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The Amazon deep sea fan is a dynamic setting in which widespread seafloor fluid vents record degassing of a gas hydrate system hosted within an upper slope thrust-fold belt linked to gravitational collapse of the depocentre. This system is to be investigated during the AMARYLLIS-AMAGAS campaign of the R/V Marion Dufresne, to take place in May-June 2023 as a collaboration between research groups in Europe and Brazil. One aim of the campaign is to study the spatial distribution and stability conditions of the shallow gas hydrate reservoir in relation to temperature changes in the past, fluid migration and venting, and to the triggering of the giant slope failures that have recurrently extended across the fan. Three heat flow transects within a 200 km2 area of the upper fan will target seismically-observed BSR (bottom simulating reflection) patches associated with fluid venting structures at the crests of thrust-fold anticlines. Temperature gradients will be measured using autonomous high-precision temperature probes attached to core barrels, while thermal conductivity will be measured onboard using a needle probe instrument on recovered sediment cores. Together with hydroacoustic and geochemical datasets to be acquired during the campaign, the results are expected to yield insights into the dynamics of the gas hydrate system in relation to subsurface fluid flow and changes in the heat flow in sediments, as well as new information on the poorly-constrained background regional heat flow regime of the Amazon deep-sea fan., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

International audience; The Nile deep-sea fan is the largest Plio-Quaternary depocentre in the Mediterranean Sea, extending over an area of >150,000 km2 within which rapid deposition drives syn-sedimentary collapse tectonics, as well as widespread seafloor venting of mixed thermogenic and biogenic gases. It is thus an ideal setting for the near-seabed accumulation of gas hydrates, which are stable below water depths of about 1100 m in the warm waters of the eastern Mediterranean. Gas hydrates have yet to be sampled on the Nile fan, although their presence has been suggested from semi-published industry seismic and well log data. Here we integrate available industry data with a regional grid of academic geophysical data (seismic and multibeam) acquired by Géoazur, in order to identify a BSR on the central Nile fan, invert it to geothermal gradients, and examine its relationship to fluid venting. The BSR is observed on several intersecting seismic profiles of varying frequency content and offset, as a discontinuous reflection of negative polarity that lies 220-330 ms below seafloor, deepening downslope in water depths of 2000-2500 m. The BSR occurs within a relatively small area (2500 km2), among a larger system of slope-parallel extensional faults. Inversion of BSR depth to temperature gradients was performed using a velocity-depth function for deep-sea sediments, a phase boundary for methane hydrate in equilibrium with seawater of 3.86% salinity (Mediterranean average), and water temperatures from MEDATLAS. The results indicate spatially varying gradients of 27-42˚C/km, up to twice background values reported in the geothermally cool eastern Mediterranean offshore. ‘Shallow’ BSRs yielding elevated thermal gradients within the Nile fan can be explained in terms of advective heat transfer by upward fluid flux. Upward fluid migration is supported by evidence of widespread seafloor fluid venting, but seafloor data indicate few or no vents in the area of the BSR. In contrast, in an area with many gas vents over 50 km from the BSR, subsurface gas hydrate accumulations are indicated by resistivity log data at two well-sites. If gas and gas hydrates are present across wider areas, why is the BSR so restricted? We hypothesise that these observations can be explained in terms of spatially varying fluid flux, such that away from the BSR a greater flux of fluids rich in dissolved gas leads to rapid gas hydrate formation and the growth of gas chimneys. We intend to test this hypothesis during a forthcoming Franco-Brazilian campaign, focused on understanding the dynamics of the fluid vents. Acknowledgement : this work has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 656821 (SEAGAS project).

International audience; Recent investigations have shown that the continental shelf west of Ireland contains sedimentary landforms recording occupation by grounded, lobate ice sheet margins that extended from Ireland during at least the last glacial cycle. This paper reviews some of the offshore evidence of past glacial events available from high-resolution bathymetry, 2D/3D seismic datasets (Fig. 1), and shallow sediment cores providing information on the sedimentology, rheology and age of glacigenic stratigraphic units. The available data suggest that the continental shelf has been repeatedly occupied by tidewater ice margins characterised by ice streaming, possibly since the mid-Pleistocene transition. The offshore record provides as yet incomplete information on the extent and timing of glaciation from multiple centres of dispersal in Ireland. The dynamics of former ice sheets in Ireland, downwind of the climatically important central North Atlantic region, makes them of wider interest in the study of partially marine based ice sheet-ocean interactions in rapidly changing environments.

