| Popis: |
The Atlas system of Morocco comprises the Mediterranean Rif orogen and three foreland intracontinental ranges: Middle, High and Anti-Atlas, which behave as mobile zones within the stable platform. For this region, a review is given on the major structural elements, the sedimentary and volcanic evolution from Triassic to recent, the crustal and upper mantle structure and the seismicity. Based on these data the following geodynamic model has been derived: The break-up of Pangaea and the subsequent formation of the North Atlantic and the western Tethys affected to some extent the northwestern margin of the African continent. During the Triassic-Dogger interval, rift grabens, pull-apart basins and accompanying volcanism are features of these moderate mobilizations. From Upper Jurassic onward the rifting processes slowed down and ceased during the Eocene. The accompanying magmatism changed from basaltic effusions to alcaline intrusions, thus indicating a change from a tensional to a compressional stress-field. The third period of the evolution, from Oligocene to recent, was governed by compressional movements between Africa and Europe, culminating in collisional structures of the Betic Cordilleras and the Rif. Compression and uplift of the Rif Atlas and of the three intracontinental belts took place within the same time intervals, thus implying a common relationship to certain plate tectonic events. As evidence by magnetotelluric and seismic studies intracrustal shearing and detachment have been facilitated by the existence of intracrustal high conductivity and low velocity layers, caused by Mesozoic and Tertiary thermal activities. A mechanism of transferring compressional stress from the active margin to its foreland is provided by the BALLY model of thin-skinned thrusting. The combination of these principles with the structural situation in northern Africa suggests the formation of an inclined thrust plain structure, which has penetrated the sedimentary cover and deeper crustal complexes. |
