Active faulting in the frontal Rif Cordillera (Fes region, Morocco): Constraints from GPS data

摘要

The southern Rif cordillera front, between Fes and Meknes, is formed by the Prerif Ridges, which constitute a thrust and fold belt, in contact with the Saiess foreland basin. Geological evidence and regional GPS network data support recent and active tectonics of this Alpine cordillera, with a top-to-the-S-SW motion with respect to stable Africa. A local non-permanent GPS network was installed in 2007 around Fes to constrain the present-day activity of the mountain front. Six GPS sites are located in the Prerif mountain front (jbel Thratt and jbel Zalarh), the Saiess basin and the foreland constituted by the tabular Middle Atlas. Measurements of the GPS network in 2007,2009 and 2012, over a five year span, seem to indicate that this region is tectonically active and is subjected to significant horizontal motions: (ⅰ) a regional displacement toward the SW with respect to stable Africa, showing an average rate of 2 mm/yr; (ⅱ) a southwestward convergent motion between the jbel Thratt with respect to the Saiess basin and the eastern Zalarh ridge, with an average rate of about 4 mm/yr; and (ⅲ) moderate NNE-SSW divergent dextral motion between the Saiess basin and the northern front of the tabular Middle Atlas with an average rate of about 1 -2 mm/yr. The regional southwestward motion is related to the activity of the NE-SW sinistral North Middle Atlas-Kert fault zone, which follows the Moroccan Hot Line. Convergence between the Prerif ridges, located at the southern edge of the Rif, and the Saiess basin is accommodated by ENE-WSW striking northward dipping reverse sinistral faults and south vergent folds. In addition, increasing deformation toward the western ridges is in agreement with the stepped mountain front and the development of the arched structures of the Prerif ridges. Normal faults located south of the Saiess basin are responsible for local extension. Whereas the most active deformation occurs in the southern front of the jbel Thratt near Fes, the Saiess foreland basin and the Middle Atlas foreland have only moderate to low tectonic activity, as evidenced by geological and GPS data.