International audience; The reconstruction of the largest ice stream to drain the British-Irish Ice Sheet at the Last Glacial Maximum (LGM) can provide essential palaeoglacial observations required for constraining numerical ice sheet models. The Irish Sea Ice Stream (ISIS) was long considered to have terminated on the mid-shelf of the Celtic Sea, based on sediment cores and seismic data collected in the 1970s. Here we summarise findings from sediment cores and geophysical data acquired since 2009, and multi-beam bathymetric data acquired since 2001, which permit an updated evolution and palaeoglacial reconstruction of the Irish and UK sectors of the Celtic Sea shelf. In near-shore areas, multi-beam data reveal over 2000 glacial features, including moraine ridges, streamlined bedrock and meltwater channels, recording the southwest advance of the ISIS towards the shelf-edge and its subsequent retreat. The mid- to outer-shelf is characterised by the largest known linear shelf sediment ridges. These vary from long and linear features, the megaridges, in the northwest to sinuous and shorter ridges in the southeast. This ridge field was initially interpreted as tidal in origin, but glacigenic sediments have been recovered from the flanks of the megaridges. Correlating decimetric-resolution geophysical data to sediment cores, the megaridges comprise three main units. 1) A superficial fining-upward drape above an unconformity, inferred to record decreasing ocean energy during marine transgression. Underlying this drape is 2), the Melville Formation (MFm), which comprises the upper bulk of the megaridges, displaying dipping internal acoustic reflections and consisting of medium to coarse sand and gravel, characteristics that could be consistent with either a tidal or glacifluvial origin. The MFm unconformably overlies 3), the Upper Little Sole Formation (ULSFm), previously proposed to be of late Pliocene to early Pleistocene age, but is here shown to contain glacigenic sediments dated to the LGM. This stratigraphy constrains the age of the MFm to between 24-14 ka BP, coeval with deglaciation and a modelled period of megatidal conditions during transgression. Stratigraphically and sedimentologically these megaridges could represent glacifluvial features eroded during the post-glacial marine transgression. However, it is argued that they comprise a partially-eroded glacial topography (ULSFm) mantled by post-glacial tidal deposits (MFm), both subsequently eroded by a proposed mechanism of enhanced wave energy during decreasing tidal energy in the later stages of transgression. Regardless of the origin of the ridges, the evidence shows that the ISIS extended to the shelf-edge of the Irish and UK sectors during the LGM.

International audience; Ice sheets have occupied the Atlantic continental margin of Europe as far south as the Celtic Sea, where the maximum extent of glaciation remains in question. The Celtic Sea contains no obvious glacial landforms, but is dominated by a system of shelf-crossing seafloor megaridges, up to 60 m high and 10 km wide, that extend seaward up to 300 km in water depths of 100-200 m. The last British-Irish Ice Sheet (BIIS) was thought to have reached a limit on the mid-shelf, based on glacigenic sediments sampled on and between megaridges in the Irish-UK sectors, but recent work in this area has shown that the BIIS extended at least 150 km farther, at minimum to the UK shelf edge at 48˚20'N. Stratigraphic analysis of newly acquired core and seismic data has indicated the megaridges to be eroded sand bodies, which can be interpreted as a) glaciofluvial ridges modified by post-glacial megatides, or b) tidal banks truncated by wave energy. Here we examine megaridge morphology using a regional bathymetric grid and multibeam data across a 25x100 km area of the mid-shelf. At regional scale, the megaridges fan seaward to meet the shelf edge near-transversely along 600 km of its length, their axes rotating by 80˚ from W-E (Irish shelf) to almost N-S (French shelf). The fan-shaped network points to an apex in the north Celtic Sea, yet axial convergences are more common to seaward than landward. Individual megaridges consist of segments tens of kms long, of differing orientation, that form bathymetric highs. On the Irish-UK mid-shelf, multibeam imagery show that en echelon megaridge segments (up to 40 km long, 7 km wide, 55 m high) give way both axially and laterally to transverse 'ribs' (up to 10 km long, 10 m high). The ribs vary in form and spacing, distinct from asymmetric regularly-spaced sand waves within the multibeam data. Glacigenic sediments have been reported near seabed on and between both the megaridges and ribs, but subglacial lineations are not observed at seafloor. We hypothesise that the rectilinear network of megaridges and ribs are the eroded remnants of giant eskerine ridges flanked by glaciofluvial De Geer moraines, formed time-transgressively along an ice sheet margin during its retreat from the shelf edge. If correct, this interpretation would have broad implications for the dynamics of the last BIIS, and for the supply of meltwater and sediment across the shelf and slope to the North Atlantic at the LGM. Glaciofluvial vs tidal models for the formation of Celtic Sea bedforms invite testing through the targeted acquisition of core data, notably from areas of existing multibeam data in both the Irish-UK and French sectors.

