Anne Le Friant
email: lefriant@ipgp.fr

Elodie Lebas

Morgane Brunet

Sara Lafuerza

Matthew Hornbach

Maya Coussens

Sebastien Watt

Michael Cassidy

Peter J Talling

Le Friant

Submarine landslides around volcanic islands: A review of what can be learned from the Lesser Antilles Arc

Keywords: Landslide, volcanic island, debris avalanche, seafloor sediment failure, tsunami, IODP

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Prodigious submarine landslides offshore volcanic islands have been widely recognized worldwide, such as offshore the Hawaiian Archipelago [START_REF] Lipman | The giant submarine Alika debris slide, Mauna Loa, Hawaii[END_REF][START_REF] Moore | Prodigious submarine landslides on the Hawaiian ridge[END_REF], La Réunion [START_REF] Labazuy | Recurrent landslides events on the submarine flank of Piton de la Fournaise volcano (Reunion Island), in Volcano Instability on the Earth and Other Planets[END_REF]Le Friant et al., 2011;[START_REF] Oehler | Growth and collapse of the Reunion Island volcanoes[END_REF], and in the Canary Islands [START_REF] Holcomb | Large landslides from oceanic volcanoes[END_REF][START_REF] Krastel | Submarine landslides around the Canary Islands[END_REF][START_REF] Masson | Slope failures on the flanks of the western Canary Islands[END_REF][START_REF] Masson | Submarine landslides: processes, triggers and hazard prediction[END_REF][START_REF] Urgeles | The most recent megalandslides of the Canary Islands : El Golfo debris avalanche and Canary debris flow, west El Hierro Island[END_REF][START_REF] Watts | A giant landslide on the north flank of Tenerife, Canary Islands[END_REF]. Individual landslide deposits can contain hundreds to thousands of cubic kilometers of material and can extend over a distance of hundreds of kilometers [START_REF] Moore | Prodigious submarine landslides on the Hawaiian ridge[END_REF]. Over the last decades, increasing numbers of marine geophysical surveys, along with modern marine geophysical techniques, have revealed our ability to identify such deposits around volcanic islands, recognizing flank collapses as a common and important process during the evolution of volcanic edifices [START_REF] Glicken | Study of the rockslide avalanche of May 18 1980[END_REF]Ida & Voight, 1995;[START_REF] Mcguire | Volcano instability: a review of contemporary themes, in Volcano Instability on the Earth and Other Planets[END_REF][START_REF] Voight | Structural stability of andesite volcanoes and lava domes[END_REF]. The landslide deposits, recognized both in oceanic intraplate setting or subduction zone context, are related to landslides that affected the subaerial and/or submarine parts of the volcanoes, and have similar features suggesting that common processes control volcano instability and landslide behavior across all settings. These catastrophic events represent a range of significant volcanic hazards, which also includes the related explosive eruptions (Siebert, 1984;[START_REF] Glicken | Study of the rockslide avalanche of May 18 1980[END_REF] and tsunamis (Løvholt et al., 2008), which can be triggered by relatively small events. Understanding where submarine landslide deposits are coming from and how they were emplaced offshore is crucial, since emplacement dynamics determine the magnitude of the associated tsunamis. The potential magnitude of volcanic landslide-generated tsunamis is the subject of an intense debate, mainly due to uncertainties associated with slide physics and dynamics to describe how collapsed material enters the ocean [START_REF] Cassidy | Multi-stage collapse events in the South Soufrière Hills, Montserrat as recorded in marine sediment cores[END_REF][START_REF] Hunt | Sedimentological and geochemical evidence for multistage failure of volcanic landslides: A case study from Icod landslide on north Tenerife, Canary Islands[END_REF]Løvholt et al., 2008;[START_REF] Masson | Submarine landslides: processes, triggers and hazard prediction[END_REF][START_REF] Mattioli | Unique and remarkable dilatometer measurements of pyroclastic flow-generated tsunamis[END_REF][START_REF] Slingerland | Occurrence and predictive models for slide-generated water waves[END_REF][START_REF] Ward | Cumbre Vieja volcano -potential collapse and tsunami at La Palma, Canary Islands[END_REF]Watt et al., 2012a[START_REF] Wall-Palmer | Late Pleistocene stratigraphy of IODP U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles[END_REF]. While some studies indicate that volcanic landslides often generate large (10-100 m high) tsunamis (e.g., [START_REF] Frohlich | Huge erratic boulders in Tonga deposited by a prehistoric tsunami[END_REF][START_REF] Nomanbhoy | Generation mechanism of tsunamis from the 1883 Krakatau eruption[END_REF][START_REF] Ward | Ritter Island volcano-lateral collapse and the tsunami of 1888[END_REF], other studies [START_REF] Gardner | The West Mariana Ridge, western Pacific Ocean: Geomorphology and processes from new multibeam data[END_REF][START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF][START_REF] Moore | Giant hawaiian landslides[END_REF] suggest that many of these volcanic landslides may deform incrementally, via creep, and therefore represent only a limited tsunami hazard. These mechanisms -creep versus catastrophic failure -are not mutually exclusive, as both might occur during volcanic flank collapse (e.g., [START_REF] Mcguire | Volcano instability: a review of contemporary themes, in Volcano Instability on the Earth and Other Planets[END_REF].

Unfortunately, the very few direct observations of volcanic island landslide processes are limited to small historical events (e.g., Stromboli, Tinti et al., 2006) and we are not able yet to directly monitor submarine landslide processes. Studies on landslide deposits geometry allow some authors to propose conceptual models on the emplacement dynamics of submarine landslides. Bull et al. (2009) presented a review of kinematic indicators from mass transport complexes, while [START_REF] Frey-Martinez | Frontally confined versus frontally emergent submarine landslides: a 3D seismic characterisation[END_REF] defined two categories of submarine landslides depending on the existence of an emergent frontal ramp. [START_REF] Watt | New insights into the emplacement dynamics of volcanic-island landslides[END_REF] focused on volcanicisland landslides and provided new constraints and models on the emplacement dynamics for down-slope propagation of submarine deformation, for different scenarii: (i) erosion and loading of seafloor sediment by volcanic debris avalanche (e.g., [START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF], (ii) loading of seafloor sediment by an over-running volcanic debris flow (e.g., Watt et al., 2012a,b), (iii) self-loading by down-slope propagating seafloor sediment failure (e.g., [START_REF] Watt | Widespread and progressive seafloor-sediment failure following volcanic debris avalanche emplacement: Landslide dynamics and timing offshore Montserrat, Lesser Antilles[END_REF], and (iv) décollement propagation from DAD loading (e.g., Watt et al., 2012a[START_REF] Wall-Palmer | Late Pleistocene stratigraphy of IODP U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles[END_REF]. However, the impacts of volcanic island landslides are not readily assessed from the final landslide deposits without a good knowledge and understanding of the components that make up the deposits.

