Inversionsstrukturen - Ein Beispiel aus dem Mittleren Atlas (Marokko)
[Inversion structures An example of the Middle Atlas (Morocco).]
Authoren : Jamal Rhrib
Abstract :
The Moroccan Atlas system consists of intracontinental mountain ranges. Its evolution was controlled by the opening of the Atlantic and by the Alpine orogeny. Fault systems along strike of these mountain ranges played a major role in this process. This study focusses on the structural evolution along one of these fault zones, the Northern Middle Atlas Fault (= Accident Nord Moyen Atlasique: ANMA). This sinistral strike-slip fault divides the Middle Atlas into two units: the northwestern Tabular Middle Atlas (= Causse Moyen Atlasique) and the southeastern Folded Middle Atlas (= Moyen Atlas Plissi). The mainly NE/SW trending fault zone bends into N/S direction along its central third. Related to the NE/SW main trend are transversal NW/SE-fault trends. The NE/SW strike-slip faults as well as their NW/SE trending transversal counterparts both represent hercynian age basement faults affecting sedimentary cover by multiple reactivation since the Triassic. During the Mesozoic and Tertiary the following important events can be distinguished:
Synsedimentary active normal faults have been proofed along the ANMA, especially for block rotations along normal faults during the Upper Triassic, for intraformational unconformities during the Carixian and Domerian, and for the generation of a Toarcian halfgraben (Phase D1).
Slump structures closely related to NE/SW striking faults provide evidence for block tilting during the Dogger (Phase D2). \item The compressive Cretaceous-Eocene period started in pre-Barremian times and reached one maximum during the Senonian. The development of facies and thickness distribution resembles intense synsedimentary tectonic activities. Basal angular unconformities and conglomerates of variable thickness indicate continuing tectonic activities (Phase D3).
Tertiary thrusts develop during renewed compression, which are contemporaneous with decollement of the sedimentary cover of the Qued Zra-Block (Phase D4).
The paroxysmal character of the compressional tectonics during Upper Miocene and Quaternary can be confirmed through the development of mega tension fractures in the Middle Atlas by blocking the NE-SW sinistral strike-slip movements. These fractures reach the deeper lithosphere and cause rising of magma (Phase D5).
The evolution of longitudinal NW/SE trending Miocene basins was related to extensional fault systems, that controlled the development of halfgrabens, thus locking strike-slip motions along the ANMA. Such motions are also indicated by the occurrence of asymmetric Z-folds with vertical axes in Upper Eocene rocks from the Ain Nokra syncline as well as amygdal-structures from Jbel Hayane. Furthermore, the amount of lateral transport (7 km) along the ANMA can be estimated due to the discovery of a tectonic friction-breccia 20 km south of Taza consisting of Toarcian carbonate and basalt components supported by a matrix of red Triassic pelites. Besides, the Z-folds permit the calculation of the main deviatoric paleostress 1 directly at the ANMA.
1 directly at the ANMA.