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Since May 2018, Mayotte island has experienced an important seismic activity linked to the on-going sismo-volcanic crisis. Although variations in the number of earthquakes and in their distribution have been observed since the start of the eruption in early July 2018, a continuous seismicity persists. It could weaken the steep submarine slopes of Mayotte, as highlighted by the high-resolution bathymetry data collected during the MAYOBS cruise in May 2019. This could trigger submarine landslides with associated tsunamis.To address the hazards associated with such events, we analyzed geomorphological data to define 8 scenarios of potential submarine landslides with volumes ranging from 11,25.106 to 800.106 m3. We simulated the resulting landslide dynamics as well as generated waves (Poulain et al. 2022). In order to estimate the uncertainty associated to the modeling approach, a hierarchy of different model approximations was tested, spanning hydrostatic, non-hydrostatic and multilayer approaches. A sensitivity analysis was also performed by varying the initial released mass, the rheological parameters describing the landslide, its interaction with the water column, the Manning friction coefficient as well as the resolution of the bathymetry description. The combination of all these elements provides an estimate of the uncertainty on simulation results. We show that, in the context of Mayotte, non-hydrostatic effects have the most prominent influence on simulated water elevation and waves velocity. Other key factors include the friction coefficient within the landslide and the resolution of the bathymetry. These results show that landslide-tsunami models should still be improved as well as the estimates of the parameters involved to reduce the related uncertainties on the water wave calculation (water elevation, velocity) that can exceed a factor two.Poulain, P., et al. (2022). Numerical simulation of submarine landslides and generated tsunamis: application to the on-going Mayotte seismo-volcanic crisis. Comptes Rendus. Géoscience, 354(S2), 1-30.

SinceMay 2018,Mayotte Island has been experiencing seismo-volcanic activities that could trigger submarine landslides and, in turn, tsunamis. To address these hazards, we use the HySEA numerical model to simulate granular flow dynamics and the Boussinesq FUNWAVE-TVD numerical model to simulate wave propagation and subsequent inundations. We investigate 8 landslide scenarios (volumes from 11.25£106 m3 to 800£106 m3). The scenario posing the greatest threat involves destabilization on the eastern side ofMayotte’s lagoon at a shallowdepth and can generate sea-surface deformations of up to 2 m.We show that the barrier reef surroundingMayotte plays a prominent role in controlling water-wave propagation and in protecting the island. The tsunami travel time to the coast is very short (a few minutes) and the tsunami is not necessarily preceded by a sea withdrawal. Our simulation results provide a key to establishing hazard maps and evacuation plans and improving early-warning systems, Depuis mai 2018, l’île deMayotte connaît une activité sismo-volcanique importante susceptible de déclencher des glissements de terrain sous-marins générant des tsunamis. Pour faire face à ces aléas, nous utilisons deux modèles numériques complémentaires : le modèle HySEA (simulant la dynamique des écoulements granulaires) et le modèle Boussinesq FUNWAVE-TVD (simulant la propagation des vagues et les inondations) pour étudier 8 scénarios de glissements sous-marins potentiels (volumes de 11,25£106 m3 à 800£106 m3). Les scénarios ayant le plus d’impact se situent à proximité de Petite Terre et à faible profondeur. Ils peuvent générer une élévation de la surface de la mer jusqu’à 2 m en zone habitée à Petite Terre. Nous montrons que la barrière de corail entourant Mayotte joue un rôle prépondérant dans le contrôle de la propagation des vagues et dans la protection de l’île. Le temps de trajet du tsunami jusqu’à la côte est très court (quelques minutes) et le tsunami n’est pas nécessairement précédé d’un retrait maritime. De telles observations sont essentielles pour construire des cartes d’aléas précises et des plans d’évacuation afin d’aider la population

The Toba volcano super-eruption on the island of Sumatra occurred about 74,000 years ago[1], close to the transition between interglacial Marine Isotope Stage (MIS) 5 and glacial MIS 4. This eruption, called Youngest Toba Tuff (YTT), is currently described as the largest cataclysmic eruption of the Quaternary. However, the impact of this super-eruption on climate is widely debated and its effects on the ocean remains poorly understood.The aim of this work is to estimate its impact on oceanic pH at a site near the eruption center. To do so, we measured δ11B values (pH proxy) on monospecific samples of planktonic foraminifera Globigerinoides ruber and Pulleniatina obliquiloculata from sediment core BAR94-25 (Andaman Sea) using a recently developed method at the Institut de Physique du Globe de Paris (IPGP)[2]. G. ruber is a species that thrives preferentially in surface waters, while P. obliquiloculata lives at the thermocline. Therefore, δ11B measurements on their shells can reconstruct pH variations in surface and thermocline waters, respectively.We selected the interval from 258 to 355 cm, corresponding to an age between 57 and 82 ka. This interval contains two clearly visible tephra layers corresponding to the YTT, at the transition from MIS 5 to MIS 4, and to a post-YTT explosive activity during MIS 4. These layers are correlated with a significant decrease in carbonate content (CaCO3). Our results indicate a complex pH response during the two concerned volcanic episodes. Thermocline seawater doesn’t show significant pH decrease during the volcanic episodes compared to the overall signal recorded throughout the studied interval. Conversely, surface seawater shows a much more important pH decrease during part of the volcanic episodes than during the all studied interval. Such decrease in pH during the transition to a glacial state is particularly surprising because an increase in pH, due to the global reduction in atmospheric CO2, is rather expected, as shown by previous foraminifera δ11B records[3].The coupling of CaCO3 and pH decrease during tephra levels suggests acidification in the Andaman Sea as a consequence of the Toba volcanic eruptive activity. The seawater surface seems much more sensitive to pH changes than the thermocline zone. However, the reduction of carbonate in the two tephra layers may also be due to dilution from ash falling into the sediment. Other analyses, such as measuring the variation of calcification intensity in planktonic foraminifera, are therefore necessary to better interpret these paleo-pH data.[1] Storey et al., 2012, PNAS, 109 (46), 18684-18688[2] Buisson et al., 2021, JAAS, 36, 2116-2131[3] Foster et al., 2008, EPSL, 271, 254-266

We investigate the dynamics and deposits of granular flows and the amplitude of the generated water waves using the depth-averaged shallow numerical model HySEA, both at the lab- and field scales. We investigate the different sources of errors by quantitatively comparing the simulations with (i) six new laboratory experiments of granular collapses in different conditions (dry, immersed, dry flow entering water) and slope angles, and (ii) numerical simulations made with the code SHALTOP that describes topography effects better than most landslide-tsunami models. In the laboratory configurations, at the limit of the shallow-approximation in such models, we show that topography and non-hydrostatic effects are crucial. However, when empirically accounting for topography effects by artificially increasing the friction coefficient and performing non-hydrostatic simulations, the model is able to reproduce the granular mass deposit and the waves recorded at gauges located at a distance of more than 2-3 times the characteristic dimension of the slide, with an error ranging from 1 % to 25 % depending on the scenario, without any further calibration. Taking into account this error estimation, we simulate landslides that occurred on Montagne Pelée volcano, Martinique, Petites Antilles as well as the generated waves. Results support the hypothesis that large flank collapse events in Montagne Pelée likely occurred in several successive sub-events. This result has a strong impact on the amplitude of the generated waves, and thus on the associated hazards. In the context of the on-going seismic volcanic unrest at Montagne Pelée volcano, we calculate the debris avalanche and associated tsunami for two potential flank-collapse scenarios.