Previous ODP Legs in Hawaii (ODP Legs 126 and 200: Garcia, 1993;[START_REF] Garcia | Turbidites from giant Hawaiian landslides: Results from Ocean Drilling Program Site 842[END_REF][START_REF] Garcia | Frequent landslides from Koolau Volcano: Results from ODP Hole 1223A[END_REF] and the Canary Islands (ODP Leg 157: [START_REF] Goldstrand | Provenance and sedimentologic variations of turbidite and slump deposits at sites 955 and 956[END_REF][START_REF] Schmincke | Volcanic evolution of Gran Canaria reconstructed from apron sediments: synthesis of vicap project drilling[END_REF][START_REF] Schneider | Du volcan au sediment: la dynamique du talus volcanoclastique sous-marin de Gran Canaria, Canaries (Atlantique oriental, Leg ODP 157)[END_REF] drilled offshore oceanic intraplate volcanic islands, but the drill sites mainly targeted distal turbidite deposits located away from the seismically chaotic landslide deposits recognized in more proximal settings. The first scientific drilling of volcanic-arc island landslide deposits was done by IODP Expedition 340 offshore the Lesser Antilles Arc (Figure 1). The newly-acquired drilled cores and downhole logging data were combined with seismic reflection data acquired during four previous cruises (AGUADOMAR 1999 -PI: C. Deplus;CARAVAL 2002 -PI: C. Deplus, GWADASEIS 2009 -PI: N. Feuillet, andJC45/46 -PI: P. Talling; e.g., [START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF], Lebas et al. 2011;[START_REF] Friant | Large scale flank collapse events during the activity of Montagne Pelée, Martinique, Lesser Antilles[END_REF], Watt et al., 2012 a,b) to reinterpret the composition and emplacement dynamics of the large mass-wasting deposits mapped offshore the volcanic islands of Montserrat and Martinique (Figure 1). In this contribution, we discuss the most important outputs of the IODP Expedition 340 concerning (i) the composition of the gigantic chaotic units interpreted as submarine landslide deposits, (ii) the type of deformation and the mechanisms of the landslide generation and propagation onto the seafloor, (iii) the geohazard potential of such systems, and finally (iv) the implications of our results for other volcanic islands. This contribution is mainly based on a review of earlier published material deriving both from the previous seismic and coring campaigns, and the IODP Expedition 340 carried out in 2012 [START_REF] Brunet | Caractérisation des processus d'instabilité affectant les îles volcaniques : Application au volcan de la Montagne Pelée à partir de l'exploitation des forages de l'Expédition IODP[END_REF][START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF][START_REF] Brunet | Numerical simulations of 30-45 ka debris avalanche flow of Montagne Pelee volcano, Martinique: from volcano flankcollapse to submarine emplacement[END_REF][START_REF] Cassidy | Timing and emplacement dynamics of newly recognised mass flow deposits at ~8-12 ka offshore Soufrière Hills volcano, Montserrat: how submarine stratigraphy can complement subaerial eruption histories[END_REF][START_REF] Cassidy | Multi-stage collapse events in the South Soufrière Hills, Montserrat as recorded in marine sediment cores[END_REF][START_REF] Cassidy | Rapid onset of mafic magmatism facilitated by volcanic edifice collapse[END_REF][START_REF] Coussens | The relationship between eruptive activity, flank collapse and sea-level at volcanic islands: a longterm (>1 Ma) record offshore Montserrat, Lesser Antilles[END_REF]Fraas et al., 2016;[START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF]Jutzeler et al., 2016Jutzeler et al., , 2014;;[START_REF] Lafuerza | Geomechanical characterizations of submarine volcano flank sediments[END_REF][START_REF] Friant | and the Expedition 340 Scientists[END_REF][START_REF] Brunet | Caractérisation des processus d'instabilité affectant les îles volcaniques : Application au volcan de la Montagne Pelée à partir de l'exploitation des forages de l'Expédition IODP[END_REF][START_REF] Palmer | Discovery of a large 2.4 Ma Plinian eruption of Basse-Terre, Guadeloupe, from the marine sediment record[END_REF][START_REF] Wall-Palmer | Late Pleistocene stratigraphy of IODP U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles[END_REF]Watt et al., 2012a[START_REF] Wall-Palmer | Late Pleistocene stratigraphy of IODP U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles[END_REF].

The following terminology is used in this paper: Landslide is used as a general term referring to any type of slope failure and resulting mass-movement, whether initiated onshore or offshore.

The term debris avalanche is used for a failure comprising material in a mobile flowage-type landslide from a volcanic edifice [START_REF] Glicken | Study of the rockslide avalanche of May 18 1980[END_REF][START_REF] Masson | Slope failures on the flanks of the western Canary Islands[END_REF][START_REF] Masson | Submarine landslides: processes, triggers and hazard prediction[END_REF][START_REF] Siebert | Landslides resulting from structural failures in volcanoes[END_REF][START_REF] Voight | Catastrophic rockslide avalanche of May 18[END_REF][START_REF] Voight | The 26 December (Boxing Day) 1997 sector collapse and debris avalanche at Soufrière Hills Volcano[END_REF][START_REF] Voight | Undrained Sediment Loading Key to Long-Runout Submarine Mass Movements: Evidence from the Caribbean Volcanic Arc[END_REF], and the resulting deposits are referred to as debris avalanches deposits (DAD). Debris avalanches are distinguished from slumps, which are coherent masses of poorly consolidated materials or rocks that move over a relatively short distance downslope, on a concave-up or planar slip surface (e.g., [START_REF] Delcamp | The influence of edifice slope and substrata on volcano spreading[END_REF]. Debris flows are watersaturated flows of disaggregated and/or reworked sediment with high sediment concentrations and strong interactions between solid and fluid components [START_REF] Iverson | The physics of debris flows[END_REF]. Finally, failures of seafloor sediment located beyond the slope break (i.e., transition between the base of the volcanic edifice and surrounding basin) are referred to as seafloor sediment failures (Watt et al., 2012a[START_REF] Wall-Palmer | Late Pleistocene stratigraphy of IODP U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles[END_REF], and the resulting deposits are referred to as submarine landslide deposits (SLD).

