Microscopic planar deformation features in quartz of the Vredefort structure: Anomalous but still suggestive of an impact origin

Abstract

The orientation and distribution of diagnostic shock-produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the Vredefort structure in South Africa. These features are widespread, occurring in all but four samples. The majority of the planar features have a basal {0001} orientation, with a few higher pressure so-called ω features and three π features occurring near the center of the structure. Their relative distribution and preservation are, however, anomalous compared to those at other large terrestrial impact structures and recorded shock pressures, calculated on the basis of planar feature orientation, do not show a regular decrease radially outward as observed at other known impact structures. The apparently anomalous development of planar features is interpreted as a result of post-shock recrystallization. If the effect of recrystallization of quartz, which generally increases inward toward the core of the structure, is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure, then higher pressures are recorded in the core compared to the collar rocks. No evidence of a previously reported, additional post-recrystallization shock event is found in this study, although previous researchers report that pseudotachylite, which may or may not be associated with the shock event, cuts recrystallized quartzites. In is concluded that, although anomalous, the evidence from planar features in quartz is still consistent with the Vredefort structure being an erosional remnant of a large, complex, impact structure.