International audience; The Irish Sea Ice Stream (ISIS) has long had one of the best documented retreat histories of the British-Irish Ice Sheet (BIIS) and was the first ice stream to be constrained by Bayesian analysis of geochronological data. These attributes made it a model system for the BRITICE-CHRONO research project, which aims to produce the best constrained retreat record of any palaeo-ice sheet contributing key observational constraints for ice sheet modelling. The project has generated a suite of new radiocarbon ages from deglacial sequences offshore in the Celtic and Irish seas and terrestrial cosmogenic nuclide and optically-stimulated luminescence ages from ice-marginal sites in the Isles of Scilly, Ireland, Wales and NW England. The ISIS was unusual within the former BIIS, in that it was a compound ice stream with two outlets, one marine terminating that flowed through the Irish Sea Basin into the Celtic Sea, and a terrestrial terminus that flowed southwards through Cheshire-Shropshire lowlands into the English Midlands around 25.5 ka. Here we assess the retreat dynamics across the entirety of the ISIS, integrating the new chronology in a revised Bayesian analysis that constrains the pattern and timing ice marginal fluctuations. The retreat chronology in the Irish Sea is better constrained than in the Celtic Sea, where the ISIS is now recognised to have extended as far as the continental shelf break to the SW of Britain and Ireland between 24 and 27 ka; this advance was synchronous with independently-dated ice-rafted detritus from ISIS in adjacent deep-sea cores. The ISIS then retreated rapidly northwards through the Celtic Sea, with evidence for readvance phases, deglaciating the Isles of Scilly at 25.5 ka, reaching St Georges Channel by 24.3 ka and the Llŷn Peninsula by 23.9 ka. The initiation of retreat from both the eastern (terrestrial) and western (marine) components of ISIS was synchronous. The eastern terrestrial lobe had vacated the Cheshire-Shropshire lowlands by 22-21 ka. The complex readvance sequences identified on the Llŷn (24-20ka) and in eastern Ireland have now been tightly constrained to register centennial-scale oscillations of the ice front driven by internal ice dynamics over topographic pinning points and constrictions of the ice-stream. Retreat northwards into the northern Irish Sea then accelerated, first evacuating the deeper water of the western Irish Sea, and developing pronounced ice margins across the northern Isle of Man by 19.1 ka. The final retreat phase, with ice margins pulling back onto terrestrial settings in the English Lake District, the north of Ireland and SW Scotland around 17 ka, was a deglaciation accomplished in a fully marine context evidenced by the preservation on the seabed of subglacial landforms and by increasing influence of local ice sources with flow realignment during draw-down and ice margin retreat.

International audience; Gas hydrates are ice-like compounds of water and volatiles (mainly methane) that are stable in deep-sea sediments due to high pressures and low temperatures. Changes in oceanographic conditions that reduce their stability field (e.g. sea level lowering, bottom water warming) have been suggested to trigger continental slope failures. The Amazon deep-sea fan is a major Plio-Quaternary depocentre associated with large-scale slope instabilities, in which the presence of gas hydrates has been reported from a discontinuous bottom simulating reflection (BSR) on the upper slope. Reductions in gas hydrate stability during lowered sea levels have been argued to trigger megaslides from the upper fan; megaslides have also been linked to tectonism within an extension- compression system on the upper fan recording its collapse above deep detachments. Here we present the first systematic mapping of the Amazon fan BSR using a regional grid of 2D/3D seismic reflection data, and argue the results to provide evidence for stability zone changes driven from below by fluid upwelling. The BSR is seen to extend over an area of at least 6800 km2 as elongate patches up to 10s km wide and >100 km long that coincide with the crests of thrust-fold anticlines. The BSR patches lie within 300 m of seafloor, in places rising beneath seafloor features that 3D seismic imagery show to be pockmarks and mud volcanoes. The BSRs are shallower than the methane hydrate stability zone calculated using regional datasets, and inversion of depths to temperatures yields spatially variable gradients up to 10 times background values in well data from the fan. We interpret the elevated BSR patches to record the upwelling of warm, gas-rich fluids via the thrust-folds. We propose that changes in heat flux due to episodic fluid flow, notably during fault movements, will result in gas hydrate dissociation to reduce pore pressures at the base of the stability zone. This mechanism could account for recurrent large-scale failures from the upper Amazon fan during its Plio-Quaternary collapse. Our model of 'bottom-up' gas hydrate dynamics driven by fluid migration is applicable to collapsing passive margin depocentres worldwide, and is independent of 'top-down' changes in gas hydrate stability due to climate-driven changes in ocean conditions.

