Extensional tectonics, graben development and fault terminations in the eastern Rif (Bokoya–Ras Afraou area)

Abstract Westward motion of the Alboran Domain between the Eurasian and African plate boundaries determined crustal thickening along the southern border of the Gibraltar Arc, forming the Rif Cordillera. This process developed major sinistral NE–SW to ENE–WSW faults (such as the Nekor Fault), inactive since the Late Miocene. However, the Neogene–Quaternary Boudinar and Nekor basins underwent very intense recent tectonic and seismic activity related to N–S faults. Kinematics of this fault set changes with depth. While at ~ 10 km faults have a sinistral strike–slip kinematics, they become normal to normal–oblique at surface (Sfeha, Trougout and Boudinar faults). Their different kinematics could be explained by the existence of a crustal detachment separating two differently pre-structured domains. Shallow transtensive N–S faults trend orthogonal to the coastline, decreasing their slip southwards until disappearing. Paleostress analysis shows a progressive change from E–W extension near the coastline up to radial extension in southern areas of major fault terminations. The behavior of each fault-bounded block is conditioned by its inherited rheological features. The sequence of horsts (Bokoya, Ras Tarf, Ras Afraou) corresponds mainly to resistant rocks (volcanics or limestones), whereas the grabens (Nekor and Boudinar basins) are generally floored by weak metapelites and flysch. The presence of liquefaction structures, interpreted as seismites, underlines the continued recent seismic activity of the region. The recent structures deforming the two Alboran Sea margins come to support the continuity, at present, of orogenic processes undergone by the eastern internal regions of the Gibraltar Arc, involving regional E–W extension in the framework of NW–SE to N–S Eurasian–African convergence.