Mobiles belts on the western part of the West African Craton and geodynamic interpretations

Until now, no satisfactory geodynamic model has been delivered concerning the three main West African orogens: Panafrican 1 (Bassaride belt), Panafrican 2 (Rokelide belt) and Hercynian (Mauritanide belt). However, since the last synthetic paper (Villeneuve, 2008), new geological, geophysical and geochronological data, from the Moroccan Sahara to Sierra Leone, allow us to propose a new geodynamical model. It includes the two Panafrican events in a single model very similar to the present western Pacific margin. An old "West African Neoproterozoic ocean" (WANO) was limited by a set of island arcs separated from the West African craton by a series of "back arc basins". The closure of this first round of back arc basins around 650 Ma led to the Bassaride belt (Panafrican 1). Then the WANO was subducting underneath the island arcs (between 650 and 550 Ma) meanwhile a new generation of "back arc basins" opened to the east between the arcs and the craton margin. The closure of the WANO and associated island arcs and back arc basins (550 to 500 Ma) led to the Rokelide belt (Panafrican 2). The Hercynian structures involving a Palaeozoic cover (made with continental material) associated to a "greeenschist facies" metamorphism is ascribed to an intracontinental belt.Original Abstract: Plusieurs syntheses ont essaye de resumer la geologie de la marge marocaine du craton Ouest-Africain, mais aucune n'a produit de modele geodynamique satisfaisant pour expliquer les trois orogeneses successives : Panafricain I, Panafricain II et Hercynien ayant conduit respectivement a l'edification des chaines des Bassarides, Rokelides et Mauritanides. Des travaux recents sur le Sahara marocain, sur les Mauritanides centrales, sur le substratum du bassin cotier, sur les Bassarides (au Nord de la Guinee) et sur les Rokelides (au Sud de la Guinee), ont permis de proposer un modele geodynamique integrant les chaines du Panafricain I et II dans un systeme de type marge < Ouest-Pacifique > avec subductions, arcs volcaniques et bassins d'arriere-arc. En revanche, la chaine hercynienne des Mauritanides serait, soit intracontinentale, soit un < front de nappes > relie a la suture appalachienne de l'Ouest Senegal.