The Celtic Sea contains the world's largest continental shelf sediment ridges. These megaridges were initially interpreted as tidal features formed during post-glacial marine transgression, but glacigenic sediments have been recovered from their flanks. We examine the stratigraphy of the megaridges using new decimetric-resolution geophysical data correlated to sediment cores to test hypothetical tidal vs glacial modes of formation. The megaridges comprise three main units, 1) a superficial fining-upward drape that extends across the shelf above an unconformity. Underlying this drape is 2), the Melville Formation (MFm) which comprises the upper bulk of the megaridges, sometimes displaying dipping internal acoustic reflections and consisting of medium to coarse sand and shell fragments; characteristics consistent with either a tidal or glacifluvial origin. The MFm unconformably overlies 3), the Upper Little Sole Formation (ULSFm), previously interpreted to be of late Pliocene to early Pleistocene age, but here shown to correlate to Late Pleistocene glacigenic sediments forming a precursor topography. The superficial drape is interpreted as a product of prolonged wave energy as tidal currents diminished during the final stages of post-glacial marine transgression. We argue that the stratigraphy constrains the age of the MFm to between 24.3 and 14 ka BP, based on published dates, coeval with deglaciation and a modelled period of megatidal conditions during post-glacial marine transgression. Stratigraphically and sedimentologically, the megaridges could represent preserved glacifluvial features, but we suggest that they comprise post-glacial tidal deposits (MFm) mantling a partially-eroded glacial topography (ULSFm). The observed stratigraphy suggests that ice extended to the continental shelf-edge.

International audience; Submarine mud volcanoes release sediments and gas-rich fluids at the seafloor via deeply-rooted plumbing systems that remain poorly understood. Here the functioning of Venere mud volcano, on the Calabrian accretionary prism in ~1,600 m water depth is investigated, based on multi-parameter hydroacoustic and visual seafloor data obtained using ship-borne methods, ROVs, and AUVs. Two seepage domains are recognized: mud breccia extrusion from a summit, and hydrocarbon venting from peripheral sites, hosting chemosynthetic ecosystems and authigenic carbonates indicative of long-term seepage. Pore fluids in freshly extruded mud breccia (up to 13 °C warmer than background sediments) contained methane concentrations exceeding saturation by 2.7 times and chloride concentrations up to five times lower than ambient seawater. Gas analyses indicate an underlying thermogenic hydrocarbon source with potential admixture of microbial methane during migration along ring faults to the peripheral sites. The gas and pore water analyses point to fluids sourced deep (>3 km) below Venere mud volcano. An upward-branching plumbing system is proposed to account for co-existing mud breccia extrusion and gas seepage via multiple surface vents that influence the distribution of seafloor ecosystems. This model of mud volcanism implies that methane-rich fluids may be released during prolonged phases of moderate activity.

International audience; Marine sediments store large quantities of organic carbon derived both from the drainage basins of rivers on land and from life in the oceans. The Amazon is the world's largest river, from which sediment supply to the Atlantic Ocean has resulted over the last millions of years in the build-up of a vast deep-sea fan up to 10 km thick. Since the last glacial maximum (ca. 24,000 years before present), the fan has accumulated a mass of organic carbon comparable to that contained in the presentday Amazon Forest. Over geological timescales, the process of sedimentary burial in the Amazon and other marine depocentres sequesters organic carbon, but also allows microbial methanogenesis to convert some of it to mobile methane. Methane moving through sediments may be oxidized and converted into CO2, trapped as dissolved or free gas or, where stability conditions permit (in water depths greater than 500 m), as gas hydrates. Reservoirs of gas hydrates are favoured by high sedimentation rates, and it has been suggested that the Amazon and other rapidly deposited deep-sea fans are net carbon sinks. Here we present evidence that carbon stored in and beneath gas hydrates is escaping to the ocean through seafloor venting features. The distribution of these features suggests them to form along faults created by the gravitational collapse of the fan, and along the upper limit of the gas hydrate stability zone in response to changes in pressure (depth) and/or temperatures influenced by climate. Our findings support recent studies suggesting that seafloor gas venting in deep-sea settings where gas hydrates are stable is a more widespread phenomenon than previously recognised, suggesting estimates of global methane emissions to be conservative. A more accurate knowledge of seafloor methane emissions is thus of paramount importance for our understanding of the effects of ongoing climate-driven changes on ocean processes, as well as for the effective management of marine resources.

