Benabdellouahed Massinissa
email: massinissa.benhabdelhaoued@ifremer.fr

Baltzer Agnes
email: agnes.baltzer1@univ-nantes.fr

Rabineau Marina
email: marina.rabineau@univ-brest.fr

Aslanian Daniel

Sahabi Mohamed

Germond Fabien

Loubrieu Benoit

Biari Youssef

Slope morphologies offshore Dakhla (SW-Moroccan margin) Morphologies de la pente continentale au large de Dakhla (marge SW marocaine)

Keywords: African margin, Bathymetry, Seismic interpretation, Pockmarks, Ridges, Scarps Marge africaine, Bathymétrie, Interprétation sismique, Pockmarks, Rides, Escarpements

This study explores a portion of the West African margin at the junction between two well-known segments offshore Dakhla and offshore Mauritania with destructional architecture characterized by giant slides. In between these two segments, the Dakhla segment has historically been described as a constructional section. During an oceanographic Dakhla cruise (2002), high resolution seismic data, swath bathymetry and imagery were acquired around latitude 23° N, offshore Dakhla,. This new data set reveals the existence of varied and complex morphologies on the continental slope, interpreted as a "shallot-shaped" canyon, seafloor depressions or pockmarks, ridges and scarps. These morphologies are interpreted as clues of sedimentary transfers and rupture processes. A scenario is proposed for the development of these different sedimentary morphologies.

Introduction

The study of slope stability along passive continental margin is important to understand 48 sediment transfer from the shelf to the oceanic basin and to approach potential geo-hazards. 49 Among them, submarine landslides affect all types of continental margins and landslides are 50 relatively larger on passive margins than along active margins [START_REF] Georgiopoulou | Linked turbidite-debrite resulting from recent Sahara Slide headwall reactivation[END_REF] Compared to all passive margins, the North-western African continental margin is characterized by large submarine landslides (>400 -600 km 3 ) such as the Sahara Slide, the Cape Blanc Slide, the Mauritania Slide, and the Complex Dakar Slide, which are intensively studied since the 1970's [e.g. [START_REF] Antobreh | Morphology, seismic characteristics and development of Cap Timiris Canyon, offshore Mauritania: A newly discovered canyon preserved-off a major arid climatic region[END_REF][START_REF] Embley | Anatomy of some Atlantic margin sediment slides and some comments on ages and mechanisms[END_REF][START_REF] Gee | Passage of debris flows and turbidity currents through a topographic constriction: seafloor erosion and deflection of flow pathways[END_REF]1999;[START_REF] Georgiopoulou | Sahara Slide: Age, initiation, and processes of a giant submarine slide[END_REF]2009;[START_REF] Hühnerbach | Landslides in the North Atlantic and its adjacent seas: an analysis of their morphology, setting and behaviour[END_REF]2004;[START_REF] Jacobi | Northwest African continental rise: effects of near-bottom processes inferred from high-resolution seismic data[END_REF][START_REF] Krastel | Large-scale mass wasting on the Northwest African Continental Margin: some general implications for mass wasting on passive continental margins[END_REF][START_REF] Wynn | The Northwest African slope apron: a modern analogue for deep-water systems with complex seafloor topography[END_REF] In the northern part of the west African coast, south of the Canary Islands and offshore Dakhla, around 25°N, the margin shows a destructional margin architecture [START_REF] Ranke | Stratigraphy, facies and tectonic development of the on-and offshore Aaiun-Tarfaya Basin-A review[END_REF] with a steep irregular profile and numerous slumps and slides cutting the slope surface such as the Sahara slide [START_REF] Georgiopoulou | Sahara Slide: Age, initiation, and processes of a giant submarine slide[END_REF][START_REF] Wynn | The Northwest African slope apron: a modern analogue for deep-water systems with complex seafloor topography[END_REF]. In the southern zone, south of 21°N and offshore Mauritania, two slides have been described by [START_REF] Krastel | Mapping of seabed morphology and shallow sediment structure of the Mauritania continental margin, Northwest Africa: some implications for geohazard potential[END_REF] and [START_REF] Antobreh | Morphology, seismic characteristics and development of Cap Timiris Canyon, offshore Mauritania: A newly discovered canyon preserved-off a major arid climatic region[END_REF], the Cap Blanc slide and the Mauritania slide.

Between these two zones, where instabilities are related to relatively high accumulation rates but rare triggers, [START_REF] Ranke | Stratigraphy, facies and tectonic development of the on-and offshore Aaiun-Tarfaya Basin-A review[END_REF] described a constructional section of the margin with smooth convex/concave shape and evidences of shelf and slope progradation.

During the Dakhla cruise [2002] devoted to deep investigation of the margin on the R/V Suroit, high resolution seismic data together with swath bathymetry were additionally acquired on a relatively unknown portion of the margin, at 23°N offshore Cap Blanc and Dakhla (Fig. 1). This new data set reveals new morphologies linked to different types of sedimentary transfer processes, that are described in this paper. A scenario is proposed to link these different morphologies, with relative ages.

