Northern margin of the Limpopo mobile belt, southern Africa

Abstract

The Limpopo mobile belt is a zone of complexly deformed Archean gneiss units and rocks of high metamorphic grade. The Limpopo belt separates Archean greenstone belts and gneiss units of lower metamorphic grade in the Rhodesian craton to the north from similar lower grade rocks in the Kaapvaal craton to the south. At the northern edge of the Limpopo belt (that is, the southern edge of the Rhodesian craton), volcanic and sedimentary rocks of the greenstone belts, dated at 2,600 m.y. B.P., were deposited on gneissic basement, dated at 3,500 m.y. B.P. The greenstone belts were intensely deformed (F 1 ) into recumbent folds and thrust slices before being intruded by large diapiric masses of granite. The subsequent deformation (F 2 and F 3 ) can be considered in terms of intracratonic block tectonics. During the main phase of deformation, which produced a regional finite strain fabric throughout the Limpopo belt and the southern part of the Rhodesian craton, the deformation can be considered as having been produced by the Rhodesian block moving to the southwest, parallel to (not across) the trend of the Limpopo belt. This movement produced horizontal shortening across the cleavage, normal to the Limpopo trend, by about 50 percent in southwest Rhodesia and northeast Botswana; in southeast Rhodesia, however, the result was northeast-trending zones of heterogeneous simple shear with nearly horizontal sinistral movement parallel to the Limpopo trend. During a later (F 4 ) phase, the deformation involved moving the Messina block, located in the central part of the Limpopo belt, west against the Rhodesian block, again nearly parallel to (not across) the Limpopo trend. In southern Rhodesia the junction between these two blocks is a zone of gently dipping mylonite, but in Botswana the junction is a steep shear zone. From the intensity of deformation within the mylonite zone, estimates of more than 40 km of displacement during this phase have been made.