Paleomagnetic Evidence Relevant to the Origin of the Vredefort Ring

Abstract

The time of emplacement of three masses of alkali granite and associated syenites, intruded into the outward-overturned collar of sedimentary rocks is crucial to choosing between a meteorite-impact or terrestrial origin for the Vredefort structure. These intrusives contain pseudotachylite, shatter cones, and microscopic evidence of shock deformation, and their solidification must be preimpact if the structure is an astro-bleme. However their field relationships are considered to indicate that they were emplaced after the overturning, and hence that their shock features are unrelated to the mechanism of overturning. Radiometric age measurements have shown that the alkali granites are at most 70 m.y. older than the basic granophyre (1,970 m.y.) of a poststructure ring-dike complex. Relatively little polar wander might be expected in such a short interval. The paleomagnetism of this granophyre, the alkali granites, and other relevant intrusive bodies has been investigated to determine whether the paleomagnet...