International audience; This work focuses on the use of thin-layer models for simulating fast gravitational flows for hazard assessment. Such simulations are sometimes difficult to carry out because of the uncertainty on initial conditions and on simulation parameters. In this study, we aggregate various field data to constrain realistic initial conditions and to calibrate the model parameters. By using the SHALTOP numerical code, we choose a simple and empirical rheology to model the flow (no more than two parameters), but we model more finely the geometrical interactions between the flow and the topography. We can thus model both a rock avalanche, and the subsequent remobilization of the deposits as a high discharge debris flow.Using the Prêcheur river catchment (Martinique, Lesser Antilles) as a case study, we focus on extreme events with a high potential to impact populations and infrastructures. We use geological and geomorphological data, topographic surveys, seismic recordings and granulometric analysis to define realistic simulation scenarios and determine the main characteristics of documented events. The latter are then reproduced to calibrate rheological parameters. With a single rheological parameter and the Coulomb rheology, we thus model the emplacement and main dynamic characteristics of a recent rock avalanche, as well as the travel duration and flooded area of a documented high discharge debris flow. Then, in a forward prediction simulation, we model a possible 1.9x10^6 m^3 rock avalanche, and the instantaneous remobilization of the resulting deposits as a high-discharge debris flow. We show that successive collapses allow to better reproduce the dynamics of the rock avalanche, but do not change the geometry of the final deposits, and thus do not influence the initial conditions of the subsequent debris flow simulation. A progressive remobilization of the materials slows down the debris flow and limits overflow, in comparison to instantaneous release. However, we show that high discharge debris flows, such as the one considered for model calibration, are better reproduced with an instantaneous initiation. The range of travel times measured for other significant debris flows in the Prêcheur river is consistent with our simulation results, with various rheological parameters and the Coulomb or Voellmy rheology.

Co-auteur étranger; International audience; Deep-sea submarine eruptions are the least known type of volcanic activity, due to the difficulty of detecting, monitoring, and sampling them. Following an intense seismic crisis in May 2018, a large submarine effusive eruption offshore the island of Mayotte (Indian Ocean) has extruded at least 6.5 km3 of magma to date, making it the largest monitored submarine eruption as well as the largest effusive eruption on Earth since Iceland's 1783 Laki eruption. This volcano is located along a WNW-ESE volcanic ridge, extending from the island of Petite Terre (east side of Mayotte) to about 3,500 m of water depth. We present a detailed petrological and geochemical description of the erupted lavas sampled by the MAYOBS 1, 2, and 4 cruises between May and July 2019 and use these to infer characteristics and changes through time for the whole magmatic system and its dynamics from the source to the surface. These cruises provide an exceptional time-series of bathymetric, textural, petrological, and geochemical data for the 2018-2019 eruptive period, and hence bring an invaluable opportunity to better constrain the evolution of magma storage and transfer processes during a long-lived submarine eruption. Integrating the petrological signatures of dredged lavas with geophysical data, we show that the crystal-poor and gas-rich evolved basanitic magma was stored at mantle depth (>37 km) in a large (≥10 km3) reservoir and that the eruption was tectonically triggered. As the eruption proceeded, a decrease in ascent rate and/or a pathway change resulted in the incorporation of preexisting differentiated magma stored at a shallower level. Magma transfer from the deep mantle reservoir is syn-eruptive, as indicated by transfer times estimated from diffusion in zoned olivine crystals that are much shorter than the total eruption duration. Our petrological model has important hazard implications concerning the rapid and stealthy awakening of a deep gas-rich magma reservoirs that can produce unusually high output rates and long-lived eruption. Sudden tapping of large crystal poor reservoirs may be the trigger mechanism for other rarely witnessed high-volume (>1 km3) effusive events.

This work focuses on the use of thin-layer models for simulating fast gravitational flows for hazard assessment. Such simulations are sometimes difficult to carry out because of the uncertainty on initial conditions and on simulation parameters. In this study, we aggregate various field data to constrain realistic initial conditions and to calibrate the model parameters. By using the SHALTOP numerical code, we choose a simple and empirical rheology to model the flow (no more than two parameters), but we model more finely the geometrical interactions between the flow and the topography. We can thus model both a rock avalanche, and the subsequent remobilization of the deposits as a high discharge debris flow.Using the Prêcheur river catchment (Martinique, Lesser Antilles) as a case study, we focus on extreme events with a high potential to impact populations and infrastructures. We use geological and geomorphological data, topographic surveys, seismic recordings and granulometric analysis to define realistic simulation scenarios and determine the main characteristics of documented events. The latter are then reproduced to calibrate rheological parameters. With a single rheological parameter and the Coulomb rheology, we thus model the emplacement and main dynamic characteristics of a recent rock avalanche, as well as the travel duration and flooded area of a documented high discharge debris flow. Then, in a forward prediction simulation, we model a possible 1.9x106 m3 rock avalanche, and the instantaneous remobilization of the resulting deposits as a high-discharge debris flow. We show that successive collapses allow to better reproduce the dynamics of the rock avalanche, but do not change the geometry of the final deposits, and thus do not influence the initial conditions of the subsequent debris flow simulation. A progressive remobilization of the materials slows down the debris flow and limits overflow, in comparison to instantaneous release. However, we show that high discharge debris flows, such as the one considered for model calibration, are better reproduced with an instantaneous initiation. The range of travel times measured for other significant debris flows in the Pr\^echeur river is consistent with our simulation results, with various rheological parameters and the Coulomb or Voellmy rheology.

Since May 2018, Mayotte island has experienced an important seismic activity linked to the on-going sismo-volcanic crisis. The epicenters of the seismic swarms are located between 5 and 15 km east of Petite Terre for the main swarm, and 25 km east of Petite Terre for the secondary swarm. Although variations in the number of earthquakes and their distribution have been observed since the start of the eruption in early July 2018 [Lemoine A.(2020), Cesca et al.(2020)], a continuous seismicity persists and could generate several earthquakes of magnitudes close to M4 widely felt by the population. This recurrent seismicity could weaken the steep submarine slopes of Mayotte, as highlighted by the high resolution bathymetry data collected during the MAYOBS cruise in May 2019 (Feuillet et al.,submitted) and trigger submarine landslides with associated tsunamis.To address the hazards associated with such events, we analyzed morphological data to define 8 scenarios of potential submarine slides with volumes ranging from 11,25.106 to 800.106 m3 and we simulate the landslide dynamics and generated waves. We use two complementary numerical models: (i) the code HYSEA to simulate the dynamic of the submarine granular flows and the water wave generation, and (ii) the Boussinesq FUNWAVE- TVD model simulate the waves propagation and the inundation on Mayotte. The effect of the time at which the models are coupled is investigated.The most impacting submarine slide scenarios are located close to Petite Terre at a shallow depth. They can locally generate a sea surface elevation more than a meter in local areas especially at Petite Terre. The various simulations show that parts of the island are particularly sensitive to the risk of tsunamis. Indeed, some scenarios that does not cause significant coastal flooding still seems to cause significant hazards in these exposed areas. The barrier reef around Mayotte has a prominent role in controlling the wave propagation towards the island and therefore reducing the impact on land. It should be noted that the arrival of tsunamis on the coastline is not necessarily preceded by a retreat from the sea and the waves can reach the coasts of Mayotte very quicky (few minutes). Cesca, S., Letort, J., Razafindrakoto, H.N.T. et al. Drainage of a deep magma reservoir near Mayotte inferred from seismicity and deformation. Nat. Geosci. 13, 87–93 (2020). https://doi.org/10.1038/s41561-019-0505-5Feuillet, N, Jorry, S. J., Crawford, W, Deplus, C. Thinon, I, Jacques, E. Saurel, J.M., Lemoine, A., Paquet, F., Daniel, R., Gaillot, A., Satriano, C., Peltier, A., Aiken, C., Foix, O., Kowalski, P., Laurent, A., Beauducel, F., Grandin, R., Ballu, V., Bernard, P., Donval, J.P., Geli, L., Gomez, J. Guyader, V., Pelleau, P., Rinnert, E., Bertil, D., Lemarchand, A., Van der Woerd, J.et al. (in rev). Birth of a large volcano offshore Mayotte through lithosphere-scale rifting, Nature.Anne Lemoine, Pierre Briole, Didier Bertil, Agathe Roullé, Michael Foumelis, Isabelle Thinon, Daniel Raucoules, Marcello de Michele, Pierre Valty, Roser Hoste Colomer, The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption, Geophysical Journal International, Volume 223, Issue 1, October 2020, Pages 22–44, https://doi.org/10.1093/gji/ggaa273

