Seismo‐Tectonic Model for the Southern Pre‐Rif Border (Northern Morocco): Insights From Morphochronology

Abstract

In intraplate tectonic regions characterized by moderate seismic activity, the recognition of active zones of deformation has long been a challenging task (e.g., Landgraf et al., 2017). Like Northern Africa, such regions are generally characterized by low hazard but high risk due to the concentrations highly vulnerable population and/or infrastructures (Moreno et al., 2004). Moreover, when low strain rates are combined with meteorological and anthropogenic overprints, the geomorphic signatures associated with active faults fade away as fault slip rates decrease. Consequently, diagnostic criteria established in areas of high strain rates may not be effectively applied. Furthermore, low-levels of seismic strain induce low displacement rates, which may be distributed over numerous fault segments rather than localized on a single fault. At the regional scale, such a distribution of the tectonic deformation can also obscure the seismogenic potential of any given single structure, as, for example, the La Rouvière Fault reactivation during the 2019 M w 4.9 Le Teil Earthquake (Ritz et al., 2020), in southeastern France, which was previously considered as inactive (Jomard et al., 2017).Part of the peri-Mediterranean Alpine chain surrounding the Alboran Sea, the Rif Mountain belt in Morocco is an area of moderate tectonic activity. Located within the diffuse convergence zone between the Nubia Abstract Located at the southern boundary of the Alpine chain in Morocco, the deformation front of the Southern Rif Mountains is a region of moderate tectonic activity, which makes it a good natural laboratory to understand whether, and how, low compressional strains are located on specific structures. Along the ≈80 km-long left-lateral, transpressive and reverse fault zone that runs at the toe of the Pre-Rif Ridges, an analysis of high-resolution digital topography provides new geomorphic lines of evidence supporting Quaternary activity along, 20 km-long fault segments. The fault zone can be divided into the Meknès and the Fès segments, which are constrained at depth by reactivated, NE-trending basement faults, delimitating paleo-grabens associated with the Late Triassic-Jurassic opening of the Atlantic Ocean. For selected sites, we used in situ-produced 36 Cl, 10 Be, and 26 Al and high-resolution topography to infer the timing of abandonment of fluvial markers, which suggest incision rates on the order of 0.6-2 mm/yr. Given their lengths, scaling laws suggest that the identified fault segments should root at about 7-12 kmdepth, possibly reactivating former basement normal faults and making them potential seismogenic sources capable of generating M w 6+ earthquakes, with return times of the order of several hundreds of years. Our new morphochronological data set confirms that the Southern Rif deformation front is a key structure that may have accommodated most of the lateral extrusion of the Rif between the Nubia and Iberia tectonic plates.Plain Language Summary In Morocco, the population may suffer from the dramatic consequences of ...

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