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Understanding and quantifying the migration of free-gas in hydrate-bearing sediments through time is particularly compulsive along continental margins, where gas hydrate dissociation could have triggered some of the largest submarine landslides observed on Earth. Offshore Romania, high-resolution seismic profiles reveal low reflective or low-velocity zones, which are indicative of free gas, beneath vertical stacked Bottom Simulating Reflectors (BSRs). To further understand the occurrence of double BSRs in the area and the possible effect of gas hydrate dynamics on slope instability and free gas releases, we performed a numerical 2D transient modelling of the evolution of the thermodynamic stability of gas hydrates, integrating in-situ measured physical data and indirect assessments of sea-bottom temperature, thermal conductivity, salinity and sea-level variations. We found that the shallowest BSR matches well with the current Base of the Gas Hydrate Stability Zone (BGHSZ) and the deeper one with the Last Glacial Maximum (LGM) base of GHSZ. The reduction of the GHSZ extension subsequently led to widespread gas hydrate dissociation associated with warming conditions and an increase in Black Sea salinity. However, this dissociation is only responsible of some very superficial submarine landslides (< 30 mbsf and 3 m thick in average) that occurred during this same period. These new constraints improve our understanding of the sliding mechanisms on the Romanian slope that have been ongoing since the LGM and support less catastrophic scenarios than those suggested previously in the case of active gas hydrate dissociation. These results also allow solving the mystery of the double BSR, which here corresponds to a relic of the LGM BGHSZ.

International audience; The provenance of Early Cretaceous sandstones dredged on the northern margin of the Demerara Plateau, offshore French Guiana and Suriname, reveals the sediment routing system that prevailed through the Equatorial Atlantic rifting. Fission-track analysis and U–Pb dating of 310 and 111 detrital zircons, respectively, have been performed. Microfacies analysis and inherited cooling ages suggest that the sandstones were deposited in shallow marine environments during the Early Cretaceous, before the Late Albian drowning of the marginal plateau. Most of the U–Pb zircon crystallisation ages are comprised of between 700 and 600 Ma and are attributed to the Pan-African-Brasiliano orogeny. Statistical and chronological evidence suggest that the zircon fission-track cooling ages were inherited from source materials. Triassic peak ages (>50% of the population) are attributed to the early phase of Central Atlantic rifting. One sample records a cooling phase at ca. 170 Ma, presumably following volcanic hotspot activity and the opening of the Central Atlantic Ocean. Two other samples record the rapid exhumation of the French Guiana transform margin during the Equatorial Atlantic rifting (127 ± 11 and 106 ± 8 Ma). We propose a source-to-sink model in which the Pan-African-Brasiliano basement of the margin was eroded as a result of flexural uplift along the French Guiana margin, and the detrital material funnelled in the Cacipore graben sustained the Early Cretaceous syn-rift sedimentation on the marginal plateau.

The Great Amazon Reef System is a living biogenic mesophotic reef ecosystem that has been recently described along the shelf break of Brazil. An exploration mission was carried out in 2019 along the outer edge of the French Guiana Shelf. A side-scan sonar survey was conducted to locate reef outcrops and allowed twelve in situ 80 to 120 m depth dives and sampling of the reef rocks and peripheral sands. The majority of the hard-rocks are composed of biological concretions. However several fragments revealed the inside presence of sandstone clasts. These clasts, more or less enveloped by biogenic coatings, probably represent destroyed clasts of underlying or neighboring beachrock banks. Their dominant cement is micritic (high-magnesian calcite); the intergranular or extragranular porous field was later filled with low-magnesian sparry calcite. The sand or gravel that accumulated near the barrier mainly consist of the blunt debris of coastal fauna and flora associated with different carbonate or ferruginous neoformed ooids. At 104 m depth, ooids extracted from Dive 11 samples dated from the start of MIS2 (27,370 cal yr BP) and attest to the presence of a significant coastal accumulation. At this same site, cementing did not take place until about 3,500 years later (23,990 cal yr BP). The cement of a nearby beachrock indicates a much more recent age (16,170 cal yr BP). Lastly, the age of 4,100 year BP measured on the barnacles attached to the top of the reef attests to the late Holocene reef's biological activity.

Marine landslides, which are observed worldwide along continental slopes, constitute the most important processes reworking sedimentary deposits and a major geohazard for marine and coastal domains. They can generate potentially the destruction of marine infrastructures through the formation of turbidity currents and/or hazardous tsunamis. In the Romanian sector of the Black Sea, high amounts of methane are detected in the sediments and at the seafloor through the identification of gas seeps in the water column. They occur on the upper slope, mostly outside the large gas hydrates system occurring in the sediment below -660 m water depth, where methane is trapped in ice cages that act as a buffer zone hampering methane to reach the water column. New geophysical and geotechnical dataset acquired along the Romanian margin reveals that the active seepage zone is associated with numerous slope failures, which incised the continental upper slope. Is there a possible relationship between gas hydrate system and recent slope instabilities? Could intense free gas emissions and/or gas hydrates dissociation have triggered such geohazards? To answer these questions, we present (1) an high resolution mapping and, more important, dating of landslides since the last glacial maximum (35 ka), (2) results of numerical modelling showing the evolution of gas hydrates stability zone inside the sediments since the last 35 ka taking into account the environmental variations that occurred during this time lapse (geothermic gradient, temperature, salinity and sea level).The models highlight the major effect of environmental changes and particularly the glacial/interglacial cycles and salinity variations on the extent of the gas hydrates. The confrontation of those models with slide extensions, depths and ages allows to better discuss the respective influences of gas hydrates dissociation, stratigraphic overpressure and seepage on slope instability since the last glacial maximum.

International audience; The Demerara and Guinea plateaus are conjugate Transform Marginal Plateaus (TMPs) formed at the junction of the Jurassic Central Atlantic Ocean and the Cretaceous Equatorial Atlantic Ocean. We compare their structure and evolution through a combined industrial/academic seismic dataset tied by well data. We show that these TMPs record a complete evolution history from the Jurassic Central Atlantic to the Equatorial Atlantic breakup and seafloor spreading phases. Both plateaus first formed as volcanic margins displaying successive SDR wedges migrating towards the newly forming Central Atlantic domain. In this context, Demerara and Guinea, conjugates of the Bahamas, seem to have formed in relation to plume activity at the southern end of the Central Atlantic domain. Our dataset suggests that the Demerara and Guinea plateaus initially represented two distinct extrusive centres separated by a crustal basement high. Later, during the Cretaceous Equatorial opening phase, both plateaus separated in a transform mode following this discontinuity. Deformation is notably asymmetric during this phase: Aptian to Albian folding, strike-slip, transtensive deformation and complex vertical movements on the Demerara side and only minor deformation on the Guinean side, except on its divergent Sierra Leone termination. The deformation is sealed on both plateaus by a regional upper Albian erosion unconformity. Extensional reactivation occurs on the Guinea side (probably through a general collapse?). To conclude, this study provides new insights into the nature and origin of TMPs and the key tectono-sedimentary archives they may contain to understand the polyphase breakup conditions of Gondwana.

