Geodinámica de las Cordilleras del Alto y Medio Atlas: síntesis de los conocimientos actuales

Zusammenfassung

We discuss recent developments on the geodynamics of the High and Middle Atlas of Morocco, two high-topography mountain belts developed within the African plate. The NE-trending Middle Atlas and the nearly E-trending High Atlas derived from inversion of an orthogonal and an oblique rift respectively, both essentially Jurassic in age. NNW-SSE-directed tectonic shortening is modest, decreasing from E to W along the High Atlas with a maximum value of 24%, and reaching only 10% in the Middle Atlas. The age of deformation is Oligocene to Quaternary in the frontal thrust belt of the southern High Atlas and the Ouarzazate basin, where there is the best tectonosedimentary record of the Atlas orogenic system. First foreland deposits suggest however that orogenic growth may have started already in Mid Eocene times. Deformation is heterogeneously distributed in time and space. Despite the modest shortening values, the mean elevation exceeds 2000 m over large areas. Gravity modelling suggests the existence of a relatively thin crust (< 40 km) that implies a lack of isostatic compensation at crustal level. Topography appears to be equilibrated at the upper mantle by a 100-km scale lithospheric thinning deduced from potential field modelling. This thinning can be attributed to a buoyant mantle upwelling, independent from the local tectonic regime, and may explain the occurrence of abundant alkaline magmatism of Cenozoic age. The timing of the last event of lithospheric thinning is constrained between 15 Ma to recent on the basis of the related alkaline magmatism, but direct geomorphic evidence based on paleoelevation markers suggests that the bulk of the mantlerelated, thermal uplift occurred late with respect to magmatism and shortening, that is, in post-Miocene times (last 5 Ma). A general small amount of erosion prevents to detect Cenozoic central ages in apatite fission-track thermochronology, except from a narrow segment in the Atlas of Marrakech. Thermal modelling of Jurassic intrusives from the central High Atlas constrained by (U-Th)/He suggests a continued post-magmatic/ post-rift cooling episode from the intrusion time to 40 Ma ago (central AFT ages of 80-90 Ma are within this trend); then rapid exhumation to surface took place in the past 20 Ma, coherent with central ages of 17- 25 Ma in the Atlas of Marrakech. These Miocene ages are attributed to mountain building by crustal shortening, previous to the magmatic surge and the long-wavelenth surface uplift. Although roughly coeval, the precise timing relationships of deformation, uplift and exhumation are complex.