New palaeomagnetic results from the Mesoarchaean Nsuze flood basalts, South Africa

Abstract

Abstract The Pongola Supergroup is a largely undeformed ca. 2.99 to 2.87 Ga succession of volcanic and sedimentary rocks on the southeastern Kaapvaal Craton. Understanding the palaeogeographic context of the Pongola Supergroup could shed light on the tectonic setting of laterally correlative gold-bearing Witwatersrand Supergroup and have implications for Archaean geodynamics. Two previous studies were limited in spatial coverage and yielded purported primary magnetisations that were strikingly different from one another. Here we report new palaeomagnetic results from volcanic rocks of the Nsuze Group, i.e., the lower Pongola Supergroup, that are based on broad geographic sampling of a total of 57 sites making provision for several stability field tests. We report several directional components of remanent magnetisation, including widespread Karoo (ca. 180 Ma) and sporadic Namaqua-Natal (Mesoproterozoic) remagnetisation; but two additional ancient components are also commonly observed. The first of these is a south down characteristic remanence from 16 sites, of which 14 sites were included in the calculation of a mean, which we interpret as a magnetic overprint associated with intrusion of the 2.65 Ga White Mfolozi dyke swarm based on an inverse baked contact test and poorer clustering upon application of structural corrections to restore bedding to palaeohorizontal. The south down pole is located at 75.4°S and 334.5°E with an A95 of 6.4°. Recorded over a much more geographically widespread area from 15 sites is a west down characteristic remanence. The west down remanence is constrained to be older than 2.65 Ga by the same inverse baked contact test on a White Mfolozi dyke. Clustering of the west down remanence improves when bedding is restored. Fold tests are statistically indeterminate, but best grouping at ~60% unfolding was illustrated from the Amsterdam syncline. The precise age of folding is unknown. Emplacement relationships between the ~2.87 Ga Thole Complex and the folded Mozaan Group suggest that the syncline could be older than 2.87 Ga, but folding has also been ascribed to emplacement of the ca. 2.72 Ga Nhlangano gneiss dome. The age of the west down remanence and pole (15.6°S, 340.2°E and A95 = 9.3°, assuming 60% unfolding of sites at the Amsterdam syncline; or 16.6°S, 338.7°E and A95 = 10.3°, assuming 100% unfolding of sites at the Amsterdam syncline), which could be either syn-folding or pre-folding, is constrained at 2.98 to 2.72 Ga given presently known data.