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Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: The Khayr al Din bank |
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| Authors: | Abdelkarim Yelles Jacques Déverchère Rabah Bracène Pierre Strzerzynski Guillaume Bertrand Thierry Winter Pascal Le Roy Hamou Djellit |
| Institution: | a Centre de Recherche en Astronomie, Astrophysique et Géophysique (C.R.A.A.G.), BP 63, Bouzareah, Algiers, Algeria b Université Européenne de Bretagne, France c Université de Brest; CNRS, UMR 6538 Domaines Océaniques; Institut Universitaire Européen de la Mer, Place Copernic, 29280 Plouzané, France d Sonatrach Exploration, avenue du 1er Novembre, Boumerdès, Algeria e Géosciences Azur, CNRS-UMR 6526, 250 rue Albert Einstein, bât. 4, 06560 Valbonne, France f BRGM, 3 avenue Claude Guillemin, BP 36009, 45060 Orléans Cedex 2, France |
| Abstract: | The Algiers region, northern Algeria, is known to be seismically active, with recurrent large (M>6) earthquakes. Because of the lack of high-resolution bathymetry, the offshore structures remained for a long time poorly known. Thanks to a new marine data base (MARADJA 2003 cruise), the offshore part of the margin is accurately mapped, and new active and recent structures are described. West of the bay of Algiers, the margin enlarges, forming the Khayr al Din bank, interpreted as a tilted block of the passive margin born during the opening of the Algero-Provençal basin. At the slope break, a 80 km-long fault-tip Quaternary fold, namely the Khayr al Din fault, extends at the foot of the margin off NW Algiers and represents the largest active structure of the coastal area, together with the Sahel anticline. We also map for the first time a set of overlapping, en echelon active folds in the upper part of the Khayr al Din bank, located off previously known active structures on land. Most of these faults represent actually a threat for the Algiers region in terms of seismic hazard but also geological hazards, such as tsunamis, as most of them depicts significant dimensions and slip rates. The highest long-term horizontal shortening rate is found on the Khayr al Din fault and is estimated at 0.5 ± 0.1 mm/yr, with a maximal magnitude of 7.3, which provides one of the highest seismogenic potential in the region.A new tectonic framework for the Algiers region is proposed, in which the main south-dipping offshore structure, of opposite vergence relative to most thrusts on land, appears to be nowadays the main driving fault system, as also found further east in the Boumerdès (M 6.8) 2003 rupture zone. The overall apparent pop-up structure of the recent and active faults may result from a progressive migration of the plate limit from the Late Miocene, north-dipping suture zone on land, to the Quaternary, south-dipping main Khayr al Din fault at sea, suggesting a process of subduction inception. |
| Keywords: | Algerian margin Contractional reactivation Active fault-related folds Seismic hazard Seismic reflection Africa-Eurasia convergence Tilting Ocean-continent transition |
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