GENERAL SETTINGS

The West African margin is one of the oldest passive margins in the world: the rifting phase of this margin occurred between the late Triassic and the Early Jurassic and was followed by oceanic spreading initiated around 195 Ma [Labails, 2007;[START_REF] Labails | An alternative early opening scenario for the Central Atlantic Ocean[END_REF][START_REF] Sahabi | Un nouveau point de départ pour l'histoire de l'Atlantique central[END_REF]. Earthquakes are rare and have been recorded in the old weakness zones created during the opening of the Atlantic Ocean [START_REF] Hayes | -Mesozoic magnetic lineations and the magnetic quiet zone off northwest Africa[END_REF]. From Jurassic to Early Cretaceous, carbonate platforms have been constructed, followed by marine clastic infill between Cretaceous and Tertiary. Since the late Cretaceous, erosive cycles have shaped the margin [Labails, 2007;[START_REF] Labails | An alternative early opening scenario for the Central Atlantic Ocean[END_REF] and a major eocene erosional surface [Labails, 2007], observed on the opposite American margin by [START_REF] Mountain | Mesozoic and Cenozoic geology of the U.S. Atlantic continental slope and rise[END_REF], is associated to a succession of gullies and canyons perpendicular to the margin.

On the Dakhla coast (Fig. 1), the shelf-break is located at 100-200 m of water depth. Beyond, the continental slope has a width of 50-250 km [START_REF] Wynn | The Northwest African slope apron: a modern analogue for deep-water systems with complex seafloor topography[END_REF], and displays slope angles of 1-6°. In some areas, e.g. adjacent to the Sahara Canyon System, slope angles may reach 14°. The slope evolves to the continental rise at water depths of 1500-4000 m, with gradients decreasing from about 1° on the lower slope/upper rise, to 0.1° on the lower rise [START_REF] Masson | Flow processes and sediment deformation in the Canary debris flow on the NW African continental rise[END_REF]. The rise is generally 100-1500 km wide and terminates at water depth of 4500-5400 m, the deep abyssal plains.

Offshore the Dakhla margin, quaternary deposits are constituted of pelagic and hemipelagic sediments [START_REF] Georgiopoulou | Sahara Slide: Age, initiation, and processes of a giant submarine slide[END_REF][START_REF] Wynn | The Northwest African slope apron: a modern analogue for deep-water systems with complex seafloor topography[END_REF]. This fine-grained clastic slope apron, with pelagic/hemipelagic 'background' sedimentation is overprinted by downslope gravity flows, shaped by along slope bottom currents [START_REF] Wynn | The Northwest African slope apron: a modern analogue for deep-water systems with complex seafloor topography[END_REF]. [START_REF] Zühlsdorff | Persistent quasi-periodic turbidite activity off Saharan Africa and its comparability to orbital and climate cyclicities[END_REF] showing successions of turbiditic and hemipelagic deposits in the Cap Timiris Canyon system that were correlated to the alternation of arid and humid climatic conditions. They explained that both atmospheric systems: the northerly Trade winds and the southerly African monsoon, interact with each other and with seasonal shifts: northwards during times of boreal summer (or climatically forced on larger time scales) due to Trade winds weakening and southwards when the Trade winds strengthen. The effect for western Africa is humidification when monsoonal pattern prevails and aridification when Trade winds become stronger [START_REF] Zühlsdorff | Persistent quasi-periodic turbidite activity off Saharan Africa and its comparability to orbital and climate cyclicities[END_REF].

Offshore Mauritania, high accumulation areas of biogenic detritus have also been observed at water depths of 1000-1500m in relation with intense upwelling activity in the area [START_REF] Fischer | Offshore advection of particles within the Cape Blanc filament, Mauritania: Results from observational and modelling studies[END_REF][START_REF] Henrich | Architecture and sediment dynamics of the Mauritania Slide Complex[END_REF]. Oceanic upwelling is seasonally and spatially variable on the northwest margin [START_REF] Mittelstaedt | The ocean boundary along the northwest African coast: Circulation and oceanographic properties at the sea surface[END_REF][START_REF] Van Camp | -Upwelling and boundary circulation off Northwest Africa as depicted by infrared and visible satellite observations[END_REF] with a major upwelling zone as seen on figure 1. The region off Cap Blanc is dominated by a persistent upwelling and strong activity of small-scale eddies, filament and jets [START_REF] Karakaş | High-resolution modeling of sediment erosion and particle transport across the northwest African shelf[END_REF]. The area between 26°N and 21°N shows that a permanent upwelling has been recorded since 2002 and reported by the Bulletin of Upwelling, [2015]. High sediment accumulations areas of biogenic detritus are also observed at 1000-1500 m water depths (mwd) in relation with intense upwelling activity in the area [START_REF] Fischer | Offshore advection of particles within the Cape Blanc filament, Mauritania: Results from observational and modelling studies[END_REF].

Thus, the large open-slope landslides (>400-600 km 3 ) occurring on this continental slope (Fig. 1), like the Mauritania, Cap Blanc and Sahara Slides [START_REF] Embley | Anatomy of some Atlantic margin sediment slides and some comments on ages and mechanisms[END_REF][START_REF] Gee | The Saharan debris\pounds ow: an insight into the mechanics of long runout submarine debris\pounds ows[END_REF][START_REF] Georgiopoulou | Linked turbidite-debrite resulting from recent Sahara Slide headwall reactivation[END_REF][START_REF] Georgiopoulou | Repeated instability of the NW African margin related to buried landslide scarps[END_REF][START_REF] Henrich | Architecture and sediment dynamics of the Mauritania Slide Complex[END_REF][START_REF] Krastel | Mapping of seabed morphology and shallow sediment structure of the Mauritania continental margin, Northwest Africa: some implications for geohazard potential[END_REF] largely consist in remobilized hemipelagic sediments [START_REF] Gee | The Saharan debris\pounds ow: an insight into the mechanics of long runout submarine debris\pounds ows[END_REF][START_REF] Krastel | Mapping of seabed morphology and shallow sediment structure of the Mauritania continental margin, Northwest Africa: some implications for geohazard potential[END_REF].

