THE PALAEOPROTEROZOIC (BIRIMIAN) OF HAUTE-COMOE IN THE WEST-AFRICAN CRATON, IVORY-COAST - A TRANSTENSIONAL BACK-ARC BASIN

Deformation in the Palaeoproterozoic of Haute-Comoe, 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 N-S, N 25 degrees E, and N 130 degrees E, of which at least the first and third sets were essentially transcurrent faults. The volcanosedimentary and subordinate, mainly tholeiitic to calc-alkaline, volcanic rocks of the Tehini succession (Serie de Tehini) that characterise the Haute-Comoe belt, overlie detrital sedimentary rocks and are overlain by the thick Comoe succession of sedimentary rocks (Serie de Ia Comoe). Geochemically, the Tehini succession has affinities with ensialic rift rocks in the south and with rocks of back-are basins in the north. This model differs from the oceanic plateau setting proposed by Abouchami et al. (1990) for the Birimian of West-Africa. Tehini and Comoe successions have been affected throughout only by transcurrent-type Eburnean deformations (D2 and D3 of Milesi 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-are basin established after similar to 2200 Ma and within the Burkinian crust that had previously been deformed in an early Palaeoproterozoic orogenic cycle at similar to 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 similar to 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 degrees E faulting and crenulation cleavage (S4) and SSW thrusting upon the earlier deformation fabrics.