Transform marginal plateaus (TMPs) are large and flat structures commonly found in deep oceanic domains, but their origin and relationship to adjacent oceanic lithosphere remain poorly understood. This paper focuses on two conjugate TMPs, the Demerara Plateau off Suriname and French Guiana and the Guinea Plateau, located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic oceans. The study helps to understand (1) the tectonic history of both Demerara and Guinea plateaus and (2) the relationship between the Demerara Plateau and the adjacent oceanic domains, and finally, (3) throws light on the formation of TMPs. We analyse two existing wide-angle seismic-derived velocity models from the MARGATS seismic experiment (Demerara Plateau), and adjacent composite industrial seismic lines covering the Demerara and Guinea plateaus. The Demerara Plateau displays a 30 km thick crust, subdivided into three layers, including a high-velocity lower crust. The velocities and velocity gradients do not fit the values of typical continental crust but instead correspond to volcanic margin- or large igneous province-type crusts. We propose that the, possibly continental, lower crust is intruded by magmatic material and that the upper crustal layer is made from extrusive volcanic rocks of the same magmatic origin, forming thick seaward (westward)-dipping reflectors (SDRs) sequences. This SDR complex extends to the Guinea Plateau as well and was emplaced during hotspot (Sierra Leone)-related volcanic rifting preceding the Jurassic opening of the Central Atlantic and forming the western margin of the plateau. North–south composite lines linking the Demerara and Guinea plateaus reveal the spatial extent of the SDR complex but also a pre-existing basement ridge separating the two plateaus. The entire Demerara–Guinea margin would therefore be an inherited Jurassic volcanic margin bordering the Central Atlantic Ocean to the east, with a possible conjugate being the Bahamas Plateau on the other side of the ocean. This margin was then reworked during a non-coaxial Cretaceous second phase of rifting potentially accompanied by a magmatic event. Opening of the northern margin occurred in a transform mode splitting the Jurassic volcanic margin into two parts (the Guinea and Demerara TMPs), conceivably along a pre-existing basement ridge. Rifting of the eastern part of the Demerara Plateau occurred surprisingly along the eastern limit of the Jurassic SDR complex, forming the present-day eastern divergent margin of the Demerara Plateau. After that stage, the Demerara and Guinea plateaus are individualized on each side of the Equatorial Atlantic. This study also highlights the major contribution of thermal anomalies related to hotspots and superposed tectonic phases in the case of other TMPs that share numerous characteristics with the Demerara Plateau.

During the end of the lower Cretaceous, the connection between the South and the Central Atlantic accretionary axis led to the oblique opening of the Equatorial Atlantic, and to the separation of Africa and South America by alternating transform and rift margins. At the western end of the Equatorial Atlantic, we investigate the structure of the Cretaceous margins surrounding the Demerara plateau, north of French Guiana and Suriname. These margins were previously described as transform northward and divergent eastward. From the bathymetry and deep structures, we propose to divide the northern transform into three margin segments, with two transform segments separated by a divergent one. These two transform margins are very different, the northwestern one being linear and associated with a steep and erosive continental slope, the northeastern one consisting of several faults and ridges en echelon disposed. In between, the divergent margin appears to be a pull-apart basin localized by structures inherited from the previous Jurassic rifting. Additionally, the eastern divergent margin may have been localized by a thermal anomaly tentatively related to a hotspot. It is proposed that the deformation was first localized in divergent (rift) basins, subsequently connected by transform faults. The structure of the transform fault varies with the offset between adjacent rift basins: a large offset forms a linear transform and a short (less than 200 km) offset forms en echelon structures.

International audience; The Demerara Plateau is a marginal plateau which forms a bathymetric relief on the sea floor. Here, contourite deposits have been studied in detail, following the recent discovery of contourite sequences likely related to the bottom currents and linked both to contour current and peculiar sea-floor morphology. A chronostratigraphic framework, based on δ18O relative variations and palaeomagnetic events in sediment cores allows correlating sediment processes to current intensity changes and major climate phases (glacial or interglacial). The studied sediments are enriched in glauconitic grains. In addition, the glauconite mineralogical maturity can easily correlate to low sedimentation rate and slightly energetic bottom currents on the seafloor. Based on these data and using the glauconitic authigenic mineral as proxy for inferring the degree of winnowing at the sediment-water interface, we might put forward the hypothesis that the intensity of NADW is higher during the glacial stages and lower during interglacial periods.

International audience; The Lower Congo Basin is known to discharge a substantial volume of oil towards the sea surface, from more than one hundred seafloor seep sites distributed throughout the deep province of the Lower Congo Basin. A large geochemical coring survey confirmed the presence of oil on the seabed. The combination with the seismic data considerably improved the identification of the origin of the oil slicks on the seabed. Multiple specific geophysical characteristics of thermogenic hydrocarbon seep sites were highlighted in a previous detailed analysis of seismic datasets. This study aims to test the characteristics previously identified at regional-scale. The active discharge zone is limited to the distal province of the basin. It is characterised by strong compression/shortening due to the sliding of the post-salt super-sequence, resulting in numerous salt diapirs that control the location of seafloor oil seep sites. The paper describes the bathymetric, reflectivity, amplitude and sub-bottom profiler characteristics of a group of thermogenic seep sites. They correspond essentially to submarine mounds or pockmarks of complex and irregular shape surrounded by hummocky mounds. Active oil pockmarks are systematically associated with positive amplitude anomalies on the seabed and are linked to vertical high-amplitude columns rooted in the seismic reflector and associated with the base of the gas hydrate stability zone. The sub-bottom profiler data shows that the hummocky mounds are connected by a network of faults to high-amplitude bodies buried under a consistent sediment thickness. Based on the definition of specific geophysical features (seafloor mounds, complex shape and irregular pockmarks, positive anomalies of seafloor amplitude, high-amplitude vertical pipes), we identified a series of potential oil seep sites at basin scale. The mounds are particularly recognizable using the seismic curvature attribute; we have identified 2946 individual hummocks that are grouped in 50 zones with a density of 35–240 per km2. They are believed to be associated with asphalt storage on the seafloor and related to the biodegradation of heavy oils during hydrocarbon dysmigration through the sedimentary pile. Unusual double BSRs occur over the study area; these are also a specific feature of thermogenic seep sites. Barely half of the potential sites identified on the seismic datasets are associated with recurrent oil slicks at the sea surface. The proportion of remaining anomalies may be associated with inactive seep sites over the period of satellite-based monitoring or gas-dominated seep sites.

