Neotectonics of the Middle Atlas Mountains (Morocco):  Intracontinental Deformation in Response to the Tectonic Interactions of Africa and Europe

Francisco Gomez, Muawia Barazangi, Richard W. Allmendinger (All at: Institute for the Study of the Continents, Cornell University, Ithaca, New York  14853; e-mail:  fgomez@geology.cornell.edu)

Mohammed Bensaid, Mohammed Dahmani (Direction de la Geologie, Ministere de l'Energie et des Mines, Rabat, Morocco)

Ahmed Er-Raji (Universite Mohammed V, Rabat, Morocco)

 The western Mediterranean collision between North Africa and Spain resulted in the accelerated tectonic development of the intracontinental Atlas mountains of Morocco during the Neogene and Quaternary, a process involving the reactivation of inherited structures and inversion of early Mesozoic rifts.  The NE-trending Middle Atlas mountains of Morocco are a part of this system, obliquely oriented to the direction of this convergence.  A multidisciplinary approach involving field mapping, analysis of remote sensing and digital topography, and seismicity patterns is used to characterize the kinematic scheme and its relation to the paleogeography.
 The Middle Atlas mountains consist of two geomorphic provinces, the Folded and Tabular Middle Atlas, that also represent  the paleo-rift and the flank of the paleo-rift, respectively.  Topographic roughness and tectonic lineaments mapped from remote sensing imagery reveal subtle differences between the two regions.  Seismicity attests to present tectonic activity, and evidence for Late Quaternary uplift is provided by geologic and geomorphic observations along major fault zones in the Middle Atlas.  Kinematic analysis of Middle-Late Quaternary fault-slip data and earthquake focal mechanisms show remarkable consistency and demonstrate that both crustal shortening and extension are observed in the folded and tabular regions, respectively.  The observed extension is also consistent with the extrusion of alkali basalt during the Middle and Late Quaternary in the Tabular Middle Atlas and the southern margin of the Folded Middle Atlas.  Detailed geologic mapping conducted in the vicinity of a Late Pleistocene volcanic field along the southern front of the Folded Middle Atlas revealed that volcanism postdates thrusting and is partly coeval with or younger than extension.  Furthermore, no evidence for significant strike-slip faulting was observed.
 These kinematic variations illustrate the important influence of inherited structures on intracontinental deformation.  Our observations require a reassessment of proposed tectonic models for central Morocco.