Previous Work on the Lesser Antilles Arc

The Lesser Antilles Arc results from the subduction of the Atlantic oceanic lithosphere beneath the Caribbean plate (Figure 1), at a recent convergent rate of ~ 2 cm yr -1 [START_REF] Feuillet | Arc parallel extension and localization of volcanic complexes in Guadeloupe, Lesser Antilles[END_REF]. Compared to other subduction zone volcanic chains, magma productivity in the Lesser Antilles Arc is relatively low, and estimated at 3 to 5 km 3 Ma -1 km -1 (MacDonald et al., 2000; Sigurdsson et al., 1980; Wadge, 1984). South of Dominica, the volcanic islands are bordered to the west by the 2900 m-deep, back-arc Grenada Basin (Figure 1), which has been a major depocentre for large mass-wasting deposits, volcanogenic turbiditic deposits, pyroclastic flow deposits, and hemipelagic sediment [START_REF] Boudon | Volcano flank instability in the Lesser Antilles Arc: diversity of scale, processes, and temporal recurrence[END_REF][START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF][START_REF] Picard | Contrasting sedimentary processes along a convergent margin: the Lesser Antilles arc system[END_REF][START_REF] Sigurdsson | Volcanogenic sedimentation in theLesser Antilles Arc[END_REF]. Fifty-two individual landslide deposits or flank collapse structures have been identified onshore and offshore on the flanks of the Lesser Antilles Arc volcanoes (Figures 1 and2) (e.g., [START_REF] Boudon | Flank failure-directed blast eruption at Soufrière, Guadeloupe, French West Indies: a 3,000-yr-old[END_REF][START_REF] Boudon | Magma and hydrothermally driven sector collapses: the 3100 and 11500 Y. B.P. eruptions of La Grande Découverte volcano, Guadeloupe, French West indies[END_REF][START_REF] Boudon | Volcano flank instability in the Lesser Antilles Arc: diversity of scale, processes, and temporal recurrence[END_REF][START_REF] Cassidy | Multi-stage collapse events in the South Soufrière Hills, Montserrat as recorded in marine sediment cores[END_REF][START_REF] Crutchley | Insights into the emplacement dynamics of volcanic landslides from highresolution 3D seismic data acquired offshore Montserrat, Lesser Antilles[END_REF][START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF][START_REF] Karstens | Insights into pyroclastic flow emplacement from high-resolution 3D seismic data offshore Montserrat, Lesser Antilles[END_REF][START_REF] Komorowski | Guadeloupe[END_REF][START_REF] Friant | L'île de la Dominique, à l'origine des avalanches de débris les plus volumineuses de l'arc des Petites Antilles[END_REF], 2003a,b, 2004, 2009[START_REF] Gardner | The West Mariana Ridge, western Pacific Ocean: Geomorphology and processes from new multibeam data[END_REF][START_REF] Lebas | Multiple widespread landslides during the long-term evolution of a volcanic island: insights from high-resolution seismic data, Montserrat Lesser Antilles[END_REF][START_REF] Trofimovs | What happens when pyroclastic flows enter the ocean[END_REF][START_REF] Trofimovs | Evidence for carbonate platform failure during rapid sea-level rise; ca 14 000 year old bioclastic flow deposits in the Lesser Antilles[END_REF][START_REF] Trofimovs | Timing, origin and emplacement dynamics of mass flows offshore of SE Montserrat in the last 110 ka: implications for landslide and tsunami hazards, eruption history, and volcanic island evolution[END_REF][START_REF] Vincent | The primitive volcano of Mount Pelée: its construction and partial destruction by flank collapse[END_REF]Watt et al., 2012a,b). Combined analysis of marine geophysical datasets collected along the arc has resulted in a series of hypotheses regarding the composition of the submarine landslide deposits, their origins and emplacement mechanisms. [START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF] and [START_REF] Friant | L'île de la Dominique, à l'origine des avalanches de débris les plus volumineuses de l'arc des Petites Antilles[END_REF], 2003a,b, and 2004) mapped, for the first time, numerous submarine landslide deposits along the arc and proposed that the landslide deposits correspond to DAD originating from flank collapses affecting the volcanoes onshore. The authors suggested that during the flow, the debris avalanche erodes and mobilizes pre-existing seafloor sediment, and thus disturbs underlying pre-existing sedimentary layers. They proposed that the upper part of the seismically chaotic deposits contains both volcanic debris from the original volcano flank collapse and remobilized pre-existing seafloor sediment. In contrast, the lower part would mainly be composed of deformed pre-existing seafloor sediment. Lateral transitions from well-bedded seismic reflections to chaotic facies within the landslide units in the seismic data, indicate that some landslides internally contain substantial components of seafloor-derived sediment (Figure 2). [START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF] also emphasized that a substantial amount of material deriving from the submarine part of the volcanic edifice is commonly involved in the formation of the deposits, as shown by the chutes cutting through the submarine flanks, caused by removal of volcanic debris during avalanches incising the chutes. [START_REF] Friant | Geomorphological evolution of Montserrat (West Indies): importance of flank collapse and erosional processes[END_REF] integrated this information to outline how terrestrial flank-collapse scars, submarine chutes and offshore debris avalanche deposits were linked to the geomorphological evolution of Montserrat. Moreover, Le [START_REF] Friant | Geomorphological evolution of Montserrat (West Indies): importance of flank collapse and erosional processes[END_REF] and [START_REF] Lebas | Multiple widespread landslides during the long-term evolution of a volcanic island: insights from high-resolution seismic data, Montserrat Lesser Antilles[END_REF] also used seismic data to define the extent and volume of the landslide deposits offshore Montserrat. They demonstrated that the volume of seismic chaotic units associated with landslide deposits may be much greater than the volume of the associated subaerial flank-collapse scars, thus requiring incorporation of a significant proportion of the volcano's submerged flank, or of the adjacent seafloor sediment into the deposits. Truncation of in-situ bedded sediment packages was attributed to erosional processes during debris avalanche emplacement. [START_REF] Lebas | Multiple widespread landslides during the long-term evolution of a volcanic island: insights from high-resolution seismic data, Montserrat Lesser Antilles[END_REF] also proposed that strong seismic reflectors seen at the base of some of the landslide deposits may result from slide-derived shear zones. Watt et al. (2012a[START_REF] Wall-Palmer | Late Pleistocene stratigraphy of IODP U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles[END_REF] proposed that the emplacement of volcanic debris avalanches caused erosion and triggered failure of seafloor sediment. Such seafloor sediment failure could be very widespread on low (< 1°) gradients and may dominate the total landslide volume. Their working hypothesis is consistent with the concept that rapid emplacement of volcanic edifice material onto the seafloor led to undrained shear loading and failure of underlying seafloor sedimentary layers [START_REF] Voight | Failure of volcano slopes[END_REF][START_REF] Voight | Undrained Sediment Loading Key to Long-Runout Submarine Mass Movements: Evidence from the Caribbean Volcanic Arc[END_REF]. Watt et al. (2012a,b) further remarked on the lack of frontal compression and shear break-out (e.g., Frey Martinez et al., 2006) offshore Montserrat, which suggested that much of the deformed seafloor material they observed did not move over extensive distances. The authors also proposed that some of the landslide deposits may have incorporated large blocks of relatively intact seafloor sediment.

IODP Expedition 340

The IODP Expedition 340 conducted aboard the R/V JOIDES Resolution (March-April 2012) aimed at documenting the eruptive history and edifice collapses in the Lesser Antilles Arc over the last million years (Le [START_REF] Friant | and the Expedition 340 Scientists[END_REF]. The main objective of Expedition 340 was to understand the timing, emplacement processes and dynamics of submarine landslide deposits offshore the Lesser Antilles Arc, with implications for tsunami hazards. Nine sites were drilled offshore Montserrat and Martinique during the IODP Expedition 340 (Figure 1), recovering 434 core sections containing 2,384 m of seafloor sediment samples (Table 1 in Le [START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF]. Drilling directly offshore Dominica through the most spectacular debris avalanche deposit was not done, because of the size of the blocks, and lessons from previous drilling within blocky deposits offshore Montserrat and Martinique (Le [START_REF] Friant | and the Expedition 340 Scientists[END_REF]. A wide range of physical properties (thermal conductivity, shear strength, natural gamma ray emissions, P-wave velocities, magnetic susceptibility, and density) have been directly acquired on the cores. Additional information was obtained in-situ by downhole logging operations (gamma rays, P-wave velocities, S-wave velocities, magnetic susceptibility, and resistivity; [START_REF] Lafuerza | Geomechanical characterizations of submarine volcano flank sediments[END_REF][START_REF] Friant | and the Expedition 340 Scientists[END_REF][START_REF] Manga | Heat Flow in the Lesser Antilles island arc and adjacent back arc Grenada basin[END_REF][START_REF] Wall-Palmer | Late Pleistocene stratigraphy of IODP U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles[END_REF]. The cores contain some of the most sand-rich sequences yet recovered by an IODP Expedition, where piston coring disturbances such as basal flow-in and fall-in are the most likely to occur, thus affecting the original texture of the sediment [START_REF] Jutzeler | Coring disturbances in IODP piston cores with implications for offshore record of volcanic events and the Missoula megafloods[END_REF]. Particular attention has been paid to distinguishing disturbed sediment facies from in-situ facies. To minimize these coring disturbances and provide better stratigraphic correlations between the sections, a minimum of two holes were drilled at each site (cf. Table 1 in Le [START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF].

