Transcurrent tectonics and polycyclic evolution in the lower proterozoic of Senegal-Mali

Abstract

Outcrops of the Early Proterozoic Birimian series within the Kedougou inlier (straddling the Mali-Senegal border) have traditionally been described as being composed of a basal volcanic Mako series, overlain by a detrital/carbonate Dialle-Dalema series. This geosynclinal context was later reinterpreted in terms of oceanization and subduction, as the Mako series contains, successively, basaltic pillow lavas of tholeiitic affinity and a calc-alkaline complex. However, the sedimentary Dialle-Dalema series lie in the core of a major anticlinorium and the rocks show overprinting of an earlier by a later tectonic phase; the volcanic and sedimentary rocks of the Mako series were affected only by the later tectonic phase. In addition, a fan-delta deposit intercalated between the tholeiitic and calc-alkaline rocks contains pebbles of schistose and folded sedimentary rock that clearly belong to the Dialle-Dalema series. These observations necessitate a complete revision of the local stratigraphy. Thus, the clastic deposits of the Dialle-Dalema are attributed to the Lower Birimian and are interpreted as having been deposited in intra-cratonic basins. The first tectonic phase took place before emplacement of the volcanic sereis, which shows evidence of an extensional setting with major crustal fracturing. The second deformation affected both the sedimentary and volcanic series. Structural analysis furthermore shows that the formation of the second cleavage (S 2) was contemporaneous with the emplacement of major granite bodies, as well as with major transcurrent sinistral shear zones. Finally, this evolution is shown to be comparable with that in other Lower Proterozoic terranes of the West African craton.