Carporzen L., Weiss B.P., Gilder S.A., Pommier A., Hart R.J.
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, United States; Équipe de Paléomagnétisme, Institut de Physique du Globe de Paris, University of Paris VII-Denis Diderot, 1 rue Jussieu, Paris F-75005, France; Department of Earth and Environmental Sciences, Ludwig Maximilians University, D-80333 Munich, Germany; School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States; IThemba Labs, Private Bag 11 WITS, Johannesburg 2050, South Africa
Carporzen, L., Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, United States, Équipe de Paléomagnétisme, Institut de Physique du Globe de Paris, University of Paris VII-Denis Diderot, 1 rue Jussieu, Paris F-75005, France; Weiss, B.P., Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, United States; Gilder, S.A., Department of Earth and Environmental Sciences, Ludwig Maximilians University, D-80333 Munich, Germany; Pommier, A., Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, United States, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States; Hart, R.J., IThemba Labs, Private Bag 11 WITS, Johannesburg 2050, South Africa
The Vredefort impact crater in South Africa is one of the oldest and largest craters on Earth, making it a unique analog for planetary basins. Intense and randomly oriented remanent magnetization observed in surface samples at Vredefort has been attributed to impact-generated magnetic fields. This possibility has major implications for extraterrestrial paleomagnetism since impact-generated fields have been proposed as a key alternative to the dynamo hypothesis for magnetization on the Moon and asteroids. Furthermore, the presence of single-domain magnetite found along shock-generated planar deformation features in Vredefort granites has been widely attributed to the 2.02 Ga impact event. An alternative hypothesis is that the unusual magnetization and/or rock magnetic properties of Vredefort rocks are the products of recent lightning strikes. Lightning and impact-generated fields can be distinguished by measuring samples collected from below the present surface. Here we present a paleomagnetic and rock magnetic study of samples from two 10 m deep vertical boreholes. We show that the magnetization at depth is consistent with a thermoremanent magnetization acquired in the local geomagnetic field following the impact, while random, intense magnetization and some of the unusual rock magnetic properties observed in surface rocks are superficial phenomena produced by lightning. Because Vredefort is the only terrestrial crater that has been proposed to contain records of impact-generated fields, this removes a key piece of evidence in support of the hypothesis that paleomagnetism of the Moon and other extraterrestrial bodies is the product of impacts rather than past core dynamos. Copyright 2012 by the American Geophysical Union.