Sites U1393, U1394, U1399, U1400 and U1401, which will be the focus of the paper, were selected to investigate the landslide deposits, their origins and associated mechanisms of emplacement; sites U1396 and U1397 to study the eruptive history and magmatic evolution of the volcanic centres; and sites U1395 and U1398 to study the differences existing in the sedimentation processes in distal environments between the northern and southern part of the arc ( Figures 1, 2, and 3;[START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF][START_REF] Wall-Palmer | Late Pleistocene stratigraphy of IODP U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles[END_REF]. As mentioned above, attempts to drill through the chaotic proximal landslide Deposit 1 offshore Montserrat (U1393) and Deposit 3 offshore Martinique (U1401) were unsuccessful due to the heterogeneous and cohesionless character of the deposits. The only material recovered at these sites consisted of a combination of hemipelagic mud with interbedded tephra layers and/or volcaniclastic turbiditic deposits, overlying (and therefore post-dating) the chaotic deposits (Le [START_REF] Friant | and the Expedition 340 Scientists[END_REF][START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF][START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF]. A more distal site offshore Montserrat (U1394), intersected a widely dispersed landslide, Deposit 2, identified as a chaotic facies in the seismic reflection data with discontinuous local bedded reflections (Figure 2b). The core showed this apparently chaotic deposit to be dominated by a stacked sequence of predominantly thick, massive, and coarsegrained turbiditic deposits, ranging from bioclastic to volcanoclastic in composition. However, it should be noted that core recovery was restricted to the lower half of Deposit 2. No signs of deformation were recognized [START_REF] Coussens | The relationship between eruptive activity, flank collapse and sea-level at volcanic islands: a longterm (>1 Ma) record offshore Montserrat, Lesser Antilles[END_REF][START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF]. Sediment retrieved from Sites U1399 and U1400 (both offshore Martinique), in the medial and distal parts of widely dispersed landslide deposits (D1 and D2), consisted of deformed seafloor sediment. This sediment was mainly composed of hemipelagic mud, with interbedded tephra layers and volcaniclastic turbiditic deposits (Figures 2 and3). Deformed sediment at Site U1400 is characterized by many planar beds inclined in average by 40°, with some dips being as steep as 70°. Interestingly and surprisingly, these widely dispersed landslide deposits D1 and D2 were not unambiguously identified in the cores retrieved in Site 1400, since no difference in composition was observed between the two landslides. Consequently, [START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF] suggested that they belong to the same deposit, and referred to it as SLD. Of particular significance is the absence of debris avalanche deposits comprising blocky material from the subaerial volcanic edifice in the cores.

More details about the lithologies encountered in the deposits both offshore Montserrat and Martinique can be found in [START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF]

New Model of Landslides Generation Offshore the Lesser Antilles Arc

Two Different Instability Processes

Two different types of landslide processes were recognized along the Lesser Antilles Arc:

(i) the subaerial instabilities, which correspond to volcanic flank collapses that affected the subaerial part of edifices, generating DAD accumulating offshore, and (ii) the submarine instabilities, which correspond to seafloor sediment failures at the origin of the large SLD identified in the seismic data.

The Deposit 1 offshore Montserrat (Site U1393) and the Deposit 3 offshore Martinique (Site U1401) (Figure 1) display very blocky surface morphologies, which were difficult to drill. They are located on the submarine flanks of the volcanic edifices or near the slope break, and typically characterized by hummocky topography in the bathymetry (e.g., [START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF][START_REF] Friant | Large scale flank collapse events during the activity of Montagne Pelée, Martinique, Lesser Antilles[END_REF], 2004[START_REF] Moore | Prodigious submarine landslides on the Hawaiian ridge[END_REF][START_REF] Urgeles | Recurrent large-scale landsliding on the west flank of La Palma, Canary Islands[END_REF]. These landslide deposits correspond to debris avalanches associated to volcanic flank collapses as recognized on land (e.g., Mount St Helens, [START_REF] Glicken | Study of the rockslide avalanche of May 18 1980[END_REF] or offshore (Canary Islands, [START_REF] Urgeles | Recurrent large-scale landsliding on the west flank of La Palma, Canary Islands[END_REF].

The seismically chaotic deposits drilled at Sites U1394 (Deposit 2, Montserrat), U1399 and U1400 (SLD, Martinique) reveal, unexpectedly, the absence of volcanic debris avalanche deposits, though the deposits are located in the prolongation of the flank-collapse structures (horseshoe shapes) observed on the volcanoes. The chaotic deposits correspond to the second type of instabilities recognized along the arc, i.e. submarine landslides. Characteristics of these deposits (i.e., composition, deformation, vertical truncation with adjacent sediments) are consistent with seafloor sediment failure deposits.

A model is proposed to explain how the subaerial volcanic flank collapses are genetically linked to the widespread sediment failures, with respect to the evolution of the volcanoes [START_REF] Watt | Widespread and progressive seafloor-sediment failure following volcanic debris avalanche emplacement: Landslide dynamics and timing offshore Montserrat, Lesser Antilles[END_REF][START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF][START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF][START_REF] Watt | Widespread and progressive seafloor-sediment failure following volcanic debris avalanche emplacement: Landslide dynamics and timing offshore Montserrat, Lesser Antilles[END_REF].

Décollement Propagation from Debris Avalanche Deposits Loading

Given the spatial relationship between the SLDs, DADs and collapse scars (subaerial and/or submarine), the new information provided by the IODP Expedition 340 allows us to test the previous hypotheses of emplacement dynamics that have been proposed for these submarine landslides (cf. Figure 9 in [START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF][START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF][START_REF] Watt | Combination of volcanic-flank and seafloor sediment failure offshore Montserrat, and their implications for tsunami generation[END_REF]a,b,2014). Results of the combined analysis of these datasets (i.e., spatial relationship between the deposits and the collapse structures, composition, type and degree of deformation of the material within the slide units, vertical truncation of the sediments on the seismic reflection profiles, physical properties providing the first insight into the in-situ stress state at the décollement and data on pore pressure) suggest that the mechanisms producing the large seafloor sediment failures observed offshore the Lesser Antilles Arc may have been triggered by subaerial and/or submarine flank-collapses. We proposed a model of décollement propagation from DADs loading to explain the existence of the large-scale seismically chaotic submarine landslides (Figure 4). Previous modelling from [START_REF] Viesca | Nucleation of slip-weakening rupture instability in landslides by localized increase of pore pressure[END_REF] suggested that local failure can generate cracks which propagate rapidly over much wider areas than the initial site of failure, similar to thrust-fault and décollement mechanics, and snow slab avalanches.

Lateral propagation can even occur where the load, in our case a debris avalanche, cannot be dissipated due to the low permeability of the seafloor sediment. This undrained load may generate excess pore fluid pressure and trigger sediment failure in slope and basin areas, which were not originally prone to failure. Our IODP studies provide the first insight into the in situ stress state at the décollement surface [START_REF] Lafuerza | Geomechanical characterizations of submarine volcano flank sediments[END_REF][START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF] and indicate both undercompacted and elevated pore fluid pressures in clay-rich layers (e.g., [START_REF] Lafuerza | Geomechanical characterizations of submarine volcano flank sediments[END_REF], as well as near-lithostatic pressures in sand-rich turbidites at the décollement [START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF]. From this, we suggest the sediment at the décollement may be near-critically stressed. As a result, one hypothesis is that any small change in pore pressure generated by compaction or minor grain reorganization (due, for example, to shaking via small earthquakes) might trigger additional failure along the décollement [START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF]. Current underconsolidated sediment at sites U1400 and U1399 would suggest that these excess pore pressures have not been fully dissipated (Figure 6). Lateral loading induced by the debris avalanche, which may have eroded the seafloor, can also promote overpressure. Basal décollement could thus propagate laterally from the emplaced volcanic debris avalanche (Figure 4) to produce widespread deformation of the seafloor sediment above the décollement surface.

This model in which the décollement propagates from proximal regions loaded and incised by the emplacement of a volcanic debris avalanche may also explain the large-scale landslide deposits mapped in the southern part of the Lesser Antilles Arc, from Dominica to St Vincent (Figure 1, [START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF]. If so, this process may operate at the scale of the entire arc.

Whether these failures occur rapidly and catastrophically over a large area, or, instead, incrementally over a smaller area is unclear, and remains a topic of ongoing lab and numerical research. Ongoing mechanical modelling with BASIN software, and consequent sediment deformation and failure with PLAXIS software (e.g., Llopart et al., 2018), have demonstrated overpressure formation by undrained loading. In addition with analogue modelling [START_REF] Brunet | Caractérisation des processus d'instabilité affectant les îles volcaniques : Application au volcan de la Montagne Pelée à partir de l'exploitation des forages de l'Expédition IODP[END_REF], these studies represent an important topic of on-going and future research which will improve the understanding of the décollement propagation in the near future.

