The two main steps of the Atlas building and geodynamics of the western Mediterranean

The Atlas system (Morocco, Algeria, and Tunisia) constitutes an important m orphologic barrier fringing the Sahara platform. Its structural style chang es along strike from a thick-skinned style in Morocco to a thin-skinned one in Algeria and Tunisia The position relative to the Tell-Rif system is als o different in eastern Algeria and Tunisia where the two systems are adjace nt and in western Algeria and Morocco where they are separated by large rig id cores (Moroccan Meseta and Algerian High Plateaux). New data, as well as a reappraisal of available data, show that the Atlas build up occurred eve rywhere during two main phases of late Eocene and Pleistocene-lower Quatern ary age, respectively. These phases are clearly distinct and do not represe nt end points of a progressive deformation. An additional Tortonian event e xists in the eastern region where the Tell-Rif is thrusting directly over t he Atlas. From Oligocene to middle Miocene the development of the Tell-Rif accretionary prism is coeval to subduction rollback of Maghrebian Tethys li thosphere and related to the opening of the western Mediterranean Sea. For kinematic and chronological reasons this process cannot account for the two specific steps of the Atlas building. They are better explained assuming t hat they record two jolts in the convergence of Africa with respect to Euro pe and correspond roughly to the initiation and the cessation of the subduc tion processes active in the western Mediterranean region.