Impact-related Structures in the Central Uplift of the Vredefort Impact

A structural investigation of the well-exposed northern and western parts of the Vredefort dome has established the geometry and chronology of pre-, syn-and post-impact deformation features. The strata display a crude polygonal geometry with zones of complex monoclinal or symmetric folding, and radial oblique-slip faulting separating relatively straight segments. Pseudotachylitic breccia is ubiquitous, but shows a morphological and volumetric correlation with rock type, in addition to an overall decrease in abundance outwards from the center of the dome. Multiple joint sets are intensely developed in all rock types and include sets displaying normal-slip displacements, which are consistent with radial and tangential collapse of the central uplift. These joints may postdate pseudotachylitic breccia veins. Introduction: The 2.02 Ga, ca. 300-km-wide Vredefort impact structure in South Africa is one of the largest impact structures on Earth [1]. Between 7 and 10 km of erosion has led to the removal of the crater and its impact breccia fill, and has exposed the deep levels of the structure. At the heart of the structure lies the ~80-km-wide Vredefort dome – the root zone of the central uplift. Recent structural mapping in the Archean gneissic basement in the 40-km-wide core of the dome by Lana et al. [2,3,4] has shown differential rotation of the rocks related to doming. Gibson et al. [5] have established that the centralmost parts of this core experienced shock pressures in excess of 30 Gpa, including local shock melting. However, comprehensive structural studies of impact-related deformation effects in the gneisses of the central core terrane are hampered by poor outcrop. In contrast to the core, exposure in the 15-20-km-wide supracrustal-dominated collar of the dome is generally good. The present study is focused on the northern and western sectors of the collar, in rocks of the 2.71-2.98 Ga Witwatersrand Supergroup. These rocks form the innermost collar, at a radial distance of ~20-30 km from the center of the dome, and have been uplifted by at least 15 km relative to the deepest part of the rim syncline [6]. They are, in turn, surrounded by less well-exposed rocks of the Ventersdorp and Transvaal Supergroups. This study: First results stem from a large-scale structural investigation of available Landsat images and aerial photos of the study area, as well as from surface mapping of several radial traverses across the Witwatersrand Supergroup. The supracrustal rocks show a broadly circular shape on a large-scale view, (although the …