Evolution of the Montagne Pelée Volcano Through the Landslides Processes (Martinique)

The volcanic history of Martinique Island began at 23 Ma [START_REF] Germa | The eruptive history of Morne Jacob volcano (Martinique Island, French West Indies): Geochronology, geomorphology and geochemistry of the earliest volcanism in the recent Lesser Antilles arc[END_REF][START_REF] Westercamp | Carte géologique au 1/20 000 de la Montagne Pelée[END_REF]) and has proceeded through different phases that formed several volcanic centres, including the well-known Montagne Pelée, where three major flank collapses removed much of the western flank [START_REF] Boudon | Martinique[END_REF][START_REF] Boudon | Volcano flank instability in the Lesser Antilles Arc: diversity of scale, processes, and temporal recurrence[END_REF][START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF][START_REF] Friant | Large scale flank collapse events during the activity of Montagne Pelée, Martinique, Lesser Antilles[END_REF]. The first flank collapse occurred at approximately 127 ka [START_REF] Germa | The eruptive history of Morne Jacob volcano (Martinique Island, French West Indies): Geochronology, geomorphology and geochemistry of the earliest volcanism in the recent Lesser Antilles arc[END_REF], from onland investigations) leaving behind the first horseshoe-shaped structure on the volcanic flank, with a collapsed volume estimated at 25 km³ (Le Friant et al., 2003a) (Figure 5). The associated debris avalanche entered the sea (DAD 1; Figure 5), flowed down the submarine flank and was deposited near the slope break. This first and largest DAD loaded the pre-existing seafloor sediment and initiated failure. The resulting submarine landslide propagated along a décollement surface, deforming an in situ succession of alternating hemipelagic sediment and turbiditic deposits. Distributed shearing at multiple levels within the sequence may have acted as potential sliding surfaces within the SLD (1 and 5;[START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF]. The SLD translated across the seafloor and stopped in a confined way.

The second flank collapse that affected the Montagne Pelée followed the construction of a new, voluminous volcanic edifice inside the scar of the first flank collapse, and probably occurred between 45 and 127 ka [START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF]. The collapsed volume has been estimated at about 13 km³ (Le Friant et al., 2003a) and also generated a debris avalanche (DAD2; Figure 5). DAD2 emplaced over DAD1 and may have remobilized unconsolidated surficial sediment within the central part of the SDL deposit, up to the morphological front (Figure 5), and be responsible for the southward drifting of the SLD in its southern part.

During a third event, the newly-formed cone was destroyed again by a flank collapse forming a smaller horseshoe-shaped structure, whose volume of collapsed material was estimated at 2 km³ [START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF]Le Friant et al., 200 a). The age assigned to the related DAD3 (Figure 5, which corresponds to D3 offshore Martinique in Figure 1), both identified on land and offshore, is still a matter of ongoing discussions and may be dated between 30 and 45 ka based on preliminary measurements of δ 18 O samples from the IODP Expedition 340 and previously published ages from terrestrial deposits (Table 2 in [START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF]. As for DAD2, DAD3 (clearly identified on the submarine flank by its hummocky morphology) may have locally remobilized sediments within SLD, by exerting an undrained load that supplied the deformation process.

Particularly noteworthy is that the older DAD1 and DAD2 from the conceptual model of Figure 5 were not previously identified offshore on Figure 1. The seismic data in steeper proximal areas, which represent the submarine flank of the island, are of poor quality and equivocal, although hyperbolic reflections could indicate the presence of some blocks (Figure 2a). However, the bathymetric data show a bulge at the base of the volcanic submarine flank, near the slope break, at the same location as DAD3. Thus, [START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF] proposed that this bulge may result from the accumulation of the older DAD1 and DAD2 related to the first and second flank collapses of the Montagne Pelée (Figure 5).

Implications on the Emplacement Mechanisms of the Landslides Deposits

The new model suggests that the large SLDs recognized offshore the Lesser Antilles Arc result from a décollement propagation of the marine sediment due to the emplacement of DADs on top [START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF][START_REF] Watt | Widespread and progressive seafloor-sediment failure following volcanic debris avalanche emplacement: Landslide dynamics and timing offshore Montserrat, Lesser Antilles[END_REF][START_REF] Watt | New insights into the emplacement dynamics of volcanic-island landslides[END_REF]. In the following section, we investigate the emplacement mechanisms of the two types of landslides deposits encountered in the arc (i.e., flank collapses and seafloor sediment failures), by first using a numerical approach and then by integrating geophysical properties and geotechnical constraints resulting from the IODP Expedition 340. We will use the example and the characteristics of the Martinique deposits to present our results. [START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF] and [START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF] recently proposed that flank collapses having affected the Montagne Pelée volcano produced debris avalanches that flowed down the volcanic edifice and the submarine flank, to stop around the slope break. This new scenario implies a much shorter runout distance of the debris avalanches than previously suggested [START_REF] Boudon | Volcano flank instability in the Lesser Antilles Arc: diversity of scale, processes, and temporal recurrence[END_REF][START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF]Le Friant et al. 2003a) and it has consequences for understanding debris avalanche dynamics. To confirm this scenario, numerical simulations have been performed by [START_REF] Brunet | Numerical simulations of 30-45 ka debris avalanche flow of Montagne Pelee volcano, Martinique: from volcano flankcollapse to submarine emplacement[END_REF] using two complementary, depth-averaged thin-layer continuum models, since no complete models were available in the literature at the time of the simulations. The first model used, SHALTOP [START_REF] Mangeney-Castelnau | Numerical modeling of avalanches based on Saint-Venant equations using a kinetic scheme[END_REF], describes precisely the topography. It accurately describes dry granular flows over a 3D topography and may be easily extended to describe the submarine avalanche. The second model, HYSEA [START_REF] Fernandez-Nieto | A new Savage-Hutter type model for submarine avalanches and generated tsunami[END_REF], has the advantage to take into account the interaction with the water column. It describes the subaerial and the submarine parts of the avalanche. These numerical simulations aimed to determine (i) a critical runout distance for the emplacement of the debris avalanches and (ii) the single or multiphase behavior of the associated flank collapse.

Debris avalanches

Numerical simulations have shown that the first and largest debris avalanche from the Montagne Pelée (DAD 1: 25 km³) may extend up to 50 km from the coastline, which is not an observation in the IODP data. The simulations have also demonstrated that the runout distance of the debris avalanche does not increase with successive flows of smaller volume (about 5 km³ each) and stays around 35 km from the coastline. [START_REF] Brunet | Numerical simulations of 30-45 ka debris avalanche flow of Montagne Pelee volcano, Martinique: from volcano flankcollapse to submarine emplacement[END_REF] noted that the debris avalanche slowed down after losing inertia to accumulate on the slope transition or relatively nearby, at about 35 km from the coastline. These simulations, along with IODP observations showing that the offshore DAD 1 could only be found up to 35 km from the coastline, led [START_REF] Brunet | Numerical simulations of 30-45 ka debris avalanche flow of Montagne Pelee volcano, Martinique: from volcano flankcollapse to submarine emplacement[END_REF] to suggest that the first and largest flank collapse that affected the Montagne Pelée may have occurred in several phases. Therefore, debris avalanche volumes for each phase probably did not exceed 5 km³, with smaller runout distances than previously thought [START_REF] Boudon | Volcano flank instability in the Lesser Antilles Arc: diversity of scale, processes, and temporal recurrence[END_REF][START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF]Le Friant et al. 2003a). Complex turbidite stratigraphies correlated with landslide deposits offshore Montserrat suggest that flank collapses may have occurred in multiple stages (e.g., [START_REF] Cassidy | Multi-stage collapse events in the South Soufrière Hills, Montserrat as recorded in marine sediment cores[END_REF]. Indeed, analysis of tephra fall and volcaniclastic turbiditic deposits in drill cores at Sites U1394 and U1395, revealed three heightened periods of volcanic activity on the island (930-900 ka, 810-760 ka, and 190-120 ka), which coincides with periods of increased volcano instability and mass-wasting events [START_REF] Coussens | The relationship between eruptive activity, flank collapse and sea-level at volcanic islands: a longterm (>1 Ma) record offshore Montserrat, Lesser Antilles[END_REF]. The youngest period 190-120 ka marks a peak in the volcanic activity of Soufrière Hills. The largest flank collapse having affected the volcano occurred toward the end of this period (~130 ka) and was followed by two younger events occurring during a period of time where the volcanic activity was relatively elevated [START_REF] Coussens | The relationship between eruptive activity, flank collapse and sea-level at volcanic islands: a longterm (>1 Ma) record offshore Montserrat, Lesser Antilles[END_REF]. Multiphase flank collapses have also been described in the Canarian and Hawaiian Islands, where they generated distinctive turbidites that comprise multiple fining-upward subunits [START_REF] Garcia | Turbidites from giant Hawaiian landslides: Results from Ocean Drilling Program Site 842[END_REF][START_REF] Hunt | Sedimentological and geochemical evidence for multistage failure of volcanic landslides: A case study from Icod landslide on north Tenerife, Canary Islands[END_REF][START_REF] Hunt | Multistage collapse of eight western Canary Island landslides in the last 1.5 Ma: sedimentological and geochemical evidence from subunits in submarine flow deposits[END_REF][START_REF] Wynn | Canary Islands landslides and tsunami generation: can we use turbidite deposits to interpret landslide processes?[END_REF]. These observations suggest that flank collapses may occur in multiple stages. Recently, [START_REF] Hunt | Multi-stage volcanic island flank collapses with coeval explosive caldera-forming eruptions[END_REF] related volcano flank failures to volcanic activity (i.e. explosive eruptions) in the Canary islands, and suggested that these multiple stages events might have been separated by days to weeks.

