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

• A. Teixell ^[1] ; P. Ayarza ^[2] ; E. Tesón ^[1] ; J. Babault ^[1] ; F. Alvarez-Lobato ^[2] ; M. Charroud ^[3] ; M. Julivert ^[1] ; L. Barbero ^[4] ; M. Amrhar ^[5] ; M.L. Arboleya ^[1]
  1. [1] Universitat Autònoma de Barcelona

     Universitat Autònoma de Barcelona

     Barcelona, España

     
  2. [2] Universidad de Salamanca

     Universidad de Salamanca

     Salamanca, España

     
  3. [3] Sidi Mohamed Ben Abdellah University

     Sidi Mohamed Ben Abdellah University

     Fes-Medina, Marruecos

     
  4. [4] Universidad de Cádiz

     Universidad de Cádiz

     Cádiz, España

     
  5. [5] Cadi Ayyad University

     Cadi Ayyad University

     Marrakech-Medina, Marruecos

     

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• Localización: Revista de la Sociedad Geológica de España, ISSN 0214-2708, Vol. 20, Nº. 3-4, 2007, págs. 333-350
• Idioma: español
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• Resumen
  • español

    Se presenta una síntesis geodinámica de las cadenas de intraplaca del Alto y Medio Atlas de Marruecos. Estas cadenas derivan de la inversión tectónica de cuencas extensionales Jurásicas, con un acortamiento orogénico modesto (máximo del 24% en el Alto Atlas y del 10% en el Medio Atlas) heterogéneamente distribuido en el tiempo y en el espacio. Las relaciones entre tectónica y sedimentación en el margen sur del Alto Atlas indican que la deformación compresiva tuvo lugar entre el Eoceno medio y el Cuaternario. Consecuentemente con los valores de acortamiento, el espesor de la corteza detectado por gravimetría no supera los 40 km. A pesar de ello, la topografía de las cadenas del Atlas es elevada, y el modelado de campos potenciales sugiere que está causada por un marcado adelgazamiento litosférico, independiente de la tectónica compresiva regional. Un abundante magmatismo alcalino indica que el adelgazamiento litosférico coexistió con el acortamiento en los últimos 15 Ma, aunque los indicadores geomorfológicos sugieren que la mayor parte del levantamiento de origen mantélico es relativamente reciente (últimos 5 Ma). La termocronología indica que la cantidad de erosión es generalmente pequeña en el Atlas, registrándose de manera notable solo en los últimos 20-25 Ma.

  • English

    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.