Paleomagnetic Evidence Relevant to the Origin of the Vredefort Ring

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 paleomagnetic vector in the alkali granite is similar to, or is overturned with respect to, that in the basic granophyre. Widely scattered results, possibly due to the shock deformation, were obtained from the two innermost alkali-granite plutons. Good data, however, were obtained from the third, outer alkali-granite mass, and from olivine gabbro of the Rietfontein Complex, with which it is closely associated. The magnetic vectors from these two rock masses are about 54 degrees apart, suggesting differential structural rotation. When compared with the vector of the poststructure basic granophyre, however, they are both displaced more than 90 degrees before, and less than 30 degrees after, correction for overturning. It is therefore concluded that the basic igneous complex and the outer alkali granite on the farm Rietfontein, and presumably the other two alkali-granite plutons as well, were intruded prior to the development of the Vredefort Ring structure. The ring must have formed in the 70 m.y. or less between the times of emplacement of the granites and the basic granophyre, and could have been produced by impact.