Archean Rocks unraveled: an 'exclusive' first impression on paleomagnetism of deep-drilled rocks of the Onverwacht Suite, Barberton Greenstone Belt (SA)

Abstract

Rocks from the 3.5-3.1 Ga Barberton Greenstone Belt (BGB), South Africa, may provide information about the earliest history of our Earth. In this period, early life started, plate tectonics as we know today may have started and a stable geomagnetic field could already have been in place. To get constraints on the Early Earth’s tectonics and geodynamo behaviour and development, paleomagnetic data on these Archean rocks are necessary. Previous studies on surface rocks indicate a great potential for rocks from the Onverwacht Suite of the BGB. (Near) primary geomagnetic signals indicate the presence of a stable geomagnetic field during this period. However, surface rock data are not straightforward since surface processes like lightening and weathering influence the primary signal in rocks. In 2009, the Barberton Scientific Drilling Program (BSDP) started drilling inside the BGB in a large folded structure: the Onverwacht Anticline. This study describes the first paleomagnetic research on these fresh rocks throughout the whole cored section. Despite large low temperature or low coercive overprint components, most likely caused by (re)magnetization of the core by the equipment used for cutting, data from samples of the Hooggenoeg Complex indicate the presence of a possibly (near-) primary signal. Comparison with recent studies on similar rocks from outcrops, however, gives incompatible result. Nevertheless, the new paleomagnetic data from this study support the hypothesis of the presence of a stable geomagnetic field during the mid-Archean.