The crystalline basement of Africa comprises three major suites of rocks: the granite-gneiss-greenstone association of Archaean cratonic nuclei; strongly deformed metamorphic suites in mobile belts, mainly of Proterozoic age; and anorogenic intrusions which include Phanerozoic intrusive magmatic rocks related to rifting. This heterogeneous basement is extensively concealed beneath a variable thickness of diverse, unmetamorphosed sedimentary and extrusive volcanic rocks and weathering products.
The Archaean cratons have similar geological histories, which generally culminate with anorogenic potassic granites emplaced at about 2500 Ma. The history of the Kaapval Craton is prematurely curtailed at about 3050 Ma. The Limpopo Mobile Belt is uniquely Archaean: its development is linked to differential movement between the oldest cratonic elements of southern Africa. Elsewhere in Africa, the oldest Archaean record suggests an absence of thick continental crust: early greenstone belts formed above mantle plumes on mobile, thin crust.
The development of the Proterozoic mobile belts is related both to collision of older cratons (Wilson cycle orogenesis) and to ensialic disruption of single cratons. Early extensional orogenic phases produced both active (new oceanic crust development) and failed rifts. The earliest Proterozoic mobile belts cut across Archaean cratonic domains and later belts may be superposed. Older fractures are commonly reactivated by new stress systems. Much of the Phanerozoic anorogenic magmatism is related to Mesozoic and Cenozoic continental fragmentation and rifting, possibly associated with hot spots (White & McKenzie 1989).
Notwithstanding the relative antiquity of most of the basement, it is the effects of high-level brittle fracturing and weathering which largely control groundwater storage. The fracturing varies in age from Archaean, within the greenstone belts, to Phanerozoic, for faults related to movement of the African Plate, with ongoing Quaternary faulting in the major tensional rifts such as the East African Rift System. The thickest weathering profiles occur above the oldest erosion surfaces; regional thickness variations are controlled by past and present climatic differences.