International audience; The Amazon River supplies one of the world's largest deep sea fans, a sedimentary depocentre over 10 km thick that occupies an area of nearly 330,000 km 2 from the shelf-break to water depths greater than 4000 m. Gas hydrates have been inferred to occur within the Amazon fan, based on seismic profiles showing discontinuous bottom simulating reflectors (BSRs) on the upper fan, and the presence of gassy sediments and pore water freshening in sediment cores of ODP Leg 155 on the lower fan. Gas hydrate dissociation during glacial-interglacial changes in sea level has been hypothesized to act as a trigger for slope instabilities within the fan. The Amazon fan is also characterized by remarkably high rates of sedimentation, especially during the late Quaternary when average sedimentation rates reached 50 mm/year during glacial stages. Rapid deposition favours overpressure build-up, which can drive deformation at different scales. On the Amazon fan this includes gravitational collapse above multiple deep detachment surfaces, expressed near the seafloor in a large-scale extensional-and-compressional system characterized by normal faults on the shelf and upper slope (above ~500 mbsl) and thrust-folds at greater depths (from ~1000 mbsl to 2000 mbsl). Some of these structures can reach the seafloor generating scarps up to 500 m in relief attesting to ongoing deformation. Our interpretation of a regional grid of 2D and 3D multichannel seismic reflection data, combined with modeling of the gas hydrate stability zone, provides new information on the distribution and character of discontinuous BSRs on the Amazon fan and their relation to fluid flow through structures within the compressive domain. BSRs are only observed in water depths of 1200-2000 m, mainly on the northwest part of the main depocentre, forming a series of elongate BSR 'patches' up to 140 km long and 10-50 km wide that coincide with the crests of thrust-folds. The BSRs mainly lie at 200-300 mbsf, as strong reflections of negative polarity that cut across deformed strata within the thrust-folds and fade away within intervening basins. In places the BSRs rise to shallower depths, within 150 m of the seafloor, in some cases beneath fluid escape features observed on 3D seismic, including pock-marks and probable mud volcanoes linked to migration pathways within the thrust-folds. The regional methane hydrate stability zone (RMHSZ) was modeled using the phase boundary for pure methane in equilibrium with water of 3.5% salinity and inputs for bathymetry (Gebco08), bottom-water temperatures (World Ocean Database) and geothermal gradients (International Heat Flow Commission, ODP and published sources). The results show the upper limit of the RMHSZ to lie between 500-600 mbsl, its exact position being controlled by bottom-water temperatures that vary over seasonal and longer timescales. The RMHSZ thickens rapidly downslope in response to low geothermal gradients within the upper-central fan (17-20˚C/km), to maximum thicknesses of over 1 km (in 2500-3000 m water depth) but decreases to 305 m on the lower fan (> 4,000 m water depth). On the upper fan, the modeled depth of the RMHSZ (700-900 mbsf) is several times greater than the maximum depths of the BSR observed on seismic sections. Anomalously shallow BSRs could be explained either by highly saline pore fluids, or by higher sub-seafloor temperatures. Modeling of the RMHSZ using a range of constant geothermal gradients shows the BSR patches to correspond to values that vary across their widths (of 10 km or more) from 40˚C/km at their edges to as high as 90˚C/km where shallowest. This variation in gradients corresponds to vertical variations in temperature of up to 7.5˚C across the observed sub-seafloor depth range of the BSRs (150 m). The coincidence of BSRs with thrust-folds versus their absence elsewhere (over water depths of 500-4500 m), suggests that compressive structures are key elements for the migration of fluids from depth. The fact that BSRs are only observed above thrust-folds could be explained in terms of higher fluid flux, in particular by a supply of free