International audience; Since 2018, the submarine east flank of Mayotte Island (Comoros archipelago) is the site of a major eruption located at 3.5 km depth bsl on a WNW-ESE volcanic ridge. Samples brought by oceanographic cruises carried out to monitor this seismo-volcanic crisis indicate that this volcanic ridge is built by a bimodal sodic alkaline magmatic series that includes basanites and phonolites. A petrological study of dredged samples allowed us to image the magmatic system feeding the volcanic ridge and to determine the link between basanitic and phonolitic magmas. The magmatic system feeding the volcanic ridge comprises multiple levels of magma storage. Basanitic magmas generated at 80–100 km mantle depth are stored in two or more deep reservoirs (≥ 37 km) and then in shallower basanitic and phonolitic lenses located close to the Moho interface before rising the surface. This study identifies three possible scenarios: (1) the deep basanitic magma rises directly and quickly to the surface from the deep mantle reservoir (as is currently happening 60 km offshore), (2) the basanitic magma stalls in a shallower reservoir near the Moho before resuming its ascent toward the surface and erupting as porphyritic basanite, (3) the basanitic magma stops and evolves to phonolite in these sub-crustal reservoirs. The phonolitic lavas are produced by approximately 80% fractional crystallization (34% clinopyroxene, 30% anorthoclase feldspar, 15.5% magnetite, 12.5% olivine, 5% apatite and 4% ilmenite) of a hydrous basanitic magma at mantle depths (P > 0.6 GPa) under reduced oxygen fugacity (~ FMQ-1). In this third scenario, the phonolitic magma might be reactivated by the arrival of a new batch of deeper basanitic magma.

23 pages, 12 figures, 4 tables, supporting information https://doi.org/10.1111/bre.12553.-- Data availability: Data from the IODP-340 Expedition and the seismic datasets used are available at: http://iodp.tamu.edu/scienceops/expeditions/antilles_volcanism_landslides.html, http://donnees-campagnes.flotteoceanographique.fr/search, West off Martinique (Lesser Antilles), the Grenada Basin submarine sediments were affected by the emplacement of Debris Avalanche Deposits (DAD). Montagne Pelée Volcano has experienced three major flank collapses during the last ca. 127 kyrs, resulting in a cumulated volume of up to 300 km3 offshore. Using a combination of geophysical and geotechnical data, we investigate the effect of these debris avalanches emplacements on the basin hydrogeology and their relationship with the observed sediment deformation in the seismic and coring data. The geotechnical test carried on IODP-340 cores samples reveal four sediment types within the basin with distinctive mechanical and hydraulic properties: proximal volcanoclastics, distal volcanoclastics, hemipelagic and ash-rich sediments. These results, together with margin stratigraphic models obtained from seismic reflection data, were used as inputs for the numerical finite-element model. This model shows that the coupling of the sediment properties with the mid- to low-sedimentation rates results in the development of low overpressures prior to the first flank collapse at 127 ka. However, the emplacement of the first two DADs, between 127 and 36 ka, developed high overpressures ratios (λ* > 0.9) in the easternmost part of the Grenada Basin. According to the model, the sudden compaction of the pre-existing sediments due to the DADs load created fluid flow velocities up to 7 times higher than the hydraulic conductivities, which would have thus reduced the sediment bearing capacities and shear strength, favouring their mobilization and deformation. From 127 to 36 ka, the sea-floor sediments suffered a long-term deformation driven by the combination of the weight of the emplaced material and the persistence of high overpressure ratios through time. This deformation propagated tens of kilometres away from the DAD’s emplacement and it is possible that still continues today due to the persistence of low overpressure ratios. This long-term and long-distance deformation and persisted overpressures are a key factor to take into account in the framework of a geohazards evaluation in areas recurrently affected by earthquakes and volcanic flank collapses, This study was supported by the PREST project co- funded by INTERREG Caraïbes for the European Regional Development Fund.-- With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

International audience

The Prêcheur river is located to the West of Montagne Pelée, in the Northern part of Martinique island. For several decades it has produced numerous lahars that directly threaten the Prêcheur village, at the mouth of the river. In recent years, the most important lahars have been correlated to massive collapses of the Samperre cliff, 9 km upstream from the sea, that create a reservoir of loose material at the bottom of the cliff. In 2010, a lahar started from this reservoir, destroyed a bridge and inundated part of the Prêcheur village. A new major period of collapses of the Samperre cliff started on 2 January 2018, involving more than 4 × 106 m3 of material. In the following days, intense rainfalls triggered several lahars that reached the Prêcheur village but remained confined in the river bed. Since then, lahars and collapses have continued to occur, even though their frequency has decreased with time and their intensity is smaller compared to the onset of the crisis. One single lahar overflowed the river bed on 22 February 2018 without significant impact on infrastructures.In this study, we test different possible scenarios to model the first and most important phase of the collapse of the Samperre cliff, that occurred in early January 2018, with the shallow-water model SHALTOP. We constrain the collapse geometry with photogrammetric 3D models and LIDAR topographic surveys, acquired in 2010 and in late January 2018. We also consider an intermediate volume to take into account a possible retreat of the cliff face between 2010 and 2018. The modeled traveled distances are compared to field observations. Finally, we use geomorphological and geological observations to identify potentially unstable structures within the cliff, and model the associated collapses.These simulations provide insights on the possible geometry (extent and depth) of the debris reservoir at the bottom of the cliff, after a major collapse episode. This is of prior importance in order to estimate the location and volume of future lahars. In order to investigate their dynamics, we model the major 2010 lahar, for which the initial debris reservoir volume is known (about 2 Mm3). We first simulate the progressive remobilization of the reservoir by water with the r.avaflow numerical code. In a second test, we impose instead a constant flow discharge upstream until the same volume has been released. We test different parameters to identify which ones have the most significant influence on the lahar travel time, from its initiation until it reaches the Prêcheur village.

