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Magnetic anomaly near the center of the Vredefort structure: Implications for impact-related magnetic signatures

¹ Schonland Research Centre, University of the Witwatersrand, P.O. Box 3, Wits 2050, Johannesburg, South Africa
² Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08540
³ Atomic Energy Corporation of South Africa Ltd., P.O. Box 582, Pretoria, 0001, South Africa
⁴ Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa
⁵ Bernard Price Institute for Geophysics, University of the Witwatersrand, P.O. Box 3, Wits 2050, Johannesburg, South Africa

A strong magnetic anomaly near the center of the ancient and deeply eroded Vredefort structure is attributed to remanent magnetization caused by a large meteorite impact at 2.0 Ga. The rocks underlying the anomaly are Archean gneisses thought to represent mid-crustal depths that were uplifted to the surface during the postulated impact event. Measurements of the remanent magnetization of the basement rocks yielded consistent vectors of declination = 25° , inclination = 56° , k = 18, = 16 that correspond to the paleomagnetic pole position at time of impact. Petrologic studies indicate that during impact, large volumes of rock were heated enough to cause thermal remagnetization in the ambient field. Thermal effects of all large impacts on cratons may induce a remanent magnetization of sufficient intensity to cause anomalies in the geomagnetic field that are detectable even by satellites.

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