Seafloor Sediment Failures Triggered by Debris Avalanches Loading

Using core logging, sedimentological and geotechnical data from the IODP Expedition 340, [START_REF] Lafuerza | Geomechanical characterizations of submarine volcano flank sediments[END_REF] analyzed the uppermost 200 m of cores at Sites U1397, U1398, U1399, and U1400 offshore Martinique. The authors found that shear strength-overburden stress ratios (Su/s' v ) on most of the hemipelagic sediment tested, are lower than 0.17-0.25 (values on normal consolidation conditions, gray shading in Figure 6). Contrasting with consolidation tests, the authors demonstrated that low hydraulic conductivity of hemipelagic sediment prevents their consolidation and causes low rates of dewatering in turbidity current deposits and tephra layers, allowing excess pore fluid pressures to persist at depth (Figure 6). In all cores, the uppermost 200 m showed clear evidence for excess pore fluid pressures, except at Site U1400 where overpressure only occurred after 180 m downwards. [START_REF] Lafuerza | Geomechanical characterizations of submarine volcano flank sediments[END_REF] explained this overpressure by a slow escape of pore fluid during the burial of more permeable turbidity current deposits and tephra layers, sandwiched between minimally permeable hemipelagic sediment. Higher overpressures were recognized at Sites U1397, U1398, and U1399, where higher abundance of thicker turbiditic deposits and coarser tephra layers were described. Pore pressures at Site U1399 were also investigated by [START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF] using integration of physical properties, downhole logging measurements from the IODP Expedition 340, and rock physics models (Hornbach & Manga, 2014). The authors indicated that high pore pressures existed throughout much of the entire cores at the same locations and concluded that unusual, elevated pore fluid pressures occurred at present in multiple sand-rich zones within the SLD deposit, specifying that only small changes in the current stress regime may trigger motion. In their work, [START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF] also discussed the most likely plausible mechanisms responsible for creating excess pore pressures at Site U1399, and concluded that shearing processes resulting from regional strain and grain reorganization during sediments contraction, may best explain increasing in pore fluid pressures. As also pointed out by [START_REF] Lafuerza | Geomechanical characterizations of submarine volcano flank sediments[END_REF], [START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF] showed that excess pore pressures reside in thicker sand-rich intervals, while intervening mud-rich zones -within and outside the SLD deposit -show very low permeabilities, thus acting as seals. All these observations suggest that highly elevated pore pressures can persist over long time spans within the SLD deposit, long after the initial failure. This implies that deformation within SLD could continue at present or in the near future [START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF]. The work presented by [START_REF] Hornbach | Permeability and pressure measurements in Lesser Antilles submarine slides: Evidence for pressure-driven slow-slip failure[END_REF] is coherent with the observations of multiple, post-deposition deformation zones within cores U1399-A and U1400-C, suggesting the existence of a series of failure prone horizons within SLD and not solely a single layer at the base of the deposit [START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF]. These failure prone horizons may coincide with sand layers, or boundaries between sand and mud layers, which exist in many places inside these drill cores. It is also possible that homogenized muddy sand layers result from previous distributed shearing at multiple levels, and that these muddy sand layers act as pressure seals. [START_REF] Ogata | Shear-zone liquefaction in mass transport deposit emplacement: A multi-scale integration of seismic reflection and outcrop data[END_REF] highlighted the fundamental role of shearing-related liquefaction as one of the main factors controlling slide mobility though the "lubrication" of internal and basal friction forces. Thus, these layers may be regarded as multiple décollement surfaces that, when solicited, may act together to promote and enhance deposit mobility, and longer runout distances.

Implications for Tsunami Hazards

Assessing landslide-related tsunami hazards ultimately requires detailed knowledge of slide physical properties, volumes, water depths, speeds, and evolution with time (e.g., [START_REF] Masson | Submarine landslides: processes, triggers and hazard prediction[END_REF][START_REF] Ward | Cumbre Vieja volcano -potential collapse and tsunami at La Palma, Canary Islands[END_REF], Watt et al., 2012a). Unfortunately, in many instances, these basic physical properties are very poorly constrained. By providing direct observations and measurements of slide physical properties (such as stress state, thickness, and sediment type), IODP drilling data ground-truths seismic interpretations, improves our understanding of slide physical properties, and from this, enhances our understanding of landslide-derived tsunami hazards.

The results derived from the IODP Expedition 340 confirmed that the landslides identified around the Lesser Antilles volcanoes result from two fundamentally different processes: (i) volcanic flank collapses, generating blocky DADs and (ii) seafloor sediment failures, generating large and voluminous SLDs. [START_REF] Brunet | Numerical simulations of 30-45 ka debris avalanche flow of Montagne Pelee volcano, Martinique: from volcano flankcollapse to submarine emplacement[END_REF] suggested through numerical simulations that the largest flank collapse that affected the Montagne Pelée volcano may have occurred in several phases, with individual debris avalanches volumes not exceeding 5 km³.

The multistage character of the volcanic flank collapses is crucial for evaluating the resulting tsunamigenic hazard not only in Martinique, but also throughout the Lesser Antilles Arc in general. Both types of landslide may generate a tsunami, but the magnitude of the landslidederived tsunamis will be different.

Using Deposits 1 and 2 offshore Montserrat as examples, Watt et al. (2012a) approximated the tsunami magnitudes that might be expected for the deposits, taking into account different possible triggering, i.e., from a failure of: (i) the subaerial flank of the volcanic edifice, (ii) the submerged slopes of the volcano, or (iii) deep-water seafloor sediment neighboring the volcano. Watt et al. (2012a) used a series of relationships based on theory or extrapolation of small-scale laboratory experiments previously proposed in the literature and whose assumptions are discussed in detail by the authors. A key point is that tsunami magnitude strongly depends on the relative landslide and tsunami wave speeds, with tsunami magnitude increasing if the tsunami wave and the submerged landslide travel at comparable speeds. Such a case will typically only occur in shallow water depths (<200 m), implying that seafloor sediment failures occurring in deep water depths tend to produce much smaller tsunami magnitudes [START_REF] Ward | Landslide tsunami[END_REF]. Watt et al. (2012a) suggested that a seafloor sediment failure of ~2 km 3 would produce a maximum tsunami amplitude approximately 100 times smaller than one involving a subaerial or submerged failure of a volcanic flank of comparable volume (see Watt et al., 2012a more particularly Figure 7). Even a seafloor sediment failure with a much larger volume, such as the SLD deposit offshore Martinique (25 km 3 ), would likely produce a much smaller tsunami magnitude than a failure of the subaerial or submerged parts of the volcanic edifice, for a similar volume.