The early development and growth of seamounts are poorly known as the birth of a volcano on the sea bottom has been rarely observed. The on-going Mayotte seismo-volcanic crisis is associated with the formation of a new seafloor volcano at a water depth of 3300 m and provides the opportunity to study its early development.Four oceanographic cruises, MAYOBS 1 to 4, were carried out between May and July 2019 aboard the French R/V Marion Dufresne. High resolution bathymetry and backscatter data as well as sub-bottom profiler, gravity and magnetic profiles were collected during each cruise. A dense network of profiles has been achieved over the new volcano at different epochs, allowing to assess its detailed morphology and the evolution through time. During MAYOBS4, a deep-towed underwater camera provided sea bottom videos and photos on the volcano.First results indicate that the new volcano is still growing at the end of July 2019. Repetitive surveys in May, June and July 2019 allow to document the morphological evolution of the volcano, to estimate the volume of material emplaced between each epoch and to discuss the emitted lava rate.The new volcano has a starfish shape and is now 820 m high. Steep slopes are observed close to the summit and several radial ridges developed from its central part, displaying hummocky morphology similar to the ones observed along mid oceanic axial volcanic ridges. At the bottom, flat areas with high backscatter could indicate channelized lava flows emplaced at higher effusion rates. The morphological analysis combined with video imagery brings constraints to the eruptive processes yielding to the formation of a nascent volcano.

After 25 years of gradual increase, volcanic unrest at La Soufriere of Guadeloupe reached its highest seismic energy level on 27 April 2018, with the largest felt volcano-tectonic (VT) earthquake (M-L 4.1 or M-W 3.7) recorded since the 1976-1977 phreatic eruptive crisis. This event marked the onset of a seismic swarm (180 events, 2 felt) occurring after three previous swarms on 3-6 January (70 events), 1 st February (30 events, 1 felt) and 16-17 April (140 events, 1 felt). Many events were hybrid VI's with long-period codas, located 2-4 km below the volcano summit and clustered within 2 km along a regional NW-SE fault cross-cutting La Soufriere. Elastic energy release increased with each swarm whereas inter-event time shortened. At the same time, summit fractures continued to open and thermal anomalies to extend. Summit fumarolic activity increased significantly until 20 April, with a maximum temperature of 111.4 degrees C and gas exit velocity of 80 m/s, before declining to similar to 95 degrees C and similar to 33 m/s on 25 April. Gas compositions revealed increasing C/S and CO2/CH4 ratios and indicate hydrothermal P-T conditions that reached the critical point of pure water. Repeated MultiGAS analysis of fumarolic plumes showed increased CO2/H2S ratios and SO2 contents associated with the reactivation of degassing fractures (T = 93 degrees C, H2S/SO2 approximate to 1). While no direct evidence of upward magma migration was detected, we attribute the above phenomena to an increased supply of deep magmatic fluids that heated and pressurized the La Soufriere hydrothermal system, triggering seismogenic hydro-fracturing, and probable changes in deep hydraulic properties (permeability) and drainage pathways, which ultimately allowed the fumarolic fluxes to lower. Although this magmatic fluid injection was modulated by the hydrothermal system, the unprecedented seismic energy release and the critical point conditions of hydrothermal fluids suggest that the 2018 sequence of events can be regarded as a failed phreatic eruption. Should a similar sequence repeat, we warn that phreatic explosive activity could result from disruption of the shallow hydrothermal system that is currently responsible for 3-9 mm/y of nearly radial horizontal displacements within 1 km from the dome. Another potential hazard is partial collapse of the dome's SW flank, already affected by basal spreading above a detachment surface inherited from past collapses. Finally, the increased magmatic fluid supply evidenced by geochemical indicators in 2018 is compatible with magma replenishment of the 6-7 km deep crustal reservoir feeding La Soufriere and, therefore, with a potential evolution of the volcano's activity towards magmatic conditions.

International audience

Site U1396 was piston cored as a part of Integrated Ocean Drilling Project Expedition 340 to establish a long record for Lesser Antilles volcanism. A ∼150 m sediment succession was recovered from three holes on a bathymetric high ∼33 km southwest of Montserrat. A series of shipboard and newly-generated chronostratigraphic tools (biostratigraphy, magnetostratigraphy, astrochronology, and stable isotope chemostratigraphy) were employed to generate an integrated age model. Two possible chronostratigraphic interpretations for the Brunhes chron are presented, with hypotheses to explain the discrepancies seen between this study and Wall-Palmer et al. (2014). The recent Wade et al. (2011) planktic foraminiferal biostratigraphic calibration is tested, revealing good agreement between primary datums observed at Site U1396 and calibrated ages, but significant mismatches for some secondary datums. Sedimentation rates are calculated, both including and excluding the contribution of discrete volcanic sediment layers within the succession. Rates are found to be 'pulsed'or highly variable within the Pliocene interval, declining through the 1.5-2.4 Ma interval, and then lower through the Pleistocene. Different explanations for the trends in the sedimentation rates are discussed, including orbitally-forced biogenic production spikes, elevated contributions of cryptotephra (dispersed ash), and changes in bottom water sources and flow rates with increased winnowing in the area of Site U1396 into the Pleistocene.

Over the past 9,150 years, at least 9 flank collapses have been identified in the history of La Soufrière of Guadeloupe volcano. On account of the volcano’s current unrest, the possibility of such a flank collapse should not be dismissed in assessing hazards for future eruptive magmatic as well as non-magmatic scenarios. We combine morphological and geophysical data to identify seven unstable structures (volumes ranging from 1 × 106 m3 to 100 × 106 m3), including one that has a volume compatible with the last recorded flank collapse in 1530 CE. We model their dynamics and emplacement with the SHALTOP numerical model and a simple Coulomb friction law. The best-fit friction coefficient to reproduce the 1530 CE event is tan(7°) = 0.13, suggesting the transformation of the debris avalanche into a debris flow, which is confirmed by the texture of mapped deposits. Various friction angles are tested to investigate less water-rich and less mobile avalanches. The most densely populated areas of Saint-Claude and Basse-Terre, and an area of Gourbeyre south of the Palmiste ridge, are primarily exposed in the case of the more voluminous and mobile flank collapse scenarios considered. However, topography has a prominent role in controlling flow dynamics, with barrier effects and multiple channels. Classical mobility indicators, such as the Heim’s ratio, are thus not adequate for a comprehensive hazard analysis.

Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass-wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (∼930 to ∼900 ka, ∼810 to ∼760 ka, and ∼190 to ∼120 ka) that coincide with periods of increased volcano instability and mass-wasting. The youngest of these periods marks the peak in activity at the Soufriere Hills volcano. The largest flank collapse of this volcano (∼130 ka) occurred toward the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufriere Hills edifice, and their timing also coincides with periods of rapid sea level rise (>5 m/ka). Available age data from other island arc volcanoes suggest a general correlation between the timing of large landslides and periods of rapid sea level rise, but this is not observed for volcanoes in intraplate ocean settings. We thus infer that rapid sea level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean-island settings.

