weak response where the outcrop is nearly at the same elevation as the surrounding topography. The magnetic anomaly is thus predominantly due to the outcrop protruding above ground level, suggesting a limited volume of dyke material in the subsurface and a small penetration depth. The resistivity survey performed on two profiles, set perpendicularly across the DGD, indicated a shallow penetration depth (<3 m), consistent with the magnetic interpretation. Thus, our geophysical study demonstrates that the DGD is currently at the very bottom of its original emplacement. This may either be an erosional coincidence, or it may be controlled by a fundamental process of impact cratering. Further studies are warranted to determine if other granophyre dykes at Vredefort are similarly at their lowermost terminations.