Thermal-metamorphic signature of an impact event in the Vredefort dome, South Africa

Abstract

Pseudotachylitic breccias and shock deformation features related to the 2.02 Ga formation of the Vredefort dome by meteorite impact were overprinted by a static metamorphic event, the intensity of which decreased from granulite-facies ( T ≥700 °C) in the center of the dome to greenschist-facies ( T ≤400 °C) around its margins. Geobarometric estimates of 0.2–0.3 GPa for the metamorphic parageneses indicate some 8–11 km of erosion since the impact event. The strong lateral thermal gradient implied by these P-T results is attributed to the combined effects of differential uplift of mid-crustal rocks heated along a pre-impact geotherm and increased shock heating of the target crust toward the center of the impact structure. We suggest that the exceptionally high grade of metamorphism in the center of the dome may, in part, reflect an elevated regional geothermal gradient of ∼25 °C/km in the target crust due to lingering thermal effects related to the 2.05–2.06 Ga Bushveld magmatic event.