The Canary Islands contribute to the sedimentation by an input of volcaniclastic material by both large-scale landsliding [START_REF] Wynn | -Turbidite depositional architecture across three interconnected deep-water basins on the north-west African margin[END_REF] and slow fallout through the water column into the hemipelagic sediments.

Together with the presence of these islands and seamounts, the existence of deep-water bottom currents locally influence the sedimentation pattern of this margin.

The general water mass structure of the northwest African margin includes at about 600-700 mwd the southward-flowing North Atlantic Central Water (NACW), at 2000-3800 mwd, the North Atlantic Deep Water (NADW) flowing in a southerly direction, and below 3800m, the Antarctic Bottom Water (AABW) flowing in a northeasterly direction. These bottom currents have velocities ranging from 1 to 6 cm/s [START_REF] Lonsdale | Sediment drifts of the Northeast Atlantic and their relationship to the observed abyssal currents[END_REF][START_REF] Sarnthein | Atmospheric and oceanic circulation patterns off Northwest Africa during the past 25 million years[END_REF].

Oceanic upwelling is seasonally and spatially variable on the northwest margin [START_REF] Mittelstaedt | The ocean boundary along the northwest African coast: Circulation and oceanographic properties at the sea surface[END_REF][START_REF] Van Camp | -Upwelling and boundary circulation off Northwest Africa as depicted by infrared and visible satellite observations[END_REF] (blue dashed line on Fig. 1). A major and persistent upwelling center in the region off Cap Blanc is characterized by strong activity of small-scale eddies, filament and jets [START_REF] Karakaş | High-resolution modeling of sediment erosion and particle transport across the northwest African shelf[END_REF] and between 26°N and 21°N a permanent upwelling has been recorded since 2002 and reported by the Bulletin of Upwelling [2015].

DATA AND METHODS

EM300

The EM300 is a multi beam swath sonar mapping system which gives a high-resolution bathymetry and an acoustic backscatter imagery. This system uses a single 30 kHz Mills Cross array and is able of swath mapping at depth ranges between 10 and 5000 m. When operated down to approximately 5000 m depth swath widths are about 5000 m. It runs with up to +/-75° angular swath with 135 beams per ping, and features dynamically variable beam with configuration range from 1° to 4°, to achieve both high spatial resolution in deep and shallow waters. The Bathymetry of Dakhla Cruise is processed with Caraïbes software (developed by Ifremer) and have a spatial resolution better than 50 m (in water depths <2000 m).

Chirp

The Chirp is a hull-mounted sediment echosounder. The signal is characterized by a long impulse (50 ms), lineari modulated in time and frequencies. Thus, its frequency spectrum ranges 2-5.2 kHz with an average at 3.2 kHz. This frequency bandwidth allows a very high resolution (dm) in surficial layers (high frequency) and a deep penetration with the lower frequency that may reach about 100 m. 39 profiles (Fig. 1) have been recorded equivalent to 229 hours of data. For display purposes the envelope of the signal is computed and low-pass frequency filtered (2000 Hz). No automatic gain control is applied to the data, to avoid blanking features close to the seafloor reflection.

Multichannel seismic data

A total of 1500 km of multichannel seismic profiles (Fig. 1) were shot during the Dakhla cruise on board the N/0 Suroit in 2002 [START_REF] Klingelhoefer | Crustal structure of the SW-Moroccan margin from wide-angle and reflection seismic data (the DAKHLA experiment) Part A: Wide-angle seismic models[END_REF]Labails 2007;[START_REF] Labails | An alternative early opening scenario for the Central Atlantic Ocean[END_REF]. The system was composed of a 4.5 km long streamer of 360 channels, with a 12.5 m group interval, which was towed at 15 m immersion. The data were sampled at 4 ms.

Two different marine seismic sources were used during the cruise. For the deep crustal targets a large (8100 in 3 ) airgun array was used in the single bubble mode consisting of 12 airguns between 250 in 3 and 976 in 3 (16 l). The airgun array was shot at a constant interval of 150 m.

Processing of the multichannel seismic data was performed using the Sispeed package. It included spherical divergence correction, FK-filtering, band-pass filtering (3-5-50-60 Hz), internal mute and dynamic corrections. The last processing steps included applying an automatic gain control and a Kirchoff migration using water or stacking velocities.

RESULTS

Morpho-sedimentary structures

The study area is characterized by a broad continental shelf, with a width ranging from 50 to 80 km and a shelf break at a water depth of 100-120 m. The swath bathymetry map of the continental slope, with a 100 m grid cell size resolution, reveals a complex seafloor morphology from the shelf break at 120 m to 3000 m (Fig. 2).

