Mass-transport deposits on the Algerian margin (Algiers area) : morphology, lithology and sedimentary processes

International audienceOn 21st May 2003, a damaging earthquake of Mw: 6.9 struck the region of Boumerdès 40 km east of Algiers in northern Algeria. The seismic shocks had devastating effects in the offshore area between the cities of Algiers and Dellys, where numerous cable breaks were observed. The submarine area encompassing the 2003 Boumerdès epicenter (central Algerian margin) was recently investigated using geophysical and sampling tools. It has been observed that small-size slides and mass-transport deposits occur across this area. Thus, a detailed analysis was performed on 25 submarine slides identified in water depth from 500 m to 2700 m, by measuring different morphological parameters (i.e. surface, head-scarp height, slope in the source area and in adjacent areas).Various deposits consisting of matrix supported mud-clast, distorted stratified sediments and sandturbidite beds were observed in gravity cores near and within the most significant-size slide. Based on this work and previous studies it can be presumed that the recurrent seismic activity can be considered as the main triggering factor for failure in the central Algerian margin

Searching for the Africa-eurasia Miocene Boundary offshore western algeria (Maradja'03 cruise)

International audienceWe 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. To cite this article: A. Domzig et al., C. R. Geoscience 338 (2006). 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 ouest-algérienne, orientée NE–SW, indique la présence d'une ancienne activité en décrochement ayant accommodé la translation des Zones internes vers l'ouest

THE KRAMIS DEEP-SEA FAN OFF WESTERN ALGERIA: ROLE OF SEDIMENT WAVES IN TURBIDITIC LEVEE GROWTH

International audienceThe Kramis deep-sea fan extends over 45 km at the base of the western Algerian continental slope between 2000 and 2550 m water depth and covers an area of approximately 1200 km2. The Kramis Fan was initiated after Messinian time, evolved during the Plio- Quaternary, and, is still active, as proved by submarine cable breaks during the 1954 Orléansville earthquake. The Kramis Fan is fed by two perpendicular canyons: the Kramis Canyon and the Khadra Canyon, merging in a single E-W-oriented channel confined at the foot of the slope. It is strongly asymmetric with a super-developed levee on the right-hand side of the channel, the Kramis Ridge. Based on recent multibeam, side-scan sonar, and sediment core data (Maradja, 2003 and 2005, Prisma, 2004, and Prisme, 2007 cruises), we describe the morphology and internal structure of the fan and particularly the sediment ridge, showing marked lateral changes in the sediment-wave morphology and their association with a series of large scours in the intermediate part of the ridge aligned in the continuity of the Khadra Canyon direction. Overall, the Kramis Ridge is formed by turbidity currents overspilling the ridge crest, which is 100 m above the channel floor, with two exceptions. In the distal part of the ridge the subdued ridge-crest height probably causes continuous overspill, testified by sediment waves migrating parallel to the channel. The scours occur in the intermediate part of the ridge where the ridge height is only 50-60 m; scours are interpreted as the result of cyclic steps due to flow stripping of currents provided by the intersection of the Khadra Canyon with the Kramis Canyon and Channel system. The scours probably postdate the main growth of the Kramis Ridge and induce the local erosion of the ridge, which could correspond to a new channel initiation cutting the ridge. The superposition or the interaction of flows with different directions is responsible of the amplification of the size of the sediment waves with erosional downside flanks and their transformation in scours. The Kramis Fan provides a clear example of flow interaction to explain the presence of large sediment waves and scours on modern submarine fans

Quantifying the role of sandy–silty sediments in generating slope failures during earthquakes: example from the Algerian margin