Landslides are common features in the vicinity of volcanic islands. In this contribution, we investigate landslides emplacement and dynamics around the volcanic island of Martinique based on the first scientific drilling of such deposits. The evolution of the active Montagne Pelee volcano on this island has been marked by three major flank-collapses that removed much of the western flank of the volcano. Subaerial collapse volumes vary from 2 to 25 km3 and debris avalanches flowed into the Grenada Basin. High-resolution seismic data (AGUADOMAR-1999, CARAVAL-2002, and GWADASEIS-2009) is combined with new drill cores that penetrate up to 430 m through the three submarine landslide deposits previously associated to the aerial flank-collapses (Site U1399, Site U1400, Site U1401, IODP Expedition 340, Joides Resolution, March–April 2012). This combined geophysical and core data provide an improved understanding of landslide processes offshore a volcanic island. The integrated analysis shows a large submarine landslide deposit, without debris avalanche deposits coming from the volcano, comprising up to 300 km3 of remobilized seafloor sediment that extends for 70 km away from the coast and covers an area of 2100 km2. Our new data suggest that the aerial debris avalanche deposit enter the sea but stop at the base of submarine flank. We propose a new model dealing with seafloor sediment failures and landslide propagation mechanisms, triggered by volcanic flank-collapse events affecting Montagne Pelee volcano. Newly recognized landslide deposits occur deeper in the stratigraphy, suggesting the recurrence of large-scale mass-wasting processes offshore the island and thus, the necessity to better assess the associated tsunami hazards in the region.

International audience; We simulate here the emplacement of the debris avalanche generated by the lastflank collapse event of Montagne Pele´e volcano (30–45 ka), Martinique, Lesser Antilles.Our objective is to assess the maximum distance (i.e., runout) that can be reached by thistype of debris avalanche as a function of the volume involved. Numerical simulations areperformed using two complementary depth-averaged thin-layer continuum models becauseno complete models were available in the literature. The first model, SHALTOP, accuratelydescribes dry granular flows over a 3D topography and may be easily extended todescribe submarine avalanches. The second model, HYSEA, describes the subaerial andsubmarine parts of the avalanche as well as its interaction with the water column. However,HYSEA less accurately describes the thin-layer approximation on the 3D topography.Simulations were undertaken testing different empirical friction laws and debris avalanchevolume flows. Our study suggests that large collapses (~25 km3) probably occurred inseveral times with successive volumes smaller than about 5 km3 entering the sea. Thisresult provides new constraints on the emplacement processes of debris avalanches associatedwith these collapses which can drastically change the related hazard assessment suchas the generated tsunami, in a region known for its seismic and volcanic risks.

Bioclastic flow deposits offshore from the Soufriere Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south-west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse-grained and either ungraded or poorly graded, and were deposited by non-cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub-units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub-units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi-stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea-level change.

Major scarp features have been previously identified on Soufriere volcano (St. Vincent, Lesser Antilles) and recently interpreted as flank-collapse structures although no associated debris avalanche deposits were recognized. New offshore high-resolution bathymetry and geophysical data have been collected during the Caraval cruise (N/O L'Atalante, February–March 2002) offshore of St. Vincent. Recent analysis of these data leads us to identify and characterize a debris avalanche deposit on the western flank of Soufriere volcano, which extends down to the Grenada Basin. The origin of the submarine deposit by submarine slump or flank-collapse event is discussed but data analysis tends support the occurrence of a flank-collapse event during the evolution of Soufriere volcano. The collapse volume has been estimated to about 9 km 3 and the age of the event is less than 50,000 years (using marine geophysical constraints). The origin of the flank-collapse can be related to local causes (weakening of the flank by intense hydrothermal alteration, magmatic injections, increase of the load by accumulation of volcanic products) and/or to regional causes (asymmetry of the arc, sea level variations induced by climate change that may have an effect on the stability of the volcano). Taking into account this event, we proposed a model for the evolution of Soufriere volcano including a new estimation of the magma production rate. The estimation indicates that Soufriere volcano is one of the most active volcanoes of the arc. A future flank-collapse event could have catastrophic consequences for the entire area due to the emplacement of voluminous, potentially tsunamigenic, debris avalanches.

Recent studies hypothesize that some submarine slides fail via pressure-driven slow-slip deformation. To test this hypothesis, this study derives pore pressures in failed and adjacent unfailed deep marine sediments by integrating rock physics models, physical property measurements on recovered sediment core, and wireline logs. Two drill sites (U1394 and U1399) drilled through interpreted slide debris; a third (U1395) drilled into normal marine sediment. Near-hydrostatic fluid pressure exists in sediments at site U1395. In contrast, results at both sites U1394 and U1399 indicate elevated pore fluid pressures in some sediment. We suggest that high pore pressure at the base of a submarine slide deposit at site U1394 results from slide shearing. High pore pressure exists throughout much of site U1399, and Mohr circle analysis suggests that only slight changes in the stress regime will trigger motion. Consolidation tests and permeability measurements indicate moderately low (~10-16–10-17 m2) permeability and overconsolidation in fine-grained slide debris, implying that these sediments act as seals. Three mechanisms, in isolation or in combination, may produce the observed elevated pore fluid pressures at site U1399: (1) rapid sedimentation, (2) lateral fluid flow, and (3) shearing that causes sediments to contract, increasing pore pressure. Our preferred hypothesis is this third mechanism because it explains both elevated fluid pressure and sediment overconsolidation without requiring high sedimentation rates. Our combined analysis of subsurface pore pressures, drilling data, and regional seismic images indicates that slope failure offshore Martinique is perhaps an ongoing, creep-like process where small stress changes trigger motion.

The past history of recurrent flank collapses of la Soufriere volcano of Guadeloupe, its structure, its well-developed hydrothermal system and the current activity constitute factors that could promote a future flank collapse, particularly in the case of a significant increase of activity, with or without shallow magmatic input. To address the hazards associated with such a collapse, we model the emplacement of the debris avalanche generated by a flank-collapse event in 1,250 BC (3,100 years B.P.). We use a finite-difference grain-flow model solving mass and momentum conservation equations that are depth-averaged over the slide thickness, and a Coulomb-type friction law with a variable basal (minimum) friction angle. Using the parameter values determined from this simulation, we then simulate the debris avalanche which could be generated by a potential collapse of the present lava dome. We then discuss the region which could be affected by such a future collapse, and additional associated hazards of concern.

Results from recent fieldwork and the Aguadomar marine survey in the Lesser Antilles clearly indicate that the volcanic field of southern Dominica has experienced three major edifice collapse events. This led to formation of the most voluminous debris avalanches known in the Caribbean Arc. Submarine hummocky morphology with plurikilometric megablocks is characteristic of debris avalanche deposits. We propose that steep slopes on the western Caribbean side of the island and intense hydrothermal alteration lead to recurrent large-scale edifice collapses. Therefore islands in the Lesser Antilles face a non-negligible risk from generation of tsunamis associated with potential future edifice collapse. To cite this article: A. Le Friant et al., C. R. Geoscience 334 (2002) 235–243.

International audience; Piston cores collected from IODP drilling platforms (and its predecessors) provide the best long-term geological and climatic record of marine sediments worldwide. Coring disturbances affecting the original sediment texture have been recognized since the early days of coring and include deformation resulting from shear of sediment against the core barrel, basal flow-in due to partial stroke, loss of stratigra-phy, fall-in, sediment loss through core catchers, and structures formed during core recovery and on-deck transport. The most severe disturbances occur in noncohesive (sandy) facies, which are particularly common in volcanogenic environments and submarine fans. Although all of these types of coring disturbances have been recognized previously, our contribution is novel because it provides an easily accessible summary of methods for their identification. This contribution gives two specific examples on the importance of these coring disturbances. We show how suck-in of sediments during coring artificially created very thick volcani-clastic sand layers in cores offshore Montserrat and Martinique (Lesser Antilles). We then analyze very thick, structureless sand layers from the Escanaba Trough inferred to be a record of the Missoula megafloods. These sand layers tend to coincide with the base of core sections, and their facies suggest coring disturbance by basal flow-in, destroying the original structure and texture of the beds. We conclude by outlining and supporting IODP-led initiatives to further reduce and identify coring disturbances and acknowledge their recent successes in drilling challenging sand-rich settings, such as during IODP Expedition 340.

