Precambrian mobile belts of southern and eastern africa — ancient sutures or sites of ensialic mobility? a case for crustal evolution towards plate tectonics

Abstract

There is still considerable controversy among earth scientists whether plate-tectonic processes as envisaged for the Phanerozoic have been operative since the generation of first sialic crust in the Archaean or whether global tectonics underwent an evolutionary process during the Precambrian history of the earth thereby gradually leading, through various stages, to the present plate-tectonic phase. Southern and eastern Africa represent excellent examples to study the development of a variety of mobile belts ranging in age from more than 3.0 b.y. to the end of the Precambrian and it is attempted here to test whether field geology, tectonics, geochronology and palaeomagnetic data in such belts as the Limpopo, Ubendian, Irumide and Pan African are compatible with plate-tectonic processes, notably the continental-collision model. Most of the above belts have in common that orogenic reworking of pre-existing sialic crust has taken place on a grand scale and that mafic volcanic rocks of ophiolite affinity are completely lacking, even in those belts which are not eroded to their roots. It is seen as significant that some linear belts cut older ones without completely destroying the ancient geologic record and that some belts end within the continent and were apparently not associated with a plate margin. It is also demonstrated that considerable continental thickening, following collision in a plate-tectonic model, has obviously not taken place behind some belts and that potash granites are often not generated in the lower crust through fractional crystallization of calc-alkaline magma but through melting and mobilization of the geosynclinical fill and its underlying sialic basement. Features in accordance with modern plate-tectonic concepts such as well developed suites of mafic volcanics, including ophiolites, and clearly defineable plate margins are found for the first time in Africa during the development of some Pan-African belts. It is proposed that ensialic orogeny is caused by the same subcrustal forces which are operative during plate separation, ocean opening and continental collision. If these forces are not strong enough to cause complete continental rupture, linear zones of weakness and intraplate graben systems, aulacogens or geosynclinal basins develop in the crust. The orogenic cycle is set in motion through compressive stress in the lithosphere, gravitational instability, and increased heat flow through the fractured crust in the zone of weakness. All transitional stages from non-orogenic intraplate graben systems through ensialic mobile belts to collision belts following plate separation and ocean closure should therefore exist. It is further proposed that horizontal mobility of large crustal plates increased through geologic time. An early phase of predominantly ensialic orogenic evolution up to the mid-Proterozoic was followed by a phase where plate separation and modern plate-tectonic processes became increasingly common (late Precambrian/early Paleozoic), thus leading over to the Phanerozoic phase of widespread ocean opening and plate collision.