Submarine Landslides Along the Algerian Margin: A Review of Their Occurrence and Potential Link with Tectonic Structures
The Algerian continental margin, a Cenozoic passive margin along the plate boundary between Eurasia and Africa presently reactivated in compression, is one of the most seismically active areas in the Western Mediterranean, having experienced several moderate to strong earthquakes in the coastal zone during the last century. The morphology of the continental slope offshore Algeria is steep and dominated by the presence of numerous canyons of variable size and sea-floor escarpments that are probably the seafloor expression of active thrust-folds. Numerous submarine landslides are present along these structures, as well as asso-ciated with salt diapirs in the abyssal plain. Submarine landslides are expressed by seafloor scars (usually of small size) and subsurface or buried acoustically chaotic/ transparent units interpreted as mass transport deposits (MTD). Compared with the most recent (superficial) small size of slide scars, buried MTDs seem to be of larger size, possibly suggesting a change in the functioning of gravity-driven processes in the margin throughout the Plio-Quaternary.
KeywordsSubmarine landslides multibeam seafloor morphology seismic stratigraphy Algerian margin
The Algerian margin is a Cenozoic passive margin along the plate boundary between Eurasia and Africa, presently reactivated in compression. The deformation is expressed by ESE-WNW-aligned seafloor escarpements that represent the seafloor expression of thrust-folds (Déverchère et al. 2005; Domzig et al. 2006). These structures are associated with crustal shortening and moderate to large earthquakes (Nocquet and Calais 2004). The Algerian continental margin is one of the most seis-mically active areas in the Western Mediterranean, having experienced several moderate to strong earthquakes and tsunamis during the last century. The most violent instrumentally recorded earthquake occurred on October 10, 1980 in El Asnam (currently Chlef), and reached a magnitude (Ms) of 7.3 (El Robrini et al. 1985). More recently, on May 21, 2003 an earthquake with a magnitude (Ms) of 6.9 struck the city of Boumerdès, on the coast near Algiers, and generated significant gravity flows recognized by numerous submarine cable breaks offshore (Ayadi et al. 2003).
2 Data and Methods
This paper summarizes data from four campaigns led from 2003 to 2007 in the Algerian offshore area: MARADJA 1 and 2 (2003 and 2005, R/V Le Suroît), PRISMA (2004, R/V Marion Dufresne), PRISME (2007, R/V L'Atalante). The dataset includes multibeam bathymetry, seafloor imagery (SAR side scan sonar), high resolution seismic reflection profiles (multichannel and CHIRP sonar; Fig. 1a). The multibeam bathymetric dataset was acquired in 2003 and 2005 with a Kongsberg Simrad EM300 multibeam system operating at 32-kHz with a horizontal resolution of 15 × 35 m at 1,000 m depth and a vertical accuracy of 2 to 10 m (central and lateral beams, respectively). Seafloor samples and in situ tests, cited from recent publications based on the same dataset, are not discussed extensively in this paper. Submarine landslides were identified visually as follows: (1) seafloor morphology evidence of slide scars and/or of convex-up slide deposits with distinct backscatter signature; (2) seismic reflection evidence of acoustically transparent units interpreted as MTD either buried or at the seafloor. In the maps presented in this article, slide scars are outlined in maps with lines drawn along the zone of maximum slope angle, while the acoustically-transparent deposits (MTDs) are identified by their areal extent.
3 Margin Physiography
The Algerian margin occupies the southern edge of the Western Mediterranean between 1°W and 8°E in longitude for a total coastal length of 1,280 km. It faces the Alboran and the Algero-Provençal basins. The Algerian margin is in general quite steep, with narrow plains and a few large embayments (Oran, Arzew, Algiers, Bejaia, Skikda, Annaba), where the largest Algerian cities grew (Fig. 1a, b). The Algerian continental shelf is relatively narrow (5 to 10 km wide on average). It is virtually inexistant (< 2 km) in areas offshore Dellys and Tenes, where the mountain ranges are close to the coastline, while it reaches up to 40 km in width offshore the main embayments. The continental shelf break is comprised between 100 and 200 m water depth and is indented where submarine canyon heads cut into the shelf.
