Abstract The metamorphic basement units of the Upper Ouémé watershed in Benin have been investigated to identify the structural controls on aquifer properties, groundwater flow and water balance at large scale. Spatial analysis of borehole and hydrogeophysical data suggests that large-scale weathering profiles, aquifer transmissivity and storage properties are better correlated to a palaeo-weathering surface. Multi-model analysis, combined with assessment of nine transient numerical groundwater models against observations, suggests the best conceptualizations are those where hydraulic conductivity and specific yield are distributed within a weathered zone determined through interpolation of weathered zone thickness. When compared to previous studies, the general groundwater balance of simulated models suggests the groundwater system contributes, on average, 49.8 m 3 s −1 to the river flow (mostly during the rainy season). The same volumetric flow would be lost to groundwater evapo-transpiration and deep/lateral drainage of the catchment. Borehole abstraction (about 7.5 m 3 s −1 ) represents only 6% of the average groundwater recharge and 1% of the average rainfall. This suggests that despite relatively low borehole productivity, the basement aquifer system still has an important unused potential for rural to mid-scale water supply and that, at present, the main external drivers for groundwater resource sustainability are changes in climate and land use.

Precambrian terranes and units of the Dahomeyan (now Beninian belt) from Togo, Benin, and southwest Nigeria are described and compared with those from the northernmost part of the Brasiliano belt. The major phases of deformation and metamorphism occurred about 600 Ma in both continents. Foreland nappes derived from passive margin sedimentary beds of the West African craton display high-pressure metamorphism. Parautochthons include slices of an ophiolitic-type assemblage. The suture zone metabasic rocks are only recorded in the Beninian belt. They may represent the granulitized and eclogitized mafic root of ensimatic arc terranes, and have no equivalent in northeast Brazil. The Nigerian Province includes to the west a narrow elongated belt of high-pressure granulites, also found in the northernmost part of the Borborema Province of northeast Brazil. Two main lithostratigraphic units can be recognized in both provinces: (1) gray gneisses that derive from Archean plutonic rocks, thoroughly deformed, recrystallized, and remobilized during the Pan-African–Brasiliano thermo-tectonic events; and (2) Proterozoic monocyclic units displaying the same petrostructural evolution as reworked Archean, and in which we tentatively recognize a Lower Proterozoic group consisting of aluminous metaquartzites and pelitic schists, which were intruded by 2(?) to 1.8 Ga anorogenic granites, and a younger disconformable flysch-type unit of assumed Late Proterozoic age. Large-scale horizontal movements responsible for a flat-lying foliation and nappes affected several domains of the Nigerian and Borborema Provinces, whereas steep structures formed penecontemporaneously along synmetamorphic shear zones. Second-order late-metamorphic steep shear zones are considered as trans-continental lithospheric fractures that overprinted collisional structures. Among these, the 4°50′ Kandi fault and the Sobral fault represent a particularly good correlation of northeast Brazil and southwest Nigeria-Benin, and are used to propose a rigorous Precambrian fit across the south Atlantic Ocean.

Although granulite facies rocks are present in zones of orogenesis ranging in age from 300 to 3,000 m.y. in Africa, only two distinct ages of granulite facies metamorphism, at about 1,200 and 3,000 m.y., have thus far been recognized. Granulite facies suites in regions of orogenesis of other ages — 1,850 ± 250 m.y., 600 ± 100 m.y., and Phanerozoic — represent older remnants since affected by regional retrograde metamorphism. The retrogression is particularly widespread in the 600 ± 100 m.y. orogenic zone, an important part of which is interpreted in terms of a former zone of granulite facies rocks, remnants of which are now preserved as isolated masses in amphibolite facies rocks. In most African examples, hypersthene granulites of silicic to mafic and ultramafic composition are associated with undoubted metasedimentary rocks and a variety of other rock types, including anorthosite; intrusive chamockite also occurs in a number of regions. The alkali-silica plot for 240 analyses of pyroxene granulite and chamockite suggests a chemical affinity with high-alumina basalt and its calc-alkaline derivatives; the AFM trends are similar to those of calc-alkaline and alkaline igneous suites. With few exceptions, rocks of appropriate composition in the granulite facies contain hypersthene + diopside + plagioclase ± garnet, reflecting medium to high pressure. Available Sr-isotopic data for granulite-chamockite suites of Africa and elsewhere in the world yield a crude “evolution path” that may reflect Sr-isotopic development in the deeper parts of the Earth’s crust or in the mantle beneath the continental crust.