Neue Ergebnisse zur Evolution der prÄkambrischen Kruste im südlichen Afrika

Abstract:

The structural history of the Precambrian shield areas points to an early crustal evolution which differed markedly from the development in later times, and which cannot be interpreted in terms of the classical model of Phanerozoic orogenic belts. Examples from southern Africa, where there exists an almost complete record of Precambrian geology, are presented to illustrate the development which took place in five major tectogenetic cycles. There is some evidence for the existence of a thin sialic crust some 4000 m. y. ago on which the Early Precambrian greenstone belts formed during the first tectonic event (>3500-3000 m. y.) as a result of vertical movements caused by convection processes in the upper mantle. However, the early crust in southern Africa also has analogies with present-day abyssal oceanic regions and the greenstones may have developed as primitive island arcs on a primordial ultramafic lithosphere which underwent partial melting. The greenstone belt evolution was accompanied by enormous granite intrusions, thickening and stabilizing the crust sufficiently to permit the development of ancient nuclei on which the first cratonic-type sedimentary basins formed. The Middle Precambrian crustal development is characterized by the formation of mobile belts which surround the nuclei and whose geologic evolution contrasts strongly with the greenstone belts, as shown here for the Namaqualand Gneiss Complex. The mobile belts have been subjected to multiple deformation in three distinct tectonothermal events (>2500-900 m.y.), resulting in high-grade metamorphism, granitization and complex fold patterns. Finally, they were added to the ancient nuclei to form the first continental shields. The Late Precambrian tectogenetic event begins with the development of extensive subduction zones along the marginal parts of some shields. These basins were filled with typical geosynclinal deposits which subsequently suffered alpine-type deformation during the Pan-African Orogeny (700-450 m.y.). A plate tectonics model is presented for the evolution of the Gariep geosyncline along the west coast of southern Africa. © 1973 Ferdinand Enke Verlag Stuttgart.

Año de publicación:

1973

Keywords:

Fuente:

scopus