INVERSION TECTONICS OF INTRACONTINENTAL RANGES - HIGH AND MIDDLE ATLAS, MOROCCO

The High and Middle Atlas are intracontinental mountain belts situated within the mobile foreland of the Mediterranean Rif orogen. They deve loped in three stages. The first period (Permian - Bathonian) culminat ed during the Lias with extended rift grabens and tholeiite extrusions . From Callovian to Eocene, the tectonic activity and the rates of sed imentation were reduced, both pointing to a cooling of the lithosphere . Since the Oligocene, the whole region is submitted to compressional stress. The High and the Middle Atlas were uplifted within two phases, which were correlated with main phases of Rif orogenesis. Refraction seismic measurements have recently revealed there a flat layered struc ture of the crust with several low velocity zones. The deepest one coi ncides with a layer of high electric conductivity, which is interprete d as a zone of detachment. From the geotectonic evolution of the High and Middle Atlas and from the structure of the crust, the following mo del was deduced: During Early Mesozoic rifting, the crust on top of th e mantle elevations was thinned by both extensional fracturing and by gliding along intracrustal detachment planes. During the Cenozoic coll isions of the Rif, these shear planes were reactivated by thrusting in opposite directions. Compressional deformation of the graben fillings led now to a moderate thickening of the crust, e.g. up to 40 km benea th the High Atlas. Subsequent uplift and inversion was not only caused by isostasy, but also by squeezing upward due to thick- and thin-skin ned tectonics.