New high-resolution multichannel seismic data (GWADASEIS-2009 and JC45/46-2010 cruises; 72 and 60 channels, respectively) combined with previous data (AGUADOMAR-1999 and CARAVAL-2002; 6 and 24 channels, respectively) allow a detailed investigation of mass-wasting processes around the volcanic island of Montserrat in the Lesser Antilles. Seven submarine deposits have sources on the flanks of Montserrat, while three are related to the nearby Kahouanne submarine volcanoes. The most voluminous deposit (∼20 km3) within the Bouillante-Montserrat half-graben has not been described previously and is probably related to a flank instability of the Centre Hills Volcano on Montserrat, while other events are related to the younger South Soufrière Hills-Soufrière Hills volcanic complex. All deposits are located to the south or southeast of the island in an area delimited by faults of the Bouillante-Montserrat half-graben. They cover a large part of the southeast quarter of the surrounding seafloor (∼520 km2), with a total volume of ∼40 km3. Our observations suggest that the Bouillante-Montserrat half-graben exerts a control on the extent and propagation of the most voluminous deposits. We propose an interpretation for mass-wasting processes around Montserrat similar to what has happened for the southern islands of the Lesser Antilles.
The island of Montserrat, located in the north of the Lesser Antilles Arc (Figure 1), has undergone multiple lava dome collapse events during the ongoing eruption (1995 to present). Previous work shows that the main part of material produced by the present eruption has been deposited underwater [Hart et al., 2004; Le Friant et al., 2004, 2009, 2010; Trofimovs et al., 2006, 2008; Wadge et al., 2010]. Older and much larger submarine debris avalanche deposits there have been partially described by Le Friant et al. , but the limited resolution and coverage of the data available then did not allow a complete view of the seafloor and subseafloor morphology around Montserrat. New high-resolution seismic data have been collected recently (GWADASEIS cruise 2009, 72 channels; JC45/46 cruise 2010, 60 channels) to investigate in detail the surrounding environment of Montserrat and to understand mass-wasting processes around the island. These data complement other data previously acquired (AGUADOMAR cruise 1999, 6 channels; CARAVAL cruise 2002, 24 channels), and the combined Montserrat data set is now the most complete of any Lesser Antilles volcano. The combined data are important for understanding tectonic and sedimentary processes around a volcanic island in an oceanic context. These data may be used to address such questions as: How does the formation of a volcanic island influence local sedimentation processes? What is the range of mass-wasting processes around the island? Can we quantify the proportion and distribution of volcanic material that enters the sea? What is the link between tectonic structures and volcanic instabilities and dispersal of mass movement products?
In this paper, we study the mass-wasting processes around Montserrat (Lesser Antilles) using the complete marine geophysical data set collected during the four oceanographic cruises mentioned above. We first redefine the lateral extents of previously identified deposits [e.g., Le Friant et al., 2004] and then highlight major new deposits discovered within the Bouillante-Montserrat half-graben and near the Kahouanne Volcanoes. We then discuss the characteristic features, the volumes, the origins and the distribution of submarine deposits identified around the island. Last, we propose an interpretation for mass-wasting processes around Montserrat by considering the influence of the Bouillante-Montserrat half-graben and by comparing Montserrat with the southern islands of the Lesser Antilles Arc.
2. Data Collection and Analysis
This paper is based on marine geophysical data collected during four oceanographic cruises, as mentioned previously (Figure 2). Swath bathymetry and backscatter data were collected using the Simrad EM12D system during the AGUADOMAR (1999) and CARAVAL (2002) cruises, the Simrad EM300 system during the GWADASEIS (2009) cruise, and the Simrad EM120 system during the JC45/46 (2010) cruise. Swath data presented here were processed using the CARAIBES software developed by Ifremer. Digital terrain models were constructed with resolutions of 100 m, 50 m or 25 m according to the depth of the seafloor surrounding the island. In addition, we collected 3.5 kHz echo sounder profiles during the AGUADOMAR and CARAVAL cruises and chirp subbottom profiles during the GWADASEIS and JC45/46 cruises.
