Shock pressure distribution in the Vredefort impact structure, South Africa
Special Paper of the Geological Society of America
Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Johannesburg 2050, South Africa
A petrographic study of Archean gneisses exposed in the core of the 80-km-wide central uplift of the Vredefort impact structure has revealed widespread evidence of shock-related metamorphic effects in feldspar and ferromagnesian minerals in addition to the already documented quartz microdeformation features. The shock features are variably overprinted by annealing effects, which increase in intensity toward the center of the structure. The shock effects show a general increase in intensity toward the center of the dome but are most characteristically marked by extreme heterogeneity on a small (centimeter to millimeter) scale, indicating highly localized shock pressure heterogeneity. Closest to the center, the pre-impact Archean fabrics and textures have been locally partially destroyed by comprehensive melting and melt mobilization, which has given rise to distinctive granofelses and clast-laden melt breccias. Based on the features observed, background shock pressures in the currently exposed level of the core of the dome range from ∼10 GPa at distances of ∼20 km from the center to >30-35 GPa within 5 km of the center. Pressures responsible for localized melting and for the formation of the melt breccias in this central zone may have exceeded 45 GPa, although it is difficult to constrain this value more precisely owing to uncertainty about the pre-impact temperature of the rocks and the role of local syn-shock frictional heating in raising temperature. Apart from the localized heterogeneity in shock pressures, the background shock pressure gradient appears to increase toward the center of the dome, approaching ∼4 GPa/km in the central parts. © 2005 Geological Society of America.
annealing; Archean; dome; feldspar; heterogeneity; impact structure; melting; mobilization; pressure gradient; quartz; shock metamorphism; uplift; Free State; South Africa; Vredefort Dome