[1] We investigate the active seismogenic fault system in the area of the 2003 Mw 6.9 Boumerdes earthquake, Algeria, from a high-resolution swath bathymetry and seismic survey. A series of 5 main fault-propagation folds $20-35 km long leave prominent cumulative escarpments on the steep slope and in the deep basin. Fault activity creates Plio-Quaternary growth strata within uplifted areas such as a rollover basin on the slope and piggyback basins in the deep ocean. Most thrusts turn to fault-propagation folds at the sub-surface and depict ramp-flat trajectories. We find that the two main slip patches of the 2003 Mw 6.9 Boumerdes earthquake are spatially correlated to two segmented cumulative scarps recognized on the slope and at the foot of the margin. The overall geometry indicates the predominance of back thrusts implying underthrusting of the Neogene oceanic crust. Citation: Déverchère, J., et al.

No abstract

We present new results from the MARADJA'03 cruise depicting the geological structures offshore central and western Algeria. Using swath bathymetry and seismic reflection data, we map and discuss the offshore limits of the Internal Zones corresponding to relics of the AlKaPeCa domain that drifted and collided the African plate during the Miocene. We identify large reverse faults and folds that reactivate part of these limits and are still active today. The morphology of the westernmost NE-SW margin suggests a former strike-slip activity accommodating a westward block translation responsible for the shift of the Internal Zones towards the Moroccan Rif.Résumé: Nous présentons les résultats récents de la campagne MARADJA'03, qui visent à mettre en évidence les structures géologiques dans le domaine marin au nord-ouest de l'Algérie. Grâce aux données de bathymétrie multifaisceau et de sismique réflexion, nous cartographions et discutons les limites en mer des Zones internes correspondant aux reliques du domaine AlKaPeCa qui a dérivé, puis est entré en collision avec la plaque africaine au Miocène. De grandes failles inverses et plis, actifs dans le champ de contrainte actuel, réactivent certaines de ces limites. La marge ouestalgérienne, orientée NE¿SW, indique la présence d'une ancienne activité en décrochement ayant accommodé la translation des Zones internes vers l'ouest.

[1] A paleomagnetic study of Penninic units in the southern part of the Alpine arc has been carried out. More than 200 samples (23 sites) were collected in Briançonnais Ammonitico rosso limestones of the high Ubaye valley and the Ligurian Alps. A characteristic component of magnetization of reverse polarity was isolated on most of the sites. This component does not pass the fold test and is interpreted as a Tertiary overprint related to Alpine metamorphism. Mean directions in geographic coordinates are D = 121°, I = À52°, a 95 = 11°, and D = 72°, I = À48°, a 95 = 15°for the Ubaye and the Liguria localities, respectively, indicating large counterclockwise rotations about vertical axis of 68°and 117°relative to stable Europe. These rotations, in agreement with a previous study conducted in the Briançon area, together with other paleomagnetic data from the western Alps, show that the internal Alps suffered a large but nonhomogeneous counterclockwise rotation since the Oligocene. The rotations are in agreement with the combination of earlier separately proposed processes: the rotation of the Adriatic plate accounts for about 25°of rotation, remaining rotation and southward gradient would be related to left-lateral shear accommodating the displacement between Adria and Europe at the southern border of the western Alps. Furthermore, the southward extrusion of the western Alps south of the Simplon fault zone may account for up to 10°of rotation. Rotations appear therefore as a major process accommodating deformation in the western Alps since the Oligocene.