International audience; We report the geophysical characterisation of natural oil seep sites through a combination of sea surface evidence of oil leakage from spatial imagery with a large collection of seafloor and subsurface geophysical data. This paper provides a detailed characterisation of one selected active seep site and identifies possible specific feature of oil seep sites. The oil seep is a complex-shaped feature on the seafloor consisting of a cluster of heterometric pockmarks inside a main depression area and peripheral metre-scale seafloor mounds. A strong deformation related to salt tectonics controls the location of the seafloor source by fracturing the overburden. The associated thermal anomaly induces a vertical modification position of the base of the gas hydrate stability zone (BGHSZ) that is used as a fluid migration route towards the crest of the diapir. The combination of local depressions and seafloor amplitude anomalies linked with vertical high-amplitude pipes rooted on the BGHSZ suggests a focused fluid flow towards the seafloor. In peripheral areas, the seafloor mounds are linked by shallow faults to buried high amplitude patches on sub-bottom profiler sections. The combination of restricted-size seafloor mounds with a progressive deepening of the high amplitude from the seafloor suggests a substantial decrease of the hydrocarbon flow towards peripheral areas. The proximity of actively oil-supplying seafloor depressions and seafloor mounds shows that the hydrocarbon flow rapidly decreases laterally. The thermogenic seep site is affected by two consistent and sub-parallel reflections with negative polarity. The first is interpreted as the methane-related BGHSZ, the second could correspond to the base of a thermogenic BGHSZ produced by a mixture of heavier gas. The seafloor roughness and double BSR appear to be specific features of oil seep sites. The geophysical features revealed at a localised study area will be extrapolated towards a larger province for relevance validation.

Off French Guiana and Suriname, North Atlantic Deep Water and Antarctic Bottom Water oceanic currents contour the Demerara marginal plateau, which promotes the formation of contourites. We have studied these contourites thanks to a new compilation of high-resolution subbottom profiles calibrated by sedimentary cores. The echofacies and isopach maps that we constructed highlight a sedimentary distribution parallel to the isobaths. The presence of moats along the slope is confirmed by the observation of parallel, elongated, sedimentary depleted zones and echofacies strongly affected by diffraction hyperbola and transparent echofacies. We interpret these features to be related to eroded slopes and mass-transport deposits. In contrast, the sedimentary drifts that we mapped are characterized by elongated and thick slope-parallel depocenters displaying bedded echofacies with wave-like bedforms. According to our interpretation, they result from interactions between the currents and the seafloor. Seismic wipeouts frequently affect those drifts, possibly resulting from high water or organic contents.

In order to study opening mechanisms and their variation in the Atlantic ocean basins, we compiled existing wide-angle and deep seismic data along conjugate margins and performed plate tectonic reconstructions of the original opening geometries to define conjugate margin pairs. A total of 23 published wide-angle seismic profiles from the different margins of the Atlantic basin were digitized, and reconstructions at break-up and during early stages of opening were performed. Main objectives were to understand how magma-rich and magma-poor margins develop and to define more precisely the role of geologic inheritance (i.e., preexisting structures) in the break-up phase. At magma-poor margins, a phase of tectonic opening without accretion of a typical oceanic crust often follows initial rupture, leading to exhumation of serpentinized upper mantle material. Along volcanic margins the first oceanic crust can be overthickened, and both over- and underlain by volcanic products. The first proto-oceanic crust is often accreted at slow to very slow rates, and is thus of varied thickness, mantle content and volcanic overprint. Accretion of oceanic crust at slow to very slow spreading rates can also be highly asymmetric, so the proto oceanic crust at each side of conjugate margin pairs can differ. Another major aim of this study was to understand the mechanisms of formation and origins of transform marginal plateaus. These are bathymetric highs located at the border of two ocean basins of different ages and are mostly characterized by one or several volcanic phase during their formation. They often form conjugate pairs along a transform margin as it evolves and might have been the last land bridges during breakup, thereby influencing mammal migration and proto-oceanic currents in very young basins. At these plateaus, volcanic eruptions can lead to deposits of (at least in part subaerial) lava flows several km thick, better known by their geophysical signature as seaward dipping reflectors. Continental crust, if present, is heavily modified by volcanic intrusions. These marginal plateaus might form when rifting stops at barriers introduced by the transform margin, leading to the accumulation of heat in the mantle and increased volcanism directly before or after the cessation of rifting.

The Demerara plateau (offshore Suriname and French Guiana) is an original transform marginal plateau located at the junction between the central and the equatorial Atlantic domains. New results combining the interpretation of several datasets of high-penetration industrial MCS, academic MCS and wide-angle seismic data image a 30 km thick crust in the plateau, evolving towards three different margins to the two adjacent oceanic domains.This work shows that this oceanic relief is a Jurassic volcanic margin located at the southern termination of the Central Atlantic rifting, and forming the divergent western margin of the Demerara plateau. New result from dredges also show the influence of a hotspot in this rifting phase. The resulting transitional domain is unusual, characterized by a progressive thinning of the margin toward the west and the presence of SDRs outer bodies on a reworked unit probably of continental origin. Unambiguous oceanic crust is identified at about 100 km from the slope break of the shelf. Toward the plateau, the outer SDR body let place to several thick superimposed inner SDR.Then, this Jurassic domain was remarkably reworked during the Cretaceous rifting phase linked to the opening of the Equatorial Atlantic. This second event restructured this volcanic object, forming a transform northern margin and a divergent eastern margin, each with a specific transitional domain.The presence of a volcanic margin which subsequently undergoes a non-coaxial opening with transform constraints is relatively unusual. Our data help to better constrain the transitional domains and the TOC of the Equatorial Atlantic Cretaceous margins. The characterization of the northern and eastern extension limit of the SDRs formations and of the high velocity lower crust observed in the plateau is an important regional issue. This knowledge is necessary in particular to characterize the volumes and structures associated with the Jurassic volcanic episode, which control the thermo-structural Cretaceous evolution of the plateau and the adjacent domains.

International audience; Fluorescence sensors mounted on unmanned underwater gliders open new ways of investigation to detect dissolved hydrocarbons in seawater. A glider was deployed for 20 days to monitor biogeochemical and physical signals associated with natural hydrocarbon seepages within the first 700 m in the Angolan waters. The glider was equipped with fluorometers (MiniFluo-UV) to measure the concentrations of polycyclic aromatic hydrocarbons (PAH) of interest, i.e. naphthalene, phenanthrene, fluorene and pyrene. A continuous PAH-like signal detected within the 70 m layer below the sea surface is associated with high chlorophyll concentration in the deep chlorophyll maximum. Vertical PAH-like anomalies forming either strong spikes or diffuse columns down to 700 m are observed at the exact location of oil seep sites identified on Synthetic Aperture Radar satellite images. An ~200 m thick layer of enhanced PAH-like concentration, topped by a thermo/pycnocline identified at 280–300 m water depth, is measured in concomitance with the decrease in oxygen concentration. The concomitance of these signals suggests that lower oxygen concentrations induce a preservation of hydrocarbons within the eastern Atlantic oxygen minimum zone. Even if the absence of in-situ measurements limits the understanding of physical and biogeochemical processes affecting PAH concentrations, the measurements conducted at the edges of the OMZ suggests a relationship with microbial activity and organic matter dynamics in this layer. The results presented here show that gliders equipped with PAH sensors represent a promising means for monitoring hydrocarbons in the oceans, especially when they are coupled with other systems (i.e. Synthetic Aperture Radar).

