Brittle tectonics within the Jurassic formations of the Ouarsenis culminating area, northwestern Algeria

• Analysis of the deformation within the Ouarsenis massif shows NW–SE and NNE–SSW compressive phases.
• The tectonic contacts girdle the whole massifs and are marked out by reverse faults.
• The limits of the massifs are often characterized by dextral strike-slips.
• A sinistral shear, N120° oriented, is at the origin of block rotations and tilts.

Brittle structures within the Ouarsenis culminating zone were studied in order to characterize the polyphased tectonics. Field observations revealed that the dominant structures are highly tilted. They are represented by faults and joints, shears, reverse or curvilinear, locally associated with gypsum intrusions. Field data analysis shows that large scale shear faults are localized within the limits of the structural entities. Two main tectonic phases, having affected the Ouarsenis culminating zone were identified: (i) A NW–SE oriented compressive phase, represented by the contacts marked out by in-depth reverse faults and folding, mainly located in the westernmost part of Sra Abdelkader, Rokba Atba and in the western part of Batha, Fartas and Belkeiret (thrusts), respectively. This phase was also identified in reverse and sinistral faulting within Sidi Djber at the easternmost part of Sra Abdelkader, and the Grand Pic and the southern massif of Belkheiret respectively. (ii) A NNE–SSW oriented compressive phase, characterized by dextral strike-slips, is located in the easternmost part of Sra Abdelkader and Rokba Atba.North of the Grand Pic, a sinistral ∼N120° oriented shearing occurred, with a lateral slip reaching ∼2 km. The examination and the data analysis show a major N110-120° oriented fault with a sinistral signature. This movement would be responsible for block rotations and tilts. Counterclockwise rotations of the paleostress of ∼38° and ∼43° from west to east of the Sra Abdelkader massif and from Belkeiret to Batha, Fartas blocks were recorded, respectively. This local change of the stress field is mainly due to a deformation controlled by the regional post-Miocene geodynamic system, possibly explained by the proposed model.