Onshore-offshore geophysical studies conducted on Martinique have identified major flank collapse events of Montagne Pelee that generated large submarine mass wasting deposits. Here, we evaluate the preconditioning factors involved in the deformation and failure of marine sediments related to volcano-flank collapse events. We use core logging, sedimentological and geotechnical data of the upper 200 m of core at sites U1397, U1398, U1399 and U1400 drilled during the Integrated Ocean Drilling Program (IODP) expedition 340, west of Martinique. We find that the low hydraulic conductivity of hemipelagic sediment causes low rates of dewatering of turbidites and tephra layers allowing excess pore fluid pressures to persist at depth. Overpressure generation was likely enhanced during major flank collapses, leading to low shear strength and subsequent deformation of large volumes of marine sediments, as found at Site U1400.

Marine sediments around volcanic islands contain an archive of volcaniclastic deposits which can be used to reconstruct the volcanic history of an area. Such records hold many advantages over often incomplete terrestrial data sets. This includes the potential for precise and continuous dating of intervening sediment packages which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores. Here we discuss a marine record of eruptive and mass wasting events spanning ~250 ka offshore of Montserrat using new data from IODP Expedition 340 as well as previously collected cores. By using a combination of high resolution oxygen isotope stratigraphy AMS radiocarbon dating biostratigraphy of foraminifera and calcareous nannofossils and clast componentry we identify five major events at Soufriere Hills volcano since 250 ka. Lateral correlations of these events across sediment cores collected offshore of the south and south west of Montserrat have improved our understanding of the timing extent and associations between events in this area. Correlations reveal that powerful and potentially erosive density currents traveled at least 33 km offshore and demonstrate that marine deposits produced by eruption fed and mass wasting events on volcanic islands are heterogeneous in their spatial distribution. Thus multiple drilling/coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands. This multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during Expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands. © 2014. American Geophysical Union. All Rights Reserved.

Results from a recent marine geophysical survey demonstrate the importance of the process of flank collapse in the growth and evolution of volcanoes along an island arc. The Aguadomar cruise, aboard the French R/V L’Atalante, surveyed the flanks of the Lesser Antilles Arc between the islands of Montserrat and St. Lucia. Analysis of the data shows that flank collapse events occurred on active volcanoes all along the arc and resulted in debris avalanches, some of them being of large magnitude. The debris avalanche deposits display hummocky topography on the swath bathymetry, speckled pattern on backscatter images, hyperbolic facies on 3.5 kHz echosounder data and chaotic units on air gun seismic profiles. They extend from horseshoe-shaped structures previously identified on the subaerial part of the volcanoes. In the southern part of the arc, large-scale debris avalanche deposits were identified on the floor of the Grenada Basin west of active volcanoes on Dominica, Martinique and St. Lucia. The extent of debris avalanche deposits off Dominica is about 3500 km2. The debris avalanches have resulted from major flank collapse events which may be mainly controlled by the large-scale structure of the island arc and the presence of the deep Grenada Basin. In the northern part of the arc, several debris avalanche deposits were also identified around the island of Montserrat. With smaller extent (20–120 km2), they are present on the east, south and west submarine flanks of Soufriere Hills volcano which has been erupting since July 1995. Flank collapse is thus a recurrent process in the recent history of this volcano. The marine data are also relevant for a discussion of the transport mechanisms of debris avalanches on the seafloor surrounding a volcanic island arc.

International audience; Flank-collapse events are now recognized as common processes of destruction of volcanoes. They may occur several times on a volcanic edifice pulling out varying volumes of material from km3 to thousands of km3. In the Lesser Antilles Arc, a large number of flank-collapse events were identified. Here, we show that some of the largest events are correlated to significant variations in erupted magma compositions and eruptive styles.On Montagne Pelée (Martinique), magma production rate has been sustained during several thousand years following a 32 ka old flank-collapse event. Basic and dense magmas were emitted through open-vent eruptions that generated abundant scoria flows while significantly more acidic magmas were produced before the flank collapse. The rapid building of a new cone increased the load on magma bodies at depth and the density threshold. Magma production rate decreased and composition of the erupted products changed to more acidic compared to the preceding period of activity. These low density magma generated plinian and dome-forming eruptions up to the Present. In contrast at Soufrière Volcanic Centre of St. Lucia and at Pitons du Carbet in Martinique, the flank-collapses have an opposite effect: in both cases, the acidic magmas erupted immediately after the flank-collapses. These magmas are highly porphyritic (up to 60% phenocrysts) and much more viscous than the magmas erupted before the flank-collapses. They have been generally emplaced as voluminous and uptight lava domes (called “the Pitons”). Such magmas could not ascent without a significant decrease of the threshold effect produced by the volcanic edifice loading before the flank-collapse.

Long undersea debris runout can be facilitated by a boundary layer formed by weak marine sediments under a moving slide mass. Undrained loading of such offshore sediment results in a profound drop of basal shear resistance, compared to subaerial shear resistance, enabling long undersea runout. Thus large long-runout submarine landslides are not truly enigmatic (Voight and Elsworth 1992, 1997), but are understandable in terms of conventional geotechnical principles. A corollary is that remoulded undrained strength, and not friction angle, should be used for basal resistance in numerical simulations. This hypothesis is testable via drilling and examining the structure at the soles of undersea debris avalanches for indications of incorporation of sheared marine sediments, by tests of soil properties, and by simulations. Such considerations of emplacement process are an aim of ongoing research in the Lesser Antilles (Caribbean Sea), where multiple offshore debris avalanche and dome-collapse debris deposits have been identified since 1999 on swath bathymetric surveys collected in five oceanographic cruises. This paper reviews the prehistoric and historic collapses that have occurred offshore of Antilles arc islands and summarizes ongoing research on emplacement processes.

The aragonite shell–bearing thecosome pteropods are an important component of the oceanic plankton. However, with increasing pCO2 and the associated reduction in oceanic pH (ocean acidification), thecosome pteropods are thought to be particularly vulnerable to shell dissolution. The distribution and preservation of pteropods over the last 250,000 years have been investigated in marine sediment cores from the Caribbean Sea close to the island of Montserrat. Using the Limacina Dissolution Index (LDX), fluctuations in pteropod dissolution through the most recent glacial/interglacial cycles is documented. By comparison to the oxygen isotope record (global sea ice volume), we show that pteropod dissolution is closely linked to global changes in pCO2 and pH and is, therefore, a global signal. These data are in agreement with the findings of experiments upon living pteropods, which show that variations in pH can greatly affect aragonitic shells. The results of this study provide information which may be useful in the prediction of future changes to the pteropod assemblage caused by ocean acidification.