There is a growing interest in the management of seamounts of the South Western Indian Ocean (SWIO) both in waters under national jurisdictions and in the Areas Beyond National Jurisdiction (ABNJ). On the one hand, new scientific knowledge has been gathered through various oceanographic cruises during the past decade and, on the other hand, new agreements are under consideration globally to promote conservation and sustainable use of the biodiversity in the ABNJ, where the deep sea ecosystems associated to seamounts are a growing matter of concern. SWIO seamounts have attracted interests from fishing operators since the 1960s, while mining exploration contracts have been recently granted. Seamounts are known to shelter rich, fragile and poorly resilient ecosystems whose important ecological functions that are threatened by various anthropogenic pressures. While many seamounts and shoals are located in national waters, many others fall in the ABNJ with no current legal status per se. To ensure conservation of their habitats and biodiversity, it is essential that protection measures are instigated under an internationally-recognized legal and institution framework. In this paper, we review the current state of such a framework relevant to seamounts, with emphasis on fisheries and conservation in the SWIO. We select an emblematic seamount, the Walters Shoal, as a case study to discuss how this site could become a fully-protected space in the ABNJ. Since a large part of the SWIO is under the mandate of the Nairobi Convention (Regional Sea under the auspices of UNEP), guidelines are proposed to spur a dedicated seamount governance in the framework of this Convention.

We report the discovery of 173.4 Ma hotspot-related magmatic rocks in the basement of the Demerara Plateau, offshore French Guiana and Suriname. According to plate reconstructions, a single hotspot may be responsible for the magmatic formation of (1) both the Demerara Plateau (between 180 and 170 Ma) and the Guinea Plateau (circa 165 Ma) during the end of the Jurassic rifting of the Central Atlantic; (2) both Sierra Leone and Ceara Rises (mainly from 76 to 68 Ma) during the upper Cretaceous oceanic spreading of the Equatorial Atlantic ocean; (3) the Bathymetrists seamount chain since the upper Cretaceous. The present-day location of the inferred Sierra Leone hotspot should be 100 km west of the Knipovich Seamount.

Deep seismic data and plate kinematic reconstructions help understand the mechanisms of rifting and opening of new oceans, basic principles of plate tectonic cycles. In this study, available deep wide-angle seismic velocity models from the Atlantic margins are reviewed and plate reconstructions used to define conjugate model pairs. The main objective was to study the question of how magma-rich and magma-poor margins develop and the role of inheritance in the break-up. We also studied the question of the mechanism of formation and the origin of transform marginal plateaus, which are typically found at the border of two ocean basins of different ages and are mostly characterized by at least one volcanic phase during their formation. The results of the study include the comparison of crustal thickness, oceanic plate thickness and the influence of volcanism along the Atlantic margins. The conjugate profiles image different degrees of asymmetry of the Atlantic Margin rifts. Marginal plateaus might form when rifting stops at barriers leading to the accumulation of heat in the mantle and increased volcanism directly before or after halting of the rifting.

The Demerara Plateau (offshore Suriname and French Guiana) is located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic Oceans. The study of its crustal structure is fundamental to understanding its tectonic history, its relationship with the adjacent oceanic domains and to enlightening the formation of Transform Marginal Plateaus (TMPs). This study presents two wide-angle seismic velocity models from the MARGATS cruise seismic experiment, and adjacent composite seismic reflection lines. The plateau itself is characterized by a 30 km thick crust, subdivided into three layers, including a high velocity lower crust (HVLC). The velocities and velocity gradients do not fit values of typical continental crust but could fit with volcanic margin or Large Igneous Province (LIP) type crusts. We propose that the, possibly continental, lower crust is intruded by magmatic material and that the upper crustal layer is likely composed of extrusive volcanic rocks of the same magmatic origin, forming thick seaward dipping reflector sequences tilted to the west. This SDR complex was emplaced during hotspot related volcanic rifting preceding the Jurassic opening of the Central North Atlantic and forming the present-day western margin of the plateau. The internal limit of the SDR complex corresponds to the future limit of the eastern margin. The Demerara Plateau would therefore be an inherited Jurassic volcanic margin boarding the Central Atlantic. This margin was reworked during the Cretaceous at the eastern limit of the Jurassic SDR complex, creating the present-day northern transform margin and the eastern divergent margin along the Equatorial Atlantic. This study also highlights the major contribution of thermal anomalies such as hotspots and superposed tectonic phases in the history of TMPs, which share a great number of characteristics with Demerara.

International audience; Numerous submarine plateaus form highstanding bathymetric highs at continent to ocean transitions. Due to their proximity to continents, they have been frequently labelled “marginal plateaus”, although this term has not been clearly defined or associated with a specific geology or geodynamic process. Until now, these elevations have been interpreted as submerged thinned continental fragments detached from continents, basaltic buildups formed by hotspots, volcanic margins or oceanic plateaus. Many of these plateaus formed at transform margins connecting oceanic basins of contrasted ages. We propose for the first time to define and review a class of marginal plateaus related to a specific tectonic setting: “Transform Marginal Plateaus” (TMPs). Based on a compilation of 20 TMPs around the world, we show that most of them have a polyphased history and have undergone at least one major volcanic phase. Our review highlights in particular a hitherto unrecognized close link between hotspots, volcanic activity and transform margins. We also propose that, due to their polyphased history, TMPs may contain several successive basins and overlooked long-lived sedimentary archives. We finally highlight that, because these TMPs were transform plate boundaries perpendicular or oblique to surrounding rifts, many of them were close to last-contact points during final continental breakup and may have formed land bridges or bathymetric highs between continents. Therefore, we discuss broader scientific issues, such as the interest of TMPs in recording and studying the onset and variations of oceanic currents or past biodiversity growth, bio-connectivity and lineage evolution.

The Demerara Plateau, off the French Guiana and Surinam margin, corresponds to a prominent sub-horizontal marginal plateau. The interpretation of a large set of seismic data acquired along the Demerara Plateau and integration with previous seismic stratigraphy and drill studies enables a better understanding of the sedimentary evolution of the outer Demerara marginal plateau. Since the end of the transform activity (late Albian), three sedimentary evolutionary stages have been identified. 1) The Pre-contourite Stage (late Albian to early Miocene) is strongly controlled by the structure of the margin that influences the distribution pattern of post-transform deposits through differential thermal subsidence between transform and divergent segments. 2) The Transitional Stage (middle Miocene to early Pliocene) is expressed as a current-controlled erosive surface resulting from the onset of bottom-current interactions due to the establishment and strengthening of the modern thermohaline circulation that is coeval with the progressive closure of the Isthmus of Panama. 3) The Contourite Stage (middle Pliocene to Holocene) is characterized by the development of a contourite depositional system that is linked to the activity of the North Atlantic Deep Water circulation and by a complex interaction between alongslope and downslope processes. Our study highlights that marginal plateaus might be particularly prone to develop contourite features since they regionally influence ocean current dynamics in guiding and accelerating bottom contour currents. Marginal plateaus should thus be considered as a key target to study bottom current dynamics and paleocirculation patterns.