The Algerian margin is a seismically active region, where during the last century, several large magnitude earthquakes took place. This study combines geotechnical and sedimentological data with numerical modelling to quantitatively assess the present-day slope stability of the Algerian margin. Geotechnical laboratory tests, such as cyclic triaxial tests, oedometric tests and vane shear tests were carried out on sediment cores collected on the study area. The liquefaction potential of a sediment column located about 30 km from the Boumerdès earthquake epicentre of 21st May 2003 was evaluated theoretically for an earthquake of M w = 6.8. We show that thin sand and silt beds such as those described on recovered sediment cores are the main cause of sediment deformation and liquefaction during earthquakes. Numerical calculations showed that the slope failure may occur during an earthquake characterised by a PGA in excess of 0.1g, and also that, under a PGA of 0.2g liquefaction could be triggered in shallow silty–sandy deposits. Moreover, comparison of the predicted slope failure with failure geometries inferred from seafloor morphology showed that earthquakes and subsequent mass movements could explain the present-day morphology of the study area

Algerian margin sedimentation patterns (Algiers area, southwestern mediterranean)

The present study provides an overview of recent sedimentation patterns on the central Algerian continental margin. Recent sedimentation patterns were assessed from morphological analysis, which is based on swath bathymetry and echo-facies mapping. It appears that sedimentation along the Algerian margin is controlled by two processes:(1) gravity-induced processes, including both mass-transport deposits and turbidity currents, and (2) hemipelagic sedimentation. Mass-transport deposits occur on the Algerian margin at the canyon heads and flanks, in the interfluve areas between canyons, along the seafloor escarpments, and on the flanks of salt diapirs. Mass-transport deposits (MTDs) sampled by coring consist of a variety of soft and hard mud-clast conglomerate and turbidite deposits. MTDs are mostly localized at the toes of steep slopes, where thrust faults were previously identified and mapped. Analysis of the spatial distribution of MTDs and their recurrence in time help reconstruct the main predisposing factors and triggering mechanisms, and evaluate their impact on evolution of the Algerian margi

Algerian margin sedimentation patterns (Algiers area, southwestern mediterranean)

The present study provides an overview of recent sedimentation patterns on the central Algerian continental margin. Recent sedimentation patterns were assessed from morphological analysis, which is based on swath bathymetry and echo-facies mapping. It appears that sedimentation along the Algerian margin is controlled by two processes:(1) gravity-induced processes, including both mass-transport deposits and turbidity currents, and (2) hemipelagic sedimentation. Mass-transport deposits occur on the Algerian margin at the canyon heads and flanks, in the interfluve areas between canyons, along the seafloor escarpments, and on the flanks of salt diapirs. Mass-transport deposits (MTDs) sampled by coring consist of a variety of soft and hard mud-clast conglomerate and turbidite deposits. MTDs are mostly localized at the toes of steep slopes, where thrust faults were previously identified and mapped. Analysis of the spatial distribution of MTDs and their recurrence in time help reconstruct the main predisposing factors and triggering mechanisms, and evaluate their impact on evolution of the Algerian margi

Holocene turbidites record earthquake supercycles at a slow-rate plate boundary

Ongoing evidence for earthquake clustering calls for records of numerous earthquake cycles to improve seismic hazard assessment, especially where recurrence times overstep historical records. We show that most turbidites emplaced at the Africa-Eurasia plate boundary off west Algeria over the past ∼8 k.y. correlate across sites fed by independent sedimentary sources, requiring a regional trigger. Correlation with paleoseismic data inland and ground motion predictions support that M ∼7 earthquakes have triggered the turbidites. The bimodal distribution of paleo-events supports the concepts of earthquake supercycles and rupture synchronization between nearby faults: 13 paleo-earthquakes underpin clusters of 3–6 events with recurrence intervals of ∼300–600 yr, separated by periods of quiescence of ∼1.6 k.y. without major events on other faults over the study area. This implies broad phases of strain loading alternating with phases of strain release. Our results suggest that fault slip rates are time dependent and call for revising conventional seismic hazard models

The transition from Alboran to Algerian basins (Western Mediterranean Sea): Chronostratigraphy, deep crustal structure and tectonic evolution at the rear of a narrow slab rollback system