The bathymetric map (Fig. 2) shows an undulating morphology that can be described as a bulge-like structure with a large radius of curvature of around 40 km shown on the profile (ef) (Fig. 2). This undulating morphology is oriented NW-SE, perpendicular to the shoreline.

Based on the slope map and the bathymetric profiles, we divide this portion of the continental slope in three sections (Fig. 2 -profile (ab):

-the first section, between 120 and 500 m water depth, is characterized by a slope angle of 5° and corresponds to the lower slope.

-the second section, between 500 and 2100 mwd, is characterized by a slope angle ranging from 2° to 4° , with a convex longitudinal shape from 1000 to 2000 (profiles ab and cd, Fig. 2) and a complex bulged shape seen on transversal profiles (ef) and (gh) (Fig. 2).

-the third section, between 2200 and 3000 mwd, is characterized by a gentle slope, less than 2°, with well-marked scarps (S1 and S2, Fig. 2). The southern part of the study area, between 2600 and 3000 mwd, is characterized by the presence of a meandering channel.

From upslope to downslope, four sea floor features are identified, on the morphobathymetric map (Fig. 3), 1) the "shallot-shaped" canyon, 2) the sea floor depressions, 3) different ridges types and 4) scarps which will be described in the following paragraphs.

Scarps

The morphobathymetric map shows the presence of two scarps (i.e. S1 and S2, Fig. 3) that cut the downslope part respectively between 2400 and 2600 mwd and between 2100 and 2600 mwd (Fig. 3). They both show a general N-S trend, with a complex broken shape almost imbricated.

Dakhla "shallot-shaped" canyon:

This canyon deeply incises the northern portion of the studied area, between 800 and 2200 mwd (Fig. 4). This canyon, named the "shallot-shaped" canyon because of its specific shape, is globally oriented E-W and can be spitted in three sub-parts: a proximal part, a median part and a distal part.

The proximal part of the canyon runs from 800 m down to 1000 m axial depth (Fig. 4) and displays a V-shaped with a steep flank to the South (4°; 100 m-deep axial incision) and a terrace to the North (Fig. 4 profile ab).

The median part, from 1000 m to 1400 mwd (Fig. 4), corresponds to the enlargement of the incising feature reaching up to ten kilometers with smoother slope (axial incision <2°) and edges. Profiles (cd) and (ef) show a 40 m-deep axial incision.

On the distal part, from 1400 to 2200 mwd (Fig. 4), the incision narrows down to a single channel seen on profile (gh).

Ridges morphologies:

Two groups of ridges may be observed on the figure 5.

The first one, from 1000 to 1300 m, is only visible on the bathymetric map (nor on the chirp profiles because of the rough weather and neither on the slope map), in the eastern part of the central bulge (5A). These small scale ridges reveal a regular morphology of 3 to 5 m high for a wavelength of 50 m, with a N30 orientation, thus oblique to the main slope (Fig. 5A).

A second group of ridges, larger and irregular, occur between 1500 and 1800 mwd. These ridges are larger, with 1-12m high with kilometric lengths (2-20 km) and spaced by 1 to 5 km.

They show a perpendicular orientation to the main slope (Fig. 5A, western part).

Seafloor depressions:

Asymmetric seafloor depressions have been observed in the central part of the area around 1500 mwd (Fig. 5B). Three types of seafloor depressions have been mapped and detailed on figure 5B: 1) circular (Fig. 5 -profiles 3 and 4) with a diameter varying from 0.5 to 1 km, 2) troughs 0.5-1.5 km wide, 6-9 km long and up to 45 m deep; oblique to the slope, oriented SW/NE and 3) troughs parallel to the slope.

Echo Types classification

The studied area can be divided into several zones based on the dominant echo types. Five echo types (Fig. 6 -for location see Fig. 3) have been identified and grouped within the four main classes following the classification of [START_REF] Damuth | Use of high-frequency (3.5-12 kHz) echograms in the study of near-bottom sedimentation processes in the deep-sea: a review[END_REF] and [START_REF] Damuth | Echo character of the east Brazilian continental margin and its relationship to sedimentary processes[END_REF]: distinct (I), hyperbolic (II), transparent (III), indistinct (IV). Each acoustic facies is described using amplitude, lateral continuity, frequency and geometry of the sub-bottom echoes (Fig. 6).

Distinct echoes (I)

Those echo types are characterized by a distinct sharp continuous bottom echo with parallel sub-bottom reflectors. It has been further subdivided into two sub-types (Fig. 6).

Facies I-1 is a laminated facies constituted by alternation of parallel continuous thin high and low amplitude reflections up to 60 ms thick. This facies covers a large part of the study area, from the base of the continental slope to the western most part of the area. The reflections have slightly higher amplitude than in deeper water depths, between 800 to 2200 and 2200 to 3000 mwd. This facies covers the wavy morphologies (ridges area), seafloor depressions and scarps.

Facies I-2 is similar to facies I-1 with alternation of parallel continuous thin high-and low amplitude reflectors, but is topped by a transparent upper layer (0.0025 ms), with a thick and diffuse seafloor reflector. This facies is mapped between 1600 and 2200 mwd.

