The palaeoproterozoic (Birimian) of Haute-Comoé in the West African craton, Ivory Coast: a transtensional back-arc basin

Deformation in the Palaeoproterozoic of Haute-Comoé, Ivory Coast (2150 Ma to 1800 Ma) (Birimian or approximately Orosirian period according to the new Precambrian time scale of Plumb, 1991) was controlled by ductile faults trending NS, N 25°E, and N 130°E, of which at least the first and third sets were essentially transcurrent faults. The volcanosedimentary and subordinate, mainly tholeftic to calc-alkaline, volcanic rocks of the Téhini succession (Série de Téhini) that characterise the Haute-Comoé belt, overlie detrital sedimentary rocks and are overlain by the thick Comoé succession of sedimentary rocks (Série de la Comoé). Geochemically, the Téhini succession has affinities with ensialic rift rocks in the south and with rocks of back-arc basins in the north. This model differs from the oceanic plateau setting proposed by Abouchami et al. (1990) for the Birimian of West-Africa. Téhini and Comoé successions have been affected throughout only by transcurrent-type Eburnean deformations (D2 and D3 of Milési et al., 1989). The sediments are moderately folded, weakly metamorphosed, and strongly marked by ductile strike-slip deformation. The sequence could represent a rift-type or transtensional back-arc basin established after ∼ 2200 Ma and within the Burkinian crust that had previously been deformed in an early Palaeoproterozoic orogenic cycle at ∼ 2400 to 2150 Ma (Lemoine et al., 1985; Tempier, 1986). Burkinian crust (specifically Dabakalian basement granitoids of Rhyacian to Siderian age of Plumb, 1991) was the source of the detrital sediments which were deposited at the base of the Birimian, and has been almost entirely consumed during the massive intrusion of calc-alkaline plutons. The S1, S2 and S3 cleavages are marked by greenschist-facies mineral assemblages. This generally low-grade metamorphism is related to the emplacement of early granodiorites. The deformation (particularly S1 cleavage which has grown in conformity with the stratification) is closely related to granitic plutons. We regard this S1 cleavage as unrepresentative of a regional horizontal shortening phase. If the basin was oriented obliquely to the direction of closure, the presence of oceanic material and the lack of evidence for thrusting in a closed marginal basin can be explained by transcurrent shearing. It is probable that the basin, controlled by major intracontinental faults, was deformed at ∼ 2075 Ma, resulting in the Eburnean tectonometamorphic phases D2 and D3 by NW-SE compression and strike-slip shearing. A terminal phase of NE-SW compression superimposed N 120°E faulting and crenulation cleavage (S4) and SSW thrusting upon the earlier deformation fabrics. © 1994.