International audience; Numerous recurrent seep sites were identified in the deep-water environment of the Lower Congo Basin from the analysis of an extensive dataset of satellite-based synthetic-aperture radar images. The integration of current data was used to link natural oil slicks with active seep-related seafloor features. Acoustic Doppler current profiler measurements across the water column provided an efficient means to evaluate the horizontal deflection of oil droplets rising through the water column. Eulerian propagation model based on a range of potential ascension velocities helped to approximate the path for rising oil plume through the water column using two complementary methods. The first method consisted in simulating the reversed trajectory of oil droplets between sea-surface oil slick locations observed during current measurements and seep-related seafloor features while considering a range of ascension velocities. The second method compared the spatial spreading of natural oil slicks from 21 years of satellite monitoring observations for water depths ranging from 1200 to 2700 m against the modeled deflections during the current measurement period. The mapped oil slick origins are restricted to a 2.5 km radius circle from associated seep-related seafloor features. The two methods converge towards a range of ascension velocities for oil droplets through the water column, estimated between 3 and 8 cm s-1. The low deflection values validate that the sub-vertical projection of the average surface area of oil slicks at the sea surface can be used to identify the origin of expelled hydrocarbon from the seafloor, which expresses as specific seafloor disturbances (i.e. pockmarks or mounds) known to expel fluids.

International audience; Synthetic Aperture Radar (SAR) is a spaceborne tool allowing near real-time imagery over large ground footprints of areas hundreds of kilometres wide. SAR also allows persistent observations of the process of oil discharges, benefiting from (1) day and night observations, (2) independence of cloud cover and (3) high revisiting frequencies. The interpretation of 82 SAR scenes over the Lower Congo Basin for the 1994–2012 period allowed the recognition of 1400 natural seepage slicks associated with 102 individual seep sites. We acquired an additional SAR data set with a short revisit time over a selected prolific area. The data set consisted of 22 SAR scenes acquired over a 10-day period, meaning a maximum revisiting period of 12 h between subsequent SAR acquisitions. The short-term approach shows that seepage slicks were detected with wind speeds between 1.5 and 6.5 m/s (2.91 to 12.63 knots). Both long-term and short-term monitoring evidences that seepage events are intermittent, with the occurrence rate of oil release ranging from 5 to 80%. Short-term monitoring shows that the seepage pace is independent between seep sites, suggesting contrasted controlling factors. It also shows that the residence time of seepage slicks is systematically shorter than 12 h, which is far below reported values. Understanding the seepage slick residence time is essential to provide an accurate estimation of the fluid flow and to compute the volumetric outflow. The integration of in-situ mooring points measuring the current velocity 10 m below the sea surface, in addition to local wind field and slick length, allowed us to more accurately estimate the residence time of oil slicks at the sea surface before vanishing. The results show that the sea surface residence time of oil slicks is limited to a few hours, with a median value of 3 h 15 min. These new residence time estimations allowed us to propose a quantification of the regional oil output in the Lower Congo Basin, estimated at 4380 m³/year. This area may therefore be considered as the world's third biggest oil-supplying province from natural leakages.

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 echo-sounding). 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 decollements within the Cenozoic series.

The Florence rise is located southwest of the island of Cyprus and belongs to the western part of the Cyprus arc. The Florence rise is an accretionary prism, with some amount of strike slip, where the thick layer of evaporitic Messinian decouples deformation in the post-Messinian cover from that in the pre-Messinian “basement.” The basement structural highs, whether presently active or inactive, influence the deformation and displacement patterns of the salt and its overburden. Our first experiment focused on the presence, in nature, of normal faults located above basement thrusts. Deformation of the salt layer and its overburden was influenced by several processes. On one hand, the entire model was subjected to regional shortening, and basement thrusts formed. On the other hand, the local vertical rise associated with basement thrusts created local slopes down which the salt and overburden glided gravitationally, in some places leading to the formation of normal faults in an otherwise compressional regional setting. Our second experiment was designed to model the buttressing effect of the Florence rise and the Eratosthenes Seamount on thin-skinned displacement patterns during regional gravity spreading of the Nile cone. Results indicate that in the Northeastern distal region, buttressing by these two basement highs forced the salt and its overburden to flow northeastward, as a lateral escape toward the free boundary of the region.

International audience; We present a global inventory of transform continental margins, based on both a worldwide comparison of continent-ocean boundary identifications with oceanic fracture zones traces, and a compilation of published regional studies. This inventory increases the number of identified transform margins from 29 to 78. These margins represent 16% of continental margins in cumulative length and 31% of non-convergent margins. We include morphological data, published upper crustal sections, continent to ocean transition locations and published Moho shape data in the new database. This review confirms that continent to ocean transitions are sharper at transform margins than at divergent margins. It also emphasizes the structural diversity of transform margins. Associated with one third of transform margins, we define marginal plateaus as a new type of relief that corresponds to a flat but deep surface inside the continental slope, and that may be inherited from crustal thinning prior to transform faulting. Transform margin initiation appears to be favoured along propagating oceans and within cold and thick lithospheres.

Onshore slides are driven by gravitational forces that are either related to a basal or a surface slope. Resisting these driving forces are the friction at the base of the slide, and the strength to compressional failure at the downslope edge of the slide. Two distinct processes can reduce these resisting forces and thereby promote slides. On one hand, fluid overpressure at the base of low-permeability layers decreases the effective stress, shifting the Mohr circle closer to the failure envelope. On the other hand, river incision removes the downslope buttresses. We undertook analogue experiments to investigate the combined influence of both processes on promoting landsliding. We applied air pressure at the base of horizontal or tilted models made of high and low-permeability layers to induce basal overpressure, combined with local incision similar to river incision in nature. We also tested the differences in deformation as a function of whether the incision was continuous throughout the models’ evolution. No deformation occurred in the regions not subjected to overpressure. In models subjected to continuous incision, normal faults formed first along the valley flanks, then propagated upslope retrogressively. Where incision was not continuous through time, a downslope buttress progressively formed with the sliding mass comprised of an extensional domain upslope, a long, translated but non-deformed slab at mid-slope, and a shortened domain downslope. In our models, the size of the deformed area increased with incision depth and/or increasing basal slope. These results show that river incision, combined with fluid overpressure is a potential landslide-triggering factor, as suggested by field data gathered at the Waitawhiti landslide complex, North Island, New Zealand.