International audienceThe eastern Alboran basin and its transition to the Algerian basin is a key area in the Mediterranean realm where controversial kinematic and geodynamical models are proposed. Models imply striking differences regarding the nature of the crust, the prevalence of brittle faulting and ductile shear, the origin of magmatism, the style of Miocene deformation and the driving mechanisms of the Alboran plate kinematics. Combining a new chronostratigraphic chart of the Alboran and Algerian basins based on the Habibas (HBB-1) core drill, deep seismic sections striking WSW-ENE and SSE-NNW, and potential field data, we re-assess the tectonic evolution that controlled the sedimentation and basement deformation of the westernmost limit of the Algerian basin and its transition with the Alboran domain. A WSW-directed extensional tectonic phase has shaped a stretched continental crust with typical tilted blocks along ∼100 km from Burdigalian to Tortonian times, which is assumed to result from the WSW-directed migration of the Alboran block driven by a narrow slab rollback. In the Algerian basin, this event was followed by the emplacement of an oceanic-type crust. Potential field signatures of the deep basin as well as geometrical correlations with onland outcrops of inner zones suggest a minimum WSW-directed displacement of the Alboran terrane of ∼200 km. At the southern foot of the Algerian basin, the continent-ocean transition is sharp and may result from the westward propagation of a slab tear at depth, forming two segments of STEP (Subduction-Transform Edge Propagator) margins. Our results support models of intense shear tractions at the base of an overriding plate governed by slab rollback-induced mantle flow. Finally, Messinian salt tectonics affected overlying deposits until today. A late Tortonian to Quaternary dominantly transpressive tectonic episode linked to the Africa-Iberia convergence post-dates previous events, deforming the whole margin

Coastal uplift west of Algiers (Algeria): pre- and post-Messinian sequences of marine terraces and rasas and their associated drainage pattern

(IF 1.50; Q2)International audienceThe North Africa passive margin is affected by the ongoing convergence between the African and Eurasian plates. This convergence is responsible for coastal uplift, folding, and reverse faulting on new and reactivated faults on the margin. The active deformation is diffuse and thus rather difficult to locate precisely. We aim to determine how a coastal landscape evolve in this geodynamic setting and gain insights into active tectonics. More particularly, we evidence and quantify coastal uplift pattern of the Chenoua, Sahel, and Algiers reliefs (Algeria), using sequences of marine terraces and rasas and computing several morphometric indices from the drainage pattern. Upper and Middle Pleistocene uplift rates are obtained by fossil shoreline mapping and preliminary U/Th dating of associated coastal deposits. Extrapolation of these rates combined to analyses of sea-level referential data and spatial relationships between marine terraces/rasas and other geological markers lead us to tentatively propose an age for the highest coastal indicators (purported the oldest). Values of morphometric indices showing correlations with uplift rate allow us to analyze uplift variation on area devoid of coastal sequence. Geological and geomorphological data suggest that coastal uplift probably occurred since the Middle Miocene. It resulted in the emergence of the Algiers massif, followed by the Sahel ridge massif. The Sahel ridge has asymmetrically grown by folding from west to east and was affected by temporal variation of uplift. Compared to previous study, the location of the Sahel fold axis has been shifted offshore, near the coast. The Chenoua fault vertical motion does not offset significantly the coastal sequence. Mean apparent uplift rates and corrected uplift rates since 120 ka are globally steady all along the coast with a mean value of 0.055 ± 0.015 mm/year (apparent) and of 0.005 ± 0.045 mm/year (corrected for eustasy). Mean apparent coastal uplift rates between 120 and 400 ka increase eastward from 0.045 ± 0.025 to 0.19 ± 0.12 mm/year (without correction for eustasy) or from 0.06 ± 0.06 to 0.2 ± 0.15 mm/year (with correction for eustasy). In addition, the combination of structural and geomorphic data suggests a low uplift rate for the southern part of the Algiers massif