3. Geological Setting and Regional-Scale Bathymetry
3.1. The Lesser Antilles Arc
Montserrat belongs to the Lesser Antilles Arc, which results from subduction of the Atlantic oceanic lithosphere under the Caribbean Plate at a rate of ∼2 cm/yr [Wadge, 1984; DeMets et al., 2000] (Figure 1). Arc volcanism has occurred since 40 Myr [Martin-Kaye, 1969; Bouysse et al., 1990]. The northern part of the arc is divided into two chains of islands: the outer (eastern) chain corresponds to an older, inactive arc where thick carbonate platforms cover a volcanic basement, whereas the inner (western) arc has been active since 20 Myr and contains all of the presently active volcanoes [Bouysse et al., 1990]. Montserrat belongs to the northern part of the inner arc. To the south, the arc is composed of only one chain of islands on which the older and recent parts are superimposed. These southern islands are bordered to the west by the back-arc Grenada Basin, which is up to 2.9 km deep.
The subaerial volcanic activity on Montserrat covers a period from 2.6 Myr to present, and the active volcanic centers have generally migrated from north to south [Harford et al., 2002] (Figure 1). The three main volcanic centers are Silver Hills (∼2.6 Myr to 1.2 Myr), Centre Hills (at least ∼950 to ∼550 kyr) and South Soufrière Hills (∼130 kyr)–Soufrière Hills (at least ∼170 kyr to present) [Harford et al., 2002].
The island is bordered by a shallow submarine shelf of variable width (maximum 5 km around Silver Hills) that extends to a maximum water depth of ∼100 m. This shelf was formed from erosion during repeated glacioeustatic sea level fluctuations [Le Friant et al., 2004].
The steeper submarine flanks of the island extend down to water depths of 750–1000 m. Numerous canyons form tributaries to a large undersea valley west of the island (Figure 3), while deep canyons do not occur on the other submarine flanks of Montserrat. The area north of Montserrat beyond the latitude of the Silver Hills displays a particularly contrasting flat-lying seafloor. Three areas of rough, blocky topography on the southeastern and southern submarine flanks of the island, reported by Deplus et al. , correspond to the debris avalanche deposits 1, 3 and 5 described by Le Friant et al. . Similar blocky debris avalanche deposits have been observed on the submarine flanks of neighboring islands Redonda and Antigua [Le Friant et al., 2004] (Figure 1). Submarine flow deposits associated with recent (1996–present) dome collapses of Soufrière Hills Volcano occur closer to the island, especially offshore from the Tar River Valley. These deposits were formed by pyroclastic flows that traveled down either the Tar River or White River valleys before entering the sea. Descriptions of these events and their deposits are provided by Cole et al. , Sparks et al. , Hart et al. , Herd et al. , Trofimovs et al. [2006, 2008], and Le Friant et al. [2009, 2010].
Three major fault systems have been identified by Feuillet et al.  and new observations have been added by Kenedi et al.  during the SEA-CALIPSO cruise: the Montserrat-Havers Fault System (MHFS) and Redonda Fault System (RFS) northwest of Montserrat, and the Bouillante-Montserrat Fault System (BMFS) southeast of Montserrat. The MHFS continues across the island, and is interpreted to extend offshore to the southeast (Figure 3). The BMFS dominantly dips northeast and is composed of 10 to 20 km long normal fault segments, with a strike of ∼130 deg (±20 deg), arranged in a right-stepping en echelon pattern that extends to Basse-Terre on Guadeloupe. The BMFS delimits the western side of the Kahouanne Valley (10 to 17 km wide, and ∼50 km in length), which is a typical half-graben (e.g., Bouillante-Montserrat half-graben; Figure 3). Three submarine volcanic edifices have been identified within or at the edges of the Kahouanne Valley: the Kahouanne, Spring and La Perle volcanoes. The Kahouanne Volcanoes (seamounts A and B; Figure 3) are located roughly east-southeast of Montserrat (∼22 km from the coastline for the seamount B). They are crosscut and left-laterally displaced by ∼4 km from each other by the large-scale transtensional Bouillante-Montserrat Fault System [Feuillet et al., 2010].