Hyperbolic echoes (II)

This echo is characterized by irregular overlapping hyperbolae with chaotic sub-bottom reflections of relatively high amplitude. The signal shows slight even none sea-floor penetration (Fig. 6). It is restricted to the area of the shallot-shaped canyon.

Hard echoes (III)

It has been subdivided into two sub-types (Fig. 6) Facies III-1 is characterized by high-amplitude reflectiors with no sub-bottom echoes and/or diffuses returns on seismic profiles and a rough seafloor. It occurs on the upper continental slope and shelf break.

Facies III-2 displays hard echoes with subbottom reflectors, this echo type shows discontinuous and inclined subsurface reflectors, appearing as discontinuous patches. Facies III-2 is mainly represented on the continental shelf of our study area.

INTERPRETATION

Sedimentary structures

High-resolution multibeam and bathymetry data show a highly-complex superficial morphology described above. In order to understand the origin of those sedimentary structures, we use deep penetrating multichannel seismic profiles (Figs. 7 and8). The seismic profile AB (Fig. 7) can be divided in two parts: the lower part, below the cretaceous reflector [Labails, 2007], displays long amplitude undulations that are related to the presence of a tectonic bulge described by several authors [e.g. [START_REF] Jolivet | Cinématique de l'Atlantique nord et central[END_REF]Patriat and Labails, 2006]. On the other hand, the upper part of the profile AB (Fig. 7) shows undulations of the sedimentary layers with wavelengths of about 40 km, probably related to a geological heritage with the presence of numerous canyons and interfluves (Fig. 7). This large-scale undulating morphology is locally complemented by peculiar features described in previous paragraphs: two scarps, the "shallot-shaped" canyon, the ridges and the seafloor depressions. The aim of the discussion will be to try to understand each of the sedimentary process triggering these deposit features.

The scarps morphologies

The two scarps (S1 and S2) correspond to 30-40 m steps which are observed on the seismic profile CD (Fig. 8). The seabed and shallow reflectors are separated by shallow faults or hydro-fracturing due to fluid pressure that connect to surface discontinuities (sliding surfaces; Figs. 7 and8). The scarps morphologies have affected the continuous echoes i.e. seismic facies I-1 and I-2 (Fig. 8). According to the literature, Facies I-1 and I-2 generally correspond either to an alternations of pelagic/hemipelagic (muddy/silty) sediments or to turbidites deposits [e.g. [START_REF] Damuth | Use of high-frequency (3.5-12 kHz) echograms in the study of near-bottom sedimentation processes in the deep-sea: a review[END_REF]. The reflectors (pelagic/hemipelagic facies) that drape S1 are cut by S2 can be observed on the seismic profile (Fig. 9). This observation proves that Scarp S1 is older than S2.

Shallot-shaped canyon

The "shallot shaped" canyon (Fig. 4) corresponds in its proximal part, to a group of V-shaped incisions similar to the gullies described on other continental slope by [START_REF] Twichell | Morphology, distribution, and development of submarine canyons on the United States Atlantic continental slope between Hudson arid Baltimore Canyons[END_REF]. They are issued from the shelf edge and further join together into a single canyon going downslope and finally reach the abyssal plain in its distal part. The sharp incision of the canyon (ab) does not show any sedimentary pelagic drape on the chirp profiles, The median part of the shallot-shaped canyon is characterized by an hyperbolic echo (type II, Fig. 6) interpreted by several authors [START_REF] Damuth | Use of high-frequency (3.5-12 kHz) echograms in the study of near-bottom sedimentation processes in the deep-sea: a review[END_REF][START_REF] Jacobi | Sediment slides on the northwestern continental margin of Africa[END_REF] as a rough seafloor covered with coarse elements such as blocks or mass-transport deposits so-called debrites [START_REF] Lee | Gradual downslope change in high-resolution acoustic characters and geometry of large-scale submarine debris lobes in Ulleung Basin, East Sea (Sea of Japan), Korea[END_REF]].

The chaotic nature of the echo in the shallot-shaped canyon suggests a high degree of sediment remobilization and thus deposition from turbidity currents or debris flows. Collapse of the canyon flanks (see Fig. 3 for location) may contribute to the widening of the channel and to the irregular deposit morphology. Following [START_REF] Lonergan | U-shaped slope gully systems and sediment waves on the passive margin of Gabon (West Africa)[END_REF], we propose that the gullies channelize turbidity currents originating from the shelf, supplying sediments downslope in relation with intermittent activity. The coarser elements are deposited in the median part (Fig. 4), while the rest flowing down evolves into turbidity currents by incorporating water and transforming its matrix into a fluid, which goes through the canyon (profile gh -Fig. 4), in the distal part of the system like the scenario described by [START_REF] Mulder | The physical character of subaqueous sedimentary density flows and their deposits[END_REF].

No clear scar, headwall or departure zone has been identified on bathymetric data to explain mass wasting processes on the upper slope, therefore the simplest way to explain the presence of debris flow deposits would be the connection with a continental supply system with sporadic but intense and extreme events.

Turbidity current pathways where inferred in this area but not connected to permanent present day drainage system on land because of the saharian desertic climate [START_REF] Wynn | The Northwest African slope apron: a modern analogue for deep-water systems with complex seafloor topography[END_REF].

