Magnetic model of the central uplift of the Vredefort impact structure, South Africa

Abstract

The Vredefort structure is the largest known impact structure on earth, with an original diameter estimated at—at least—250 km. The central uplift of this structure is about 80 km wide and contains a 40- to 50-km wide core of crystalline basement surrounded by a so-called collar of supracrustal sequences. Previous investigations of the structure of this large impact crater, based on integrated geophysical modeling, have shown that the central rise structure is an upper crustal feature with a presently recognized uplift of some 12 km, decreasing to about 4 km at the depth of the Moho. This study concentrates on the structure of the central uplift region (the Vredefort Dome), using aeromagnetic data along a SSW–NNE profile across its center. Available rock magnetic data were used to constrain the magnetic modeling. On the basis of this modeling, it is concluded that a 60- to 70-km wide region in the center of the structure experienced a magnetic overprint in connection with the impact event. This overprint is related to the occurrence of high-coercive magnetite and its subsequent thermal magnetization. Three sources for this thermal event are envisaged—the potential occurrence of a now eroded impact melt complex, the impact-related rise of hot crustal material of the crystalline core, and the thermal energy deposited by the shock wave. Thermal demagnetization of rocks from the crystalline core has shown that magnetite, most likely formed by shock dissociation of Mg–Fe silicates, is the dominant carrier mineral of the remanent magnetization. The thermal overprint has also included those parts of the upturned metasedimentary collar rocks, which contained suitable magnetic carrier minerals. Its present manifestation is the generally negative magnetic anomaly centered on, and encircling, the central uplift of the impact structure. Within the crystalline core, the magnetic models indicate structures of outward collapse along low-angle thrust surfaces. The impact event has, thus, caused an increased magnetic complexity both by drastic changes of the magnetic properties and by the structural perturbation of the upper crystalline crust.