Basement Complexes and Regional Plateaus

Abstract

The basement complexes of East Africa are dominated by hard, crystalline, metamorphic rocks from the Archaean and Neoproterozoic Eras. The oldest rocks in this region occur in the Lake Victoria Terrane, a segment of the Central African craton with an age of 2.810–2.560 Ga. The two main components of this terrane are greenstones, a group of metamorphosed, mostly basaltic rocks, and granite-gneiss. The greenstones occur in relatively small, arcuate bodies, with the granite-gneiss forming much larger, overlapping plutons. Sections of the Serengeti Plains to the west of the Gregory Rift Valley in northern Tanzania are underlain by the Lake Victoria Terrane . The Central African craton is bordered on the eastern side by the Mozambique Belt, a Neoproterozoic-age mobile belt which extends from southern Africa into the Arabian Shield. The Mozambique Belt is dominated by quartzite and schist, together with plutons of granite. The Lake Victoria Terrane contains economic mineral deposits, particularly of gold. The Mara gold fields overlap into the Serengeti National Park and were subjected to a ‘gold rush’ in the 1930s. There are several operating mines near Lake Victoria. The Mozambique Belt contains important gemstone deposits. Locally named minerals such as tanzanite and tsavorite are marketed internationally. The gemstones developed during the high-pressure regimes of regional metamorphism on the edge of the Central African craton. The basement complexes were uplifted and eroded during the breakup of the supercontinent of Gondwana. This process commenced in the Jurassic at approximately 180 Ma and persisted into the Palaeocene. Multiple cycles of erosion during the hot and humid climatic regimes of the Jurassic through Palaeocene planed off the uplifted terranes to create the regional plateaus for which East Africa is so renowned. The most prominent is the 70-Ma-old (Cretaceous) African Erosion Surface which is associated with most of the plateaus at elevations of 1,200–1,500 m.

Photographs not otherwise referenced are by the author.