Paleomagnetic and rock magnetic study of the Vredefort impact structure and the Johannesburg Dome, Kaapvaal Craton, South Africa - Implications for the apparent polar wander path of the Kaapvaal Craton during the Mesoproterozoic

摘要

New paleomagnetic and rock magnetic results are presented for various pre-, syn-and post-impactlithologies from the region of the 2023 +/- 4 Ma, originallyca. 250 km diameter, Vredefort impact structure, on the Kaapvaal Craton of South Africa. After removal of a viscous remanent magnetization (VRM) component. the characteristic remanent magnetization (ChRM) component from 16 Vredefort Granophyre and pseudotachylitic (PT) breccia samples (D = 18.3 degrees, I=54.8 degrees, alpha(95) = 8.1 degrees) was isolated and a paleomagnetic pole at 25.1 degrees N and 43.5 degrees E (A(95) = 10.6 degrees) obtained. Since, such a ChRM component was isolated also from the Archean Basement rocks of the Vredefort Dome, we consider its nature as a primary component of the impactites proven. The pole falls onto the Paleoproterozoic part of the apparent polar wander path of the Kaapvaal Craton. Rock magnetic analysis and scanning electron microscopy (SEM) revealed that the pseudotachylitic breccia and Granophyre samples contain two distinct magnetite phases: one ultra-small (micrometersize), and one altered and larger (>50 mu m) phase. The ultra-small magnetite is interpreted as the carrier of the ChRM and the larger grains as the carriers of the VRM component. Additionally, rock magnetic and petrophysical data reveal unusually high Koenigsberger ratios (Q values) in all pre-impact lithologies, in some Vredefort impactite samples, and in the much younger (1.1 Ga) Anna's Rust Sheet (ARS) gabbro samples. As the high Q values, which had also been reported by previous studies of Vredefort lithologies, are now also seen in samples from the Johannesburg Dome, a direct link to the Vredefort impact can be ruled out. This is also supported by the hysteresis data of this study. As the observed magnetization is rather hard and shows multiple components of remanent magnetization, we exclude lightning as a cause for all observed high Q values (except in case of ARS gabbros). It is instead suggested that the cause of the high Q values could be related to the high temperatures of the rocks that were uplifted by the impact event from a mid-crustal original setting, and to fluid circulation within the two domes that made the rocks vulnerable to acquire high thermochemical remanence. Moreover, paleomagnetic analysis of the rocks around the ARS gabbro intrusion in the northern part of the Vredefort structure revealed the presence of either a shallow north or a shallow south direction, which is tentatively related to emplacement of the Umkondo large igneous province. Analysis of all rocks, including the Vredefort impactites, yields occasionally distinct great circle paths towards these shallow directions. A likely explanation for this overprint direction is the heating caused by now eroded ARS-type gabbro in the area, or by regional, ca. 1.1-1.0 Ga orogenic effects. No evidence of Karoo-type (0.18 Ga) overprint is seen. (c) 2008 Elsevier B.V. All rights reserved.