3.3. New Observations From 2009 High-Resolution Bathymetry
New high-resolution bathymetric data collected in 2009 allow us to describe the morphological characteristics of the Kahouanne Volcanoes in more detail (Figure 3). Seamount A has an elongated shape with dimensions of ∼5 by ∼3 km, and rises to a height of at least 665 m above the graben floor. Its flat summit displays several scarps. Seamount B has dimensions of ∼5.0 km by 5.3 km, and a height of at least 823 m above the graben floor. A northwest-southeast orientated depression (1.5 by 1.6 km) occurs on seamount B, which also has a fault crossing its flat summit. A smaller volcanic cone southeast of seamount B displays a crater on its summit with a diameter of ∼550 m. The flattened summits of both seamounts A and B would suggest relatively old ages for the volcanic activity. The Spring Volcano (∼1.5 by ∼1.2 km) is located on the west side of the Kahouanne Valley above the BMFS. A volcanic cone with a ∼250 m wide summit crater occurs ∼1 km northwest of the Spring Volcano and is probably associated to it. La Perle Volcano is located ∼19 km southeast of the Spring Volcano, and comprises a km diameter cone that is ∼800 m in height. A horseshoe-shaped structure open to the south (∼855 m width) is observed on its southern flank, in which a subsequent cone has grown. The relatively smooth form of La Perle Volcano, without extensive fault or erosional scarps and the absence of sedimentary layers on 3.5 kHz data, suggest a relatively young age.
The new high-resolution bathymetry survey also shows a previously unknown but well developed field of sediment waves to the southeast of the Kahouanne Volcanoes (Figure 3). The sediment waves are located downslope from an ENE-WSW elongated depression (1.7 by 3.4 km) within the Kahouanne Valley, and the sediment wavefield covers an area of ∼50 km2. The waves are roughly orientated NE-SW and comprise two sets. The first set of waves is located close to the depression, and has larger wavelengths and wave heights (∼340 m; up to 21 m, respectively). The second wavefield has smaller wavelengths of ∼115 m (∼1/3 of the first set) and heights up to ∼8 m. The maximum length of wave crests reaches ∼8 km and is observed within the first set.
The high-resolution seismic reflection data collected during 2009 and 2010 provide new information on the number, extent, shape and internal character of submarine landslide deposits around Montserrat.
The extent of deposits is deduced from a combined analysis of swath bathymetry, 3.5 kHz profiles and seismic reflection data. On seismic profiles, debris avalanche deposits display typical chaotic reflectors which differ from well-bedded sedimentary units (Figure 4). The high vertical resolution of the new data (GWADASEIS, JC45/46; Table 1) has allowed us to clearly distinguish superimposed landslide deposits at depth by highlighting well-bedded sedimentary units lying between landslide units (Figures 4b, 4c, 5a, 5b, 5c, and 6). The high resolution has also permitted us to accurately locate the upper and lower boundaries of each debris avalanche deposit, and to therefore estimate deposit thickness. Different subunits inside the deposits have also been recognized by the presence of irregular and discontinuous, strong reflectors (Figures 5b and 6).
Some of the deposits recognized in this study were described previously by Le Friant et al. . We have retained the deposit numbering system of this earlier study, extended with new numbers for the newly identified deposits (Figure 7). Some local small units visible only on one seismic profile have been referenced as slide units (slide units a and b; Figures 4c, 6 (line gwa44–40), and 7) since they could not be defined as extended deposits.
4.1. The Eastern Submarine Flank of Montserrat and the Bouillante-Montserrat Half-Graben
The new data enable us to improve the lateral extent and volume estimations for deposits 1 and 2 of Le Friant et al. , and allow even larger and more deeply buried landslide deposits to be identified.