In the Nile deep-sea fan, thin-skinned deformation detaching on a layer of Messinian salt has generated an upslope to downslope progression from growth faults, to polygonal minibasins bounded by salt ridges, to buckle folds. Such progression is common in salt-bearing passive margins, where gravity spreading of the salt–sediment system causes proximal thin-skinned extension on the shelf and upper slope, and distal contraction along and in front of the lower slope. In the Eastern Nile deep-sea fan, this structural progression seems to be restricted to a corridor bounded by NW–SE-trending lineaments more than 200 km in length. These are associated with salt ridges and record strike–slip movements. In the absence of a large grid of deep-penetrating seismic data accurately imaging the basement, different likely hypotheses have been advanced about the origin of this corridor: (1) it may result from possible deep-seated tectonics related to the Rift of Suez, combined with salt-related deformation or, (2) by complex interaction between the overburden's gravity spreading and pre-existing pre-Messinian paleo-topographic features, particularly the possible buttressing effect of a seamount located North of the eastern Nile deep-sea fan. In order to understand how this corridor could have been generated, we used a series of physical experiments to test the effect on three-dimensional spreading of a sediment lobe of the following parameters: (1) active, crustal, oblique extension, (2) a dormant subsalt graben, (3) a passive buttress, such as a seamount and (4) complex paleo-topographic features along the Egyptian margin affecting initial salt distribution. These experiments show that the presence of a distal buttress, combined with a complex Messinian topography best explain the complex deformational pattern observed in the eastern Nile deep-sea fan.

During the last 10 years several marine geological/geophysical surveys conducted on the Nile deep-sea fan have provided much new information about the large fluid seepage phenomenon, which characterizes this passive margin segment. Among the data, more than 60 sediment cores have been recovered in this area. Rock clasts were collected from eight of them as well as during deep-sea dives, notably in the area of Isis, Osiris and Amon mud volcanoes. In few areas, especially in the Western and the North-eastern provinces of the Nile fan, some clasts consist of crust fragments resulting from cementation of muddy sediment by precipitation of high-Mg calcite. These clasts correspond to a simple cementation of the mud by precipitation of high-Mg calcite: as a consequence no obvious textural or faunal difference with the ambient matrix mud can be observed. These clasts are made of autochthonous crusts derived from microbial anaerobic oxidation of methane and release of bicarbonate and sulphide in the surrounding pore water. These clasts also lack aragonite. In most of the other cases (Menes caldera, Isis, Osiris, and Amon mud volcanoes), allochthonous clasts characterized by low-Mg calcite cement, siderite or dolomite mixed with expelled mud outcrop on the seafloor. Nannofossils have allowed us to date some of these clasts. Most of the ages range from Pliocene to Pleistocene; one clast is Lower Cretaceous in age. The lithological facies often indicate coastal or deltaic environments. Some clasts are closely linked to an evaporite-rich environment (halite, gypsum, and analcime); they all provide useful information on the nature of buried sedimentary series covering the margin. Their presence on the seafloor is due to the expulsion of a pressurized mixture of sediment, water, and gas. This process is believed to have used pre-existing tectonic conduits.

A geophysical and geological survey (CHAMAK) has been carried out on the Makran accretionary wedge off Pakistan in order to understand the structure of the margin and the recent sedimentary processes in this self-maintaining prism disconnected from the modern Indus inputs (Qayyum et al., 1997; Gaedicke et al., 2002a; Schluter et al., 2002). Morphostructural analysis, based on the interpretation of bathymetric data and backscatter imagery, as well as a 3.5 kHz echo-character mapping, allow us to distinguish three structural domains, from north to south, where sedimentary processes differ: (1) the accretionary wedge to the north, (2) the trench and (3) the northern Murray Ridge at the seaward edge of the trench. The accretionary wedge is cut by canyons responsible for an important erosion of the prism especially in the eastern part of the wedge. Within the trench, sediments transported by the canyons generate sediment waves and are transported westward, parallel to the E–W axis of the trench. The eastern part of the abyssal plain is eroded by strong turbidity currents whereas important sediment deposition occurs in the western part of the abyssal plain, as a consequence of a decrease in the current energy. Nearly no mass transport deposits are recognized in the study area except near the ridges forming the accretionary wedge. Small-scale slope failure scars are described. The prevalence of turbiditic processes and the existence of a morphological barrier formed by the Murray Ridge allow the confinement of turbidites within the trench. Migrating sediment waves seem to be common sedimentary structures in this setting. These features might be produced by important velocity decrease of turbidity currents when reaching the trench.

article i nfo Multi-scale gravitational instabilities are widespread in the Coastal Ranges of the North Island of New Zealand. We document here a detailed analysis of the Waitawhiti landslide complex, located in the core of the Tawhero syncline, and investigate the potential landslides triggering factors in the area. Four contiguous large slides form the Waitawhiti complex. These slides involve fine-grained Miocene sandstones and massive fractured siltstones. Sliding occurs mostly along nearly horizontal strata. All slides are bounded laterally and/ or distally by deep-incised valleys. Three gas seeps evidencing thermogenic gas release have been discovered in the vicinity of the slides. We propose that river incision, continuously removing distal buttresses, is the main destabilizing factor in the area. However, additional factors, such as tectonic activity and intense rainfall, cannot be excluded. We also propose that fluid overpressure, reducing the effective shear strength at the base of low-permeability layers, may have influenced the triggering of landslides in the Waitawhiti area.

In the Nile deep-sea turbidite system (NDSTS), the province fed by the Rosetta branch of the Nile delta is characterised by the recurrent activity of gravity processes. Seven mass-transport deposits (MTDs) were recognised from the upper to the mid slope, downstream from imbricated scars (~ 30 km-long, ~ 200 m high) running along the shelf edge nearby the Rosetta canyon. Extending on surfaces between 200 and 5000 km2, with estimated volumes from 3 to 500 km3, these MTDs represent about 40% (up to 90% locally) of the total Pleistocene–Holocene sedimentary thickness. Three types of MTDs can be distinguished on the basis of their scale. Each has also a distinctive internal configuration and distribution within the Rosetta depositional setting. Age estimates of two MTDs point towards relationships between climate and submarine mass failures through sea-level changes, sediment supply, or a combination of both. Additionally, the presence of gas in the sediment and earthquake shaking may have concurred to trigger large-scale failures on the low slope angles (1°–2°) of the Rosetta area.

