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The Roodekraal Complex as a constraint on the size of the Vredefort impact crater, South Africa

Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, South Africa

The volcanic Roodekraal Complex, situated about 40 km from the proposed center of the ~2.02 Ga Vredefort cratering event, overlies the rocks of the Pretoria Group with an angular unconformity. The unsheared basal contact of the Roodekraal Complex, a succession of alkaline basic lava intruded by diorite sills, defines an ~2.05 Ga palaeosurface and as such constrains the size of the transient Vredefort crater to a maximum of ca. 80 km in diameter. A group of concentric thrust and normal faults with listric characteristics, the Ensel Thrust System, probably represents the expected normal faults that caused collapse of the transient crater rim during the modification stage of the cratering event. A series of semi-concentric thrust faults of the order of 200 km diameter and centered on the point of impact may define the total size of the Vredefort astrobleme (taken to be the entire area of impact-induced deformation). The indicated diameters of the transient and final craters agree well with those suggested by recent numerical models.