TECTONOMETAMORPHIC EVOLUTION OF GHANA, TOGO AND BENIN IN THE LIGHT OF THE PAN-AFRICAN BRASILIANO OROGENY

The South Pan-African belt of West Africa (Ghana, Togo and Benin province) can be divided into three domains: (1) the external nappes thrust over the West African craton, derived from passive-margin sedimentary deposits (Middle to Late Proterozoic) and displaying middle- to high-pressure metamorphism, such as the Kante and Atacora nappes; (2) the intermediate nappes including the suture-zone rocks with eclogitic assemblages (protoliths ca. 800 Ma), and mainly monocyclic metasedimentary rock and orthogneiss, that are found in central Togo and northwest Benin; (3) the internal nappes composed of high-grade commonly anatectic gneiss (anatexis ca. 600 Ma), including orthogneiss (Eburnian plutons), elongated granulite belts, reworked Archean basement, and an intrusive granite and charnockite association (ca. 600 Ma), which are exposed in Benin and Nigeria. The P-T-t-d paths calculated from rocks in these three domains show a prograde-retrograde evolution: (1) an initial clockwise evolution related to an early burial stage (H P-HT in the intermediate nappes) followed by oblique thrusting in a SW direction, that caused nappe stacking and subsequent uplift; (2) a second anti-clockwise evolution with late temperature and pressure increases, only recorded in the more internal units. This temperature increase was a consequence of the previous crustal thickening with subsequent doming, extension and anatexis. The pressure increase was related to movement along the Kandi fault, working as a dextral strike-slip fault with a transpressional component under late E-W shortening. The retrograde trend corresponds to the final uplift. The evolution of the belt can thus be divided into three successive stages: (1) a first stage was characterized by an oblique collision inducing SW oblique thrusting with associated syn-foliation reverse metamorphism; (2) during a second stage anatectic doming took place; followed by (3) a third stage at still high temperature with dextral wrench faulting and associated N-S post-foliation folding under late E-W shortening. Comparison between the West African, North Brasiliano and Central African Late Proterozoic belts shows very strong similarities between their lithology, kinematics and metamorphism, and indicates that the Pan-African/Brasiliano chain resulted from collision between three major continental domains: the West African and Sao-Francisco/Congo cratons and a Late Proterozoic mobile domain.