International audience; The Demerara plateau, located offshore French Guiana and Suriname, is part of a passive transform continental margin particularly prone to develop slope instabilities, probably in relation to the presence of a free distal border along its steep continental slope. Slope failure occurred at different periods (Cretaceous to Neogene) and shows an overall retrogressive evolution through time. Upslope these failure headscarp, an enigmatic regional Mio-Pliocene unconformity has been discovered through the interpretation of new academic and industrial datasets. The aim of this work is to describe and understand the origin of this surface. Our analysis shows that this unconformity is made of a series of valleys that cross-cut sedimentary strata. Each one of these valleys has a short lateral extent and is closed along two perpendicular directions, which suggests that it could correspond to a highly meandering system, or to some sub-circular depressions. The infill of these features is equivalent to the regional stratigraphic strata found outside the structures, but in a subdued position. This seems to imply that the structures have originated by a local loss of sediments at their base or by sliding processes. Furthermore, these depressions intersect each other through time, while migrating progressively downslope.We discuss a series of hypotheses that try to explain the onset and evolution of these depressions forming the Mio-Pliocene unconformity (Canyons? Slope failures? Contourite moats? Hydrate pockmarks?). Having established that these structures are depressions formed by collapse, and have many similarities with structures recently described in the literature as pockmarks associated with gas hydrate dissolution, we favor this hypothesis.We propose that these hydrate pockmarks form with a mass failure that was triggered by fluid-overpressure development at the base of the hydrate stability zone.

Structural analysis of the Nile deep-sea fan (NDSF) indicates that post-Miocene deformation is largely controlled by down-slope movement above a Messinian salt layer. In contrast to other similar systems, the NDSF shows significant lateral asymmetry, and three distinct provinces can be recognized. Regional variations in salt thickness and base-salt geometry within the system have led to the presence of structures typical of both gravity gliding and gravity spreading. Trends of structures within the western and central parts of the NDSF are consistent with forming due to gravity gliding processes. This pattern contrasts with that in the eastern NDSF that seem to be much more consistent with deformation due to gravity spreading. Observed differences in structural style across the NDSF can be related directly to the paleomorphological evolution of the Nile cone during Messinian time, which is perhaps partly related to features that have been inherited from early Mesozoic evolution of the North Egypt passive margin. In the east part of the NDSF, the structural evolution has been significantly influenced by the proximity of Eratosthenes Seamount. This large topographic buttress has served to both limit and deflect northeastward allochthonous advance of the Messinian evaporites and has thus severely complicated deformation of the overlying Plio-Quaternary sedimentary cover within the broad corridor that runs NNW–SSE and cuts obliquely across the modern bathymetry of the Nile cone.

The Nile deep turbiditic system displays many fluid-releasing structures on the seabed: mud volcanoes reassembling small cones (100–900 m in diameter), mud pies (5 km in diameter), and pockmarks. The cones are restricted to the western province. Mud ‘pies’ delineate a belt of apparently very active gas chimneys along the upper continental slope. Pockmarks are associated either with strongly destabilized sedimentary masses or with gas chimneys. We distinguish five main controlling parameters interacting in fluid release locations: (1) the presence, at depth, of potential source rocks and reservoirs; (2) the distribution of Messinian evaporites preventing upward fluid migration; (3) the distribution of sedimentary overloading inducing localized overpressures on under-compacted and fluid-rich sediments; (4) the presence of syn-sedimentary faults acting as potential conduits for fluid migration; and finally (5) chiefly for pockmarks and mounds, the occurrence of large-scale sedimentary instabilities.

The precise nature of the transition between the present-day compressional tectonics in central Mongolia and extensional deformation in the central Baikal rift has still to be determined. For that purpose we have built a comprehensive earthquake focal mechanism data base for the Mongolia – southern Siberia area, from which we map the variations of the stress field. We focus our detailed investigations on the largest seismic event in the transition zone, the 1950 (Mw 6.9) Mondy earthquake, for which several discordant focal mechanisms have been proposed. Using a new approach in source inversion, we resolve the focal mechanism (left-lateral strike slip type on a steep south-dipping fault) and depth (14 ± 3 km) of the Mondy earthquake with a satisfactory accuracy. This seismological information, combined with the geological observations, allows us to decipher the connections between the 1950 mainshock, the local stress tensor and the active faults, which strongly suggest a partitioning of the deformation between two faults, namely the Mondy and Ikhe–Ukghun faults.

Very high-resolution reflection seismic investigations off the Belgian coast have revealed an extensive area marked by poor seismic penetration that is probably caused by the presence of shallow gas. The gas is believed to be of biogenic origin, and its geographical distribution is bound to a wide band oriented more or less parallel to the coast. The main origin of the gas could to some extent be linked to the presence of a shallow, thin peat-rich layer of Late Pleistocene/Early Holocene age. Local high sedimentation rates furthermore favoured gas formation in the shallow fine-grained Holocene sediments. The gas-related features observed on the seismic profiles include acoustic turbidity and blanking, strong multiple reflections, and to a lesser extent bright spots and phase reversal. The sea-floor morphology does not reveal any clear gas escape from the sea bed, although there are some indications of local seepage of small bubbles or dissolved gas into the water column. The top of the acoustically turbid layer is located between 0 and 7 m below the sea-bed surface. It generally forms a sharp boundary, often marked by a varying offset probably due to different levels of gas penetration which could be related to the lithology of the overlying sediments. Seismic characteristics and velocity data seem to suggest a low concentration of gas, most likely less than 1%.

The Tyrrhenian Sea is usually interpreted to be a Neogene back-arc basin that opened by continental rifting and oceanic spreading caused by the eastward migration of the Apennine subduction system from Tortonian to Pliocene times. Rifting of the Tyrrhenian Sea started first along the Eastern Sardinian margin during the Tortonian-Messinian times. The " METYSS " project aims at better constraining the relations between crustal tectonics, salt tectonics and sedimentation from Messinian times to present-day. The " METYSS 1 " cruise (2009, R/V "Téthys II"-INSU) allowed us to obtain about 1200 km of high-resolution seismic reflection profiles along the SE Corsican margin, and along the upper and middle parts of the Eastern Sardinian margin, Western Tyrrhenian. This new dataset clearly illustrates that this area has been highly dissected during the rifting stage by N-S trending normal faults delineating ridges and basins, as previously described. The Messinian seismic markers (UU and MU) locally display fan-shaped stratal geometries, but the mechanism responsible for such geometries, i.e., salt tectonics or rifting, has yet to be carefully deciphered. We also mapped the spatial distribution of the mobile salt (MU). The highly variable thickness of the confined salt basins may be related to the initial pre-Messinian basin geometry or to the fact that sediment deposition was syn-rift. Southeastward, salt tectonics appears highly vigorous. More surprisingly, our data demonstrate that some of the major faults have been significantly reactivated during the post-rift period, up to late Quaternary time. Such recent fault activity has been recorded by bathymetric scarps and associated footwall debris flows. Moreover, some of this post-rift deformation can be evidenced within the Plio-Quaternary sequence by a regional unconformity. The post-Miocene deformation style varies greatly between different areas of the margin, and can also be strongly influenced by the presence or absence of mobile Messinian evaporites (MU).