Moreover, the analysis in Watt et al. (2012a) of a seafloor sediment failure-related tsunami is based on wave-tank experiments with a sliding block (see [START_REF] Watts | Tsunami generation by submarine mass failure. II: Predictive equations and case studies[END_REF]. Such a model may not capture the way in which the deformation propagates within the sediment offshore Montserrat and Martinique. It may be that deformation in seafloor sediment failures is slow moving compared to a sliding block, and that individual parcels of seafloor sediment move laterally over short distances. If so, such slow velocities and short particle-transport distances could further decrease the tsunami magnitude induced by seafloor sediment failures. We have calculated the tsunami wave height versus water depth for the different submarine slide volumes (5, 13, 25 km³) and velocities offshore Martinique. We estimate wave heights using the nonlinear shallow-water wave approach (e.g., [START_REF] Jiang | The coupling of a submarine slide and the surface waves which it generates[END_REF]. Although we apply a maximum slide velocity of 50 m s -1 , turbidity current velocity measurements derived from cable breaks suggest lower values, typically ranging between 5 and 30 m s -1 [START_REF] Heezen | Turbidity currents and submarine slumps, and the 1929 Grand Banks earthquake[END_REF][START_REF] Hsu | Turbidity Currents, Submarine Landslides and the 2006 Pingtung Earthquake off SW Taiwan[END_REF][START_REF] Krause | Turbidity currents and cable breaks in the western New Britain Trench[END_REF]. Both water depth and slide velocity play an important role in tsunami wave height. The largest wave heights, for a given water depth, typically form when slide velocity approaches the natural shallow water wave velocity defined by 𝑔𝑑, where g is the gravitational acceleration and d, the water depth. Figure 7 shows that for an order-ofmagnitude change either in the slide velocity or in the water depth, an order-of-magnitude change also occurs in the tsunami wave height; with deeper and slower slides generating smaller tsunami wave heights. This implies that slower moving slides in deep water offshore the Lesser Antilles may represent a lower tsunamigenic risk. The block of seafloor sediment recovered completely intact at Site U1394 is consistent with relatively slow and limited lateral movement of the landslide. Offshore Martinique, deformation of seafloor sediment appears to occur at different levels within the deposits, which may indicate somewhat more vigorous motion.

A tsunami wave produced by a widespread seafloor sediment failure may thus produce a longer tsunami wavelength than that associated with localized failure of the volcanic edifice.

These longer tsunami wavelengths from seafloor sediment failures could increase the runout distance of the tsunami and the regional risk, since longer-wavelength tsunamis can propagate further with less energy loss (Watt et al., 2012a). However, even if the volume of a flank collapse is smaller than the volume of seafloor sediment failures, it may be the most significant in terms of causing tsunami damage on nearby coastlines due to the velocity of the slide associated with the flank collapse, the shallow water depth along the coastline, and the location of the failure near the coast, leading to less warning time.

Comparison with Submarine Landslides around Other Volcanic Islands

Flank collapses on oceanic-island volcanoes have produced some of the largest landslide deposits recognized on Earth. These giant landslides can contain several hundred to several thousands of cubic kilometers of collapsed material [START_REF] Masson | Slope failures on the flanks of the western Canary Islands[END_REF][START_REF] Moore | Prodigious submarine landslides on the Hawaiian ridge[END_REF], exceeding by far the largest known subaerial DAD of Mount Shasta (45 km 3 , [START_REF] Crandell | Gigantic debris avalanche of Pleistocene age from ancestral Mount Shasta volcano, California, and debris-avalanche hazard zonation[END_REF].

Criteria for identification of landslide deposits in the submarine area have been developed by several authors in different settings over the last decades. Submarine landslide deposits associated with debris avalanches typically display hummocky terrains on the swath bathymetry, speckled pattern on backscatter images, and a hyperbolic facies on echosounder data (e.g., [START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF][START_REF] Masson | Slope failures on the flanks of the western Canary Islands[END_REF][START_REF] Moore | Prodigious submarine landslides on the Hawaiian ridge[END_REF]. On seismic data, these landslide deposits show an acoustically chaotic character with strong energy diffraction, which strongly contrasts with the continuous, subhorizontal and subparallel reflectors that define underformed, well-stratified sedimentary successions.

The most block-rich volcanic-island landslides, such as Nuuanu in Hawaii, are dominated by multiple kilometer-scale blocks corresponding essentially to intact fragments of volcanic edifices that have slid to their site of deposition [START_REF] Moore | Mapping the Nuuanu and Wailau landslides in Hawaii[END_REF]. The prevalence of angular, intact blocks in some DADs suggests a relative damping of fragmentation in the submarine environment. In the Lesser Antilles Arc, such angular blocky deposits have been observed offshore Dominica [START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF][START_REF] Friant | L'île de la Dominique, à l'origine des avalanches de débris les plus volumineuses de l'arc des Petites Antilles[END_REF]. Watt et al. (2012a[START_REF] Watt | New insights into the emplacement dynamics of volcanic-island landslides[END_REF] suggested that in several instance seafloor sediment substrate probably fails during the emplacement of volcanic landslides (e.g., Antilles: [START_REF] Deplus | Submarine evidence for large-scale debris avalanches in the Lesser Antilles arc[END_REF]Papua New Guinea: Silver et al., 2009;Aleutians: Montanaro & Beget, 2011), and would remain almost in situ as a deformed package. Furthermore, limited lateral transport distances are implied by the geometry of these seafloor sediment failures, in which the toe of landslide transitions does not emerge above the seafloor [START_REF] Frey-Martinez | Frontally confined versus frontally emergent submarine landslides: a 3D seismic characterisation[END_REF][START_REF] Watt | New insights into the emplacement dynamics of volcanic-island landslides[END_REF]. When the seafloor sediment becomes more extensively disaggregated and no longer confined within the region of initial failure, a debris flow can propagate for hundreds of kilometers from the initial point of failure, as observed for the Canary Debris Flow induced by the El Golfo debris avalanche [START_REF] Hunt | Sedimentological and geochemical evidence for multistage failure of volcanic landslides: A case study from Icod landslide on north Tenerife, Canary Islands[END_REF][START_REF] Masson | Slope failures on the flanks of the western Canary Islands[END_REF].

Drilling through the seismically chaotic deposits offshore the Lesser Antilles volcanoes confirm that chaotic units observed in seismic data do not necessarily represent extensively disaggregated or deformed material that has been transported over long distances from the volcanic edifice (cf. [START_REF] Gafeira | 3D seismic evidence of internal structure within Tampen Slide deposits on the North Sea Fan : are chaotic deposits that chaotic[END_REF] and supports the view that extensive seafloor sediment failures may be relatively commonplace around volcanic islands (Watt et al., 2012a[START_REF] Watt | New insights into the emplacement dynamics of volcanic-island landslides[END_REF]. The IODP Expedition 340 also provided a valuable opportunity to compare seismic facies with core observations. Such comparison allowed us to understand the amount, style of deformation and sediment lithologies, that produce the different seismic facies. A key issue for such comparison is the substantial differences in resolution of the two types of data, which ranges from millimeters for the sediment cores to tens to hundreds of meters for seismic data (Figure 3). For instance, we observed a good correlation between the upper and lower limits of the SLD deposit offshore Martinique identified using seismic profiles, and the extent of the deformation within drilled holes (Figures 2 and3). The well-bedded seismic facies corresponds to turbidity current deposits and/or debris flow deposits, or tephra layers interbedded with hemipelagic sediments, which are flat lying and lack deformation in the cores.