4.1.1. Deposit 1
Deposit 1 is located offshore from the Tar River Valley, and its base is clearly defined by an underlying well-bedded sedimentary unit (Figures 4b and 5b). Deposit 1 typically has a hummocky facies seen in both bathymetric and seismic data (Figures 3 and 4b). The deposit spreads radially, with a width of up to 8.7 km (comparable to its downslope length of ∼9 km) to cover an area of 51 km2 that reaches ∼12 km east-southeast from the coastline. It has a thickness of ∼50 m and a volume of 1.8 km3, much larger than estimated previously by Le Friant et al. , and extends further to the west and south with respect to its earlier mapping (Figure 7).
4.1.2. Deposit 2
Deposit 2 is located below deposit 1 and was emplaced over well-bedded sediment layers (Figures 4b, 5a, 5b, and 6). It is separated from deposit 1 by sedimentary units that are ∼35 m thick in average (Figures 4b and 5b). Deposit 2 is much larger than deposit 1 and extends for ∼30 km from the coastline in a southeast direction, covering an area of 190 km2 (Figure 7). Deposit 2 displays important thickness variations, not previously identified, along each seismic profile. Thickness varies from ∼100 m, and as much as ∼135 m in local accumulation zones, to ∼50 m on line gwa38 (Figure 6); the average is ∼62 m. The deposit volume is estimated as ∼9 km3. Strong, continuous flat reflectors are identified at the base of the chaotic unit that makes up deposit 2 (Figures 4b, 5a, 5b, and 6). Continuous reflectors are also sometimes clearly seen within deposit 2, such as on the line gwa38 (Figure 6). Below the strong basal reflectors there is a thin disturbed unit (∼17 m thick in average up to 35 m on line gwa35-a; Figure 6, inset 2). The deposit is cut by faults of the MHFS close to the island and BMFS further offshore (Figures 3 and 6). The maximum offset observed, induced by faulting within deposit 2, occurs along the BMFS and is ∼34 m (Figure 6, line gwa35-a).
4.1.3. Deposit 8
A newly identified mass flow deposit (deposit 8) represents the largest and deepest large-scale chaotic unit imaged within the Bouillante-Montserrat half-graben. A smaller chaotic unit (deposit 11) has been identified below it (Figure 6, line gwa33). Deposit 8 is located underneath deposits 1, 2 and 3 and extends in a southeastern direction (Figures 5a, 5b, 5c, 6, and 7). The deposit comprises two chaotic lobes east and west of the MHFS (Figure 6, line gwa44–40). The two lobes occur at the same stratigraphic level once this fault offset is taken into account, and the two chaotic lobes were once continuous across the graben (Figure 7). Consequently, the two lobes constitute a single debris avalanche deposit, which is the largest yet identified off the island of Montserrat. Deposit 8 ran out for ∼31 km from the coastline and covers an area of ∼277 km2 (Figure 7). The thickness of deposit 8 along the seismic profiles varies, e.g., from ∼85 to ∼115 m on line gwa38 (Figure 6), with an average of ∼94 m. Its volume is estimated to be ∼20 km3. Deposit 8 is separated from the overlying deposit 2 by ∼100 m thick units of continuous reflectors in average. The base of deposit 8 is defined by strong reflectors, as was the case for deposit 2 (Figures 5a, 5b, 5c, and 6). On seismic line gwa38, strong continuous reflectors also appear within the deposit (Figure 6). These internal reflectors display an undulating shape near the island (Figure 5c), whereas further offshore a flatter geometry is observed. Below deposit 8, a local chaotic unit (slide unit a) has been identified only on the seismic profile gwa44–40 (Figure 6). This slide unit a is separated from deposit 8 by ∼200 m of sedimentary units and covers ∼4 km on the profile. The mean thickness of this slide unit is ∼67 m. The three fault systems east and southeast of Montserrat (BMFS, MHFS, BMAF) crosscut deposit 8. The maximum offset of deposit 8 is induced by a major fault in the Bouillante-Montserrat system (BMFS; southwest side) and this offset is estimated to be ∼285 m (Figure 6, line gwa35-a).
