Deformation characterization of a regional thrust zone in the northern Rif (Chefchaouen, Morocco)

This paper provides the structural analysis of the Chefchaouen area in the northern Rif. Here the Dorsale Calcaire superposes, by means of an excellently exposed thrust fault, onto the Predorsalian succession in turn tectonically covering the Massylian Unit. Hanging wall carbonates of the Dorsale Calcaire Unit form a WSW-verging regional fold with several parasitic structures, deformed by late reverse faults in places indicating an ENE vergence. A two hundred meters thick shear zone characterizes the upper part of the Predorsalian succession, located at footwall of the Dorsale Calcaire Unit. Here the dominantly pelitic levels are highly deformed by (i) C’ type shear bands indicating a mean WSW tectonic transport and (ii) conjugate extensional shear planes marking an extension both orthogonal and parallel to the shear direction. The Massylian Unit is characterized by a strain gradient increasing toward the tectonic contact with the overlying Predorsalian succession, where the dominantly pelitic levels are so highly deformed so as appearing as a broken formation. Such as the previous succession, conjugate extensional shear bands and normal faults indicate a horizontal extension parallel to the thrust front synchronous with the mainly WSW-directed overthrusting. The whole thrust sheet pile recorded a further shortening, characterized by a NW-SE direction, expressed by several reverse and thrust faults and related folds. Finally strike-slip and normal faults were the last deformation structures recorded in the analyzed rocks. A possible tectonic evolution for these successions is provided. In the late Burdigalian, the Dorsale Calcaire Unit tectonically covered the Predorsalian succession and together the Massylian Unit. The latter two successions were completely detached from their basement and accreted in the orogenic wedge within a general NE-SW shortening for the analyzed sector of the northern Rif. At lithosphere scale the thrust front migration was driven by roll back and slab tear mechanisms producing a synchronous arching and related counterclockwise rotation of the tectonic prism along the African margin. Radial displacement involved extension parallel to the thrust front well-recorded in the analyzed rocks. The NE-SW shortening, probably acting in the Tortonian-Pliocene interval, was related to the final compression of the Rif Chain resulting in out-of-sequence thrusts affecting the whole orogenic belt.