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

TLDR

The Atlas system in Morocco, Algeria, and Tunisia forms a major morphologic barrier along the Sahara, exhibiting a transition from thick‑skinned deformation in Morocco to thin‑skinned in Algeria and Tunisia, and varying spatial relationships with the adjacent Tell‑Rif system across the region. During the Oligocene to middle Miocene, the Tell‑Rif accretionary prism grew concurrently with rollback of the Maghrebian Tethys lithosphere, driving the opening of the western Mediterranean Sea. Atlas uplift occurred in two distinct late Eocene and Pleistocene‑lower Quaternary phases, with an additional Tortonian thrust event in the east, reflecting two jolts in Africa‑Europe convergence that marked the start and stop of subduction in the western Mediterranean.

Abstract

The Atlas system (Morocco, Algeria, and Tunisia) constitutes an important morphologic barrier fringing the Sahara platform. Its structural style changes 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 also different in eastern Algeria and Tunisia where the two systems are adjacent and in western Algeria and Morocco where they are separated by large rigid cores (Moroccan Meseta and Algerian High Plateaux). New data, as well as a reappraisal of available data, show that the Atlas build up occurred everywhere during two main phases of late Eocene and Pleistocene‐lower Quaternary age, respectively. These phases are clearly distinct and do not represent end points of a progressive deformation. An additional Tortonian event exists in the eastern region where the Tell‐Rif is thrusting directly over the Atlas. From Oligocene to middle Miocene the development of the Tell‐Rif accretionary prism is coeval to subduction rollback of Maghrebian Tethys lithosphere 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 that they record two jolts in the convergence of Africa with respect to Europe and correspond roughly to the initiation and the cessation of the subduction processes active in the western Mediterranean region.