4.1.4. Deposit 9
Deposit 9 is located east of Montserrat offshore from the Centre Hills Volcano. It is covered by a thick (∼210 m) sedimentary unit (Figure 5a). The deposit extends up to 14.7 km from the coastline in an eastward direction and covers an area of ∼23 km2 (Figure 7). The maximum dimensions are 6.8 km (W-E) by 4 km (N-S). The average thickness of this chaotic unit is ∼42 m, resulting in an estimated volume of ∼0.9 km3.
4.2. The Southern and Southwestern Submarine Flanks of Montserrat
4.2.1. Deposits 3, 4, and 5
The new seismic data further constrain the extents and volumes of deposits 3, 4 and 5 that were previously documented by Le Friant et al. . Deposit 5 is located on the southwestern submarine flank of Montserrat and has a hummocky morphology (Figure 3). It extends 9 km from the coastline toward the southwest, until it abuts a small submarine hill (Figure 7). The deposit covers an area of ∼27 km2, with a maximum width of ∼3 km in the distal part. The deposit volume is approximated at 0.3 km3, taking a mean thickness of ∼10 m (based on the height of blocks estimated by Le Friant et al. ).
Deposit 3 is located offshore from the White River Valley, south of Montserrat, and also displays a hummocky morphology (Figure 3). It extends for 11.7 km from the coastline in a southerly direction and covers an area of 59 km2 (Figure 7). Its shape and hummocky character are comparable to that of deposit 1. The average thickness of deposit 3 is approximately 29 m and it has a volume of ∼1.3 km3. No resolvable sedimentary drape is observed above deposit 3 on the 3.5 kHz data [Le Friant et al., 2004].
Deposit 4 is located below deposit 3 (Figure 7). The two deposits are separated by sedimentary units that are ∼30–40 m thick. Deposit 3 reaches 9 km from the coastline in a south-southwest direction and extends for 14 km in a south direction, covering an area of ∼69 km2. The mean thickness of deposit 4 decreases offshore from ∼85 m to 41 m (Figure 4c), and it has a volume of ∼3.4 km3. No strong coherent internal reflectors are observed within this chaotic unit but important thickness variations suggest the occurrence of megablocks within deposit 4. Above deposit 4, a local thin slide unit b is identified (Figure 4c). It is separated from deposit 4 by sedimentary units that are ∼110 m thick in average. It extends to 8 km from the coastline and has a minimum width of ∼1.6 km in its distal part. The thickness of the slide unit b is ∼10 m and approximately 20 m of sediments cover it.
4.2.2. Additional Deposits Around Offshore Seamounts
Deposits have been identified around the Kahouanne Volcanoes. Deposit 10, located east of seamount B, extends for ∼5 km in a north-northeast direction with a width of 13 km, and covers an area of ∼61 km2 (Figures 5d and 7). Its thickness ranges between ∼135 m (near the seamount) to 85 m, before thinning northeastward, giving an average thickness of ∼111 m and a volume of 4.3 km3. Well-bedded sedimentary layers from ∼135 m to ∼185 m thick cover the deposit 10 (up to 220 m above the deposit end; Figure 5d).
South of seamount B a small chaotic unit (deposit 11; Figures 6 (line gwa33) and 7) is identified under deposit 8. A mean thickness of ∼215 m of sedimentary units separates the deposits. Deposit 11 covers an area of ∼10 km2 and has a volume of ∼0.4 km3. West of seamount A, deposit 12 (Figures 6 (line gwa38) and 7) has dimensions of ∼4.6 km by ∼1.2 km (∼5.5 km2), an average thickness of ∼40 m, and a volume of ∼0.2 km3.
