Neotectonic deformation stages in the central Ouarsenis culminating zone, Northwestern Algeria
Résumé
The Tellian system in the north of Algeria results from the convergence of the African and Eurasian plates which began in the Late Cretaceous. The strong tangentional tectonics at the origin of the emplacement of nappes in its external domain occurred mainly during the Early Miocene. This major tectonic episode was followed by another important compressive deformation-oriented NNE-SSW and NNW-SSE during the Pliocene and the Quaternary, respectively. The Ouarsenis culminating zone is part of the Tellian domain and is characterized by an altogether distinct orographical structure made up of: (i) Jurassic formations which overthrust Cretaceous terrains; (ii) completely upturned series; diversely oriented faults (N40°, N70°, N120°, and N160°) of different kinds (thrust, reverse, normal, and shear faults). Triassic gypsum crops out along some of these faults. Microtectonic data analysis has shown alternation of two main compressive stresses, NW-SE and NE-SW oriented. The more recent stress, probably of Pliocene age, ∼N56° oriented, is responsible for the current face of the culminating zone. It highlights a major ∼N120° sinistral shear fault-generated deformation especially in its central part, affecting ductile material represented by Albo-Aptian turbidites. This fault also generated secondary shears accommodated according to a Riedel deformation model. The central part of this area has a complex tectonic structure squeezed between two massifs composed of hard material, along the sinistral shears. It has been extruded towards the north and has evolved as a positive “flower structure.”