Thus, the origin of many submarine canyons, on this continental margin of north-west Africa between 15°N and 26°N, like Cap Timiris Canyon, are assumed to be related to a major sub aerial river system that was active in the past in relation with "green sahara" periods since Plio-Pleistocene times [START_REF] Antobreh | Morphology, seismic characteristics and development of Cap Timiris Canyon, offshore Mauritania: A newly discovered canyon preserved-off a major arid climatic region[END_REF][START_REF] Larrasoaña | Dynamics of Green Sahara Periods and Their Role in Hominin Evolution[END_REF][START_REF] Vörösmarty | Global system of rivers: Its role in organizing continental land mass and defining land-to-ocean linkages[END_REF]. Recent studies [START_REF] Mcgee | The magnitude, timing and abruptness of changes in North African dust deposition over the last 20,000 yr[END_REF][START_REF] Skonieczny | A three-year time series of mineral dust deposits on the West African margin: sedimentological and geochemical signatures and implications for interpretation of marine paleo-dust records[END_REF] revealed that eolian dust fluxes varied a lot during the last 20 ka, with maximum values during Heinrich Stadial 1 and the Younger Dryas, and minimum fluxes during the end of the African Humid Period between 8 and 6 ka.

The seafloor depressions

Sea-floor depressions observed around the 1500 m isobath reveal two types of shape (Fig. 5).

The circular shaped depressions may be assimilated to pockmarks features. Pockmarks are circular to elongated seafloor depressions which are often associated with fluid gas escape flow from the subsurface [START_REF] Judd | Shallow gas and gas hydrates[END_REF]. The bright amplitudes seen on the figure 9 may correspond to hard lithology like seep carbonates which would likely be associated with gas venting [START_REF] Cathles | The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration[END_REF][START_REF] Çifçi | -Deep and shallow structures of large pockmarks in the Turkish shelf, Eastern Black Sea[END_REF][START_REF] Çifçi | -Deep and shallow structures of large pockmarks in the Turkish shelf, Eastern Black Sea[END_REF][START_REF] Gay | Seafloor facies related to upward methane flux within a Giant Pockmark of the Lower Congo Basin[END_REF][START_REF] Hovland | Unitpockmarks and their potential significance for predicting fluid flow[END_REF][START_REF] Hovland | The significance of pockmarks to understanding fluid flow processes and geohazards[END_REF][START_REF] Judd | Shallow gas and gas hydrates[END_REF]]. An unconformity is observable on the seismic chirp profile (Fig. 9) around 1.95 twt (s) (1500 mwd).

To explain the formation of the second type of depressions, the elongated ones (Fig. 5), we examine three hypotheses. The concentration of circular pockmarks can cause the formation of the elongated pockmarks observed at 1500 m. The Fig. 5B shows an outstanding example of the evolution of pockmarks from circular to elongated shape. The addition of two pockmarks may be observed before complete coalescence on the profile 2 (Fig. 5C) and a detailed observation show that the large pockmark (Fig. 5B) is composed of a coalescence of ten to twenty smaller depressions. This first hypothesis corresponds to groups or long chains of pockmarks described by [START_REF] Hovland | Seabed Pockmarks and Seepages: Impact on Geology[END_REF].

Another hypothesis is that these linear depressions reflect the surface expression of a deeper structure such as a meandering channel or a subjacent fault draped by sediment, as suggested by the third cartoon on the top of the figure 8. Meandering paleo-channels are clearly seen on the figures 3 and 5A and may explain the occurrence of some of the elongated seafloor depressions.

The dissymmetry or "en échelon" or "festooned" appearance of this succession of holes and troughs depressions have already been observed on a portion of this margin, between Cap Timiris and Nouakchott by [START_REF] Yang | Gravity-driven faults: Migration pathways for recycling gas after the dissociation of marine gas hydrates[END_REF]. They described troughs, either parallel or oblique to the slope dip, with elongate en-echelon plan-forms. These troughs have variable length, ranging from 0.5 to 10 km and are related to 2 types of gravity-driven faults: tear faults and normal faults. Based on 3D seismic data, these authors analysed a set of times slices with amplitude maps and suggest that the "en echelon and linear" troughs were formed as a result of multiple episode of down slope creep and have propagated through 60-140 m of sediments. These observations area in close agreement with our data, suggesting that the seafloor depressions may be assimilated to a potential rupture line and represent the surface expression of tear and/or linear faults affecting the upper sedimentary column. The location of "en echelon and linear" troughs at the slope inflexion (around 1500m) may suggest the departure zone of a sliding event.

Ridges areas :

The two ridges areas are observed on both sides of the sea-floor depressions area (Figs. 5).

Upper slope ridges, smaller in size and presenting an oblique orientation to the slope are assimilated to sediment-wave morphology, which would be in agreement with the existence of numerous sediment-wave fields mapped in the northern part of the margin by [START_REF] Wynn | The Northwest African slope apron: a modern analogue for deep-water systems with complex seafloor topography[END_REF]. These morphologies are commonly reported on the continental slopes of passive margins [START_REF] Faugères | Multi-process generated sediment waves on the Landes Plateau (Bay of Biscay, North Atlantic)[END_REF]Faugères et al., 200;[START_REF] Viana | Economic and palaeoceanographic significance of contourite deposits[END_REF] and active margins [START_REF] Lee | High-resolution (2-7 kHz) acoustic and geometric characters of submarine creep deposits in the South Korea Plateau, East Sea[END_REF]. The bad weather prevent any further investigation of these small ridges features on the chirp data and thus, our observations are only based on the bathymetry map.

