Tehostekuva

2e. Paleomagnetism of Vredefort impact structure: the role of lightning strokes

Participating scientists

  • J. Salminen
  • L.J. Pesonen
  • W.U. Reimold
  • R.L. Gibson
  • P. Rochette (University of Aix-Marseille III, France) and J. Cattaccea

Project goals

We study paleomagnetism, rock magnetism, petrophysics and paleointensity of 2.023 Ga old Vredefort impact structure and surroundings. As the Vredefort structure covers lithologies of ages ranging from ca. 3.2 to 1.0 Ga, paleomagnetic results are relevant to the calibration of the apparent polar wander path (APWP) and tracing the drift history of Kaapvaal craton through Precambrian. Using the petrophysical, rock magnetic and paleointensity methods we are trying to solve the origin of the magnetization. Some Vredefort rocks, both impact and target lithologies, reveal extraordinary high Koenigsberger's values (remanent magnetization/induced magnetization). There are at least three possible reasons for this: 1. impact, 2. plasma generated intense magnetic field during the impact or 3. lightning. Rock magnetic and paleointensity studies are used to find out whether the magnetization was acquired in the Earth's magnetic field or in the plasma field. Shock experiments will be used to study the impact caused magnetization in the rocks. Artificial lightning experiments are carried out to see whether the high Q-values are due to lightning effects and not by shock.

Introduction

Methods of studying Vredefort impact structure

Figure 1: Vredefort impact structure is located in a) South Africa, with in Kaapvaal's craton, b) it's circular magnetic anomaly is typical for impact structures. c) Shatter cones are field evidence of impact origin.

Highlights

The most consistent result of this paleomagnetic study so far is the direction obtained from the impact related lithologies (granophyre and pseudotachylitic breccias), which provide a paleopole at 2.023 Ga. The other result is a widespread ca. 1.1. Ga overprint in some of the rocks in the Vredefort area. Experimental "laboratory" shock experiments demonstrate that indeed lighting will most likely induce extremely high remanences and is therefore responsible for the observed high Q-values.

Recent publications

  • Salminen, J. et al., 2010, Eos Trans. AGU, 91(26), Abst GP31E-02.
  • Salminen, J., Pesonen. L.J., Reimond, U. and Gibson, R. L., 2009. Paleomagnetism of the Vredefort Dome and Johannesburg Dome, South Africa. Precambrian Research, Volume 170, Issues 3-4, May 2009, 267.
  • Salminen, J. Pesonen, L. J., Reimold, W. U. and Gibson, R. L., 2006. New data for calibrating Kaapvaal craton's apparent polar wander path - Vredefort impact crater, South Africa. The 27th Nordic geological winter meeting, 9.-12.1.2006 Oulu, Bulletin of the Geological Society of Finland, Special Issue 1, Abstract volume, p. 138.
  • Salminen, J., Pesonen, L. J., Reimold, W. U., and Gibson, R. L., 2005. New paleomagnetic results of Vredefort impact structure - pathway to Southern Africa's geological past. IAGA 2005 - A - 00665, 18.7-29.7.2005, Toulouse CD-ROM.
  • Salminen, J., Pesonen, L. J., Reimold, W. U., and Gibson, R. L., 2005. Paleomagnetic and rockmagnetic results for the Vredefort impact structure. 4th annual meeting of GSSA (Geological Sciety of South Africa), University of Kwazulu-Natal, Durban, 4.-8.7.2005 CD-ROM: GEO 2005 p.197-198.

Other information

  • Funded by the Helsinki University Research Grant
  • The project is a collaboration between the University of Helsinki, Division of Geophysics, the Univ. of Witwatersrand, South Africa, and the Humboldt University of Berlin, Germany