Impact tectonics in the core of the Vredefort dome, South Africa: Implications for central uplift formation in very large impact structures

The 80 km wide Vredefort dome presents a unique opportunity to investigate the deep levels of the central uplift of a very large impact structure. Exposure of progressively older strata in the collar of the dome and of progressively higher-grade metamorphic rocks toward its center is consistent with differential uplift; however, the deepest levels exposed correspond to pre-impact mid- crust, rather than lower crust, as has been suggested previously. Pre-impact Archean gneissic fabrics in the core of the dome are differentially rotated, with the angle of rotation increasing sharply at a distance of ~16­19 km from the center. The present asymmetric dips of the collar strata, with layering dipping outward at moderate angles in the southeastern sector but being overturned and dipping inward in the northwestern sector, and the eccentric distribution of the pre-impact metamorphic isograds around the core of the dome can be reconciled with symmetric rotation of an initially obliquely NW-dipping target sequence during central uplift formation. The rocks in the core of the dome lack distinctive megablocks or large-slip-magnitude faults such as have been described in other central uplifts. We suggest that the large-scale coherent response of these rocks to the central uplift formation could have been accommodated by small-scale shear and/or rotation along pervasive pseudotachylitic breccia vein-fractures.