A Discussion on global tectonics in Proterozoic times - Proterozoic crustal evolution in parts of southern Africa and evidence for extensive sialic crust since the end of the Archaean

Abstract

The Proterozoic mobile belts of southern Africa were subjected to multiphase deformation since the Late Archaean and formed part of an extensive Early Proterozoic crustal segment termed here the Kalahari-Malagasy Protoshield. This shield underwent tectogenesis and differential vertical movement at various times with considerable uplift in the mobile zones and relative stability in the Archaean granitegreenstone terrains. The evolution of the Namaqua, Limpopo and Malagasy mobile segments suggests no large-scale dispersive movements of major continental fragments during the Early and Middle Precambrian. The basement complex of Angola and northern South West Africa constitutes a mobile zone which includes reworked Archaean sialic crust and a tectonized Early Proterozoic supracrustal cover. This complex may have formed part of another Early Proterozoic segment termed here the Kasai-Angola Protoshield, which was probably connected with the Kalahari-Malagasy Protoshield during the Proterozoic prior to reworking during the Kibaran—Burundian, Rehoboth—Irumide and Pan African tectogenetic cycles. It is proposed that most, if not all, Proterozoic mobile zones in southern Africa constitute ensialic belts and the widespread occurrence of sialic rock units older than ± 2.5 Ga support the conclusion that a large crustal plate of predominantly granitoid composition and of continental proportions was already in existence since the end of the Archaean. The Proterozoic evolution of southern Africa is therefore characterized by plate destruction rather than by plate accretion and progressive cratonization, and only some of the granite-greenstone ‘nuclei’ have escaped this process.