The second group of ridges, larger in size are located from 1500 to 1800 mwd. These undulations are parallel to the bathymetry and occur just below the seafloor depressions area (Fig. 5), traducing a deformation of the superficial sedimentary cover. We suggest they may be interpreted as compressional ridges.

DISCUSSION

All those sea-floor features, pockmarks, compressive ridges and scarps, canyons, reveal the existence of sediment transfer and instability processes from upslope to downslope. Here we suggest a potential link between these specific morphologies.

According to the literature, non-random pockmarks show spatial arrangement related either to complex underlying geology or to local disturbance of the seabed, whereas random distribution of pockmarks is commonly displayed on the lower slope where sediments are generally fine-grained and tectonic structures are rare [START_REF] Pilcher | -Mega-pockmarks and linear pockmark trains on the West African continental margin[END_REF].

In our study area, pockmarks are arranged along a "festooned line" on a N-S direction parallel to the bathymetry. However the seismic SMT profiles (Figs. 7 and8) do not show deep faults or tectonic structures with complex underlying geology. The only visible fault is the one displayed in the scarp S2 (Fig. 8 -cartoon 2) and it appears to shift the first three reflectors). This fault is therefore considered as superficial and might be rooted in a layer which constitutes a sliding surface (basal detachment) seen on the figure 8. [START_REF] Pilcher | -Mega-pockmarks and linear pockmark trains on the West African continental margin[END_REF] described linear pockmark trains which are associated with listric slump faults in different cases of continental slope instability. We suggest that scarps S1 and S2 are former zones of pockmarks alignments (Fig. 10).

The "festooned line" or "en-échelon" line of coalescent pockmarks could witness a weakness zone on the slope which could evolve into a future detachment area and form a new scarp S3. This phenomenon of high concentration of pockmarks (causing their fusion) is widely observed e.g. in the Congo Basin [START_REF] Gay | Seafloor facies related to upward methane flux within a Giant Pockmark of the Lower Congo Basin[END_REF][START_REF] Gay | Sinuous pockmark belt as indicator of a shallow buried turbiditic channel on the lower slope of the Congo Basin, West African Margin[END_REF], Nigeria margin [START_REF] Sultan | -Pockmark formation and evolution in deep water Nigeria: Rapid hydrate growth versus slow hydrate dissolution[END_REF][START_REF] Wei | Gas hydrate distributions in sediments of pockmarks from the Nigerian margin-Results and interpretation from shallow drilling[END_REF] in the Northwestern Atlantic [START_REF] Fader | Gas-related sedimentary features from the eastern Canadian continental shelf[END_REF], the Ionian Sea, Greece [START_REF] Hasiotis | Earthquake-induced coastal sediment instabilities in the western Gulf of Corinth, Greece[END_REF], the North Sea [START_REF] Hovland | Gas-induced erosion features in the North Sea[END_REF][START_REF] Hovland | Methane-related carbonate cements in pockmarks of the North SeaHOVLAND[END_REF][START_REF] Hovland | Characteristics of two natural gas seepages in the North Sea[END_REF] and the Mediterranean [START_REF] Dimitrov | Deep sea pockmark environments in the eastern Mediterranean[END_REF].

The occurrence of compressive ridges below the pockmarks area also evidence initiation of gravitary movement.

The main triggering mechanism for slope instability and thus gravity mass movements processes, across the NW-African continental margin, is believed to be the overload of sediment [START_REF] Georgiopoulou | Sahara Slide: Age, initiation, and processes of a giant submarine slide[END_REF][START_REF] Masson | Large landslides on passive continental margins: processes, hypotheses and outstanding questions[END_REF]. Whereas, other triggers such as earthquakes, free gas, gas hydrates or diapirism are believed to be of minor importance [START_REF] Georgiopoulou | Sahara Slide: Age, initiation, and processes of a giant submarine slide[END_REF][START_REF] Krastel | Mapping of seabed morphology and shallow sediment structure of the Mauritania continental margin, Northwest Africa: some implications for geohazard potential[END_REF].

During the late Pleistocene, the repeated slope instabilities appear as a common characteristic of NW African margin [START_REF] Antobreh | Morphology, seismic characteristics and development of Cap Timiris Canyon, offshore Mauritania: A newly discovered canyon preserved-off a major arid climatic region[END_REF][START_REF] Georgiopoulou | Sahara Slide: Age, initiation, and processes of a giant submarine slide[END_REF]2007]. This margin has also been strongly affected by Quaternary sea-level cycles [START_REF] Church | A 20th century acceleration in global sea-level rise[END_REF][START_REF] Wien | -Age models for pelagites and turbidites from the Cap Timiris Canyon off Mauritania[END_REF]. According to [START_REF] Georgiopoulou | Sahara Slide: Age, initiation, and processes of a giant submarine slide[END_REF], sea level changes appear to be important and giant slides appear to coincide with periods of deglaciation. During the latter periods, the sedimentation occurred in the vicinity of the present shore line [START_REF] Georgiopoulou | Sahara Slide: Age, initiation, and processes of a giant submarine slide[END_REF]. Thus, major sedimentary events on the northwest African margin appear coincident with glacial -to -interglacial transitions [START_REF] Church | A 20th century acceleration in global sea-level rise[END_REF][START_REF] Georgiopoulou | Sahara Slide: Age, initiation, and processes of a giant submarine slide[END_REF]. According to [START_REF] Riboulot | -Control of Quaternary sea-level changes on gas seeps[END_REF]2013;2012], the upper-Pleistocene 100 ka cyclicity sea-level changes are the predominant driving factor of pockmark formation in the Gulf of Lions and Nigeria. On the basis of these observations, we suggest that Pleistocene sea level and sediment load associated with fluids escapes are the main process for soft-sediment deformation off Dakhla.