The meandrous leveed channels of the Nile Cone show clear evidence of avulsions. Their sedimentary architecture is founded on numerous stacked lens-shaped acoustic units. In the areas of the distal fan, lobe deposits are apparent from multichannel imagery. Huge debris flow deposits, sometimes associated with pockmarks, are recognized. Mud volcanoes and gas seeping are closely associated with faulting. In the East, a very long north-trending channel, originating from the Egyptian coast, merges with a network of channels, very probably originating from the Levantine coasts. Both networks outlet in the sedimentary basin located south of Cyprus.

The Tunka basin was initiated during Oligocene, under transtensional regime (normal-sinistral) as shown by large-scale structures and geomorphology. Nevertheless, a preliminary analysis of the most recent deformations allows us to evidence transpression on several sites within the basin. These tectonic features together with focal mechanisms and preliminary GPS data, suggest that the kinematics of the Tunka basin has undergone a very recent change, which could be due to the northward propagation of the India–Eurasia collisional strain field.

We present and discuss a set of data, mainly swath bathymetry, backscatter images and a few seismic data, that have been recently recorded over large areas of the Nile deep-sea fan between water depths of 1200 and 3000 m. These data demonstrate the presence of at least three distinct morphostructural provinces where interacting sedimentary, tectonic, and salt tectonic processes control the present day morphology and the recent evolution of the major Mediterranean deep-sea fan. The role of the ongoing collision presently occurring between the continental Eratosthenes Seamount and the Cyprus Arc may have been, and still is, a deterinining parameter for the development of the Nile deep-sea fan. © 2001 Published by Elsevier Science Ltd.

International audience; We present the structure and evolution of the eastern part of the Demerara plateau, offshore French Guiana, from the analysis of geophysical data collected during GUYAPLAC cruise. This area is located at the intersection of a transform segment and a divergent segment of a continental margin related to the Early Cretaceous opening of the Equatorial Atlantic. The main structures are NNE-SSW to NNW-SSE trending normal faults on the eastern edge of the plateau, and WNW-ESE to NW-SE trending acoustic basement ridges on its northern edge. When replaced in their Albian position, these structures appear to be parallel to the coeval oceanic accretion axis and transform faults, respectively. The most striking structures are related to a post-rift but syn-transform tectonic inversion, producing E-W to WNW-ESE trending folds, sealed by a regional unconformity. This shortening can not be related to ridge push, but is probably related to a plate kinematic change 105 My ago, that modified the deformation in the vicinity of the transform fault. Late post-rift evolution also includes a significant Tertiary oceanward tilt of the edge of the Demerara plateau. The driving mechanism of this late tilt is unclear, but may be related to a lithospheric flexure resulting from the loading of the abyssal plain by the Orinoco and Amazon deep-sea fans.

International audience; The Florence Ridge, part of the western Cyprus arc, is a compressional relief that was eroded during the Messinian salinity crisis while deposition of salt occurred North (Antalya basin) and South (Herodotus abyssal plain). In order to better assess the impact of salt-tectonics in the Florence Ridge region deformations, we conducted a morpho-structural analysis of available multibeam and seismic data (Simed and Prismed II campaigns). It is indeed a crucial issue to distinguish crustal and gravity driven structures in the compressional to strike-slip belts of the eastern Mediterranean.Along the Antalya basin, we mainly observed multi-directional tectonic rafts typical of gravity gliding above salt. On the Florence Ridge itself, the base of salt evolves laterally to a Messinian erosional surface that erodes a series of stacked nappes. This surface is involved in recent faulting. South of the Florence Ridge, a nearly 100 km wide fold belt characterizes the Herodotus abyssal plain. Three different zones parallel to the Florence Ridge appear within this fold belt. Those are respectively from North to South zones A, B and C. Zone A is characterized mainly by small-wavelength folding and faulting. Approaching Zone B, a long extensional graben deforms the seafloor. Zone B stands ~ 100 to 200 m higher than zones A and C. There, salt welding seems common. In the easternmost zone B deep sub-circular bathymetric depressions are associated with extremely thick and fan-shaped depocenters probably emplaced in relation with active sub-salt thrusts. Many evidences suggest post-Messinian uplift in this zone. Zone C shows medium to high wavelength salt-cored folds. Wavelength of those folds increase approaching the distal Nile deep-sea fan. Within zone C, a nearly undeformed domain exists approaching the Eratosthenes seamount. ‘En echelon’ folds bound this flat domain suggesting lateral salt extrusion at the junction between zone C and the distal Nile deep-sea fan.To conclude, numerous evidences argue for a post-Messinian reactivation of the pre-Messinian Florence Ridge accretionary wedge (numerous folds, uplifted zones). The salt, as on the nearby Mediterranean Ridge, decouples deep and surface deformations and even allow the development of isolated grabens and associated salt ridges in the heart of the Florence compressional zone. We propose in this case that local basal slope of salt increase related to basement fault activity triggered gravity gliding and thus extensional deformation despite the regional compressional regime. Finally, this study shows that the eastern Nile deep-sea fan, submitted to extremely vigorous salt tectonics seems to have collided with the Florence Ridge fold belt and to have generated a “salt extrusion” zone.

International audience; Fluid overpressure at the base of low-permeability strata reduces effective stress, allowing for gravitational sliding of the overlying cover. The force driving sliding is the slope-parallel component of the weight of the cover, whereas the resisting forces are the friction at the base of the cover and the buttressing resistance to shortening, which can be critically reduced by incision at the base of the slope. We developed an analytical model and undertook a series of analogue experiments to better understand the evolution of a sedimentary cover sliding above a low-permeability layer subjected to fluid overpressure. Where a downslope buttress was present, the sliding sheet length decreased with increasing pore-fluid pressure. In the absence of such buttress, the slide's length increased exponentially with increasing pore-fluid pressure. Another important difference dealt with geometry and kinematics. Buttressed slides consisted of one large slope-parallel mass rigidly translated and bounded by downslope thrusts and upslope normal faults. With increasing pore-fluid pressure, the contractional structures propagated upslope. By contrast, non-buttressed slides showed intense strain: deformation started with normal faults forming near the incision, then propagating upslope throughout the slide's evolution.

5th International Symposium; International audience; The eastern Demerara Rise located offshore French Guiana was surveyed in 2003 (GUYAPLAC cruise, part of the French EXTRAPLAC program) using multibeam bathymetry and imagery, 6-channel seismic data and 3-5 kHz echosounding. Analysis of seismic data shows that the flank of the Demerara Rise endures repetitive sliding of its Paleogene to Neogene sedimentary cover towards the ocean. Fluid escapes seem to be closely associated with the activity of those slides and deep faults seems to impact the location of the main headscarp. We suspect fluid overpressures and the specific architecture of transform boundaries ("free border") to be key parameters in the development of wide MTD's retrogressively eroding the eastern Demerara Rise.

Pages 72, 1 CD; International audience