The lithospheric structure of the Gibraltar Arc and possible implications for seismic hazard in the western mediterranean

Abstract

The Gibraltar arc and associated broad seismogenic zone that includes the southern Iberian Peninsula and northern part of the Morocco, hosts the convergent boundary between the European and African Plates. The area is characterized by low to moderate magnitude shallow earthquakes, although large historical events have also occurred. The crustal deformation is reflected by the topographic features. Characteristic topographic features are the Beticorogen in Iberia and in Morocco the Atlas Mountains and the Rif Cordillera. The latter two orogenic belts are the response of different geodynamic processes acting at lithospheric scale caused by the complex plate interaction. The zone delineates an arcuate arc system known as the Gibraltar arc limiting to the west Alboran basin. The area is characterized by a relatively large amount of earthquake activity at various depths and with broad spectra of focal mechanisms. Within the last decade a large international effort has been devoted to the area. Key multi-seismic projects have been developed that aim to constrain the structure, composition and tectonic scenario from south of the Atlas to the Betics, across the Rif Cordillera and the Alboran basin. The multidisciplinary research program included: natural source (earthquakes) recording with temporal deployments of broad band (BB) instrumentation and, controlled source seismic acquisition experiments where spatially dense recording of wide-angle seismic reflection shot gathers were acquired. The natural source experiments consisted ofa transect from Merzouga across the Gibraltar Arc and into the Iberian Peninsula (until south of Toledo) and, a nearly regular grid of BB instruments. The controlled source data-sets were able to constrain the crustal structure and provide seismic P-wave propagation velocity models from the coast across the Rif and the Atlas. From south to north the crust features a relatively moderate crustal root beneath the Middle Atlas which can reach 40 km clearly differing from the 35 km thickness value observed at both sides of this root. Travel time inversion results position the crustal root just south of the High Atlas defining a thrusted mantle wedge and, also a limited crustal imbrication is suggested in the Middle Atlas. The most surprising feature is a prominent and unexpected crustal root (over 50 km) located beneath the external Rif and identified by both the wide-angle data and receiver function studies. To the east of this feature the crust thins rapidly by 20 km across the Nekkor fault zone, suggested to be related to the sharp change in crustal thickness.