Main parameters of the four types of morphologies outlined in Fig. 1b
Slope angle ()
Main feature: linear canyons with: incision 100–300 m; width 1–2 km; spacing 2–3 km
Main feature: linear canyons with: incision 400–500 m; width 3–4 km; spacing 5 km
Main feature: abundant submarine slide scars Secondary features: large, sinuous canyons with: incision>400 m; width>2 km
Main feature: smooth morphology Secondary features: superficial landslides, scattered pockmarks
The continental rise is not always identifiable along the Algerian margin. This happens in areas where submarine escarpments of probable tectonic origin terminate abruptly in the abyssal plain, like offshore the Khayr al Din bank (‘Type D’ area west of Algiers in Fig. 1b; Yelles et al. 2009), or where the continental slope has a step-like morphology with seafloor escarpments and slope basins, like offshore Algiers and Dellys (Déverchère et al. 2005). Where present, especially at the emplacement of major deep sea fans (Fig. 1b), the continental rise has an average bathymetric gradient of 2–3° within a bathymetric range of 2,000 to 2,700 m, and an overall width of 0 to 30 km.
In the Western Mediterranean, the maximum water depth of the abyssal plain is about 2,800 m water depth (Camerlenghi et al. 2008). The abyssal plain at the border with the Algerian margin was imaged only in a limited area, enough to show that the overall flat morphology is characterized by the presence of several seafloor irregularities, and in particular positive features due to the deformation of buried Messinian salt (Gaullier et al. 2008). Salt diapirs and salt walls in some cases produce a vertical displacement of the seafloor up to 200 m and are surrounded by thin-skin sediment instabilities.
4 Seafloor and Subsurface Instabilities Along the Algerian Margin
5 Discussion and Conclusion
The analysis of seafloor bathymetry and seismic reflection profiles allowed to extract some preliminary conclusions about the presence and the distribution of submarine landslides along the Algerian continental margin. On the continental slope offshore Algeria, there are numerous traces of submarine landslides visible at the seafloor, most of which are relatively small-sized compared to those described along other continental margins (Domzig et al. 2009; Dan et al. in press). These superficial submarine landslides are located in preferential areas of the Algerian margins, and in particular they are associated: (1) with the major submarine canyons: at the canyon heads, along canyon flanks, and on the seafloor between major canyons; (2) with seafloor escarpment of tectonic origin up to 25 km long; (3) with salt diapirism in the abyssal plain. The majority of these superficial landslides can be classified as ‘disintegrative slides’ since no detectable deposit is associated with the slide scar. There seems to be a relationship between the location of the tectonic escarpments and the thin-skin landslides. It is to be investigated if this relationship is due to one or a combination of factors such as: long term growth of escarpments (that are in general the frontal side of compressional anticlines with a seaward vergence); seismic activity along these escarpments; the fact that on the flanks of the escarpments the dip of the sedimentary layers increases (Nouguès et al. this volume).
There are a number of acoustically transparent or chaotic deposits along the margin, mainly at the foot of the continental slope or of smaller morphologic escarpments (Fig. 4). The volume of these units seems larger than that measured from the slide scars at the seafloor. Furthermore, in the Oran-Tenes zone the size of mass-transport deposits seems larger than in the central Algerian margin (Domzig et al. 2009).
Based on a limited number of studies in the area, a plausible triggering mechanism for submarine landslides are earthquakes and consequent liquefaction of potential weak layers (thin beds of silty-sandy sediment in overall muddy sections; Dan et al. 2008; Nouguès et al. this volume). This is logical along the seismically active Algerian margin. However, two main questions deserve further investigation: (1) the relationship between the distribution of possible preconditioning factors (escarpments, presence of thin sandy layers), the recurrence of likely triggering mechanisms (earthquakes), and the presence of submarine landslides; (2) the origin of the thin sandy layers.
The distribution of recent earthquake epicenters versus submarine landslides is of non immediate comprehension (Fig. 4): the earthquake epicenters are reported since 1973 onwards (NEIC catalog 2009), so they theoretically match only with the most recent landslides, by definition at the surface. Further work is necessary to establish more consistent correlations among seafloor instability and seismicity, for example by attributing an identified landslide to a defined and known earthquake event, if possible. Other studies in the Mediterranean area show that this relationship is not straightforward (e.g., Camerlenghi et al. this volume).
From the above observations it is possible to draw some more general preliminary conclusions, or rather working hypotheses to be tested: (1) there has been a major rearrangement in the way submarine gravity processes, and in particular submarine landslides shaped the Algerian margin during the Plio-Quaternary; (2) there is a relationship in the distribution of landslides with present day morphology (morphologic sectors) and with seafloor structures of tectonic origin.
The authors would like to thank the captain, crew and technical staff on board R/V Le Suroît (during the campaigns Maradja and Maradja2), Marion Dufresne (PRISMA) and L'Atalante (PRISME). The work was supported by ANR projects ISIS and DANACOR. We thank the reviewers E. Gonthier and A. Camerlenghi for their valuable suggestions and promptness. This work is dedicated to the memory of Bruno Savoye, an enthusiastic marine geologist, co-chief scientist of the Maradja2 cruise, and a mentor for many of us.
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