5.1. Morphological Characteristics of the Submarine Landslide Deposits
5.1.1. Deposit Morphologies
Ten large landslide deposits have now been identified offshore from Montserrat (deposit 1 to 12 excluding deposits 6 and 7 related to Redonda and Antigua islands; e.g., Le Friant et al. ). The deposits can be divided into two types. The first type (deposits 1, 3 and 5) displays hummocky topography on the seafloor on swath bathymetry and seismic reflection profiles. The second type of deposit comprises shallowly buried (deposits 2, 4 and 12) or more deeply buried (deposits 8, 9, 10 and 11) units that display chaotic seismic signals beneath well-bedded reflectors. Hummock characteristics deduced from surficial deposits (Figure 3) show a range of forms, with symmetric or asymmetric, sharp, rounded and flattened shapes. As is the case with subaerial deposits [Voight et al., 2002], hummocky morphology leads to irregularities in the thickness of deposits along seismic profiles (Figures 4c and 6). These irregularities decrease away from the island as the number and the size of blocks decrease.
5.1.2. Lateral Margins of the Deposits
On some profiles, sharp transitions between sedimentary and landslide-related units have been observed (Figure 6, line gwa44–40). Similar features were recognized by Deplus et al.  for landslide deposits within the Grenada Basin in the southern part of the Lesser Antilles Arc but they also exist in other geological settings such as the big sedimentary landslides on the Norwegian continental margin, e.g., Storegga Slide [Bull et al., 2009]. Deplus et al.  and Le Friant et al. [2003a] interpreted these sharp margins as the result of erosional processes occurring during debris avalanche emplacement. On most profiles, a predominance of progressive transitions between sedimentary units and deposits have been underlined (e.g., decrease of ∼30 m in 2.5 km for the thickness of deposit 2 on line gwa35-a; Figures 4c, 5b, 5d, and 6). The thinner deposit at the front of the chaotic unit probably results from the spreading of the debris avalanches [Le Friant et al., 2003a].
5.1.3. Strong Reflectors
The new high-resolution seismic data highlight the presence of strong reflectors hitherto unidentified within or at the base of several chaotic units (Figures 4b, 5a, 5b, 5c, and 6). Strong reflectors (generally flat and continuous) are clearly identified at the base of deposits 2 and 8. They represent a strong contrast in seismic impedance, likely the result of a lithological change between disturbed sediments immediately beneath the deposit and the chaotic unit above. We propose that these strong reflectors represent the basal shear surface of debris avalanches, as suggested by Bull et al.  for the Storegga Slide deposit (Norwegian continental margin). Strong reflectors located within the central parts of slide deposits most likely separate different chaotic subunits, since they are generally discontinuous and irregular (Figure 6, line gwa38).
It is of interest that comparable basal strong reflectors are seen within both deposits 2 and 8, and that the extent of these deposits is also similar. This suggests that similar overall flow, channelization and emplacement processes influenced both deposits, and that such events may repeat through time with a similar pattern. This emplacement process pattern is potentially controlled by submarine bathymetry, as strongly influenced by local tectonics, such as in this case by the Bouillante-Montserrat half-graben fault system (see section 5.4).
5.2. Chronology of the Deposits
Here, we briefly describe the successive emplacement of the deposits identified offshore Montserrat and around the Kahouanne Volcanoes. We propose a relative chronology based on the deposit depth and on overlying sediment thicknesses. The relationships with major fault systems (such as the BMFS, MHFS, BMAF) are also discussed.
Sedimentation rates from previous studies of the Lesser Antilles volcanic arc have been used to estimate the age of landslide deposits around Montserrat. Le Friant et al.  proposed a sedimentation rate about 2.3 cm kyr−1 deduced from micropaleontology and stable isotope stratigraphy (δ18O) studies on the CAR-MON 2 sediment core, which covers a period of ∼250 kyr (length: 575 cm). However, this core was located ∼55 km southwest of Montserrat (Figure 1). The rate they derived was thus unlikely to be representative of sedimentation occurring south of the island and within the Bouillante-Montserrat half-graben, given the volcanic activity and thick accumulations of pyroclastic deposits, turbidites and detrital sediments in this area. A maximum sedimentation rate of 29 cm kyr−1 has been estimated from sediment core analysis in the southern part of the Lesser Antilles Arc (within the Grenada Basin [Boudon et al., 2008]). The smaller scale of volcanic centers on Montserrat suggests that the maximum sedimentation rate off Montserrat is likely to be lower than that in the Grenada Basin. We therefore propose to use a nominal intermediate sedimentation rate of 15 cm kyr−1 to characterize the offshore area south of Montserrat. Although we acknowledge that there are significant uncertainties involved in applying a single sedimentation rate across this area, this exercise provides an initial indication of the relative ages of different landslide deposits offshore Montserrat.