New findings from the IODP Expedition 340 thus shed light on the necessity to revisit previous interpretations of large-scale submarine landslide deposits (SLDs) previously interpreted as resulting from catastrophic, large flank collapses of neighboring volcanic islands as these systems are significantly more complex both in space and time than previously believed. Such an approach has recently been taken by [START_REF] Lebas | Understanding the evolution of an oceanic intraplate volcano from seismic reflection data: A new model for La Réunion, Indian Ocean[END_REF] offshore the volcanic island of La Réunion (Indian Ocean). The presence of alternating undisturbed to slightly deformed seismic reflectors and chaotic reflections (Figure 8) in the seismically chaotic deposit identified offshore the Piton de la Fournaise volcano, led the authors to reinterpret this "chaotic" deposit as widespread, deformed, pre-existing seafloor sediment. This deposit recorded the emplacement of the Piton de la Fournaise DADs on the submarine flank, where hummocky terrains were previously observed on the bathymetry (Le [START_REF] Friant | A new model for the evolution of la Réunion volcanic complex from complete geophysical surveys[END_REF][START_REF] Lénat | The offshore continuation of an active basaltic volcano: Piton de la Fournaise (Réunion Island, Indian Ocean); structural and geomorphological interpretation from sea beam mapping[END_REF][START_REF] Oehler | Recurrence of major flank landslides during the last 2-Ma-history of Reunion Island[END_REF][START_REF] Oehler | Growth and collapse of the Reunion Island volcanoes[END_REF]. [START_REF] Lebas | Understanding the evolution of an oceanic intraplate volcano from seismic reflection data: A new model for La Réunion, Indian Ocean[END_REF] concluded that such large seismically "chaotic" deposits may be regarded as evidence of former flank collapse events offshore other volcanic islands that might not be identifiable on the volcanic submarine flanks (due to the age or the resolution of the data), but that clearly deformed marine sediment offshore.

Figures Fig 1:

The Lesser Antilles Arc from Montserrat to St Vincent. Extent of the main landslide deposits (chaotic units) on swath bathymetry illuminated from North (AGUADOMAR cruise, 1999 andCARAVAL cruise, 2002). Active volcanoes and drilling sites are annotated. Solid lines represent the seismic profiles shown in Figure 2. Modified from [START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF]. to undrained shear strength and σ' v to effective vertical stress. Gray shading corresponds to S u /σ' v < 0.17-0.25 associated with normally consolidated materials. Black and green colors are used to differentiate S u measurements at holes B (black) and C (green) for each drilling site, except for site U1397, where hole A is in black and hole C in green. Circles: S u -handheld penetrometer; crosses: S u -automated shear vane. From [START_REF] Lafuerza | Geomechanical characterizations of submarine volcano flank sediments[END_REF]. We extend the model to a maximum water depth of 3000 m below sea level (mbsl), as this corresponds to the maximum water depth where we observe slide deposits in the Lesser Antilles Arc. Model results show a peak wave height two minutes after slide initiation, for three different average slide velocities (red: 50 m s -1 , black: 10 m s -1 , blue: 1 m s -1 ). All the calculations assume a constant parabolic slide shape (e.g., [START_REF] Jiang | The coupling of a submarine slide and the surface waves which it generates[END_REF]. Note the long runout of this large SLD deposit, which extends beyond the base of the volcanic complex. Modified from Le [START_REF] Friant | A new model for the evolution of la Réunion volcanic complex from complete geophysical surveys[END_REF] and [START_REF] Lebas | Understanding the evolution of an oceanic intraplate volcano from seismic reflection data: A new model for La Réunion, Indian Ocean[END_REF].

  ,[START_REF] Coussens | The relationship between eruptive activity, flank collapse and sea-level at volcanic islands: a longterm (>1 Ma) record offshore Montserrat, Lesser Antilles[END_REF]), Fraas et al. (2016),[START_REF] Jutzeler | Coring disturbances in IODP piston cores with implications for offshore record of volcanic events and the Missoula megafloods[END_REF][START_REF] Jutzeler | Submarine deposits from pumiceous pyroclastic density currents traveling over water : An outstanding example from offshore Montserrat (IODP 340)[END_REF],[START_REF] Lafuerza | Geomechanical characterizations of submarine volcano flank sediments[END_REF],[START_REF] Friant | Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF],[START_REF] Palmer | Discovery of a large 2.4 Ma Plinian eruption of Basse-Terre, Guadeloupe, from the marine sediment record[END_REF][START_REF] Wall-Palmer | Late Pleistocene stratigraphy of IODP U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles[END_REF].

Fig 2 :

 2 Fig 2: Seismic reflection profiles revealing the presence of chaotic deposits offshore (a) Martinique and (b) Montserrat. Location of the profiles is shown on Figure 1. Deposits are characterized by chaotic reflectors, which contrast with the well-bedded, subhorizontal sedimentary units. Locations of drilling sites are annotated.

Fig 3 :

 3 Fig 3: Correlation between seismic and drilling data for (a) Hole U1399-A and (b) Hole U1400-C. Deformation on both data sets are well correlated despite their large-resolution differences.Adapted from[START_REF] Brunet | Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure?[END_REF].

Fig 4 :

 4 Fig 4: Schematic figure of processes which can explain the presence of huge landslide deposits offshore volcanic islands (topographic/bathymetric profile from Martinique). Volcanic flank collapses generate debris avalanches that enter the sea, incise the sediments and generate failure, propagation, and deformation of marine sediments. From Le Friant et al. (2015).

Fig 5 :

 5 Fig 5: Conceptual model of the relation between aerial instability processes (i.e., volcanic flank collapses) and submarine instability processes (i.e., seafloor sediment failures) offshore Martinique. The first debris avalanche deposit (DAD1) weakened and initiated seafloor sediment failure, that resulted in a submarine landslide propagation. Successive DADs (DAD2 and DAD3) remobilized unconsolidated surficial sediments within the proximal part of the submarine landslide deposit (SLD). From Brunet et al. (2016).

Fig 6 :

 6 Fig 6: Depth distribution of shear strength-overburden stress ratio S u /σ' v where S u corresponds

Fig. 7 :

 7 Fig. 7: Tsunami wave height versus slide velocity and water depth for three different slide volumes discussed in the text (solid lines: 25 km 3 , dashed lines: 13 km 3 , dotted lines: 5 km 3 ).

Fig. 8 :

 8 Fig. 8: (a) Combined bathymetry (FOREVER and ERODER1 cruises) and topography data of La Réunion volcanic complex illuminated from NW. Dashed thick circle indicates the base of the volcanic complex, with interpretations of mass-wasting processes. The extent of rough and chaotic terrains is mapped in green (Piton des Neiges) and in red (Piton de la Fournaise). Arrows indicates the direction of the instabilities. Secondary front instabilities are mapped in light colors. (b) Seismic profile FOR 63 illustrating the seismic signature of the material composing the large submarine landslide deposit (SLD) identified offshore Piton de la Fournaise volcano.

Acknowledgments

We thank the drillers for their relentless efforts to drill through such difficult volcaniclastic material and all R/V JOIDES Resolution staff for their collaboration and efficient work on board. We are very grateful to C. Mével who encouraged and helped us to build our drilling proposal. We thank INSU, NERC, ANR-13-BS06-0009, Labex UnivEarth, Interreg Caraibes PREST, JSPS, NSF for funding analyses and IPGP for funding post-docs and PhD.

We thank the staff of the Montserrat Volcano Observatory, the Martinique and Guadeloupe Volcanological and Seismological Observatory, for assistance during the cruise and outreach education activities. Data from the IODP Expedition 340 can be obtained through the expedition's implementing organization USIO (http://iodp.tamu.edu/database). IPGP contribution 3960.