[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

The Messinian salinity crisis (MSC) [Hsü et al., 1973] has deeply shaped the Mediterranean landscape and triggered large sedimentary deposits (evaporites and clastics) in the deep basins within a short time span. Until recently, the MSC has mainly been analyzed independently, either through outcrops located onshore (e.g. Morocco, Cyprus, Spain, Sardinia, Italy) or through marine seismic profiles in the deep offshore. Each approach bears its own limitations:(1) on the one hand, land outcrops refer to incomplete Messinian successions that are geometrically disconnected from the offshore Messinian deposits owing to tectonics (e.g. Apennines) and/or because they accumulated at an early stage of the crisis in shallow marginal basins (e.g. Spain); (2) on the other hand, seismic profiles from the upper margins down to the deep basins allow to image and explore the entire MSC event as a continuous process, but with a lower resolution and with a lack of stratigraphical and lithological control, in the absence of full recovery of scientific boreholes.We present here a synthesis of a set of modern geophysical data over the Mediterranean and Black seas allowing to image the Messinian markers (erosion surfaces, depositional units and their bounding surfaces) much better than previously and to study the spatio-temporal organisation of these markers from the inner-shelves down to the bathyal plains. The results from thirteen areas located offshore are compared, with common charts and nomenclatures. The comparative and multi-site approach developed here allows to analyse the record of the MSC on margin segments and basins that depict various structural, geodynamical and geological settings, to fix a number of local influencing factors (tectonics, subsidence, inherited topography, sedimentary fluxes...) and to partly assess their influence in facies and geometrical variations of the MSC units. We are thus able to extract from our analysis some recurrent signals related to the MSC ss., allowing us to discuss: (1) the amplitude and modalities of base-level changes during the MSC; (2) the depositional modalities of the MSC units in the deep basins; (3) the location of the erosion product of the margins and to emphasise (4) the major differences between the eastern and western Mediterranean basins. Une meilleure connaissance des enregistrements de la crise de salinité messinienne en domaine marin grâce à l'analyse sismique multi-sitesMots-clés. -Crise de salinité messinienne, Méditerranée, Profils sismiques, Evaporites, Erosion, Clastiques.Résumé. -La crise de salinité messinienne (CSM) [Hsü et al., 1973] a profondément modelé les paysages méditerra-néens et généré d'épaisses accumulations sédimentaires (évaporites et dépôts clastiques) dans les bassins profonds sur une brève période de temps à l'échelle géologique. Jusqu'à présent, la CSM a principalement été étudiée distinctement, à terre, grâce aux affleurements (ex. Maroc, Chypre, Espagne, Sardaigne, Italie…), et en domaine marin, par l'intermé-diaire de profils sismiques. Chacun...

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.

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

No abstract

The eastern Demerara Plateau offshore French Guiana was surveyed in 2003 during the GUYAPLAC cruise (multibeam bathymetry and acoustic imagery, 6-channel seismic reflection and 3.5 kHz echosounding). The data show the "post-transform" Cenozoic that the series located on the outer part of the plateau (below c. 2000 m) contain at least twelve stacked mass transport deposits (MTDs) that have recorded a history of large-scale slope failure, as well as two main normal fault sets that provide possible pathways for upward fluid migration through the series, reaching at high as the uppermost MTDs. Seabed data show that the area above the failures is characterized by circular-to-elongate (slope-parallel) depressions interpreted as fluid seeps (pockmarks), some of them have been modified by along slope currents. We suggest that the development of the MTDs to results from the combinaiton of the presence of fluid overpressure at depth the geometry of the margin's deep structure, in particular the existence of a 'free borderlateral border' on the outermost plateau. Our results also emphasise the role of stratigraphic décollements within the Cenozoic series. Highlights ► Cenozoic succession of the Demerara Plateau contains a complex of at least 12 MTDs. ► Sedimentary undulations and circular to elongated depressions occur on the seafloor. ► Faults provide fluid migration pathways through the MTDs (biogenic or thermogenic). ► Detachments occur on stratigraphic horizons that outcrop along the continental slope. ► Fluid overpressures and the distal transform free border seem to control MTD dynamics.

International audienceWe describe for the first time a set of large active thrusts and folds near the foot of the easternmost Algerian margin, Western Mediterranean, from swath bathymetry and high-resolution seismic data acquired in 2005 during the Maradja2/Samra cruise. This active system resumes a previous passive margin and creates growth strata deposition on the limbs of large folds, resulting in the development of perched basins at the foot of the margin since less than ~ 1 Ma. They form a set of overlapping fault segments verging toward the Algerian basin, in a way similar to what has been observed off eastern Algiers on the rupture zone of the 2003 Mw 6.8 Boumerdes earthquake. The horizontal shortening rate across large folds is estimated to be of the order of 1 mm/yr. Although no historical earthquakes are reported here, these fault segments could have been responsible for large (M ~ 7.5) events in the past. This young tectonic system further supports the hypothesis of subduction inception of the Neogene oceanic lithosphere in the context of the Africa–Eurasia convergence