International audience; Results from recent marine geophysical surveys offer a new perspective for characterizing the evolution processes of volcanic islands. In 2006, cruises FOREVER and ERODER 1 investigated the submarine flanks and the surrounding abyssal plain of La Réunion (Indian Ocean) to obtain for the first time a complete geophysical survey of the area. Combined analyses of these data reveal major differences in the evolution of the two emerged volcanoes, Piton des Neiges and Piton de la Fournaise. We show that debris avalanche deposits extend on the abyssal plain only offshore the active Piton de la Fournaise volcano attesting the occurrence of large flank-collapse events. The absence of such deposits offshore Piton des Neiges and the presence of compressive structures within the sedimentary unit below the edifice support a mechanism of slow deformation of this volcano, such as sliding or spreading. The slow deformation of Piton des Neiges has led to numerous secondary submarine slope instabilities and favored some unconfined turbidity flows which generated large sediment waves running downward all around the island. This study proposes a new model using the most complete marine data set available: slow deformation controls the evolution of Piton des Neiges whereas Piton de la Fournaise (formed on the flanks of a pre-existing edifice) experienced catastrophic, large flank-collapse events.

International audience; A new geophysical data set off La Réunion Island (western Indian Ocean) reveals a large volcaniclastic submarine fan developing in an open-ocean setting. The fan is connected to a torrential river that floods during tropical cyclones. Sediment storage at the coast is limited, suggesting that the sediments are carried directly to the basin. The fan morphology and turbidites in cores lead us to classify it as a sand-rich system mainly fed by hyperpycnal flows. In the ancient geological record, there are many examples of thick volcaniclastic successions, but studies of modern analogues have emphasized mechanisms such as debris avalanches or direct pyroclastic flow into the sea. Because the Cilaos deep-sea fan is isolated from any continental source, it provides information on architecture and noncatastrophic processes in a volcaniclastic deep-sea fan.

[1] New high-resolution marine data acquired aboard R/V Le Suroit was used to map active normal faults offshore Montserrat in greater detail. The main faults of the Montserrat-Havers fault zone have cumulative scarps up to 200 m high, and offset sedimentary layers by hundreds of meters. They are arranged in a right-stepping, en echelon, trans-tensional array, which confirms that they accommodate the left-lateral component of motion resulting from slip partitioning of oblique convergence along the volcanic arc. They cut across Montserrat's recent volcanic complex. Faulting and fissuring exerted control on the position of andesitic domes, which are aligned along the N110°E average fault trend. The ≈10 km-long fault segments that cross the island could produce damaging, M ≈ 6 events comparable to the shallow, 16 March 1985, Mw∼6.3 earthquake that ruptured a submarine, N140°E striking, left-lateral fault near Redonda.

International audience; [1] The 1997 Boxing Day collapse, a remarkable feature of the ongoing eruption of Soufrière Hills on Montserrat, has prompted new interest in the study of volcano stability in the Lesser Antilles. Building on a few cases documented in the literature, we have now identified at least 47 flank collapse events on volcanoes of the Caribbean arc where this type of behavior is characteristic and repetitive. About 15 events occurred on active volcanoes within the last 12,000 years. In the northern part of the arc, flank collapses are repetitive, do not exceed 1 km 3 in volume, occur in all directions, and are promoted by intense hydrothermal alteration and well-developed fracturing of the summit part of the edifices. In contrast, infrequent but large sector collapses, with volumes up to tens of km 3 , are typical of the southern volcanoes. They are always directed to the west as a result of the high overall slopes of the islands toward the deep back-arc Grenada Basin. Because Caribbean islands are small, a large part of the resulting debris avalanches have flowed into the sea thus contributing voluminous and sudden inputs of volcaniclastic sediments to the Grenada Basin. Deposits from such submarine flows have been identified during the recent AGUADOMAR and CARAVAL oceanographic cruises and traced to their source structures on land. Edifice collapses have a major influence on subsequent volcanic activity but also are of high concern because of their tsunamigenic potential.

[1] We model the submarine emplacement of a debris avalanche generated by the last flank-collapse event of Montagne Pelee volcano. We estimate the collapsed volume (1.7 km3) using both the volume of the missing material in the horseshoe-shaped structure and the volume of submarine deposits. This avalanche is treated as the gravitational flow of a homogeneous continuum. It is simulated by a finite-difference model, solving mass and momentum conservation equations, that are depth-averaged over the slide thickness. Numerical simulations show that the emplacement of this debris-avalanche can be suitably modeled by a Coulomb-type friction law with a variable friction angle below 10°. We propose that variations of the friction angle are mainly influenced by the thickness of the flowing mass.

[1] A horseshoe-shaped structure already identified on the southwestern flank of Montagne Pelee (Martinique, Lesser Antilles arc) was previously interpreted as resulting of a flank collapse event, but no debris avalanche deposits were observed at the time. New offshore high-resolution bathymetry and geophysical data (Aguadomar cruise; December 1998 to January 1999; R/V L'Atalante) lead us to identify three debris avalanche deposits on the submarine western flank of Montagne Pelee extending down to the Grenada Basin. They display morphological fronts and hummocky morphology on bathymetric data, speckled pattern on backscatter data and hyperbolic facies on 3.5 kHz and seismic profiles. New on-land geological studies lead us to identify two other horseshoe-shaped structures on the same flank of the volcano. The three submarine deposits have been traced back to the structures identified on land, which confirms the occurrence of repeated flank collapse events during the evolution of Montagne Pelee. The ages of the last two events are estimated at ∼9 ka and ∼25 ka on the basis of 14C and 238U/230Th dates. Every flank collapse produced debris avalanches which flowed down to the Caribbean Sea. We propose that the repeated instabilities are due to the large asymmetry of the island with western aerial and submarine slopes steeper than the eastern slopes. The asymmetry results from progressive loading by accumulation of volcanic products on the western slopes of the volcano and development of long-term gravitational instabilities. Meteoric and hydrothermal fluid circulation on the floor of the second flank collapse structure also creates a weakened hydrothermalized area, which favors the recurrence of flank collapses.