No core or drilling wells are available in the study area, however we propose correlations based on sea-level curves. The main goal of this correlation is to propose an age for the scarps morphologies and pockmarks formation (Fig. 10). [START_REF] Riboulot | -Sea-level change and free gas occurrence influencing a submarine landslide and pockmark formation and distribution in deepwater Nigeria[END_REF] have estimated the formation of a pockmark to last ~100 ka. Based on that assumption, we correlate morphologies with the sea-level curve from [START_REF] Waelbroeck | -Sea-level and deep water temperature changes derived from benthic foraminifera isotopic records[END_REF] and propose the following chronology (Fig. 10):

-A) During the Marine Isotopic Stage MIS8 to MIS7: formation of the first generation of pockmarks caused by sedimentary load at 2600 mwd, that induced the formation of a sliding plane of the scarp 1. This scarp is draped by hemipelagic deposits; -B) During the MIS 6 to MIS 5: formation of the second generation of pockmarks at 2200 mwd, that induced the formation of a sliding plane of the scarp 2. And folding of the hemipelagic layer.

-C) During the MIS 4-2 to MIS 1 : formation and the alignment pockmarks at 1500 mwd and future scarp S3 (?). This area of pockmarks is contemporary to Sahara Slide, El Golfo / Canary Debris Flow and the Mauritania Slide [START_REF] Siddall | -Sea-level fluctuations during the last glacial cycle[END_REF].

During low sea level phases, cannibalization of the shelf combined with "green sahara" periods since Plio-Pleistocene times [START_REF] Larrasoaña | Dynamics of Green Sahara Periods and Their Role in Hominin Evolution[END_REF] induced an overload of sediment on the slope that may have favored sediment failures. The slope destabilization might also be linked to the liberation of hydrate dissociation by variation of temperature and pressure, linked to glacial/interglacial phases.

CONCLUSION

The analysis of multibeam bathymetry, high-resolution sub-bottom and seismic data on the continental slope of the off Dakhla allowed us to identify several morphologies such as shallot-shaped canyon, pockmarks, wavy morphologies and two scarps (S1 and S2). A detailed observation of these morphologies on chirp sub-bottom profiles and seismic data shows evidences rupture processes and transfer of sediments. Compressive ridges is primary linked to deformation mechanisms and creeping movements. This deformation is probably enhanced by the presence of fluids or gas associated with internal discontinuities acting as basal shear planes for creeping events. The scarps S1 and S2 indicate at least two generations of slides on this margin. We suggest that slide development was caused by rapid sea level rise after glaciation periods when sedimentation rate was maximum on the slope. This sedimentary load induced fluid or gas migrations along unconformities and superficial faults in the sediment. Fluid migration has lead to the formation and concentration of aligned pockmarks in weakness zones. These slides are believed to be 100-ka cycle events controlled by sea level variations during the second half of the quaternary period.

However, more regional studies using high resolution but deeper seismic penetration are needed in the area as well as coring and geotechnical studies to fully assess the timing of the events, the existence of gas hydrates and to determine whether creeping is still active today.

Acknowledgement

  Special thanks are due to the captains of the N/O Suroit and Nadir and their crews together with the Genavir technical staff during the Dakhla cruise. This work was partly funded by Total in the frame work of the IFREMER/TOTAL Dakhla project. Authors would like to thank Bruno Marsset for his help on seismic processing and Frauke Klingelhoefer, Jean Nizou for their comments on an earlier version of the manuscript. The two reviewers A. Gay and P.Imbert and the editor O. Lacombe are thanks for their fruitful comments that helped us to improve this paper.

Figure 1 :

 1 FIGURE CAPTIONS:

Figure 2 :

 2 Figure 2: Bathymetric map of the offshore Dakhla from SIMRAD EM300 acquired during

Figure 3 :

 3 Figure 3: Morphobathymetric interpretation on shaded relief map showing the main

Figure 4 :

 4 Figure 4: Detailed bathymetric map of the Dakhla shallot-shaped canyon. The Dakhla canyon

Figure 5 :

 5 Figure 5: Focus on the mid-slope area with the wavy morphologies (ridges) and depressions.

Figure 6 :

 6 Figure 6: List of the main chirp "echo types" described in the investigated area: distinct

Figure 7 :

 7 Figure 7: Dakhla seismic profile (AB profile) and interpretation showing the global

Figure 8 :

 8 Figure 8: Dakhla seismic profile (CD profile) and interpretation showing the global

across the scarp S1, scarp S2 and depressions. The seismic profile shows significant bulge of the basement (location of the seismic profile CD on figure 1).