Evolution of the West African fold belts: Review, new geochronological data, new correlations and new geodynamic hypothesis

Abstract

Several synthetic papers have been published about the geological structure of the threeWest African fold belts (Rokelides, Bassarides and Mauritanides), which were attached to the western margin of the West African Craton. Owing to the paucity of radiometric ages, some orogens and tectonic events have previously been confused and little consensus exists regarding their geodynamic evolution. Matching previous geological geochronological and geophysic data with nine new radiometric U-Pb ages on zircon, allows us to propose a new geodynamic model. Apart from ages of Mesoproterozoic events, four different orogens have been distinguished: Pan-African I (ca. 900 to 640 Ma), Pan-African II (ca. 640 to 520 Ma), Mali-Rokel River (ca. 490 to 450 Ma) and Variscan (ca. 450 to 300 Ma). The pan-African I orogeny is characterized by a complex evolution, terminating with a subduction process to the West, including the formation of a volcanic arc and a collisional event between ca. 660 and 640 Ma. The pan-African II orogen corresponds to a subduction process to the East leading to the building of a new volcanic arc and the formation of a back-arc basin to the East. Thenewly evidencedMali-Rokel River orogenic event led to the formation of extensive intra-continental N-S directed basins. The latter were folded and thrusted before the deposition of early Palaeozoic molasses. In the course of the Variscan Orogeny the Palaeozoicbasins setting in Mauritania and Northern Senegal were deformed during the Carboniferous Pangaea assembly. These two later orogensare intra-continental in the area but likely linked to far-field oceanic subduction to the West.For each orogenic period in the belts there are contemporaneous sedimentary deposits on the West African Craton and, of course, related unconformities between them. These data have facilitated correlations between the West African Belts and with the belts setting in adjacent areas like the Appalachians to the West and the northern Brazilian belts to the South.