Active tectonic studies of the Atlas and Rif mountains, northern Morocco

Francisco Gomez, Muawia Barazangi, Weldon Beauchamp, Alex Calvert, Dogan Seber
Department of Geological Sciences/INSTOC, Snee Hall, Cornell University, Ithaca,   New York, 14853

Mohammed Bensaid, Mohammed Dahmani
Ministère de L'Energie et des Mines, Nouveau Quartier Administratif, Agdal-Instituts, B.P. 6208, Rabat, Morocco

Ahmed Demnati, Mohamed El-Alji
Office National de Recherches et D'Exploitations Pétrolières, Av. Al Fadila Quartier Industrial, C.Y.M. B.P. 8030, Rabat, Morocco

Ahmed Er-Raji
Department de Géologie,Université Mohammed V, B.P. 1014, Rabat, Morocco

Aomar Ibenbrahim, Nacer Jabour
Centre National de Coordination et de Planification de la Recherche Scientifique et Technique, Laboratoire de Géophysique, B.P. 1346, Rabat, Morocco

     Northern Morocco is a broad zone of varied neotectonic activity composed of the Rif mountains, near the plate boundary with western Europe, and the intracontinental Atlas mountains.    The Atlas mountains developed as a series of rift basins in the Early Mesozoic that were tectonically inverted during the Late Mesozoic and Cenozoic.  Present-day tectonic activity in the Atlas system, manifested by moderate seismicity, appears to be influenced by the older, inherited structures.  A multidisciplinary approach involving earthquake seismology, reflection seismology, drill hole data, structural geology, remote sensing, and geomorphology is used to study the neotectonics of the Middle Atlas, central High Atlas, and Rif regions.
     The Middle Atlas is characterized by moderate seismicity and alkali volcanism throughout the Quaternary.  In addition to seismicity, geomorphology provides evidence for recent tectonic activity.  Kinematic analyses of Quaternary fault-slip data indicate predominantly dip-slip movement along the main fault zones; thrusting is observed along the flanks of the Middle Atlas with some apparently localized normal faulting.  Conspicuously absent is evidence for young strike-slip deformation as suggested by the Trans-Alboran shear zone model.
     Detailed geological mapping along the eastern flank of the Middle Atlas has revealed the involvement of low-angle fault geometries, thrusting lower and middle Mesozoic rocks over upper Neogene and Quaternary sediments.  In the context of tectonic rift inversion, these structures may represent foot-wall shortcuts as described from other field areas (e.g., Pyrennees) and experimental models.  Similar shortcut structures have been identified using seismic reflection profiles to the east in the Missour basin.  More examples of thrusting along the flanks of the inverted Atlas rift system can be observed in seismic reflection profiles from the High Atlas.  Thrust faults observed in seismic reflection profiles along the southern margin of the High Atlas within the Ouarzazate basin and may correspond with fault scarps described by other workers, and similar thrusting is also observed along the northern margin of the High Atlas near Beni-Mellal and within the Tadla basin.  These low-angle structures imply considerably more intracontinental shortening has occurred than has been previously suggested.
     For a better understanding of the intracontinental tectonics, it is also necessary to examine the associated plate boundary region of the Rif mountains.  The Rif mountains are part of the Alpine collisional belt, but the present-day tectonics are quite distinct from the alpine thrusting.  Today, the Rif is a complicated region characterized by strike-slip and normal faulting, and the area of Al Hoceima along the Mediterranean coast demonstrates this well.  A study of the Al Hoceima area following a 1994 earthquake there (Mw=6.0) combined with re-analysis of published micro-earthquake focal mechanisms reveals a diffuse zone of deformation with overall strike-slip kinematics.
     A regional view of the kinematics of North Africa provided by examining published fault plane solutions for moderate and larger earthquakes demonstrates the differences between the Atlas system, the Rif domain, and the Tell Atlas of Algeria.  The crustal shortening of the intracontinental Atlas region contrasts with the strike-slip deformation of the Rif mountains.  Furthermore, the Rif mountains do not appear to be simply an extension of the Tell Atlas of Algeria as the thrust kinematics of the Tell Atlas are quite different from the strike-slip tectonics of the Rif.  Thus, it can be argued that the plate boundary between western Europe and Africa shows a complex and varied tectonic deformation distributed across a zone approximately five hundred kilometers in width.