Cette thèse s’intéresse aux processus d’instabilité affectant l’île volcanique de la Martinique (Petites Antilles) à partir de l’analyse combinée de données de géophysique marine et de forages profonds. Les premières ont été acquises au cours de trois campagnes océanographiques réalisées au large de l’île (AGUADOMAR – 1999, CARAVAL – 2002 et GWADASEIS – 2009) alors que les données de forage ont été collectées lors de l’Expédition IODP 340 (2012), première du genre à avoir foré dans des dépôts liés à l’activité volcanique d’une île. La couverture complète des données de géophysique marine a permis de caractériser l’architecture, l’extension et le volume de ces dépôts, alors que les données de forages ont permis de contraindre leur composition et a posteriori, leur origine. L’ensemble de ces données nous permet aujourd’hui de proposer un nouveau modèle, établissant un lien entre les processus d’effondrement du flanc du volcan de la Montagne Pelée et des processus de glissements sous-marins au large de l’île. Ce modèle repose sur deux hypothèses principales : 1/ la mise en place des avalanches de débris est influencée par la rupture de pente entre le flanc sous-marin du volcan et le bassin de Grenade et 2/ le poids généré par les dépôts d’avalanches de débris a pu déclencher une rupture des sédiments et la propagation d’un glissement sous-marin en aval. Afin de vérifier l’exactitude de la 1ère hypothèse, des simulations numériques ont été réalisées afin de comprendre les mécanismes de mise en place des avalanches de débris, et ce à partir de la modélisation du 3ème et dernier dépôt identifié en mer. L’utilisation de deux codes numériques (SHALTOP décrivant précisément les termes complexes de la topographie et HYSEA tenant compte de l’eau), montre que SHALTOP induit une meilleure reproduction du dépôt comparé à HYSEA. Ceci démontre l’influence notable de la topographie dans la dynamique des avalanches et notamment de la rupture de pente qui tend à ralentir leur propagation. Par ailleurs, dans le but de vérifier la 2nd hypothèse de notre modèle, des expériences de modélisation analogique ont été réalisées afin de comprendre cette fois-ci, les mécanismes de propagation des glissements sous-marins sur une pente. Elles démontrent l’influence positive des ruptures de pente et des apports de matériels sur la distance de propagation du glissement, alors que l’eau tend à maintenir le glissement sur la durée. Les apports de matériel permettent le plus souvent d’accélérer momentanément la vitesse du glissement. Enfin, nous discutons la possibilité d’appliquer le nouveau modèle proposé aux autres îles de l’arc des Petites Antilles et à d’autres îles volcaniques du monde.; This study focuses on instability processes occuring on the volcanic island of Martinique (Lesser Antilles) from the combined analysis of marine geophysical and deep drilling datasets. The first ones were gathered during three oceanographic cruises offshore the island (AGUADOMAR – 1999, CARAVAL – 2002 et GWADASEIS – 2009), while drilling data were collected during the IODP Expedition 340 (2012), the first of its kind which has drilled into deposits associated to the activity of a volcanic island. The full coverage of marine geophysical dataset allowed to characterize these deposits’s architecture, extent and volume while drilling data allowed to better constrain its composition and origin. All of these data allow us to propose a new model linking flank-collapse processes of Montagne Pelée volcano and submarine landslides processes offshore the island. This model relies on two main assumptions : 1/ debris avalanches’s emplacement is influenced by the slope break between the submarine volcano flank and the Grenada Basin and 2/ the weight of debris avalanche deposits may have triggered a seafloor sediment failure resulting in a submarine landslide propagation downstream. In order to test the veracity of the 1st assumption, numerical simulation have been carried out to better understand the emplacement mechanisms of debris avalanches, based on the 3rd and last debris avalanche deposit identified offshore. The use of two numerical models (SHALTOP accurately describing the complex terms of topography and HYSEA taking into account the water column), shows that SHALTOP better reproduce the deposit compared to HYSEA. This demonstrates the significant influence of topography into the debris avalanches’ dynamic and especially of the slope break that tends to slow down their propagation. In addition, in order to verify the 2nd assumption of our model, analogue modelling experiments have been undertaken in order to better understand this time, propagation mechanisms of submarine landslides over a slope. They demonstrate the positive influence of the slope breaks and of the material inputs over the landslide’s runout distance, while the water tends to maintain the landslide’s duration. Most of the time, material inputs allow to accelerate temporarily the landslide’s velocity. Finally, we discuss the possibility to apply the new model proposed by this study to the others volcanic islands of the Lesser Antilles arc and to others volcanic island worldwide.

Cette thèse s’intéresse aux processus d’instabilité affectant l’île volcanique de la Martinique (Petites Antilles) à partir de l’analyse combinée de données de géophysique marine et de forages profonds. Les premières ont été acquises au cours de trois campagnes océanographiques réalisées au large de l’île (AGUADOMAR – 1999, CARAVAL – 2002 et GWADASEIS – 2009) alors que les données de forage ont été collectées lors de l’Expédition IODP 340 (2012), première du genre à avoir foré dans des dépôts liés à l’activité volcanique d’une île. La couverture complète des données de géophysique marine a permis de caractériser l’architecture, l’extension et le volume de ces dépôts, alors que les données de forages ont permis de contraindre leur composition et a posteriori, leur origine. L’ensemble de ces données nous permet aujourd’hui de proposer un nouveau modèle, établissant un lien entre les processus d’effondrement du flanc du volcan de la Montagne Pelée et des processus de glissements sous-marins au large de l’île. Ce modèle repose sur deux hypothèses principales : 1/ la mise en place des avalanches de débris est influencée par la rupture de pente entre le flanc sous-marin du volcan et le bassin de Grenade et 2/ le poids généré par les dépôts d’avalanches de débris a pu déclencher une rupture des sédiments et la propagation d’un glissement sous-marin en aval. Afin de vérifier l’exactitude de la 1ère hypothèse, des simulations numériques ont été réalisées afin de comprendre les mécanismes de mise en place des avalanches de débris, et ce à partir de la modélisation du 3ème et dernier dépôt identifié en mer. L’utilisation de deux codes numériques (SHALTOP décrivant précisément les termes complexes de la topographie et HYSEA tenant compte de l’eau), montre que SHALTOP induit une meilleure reproduction du dépôt comparé à HYSEA. Ceci démontre l’influence notable de la topographie dans la dynamique des avalanches et notamment de la rupture de pente qui tend à ralentir leur propagation. Par ailleurs, dans le but de vérifier la 2nd hypothèse de notre modèle, des expériences de modélisation analogique ont été réalisées afin de comprendre cette fois-ci, les mécanismes de propagation des glissements sous-marins sur une pente. Elles démontrent l’influence positive des ruptures de pente et des apports de matériels sur la distance de propagation du glissement, alors que l’eau tend à maintenir le glissement sur la durée. Les apports de matériel permettent le plus souvent d’accélérer momentanément la vitesse du glissement. Enfin, nous discutons la possibilité d’appliquer le nouveau modèle proposé aux autres îles de l’arc des Petites Antilles et à d’autres îles volcaniques du monde.; This study focuses on instability processes occuring on the volcanic island of Martinique (Lesser Antilles) from the combined analysis of marine geophysical and deep drilling datasets. The first ones were gathered during three oceanographic cruises offshore the island (AGUADOMAR – 1999, CARAVAL – 2002 et GWADASEIS – 2009), while drilling data were collected during the IODP Expedition 340 (2012), the first of its kind which has drilled into deposits associated to the activity of a volcanic island. The full coverage of marine geophysical dataset allowed to characterize these deposits’s architecture, extent and volume while drilling data allowed to better constrain its composition and origin. All of these data allow us to propose a new model linking flank-collapse processes of Montagne Pelée volcano and submarine landslides processes offshore the island. This model relies on two main assumptions : 1/ debris avalanches’s emplacement is influenced by the slope break between the submarine volcano flank and the Grenada Basin and 2/ the weight of debris avalanche deposits may have triggered a seafloor sediment failure resulting in a submarine landslide propagation downstream. In order to test the veracity of the 1st assumption, numerical simulation have been carried out to better understand the emplacement mechanisms of debris avalanches, based on the 3rd and last debris avalanche deposit identified offshore. The use of two numerical models (SHALTOP accurately describing the complex terms of topography and HYSEA taking into account the water column), shows that SHALTOP better reproduce the deposit compared to HYSEA. This demonstrates the significant influence of topography into the debris avalanches’ dynamic and especially of the slope break that tends to slow down their propagation. In addition, in order to verify the 2nd assumption of our model, analogue modelling experiments have been undertaken in order to better understand this time, propagation mechanisms of submarine landslides over a slope. They demonstrate the positive influence of the slope breaks and of the material inputs over the landslide’s runout distance, while the water tends to maintain the landslide’s duration. Most of the time, material inputs allow to accelerate temporarily the landslide’s velocity. Finally, we discuss the possibility to apply the new model proposed by this study to the others volcanic islands of the Lesser Antilles arc and to others volcanic island worldwide.