International audience; Numerous geological-geophysical studies carried out across the Amazon offshore basin (Foz do Amazonas marginal basin), Brazilian Equatorial Atlantic, identified the occurrence of large-scale mass-transport deposits (MTDs). These studies, based mainly on analyses of acoustic imagery, include those of the Brazilian Continental Shelf Survey Programme (LEPLAC) and the more recent CAPES-IODP project "Stratigraphic and structural Cenozoic evolution of the Foz do Amazonas basin: a coupling system between depositional, gravitational and fluid migration processes"-a collaboration project between UFF, UERJ, PUC-RS and foreign universities. Such studies evidence that the occurrence of large-scale slope instabilities has been pervasive across the entire margin spanning the late Miocene to Modern. MTDs all together attain a total area of nearly 315,000 km² and involve a total volume of allochthonous masses of ~128,000 km³, constituting thus essential architectural elements for the margin sedimentary construction. MTDs can be grouped into three main gigantic regional megaslide complexes (MTCs) spreading downslope off the NW, the Central and the SE slope settings of the basin, respectively: the northwestern Amapá Complex, the Central Amazon Fan Complex and the southeastern Pará-Maranhão Complex. In each MTC, individual MTDs can mobilize up to kilometre-thick sedimentary series as allochthonous masses with distinct flow directions, degrees of sediment disruption and internal coherence, attaining dimensions comparable to the world's largest megaslides. Allochthonous masses can spread either as frontly-confined or as unconfined sediment slides, depending on the region or the stratigraphic interval considered. Unconfined MTDs can present internal seismic facies indicative of large downslope modification of their original stratification, varying from upslope slide and/or slumped blocks to dominant downslope debris flows, suggesting that these MTDs may have been generated by sudden catastrophic events. On the other hand, kilometre-thick frontly-confined masses, reflected by frontal imbricate thrusts as wide as 50 km, normally involve higher remobilized sediment volumes, probably as a result of multiphase slope failures which may have encompassed a large time span and progressively incremental volumes of eroded sediments. Active gravity tectonics is assumed to be one of the major preconditioning factors inducing slope failures across the offshore Amazon basin, since the main upslope slide scars seem likely to have been initiated by movements of gravity-related structures. However, a variety of other preconditioning causes for submarine mass-wasting process are also present across this margin, such as rapid sediment accumulation in the head area of the slide complexes, together with destabilization of gas hydrates. In this realm, the Amazon offshore basin stands thus as an excellent site for the investigation of preconditioning factors that generate an excess of pore pressure conditions able to trigger the initiation of submarine gravity-driven depositional processes in passive margin settings.

The Ionian slopes of the Calabrian continental margin have been affected by mass movements of varying style, scale and age. Here we present examples of seabed and subsurface features identified from recently acquired multibeam morpho-bathymetric data and sub-bottom profiles, being interpreted in the framework of the MAGIC project. Four different types of mass wasting phenomena are recognized: mass transport complexes (MTCs) within intra-slope basins, isolated slide scars on slopes, possible features of gravity sliding above Messinian salt and headwall scarps in canyons. An improved understanding of past and present processes of mass failure on the Calabrian margin is strategic for assessments of the geohazards they may represent.

International audience; Voluminous mass-transport deposits (MTD) have been identified on seismic profiles across the central Nile Deep-Sea Fan (NDSF). The youngest MTDs are buried under 30-100 m of well-stratified slope deposits that, in water depths of 1,800-2,600 m, are characterized by undulating reflectors correlated with slope-parallel seabed ridges and troughs. Seabed imagery shows that, in the western part of the central NDSF, short, arcuate undulations are associated with fluid venting (carbonate pavements, gas flares), while to the east, long, linear undulations have erosional furrows on their downslope flanks and fluid seeps are less common. Sub-bottom profiles suggest that the western undulations correspond to rotated fault-blocks above the buried MTDs, while those in the east are sediment waves generated by gravity flows. We suggest that fluids coming from dewatering of MTDs and/or from deeper layers generate overpressures along the boundary between MTDs and overlying fine-grained sediment, resulting in a slow downslope movement of the sediment cover and formation of tilted blocks separated by faults. Fluids can migrate to the seafloor, leading to the construction of carbonate pavements. Where the sediment cover stabilizes, sediment deposition by gravity flows may continue building sediment waves. These results suggest that complex processes may follow the emplacement of large MTDs, significantly impacting continental-slope evolution.