The hummocky deposits 1, 3 and 5 were emplaced last and relative recently, given the absence of drape over the hyperbolic facies on the 3.5 kHz profiles that represent the deposit. An age of ∼2 kyr was proposed for deposit 1 by Boudon et al.  on the basis of 14C dates obtained on noncarbonized wood samples in an island deposit provisionally correlated with this unit. Deposits 3 and 5 are probably very recent but their age cannot be constrained due to the lack of sedimentary drape. Deposit 2 is separated from deposit 1 by ∼35 m of sediments, suggesting an age of emplacement at ∼233 kyr. Regarding to the thickness of the sedimentary units separating the deposit 2 to the deposit 8 (∼100 m in average), deposit 2 appears to be ∼665 kyr younger than deposit 8, giving an approximated age of 900 kyr for deposit 8. Deposit 11 is likely to be the oldest deposit identified within the Bouillante-Montserrat half-graben because of the thickness of overlying sediments. It is separated from deposit 8 by sedimentary units with a mean thickness of ∼215 m, suggesting that a time period of ∼1.5 Myr occurred between these two events.
5.3. Origin of the Landslide Deposits
The landslide deposits identified offshore Montserrat are either related to subaerial flank collapses or failures of the surrounding submarine slopes (volcanic and/or sedimentary material).
5.3.1. Submarine Surficial Deposits (1, 3, and 5)
Deposit 1 is directly associated with the horseshoe-shaped English's Crater, as shown by the continuity between the rims of the crater and the submarine embayment C1 (as documented by Le Friant et al. ) (Figure 8a). The new seismic data (GWADASEIS, JC45/46) result in an increased volume estimate for deposit 1 of 1.8 km3. The dense-rock equivalent volume is about 1.35 km3 which suggests that the English's Crater collapse event alone (0.5 km3 [Le Friant et al., 2004]) cannot account for deposit 1. The additional volume observed in deposit 1 could result from either (1) successive flank collapses (that may include a debris avalanche deposit identified in the coastal cliff and dated at 20 kyr [Boudon et al., 2007]), rather than the previously suggested single event [Le Friant et al., 2004] that formed English's Crater, (2) underwater slope failures concomitant with the English's Crater debris avalanche, and/or (3) eroded seafloor sediments incorporated within the mass flow during emplacement [Le Friant et al., 2009]. The latter mechanism is common with gravitational mass flows and is referred to as bulking.
Deposit 3 occurs adjacent to submarine embayment C3 (Figure 8a), and this association suggests that deposit 3 originated through a submarine slope failure [Le Friant et al., 2004]. However, deposit 3 has a similar morphology to deposit 1 (see section 4), which has a subaerial collapse component. Deposit 3 may therefore also include failure of the subaerial as well as the submarine volcanic flanks, although any subaerial scar is now concealed by subsequent volcanic activity. Deposit 5 probably originates from a submarine slope failure linked to embayment C4 [Le Friant et al., 2004] (Figure 8a).
5.3.2. Shallowly Buried Deposits (2, 4, and 12)
The high resolution of the new seismic reflection data shows more closely the link between deposit 2 and submarine embayment C2, suggesting that deposit 2 indeed resulted from failure of the submarine island flank [Le Friant et al., 2004] (Figure 8b). The size and the extent (∼30 km) of deposit 2 would suggest evolution of an initial debris avalanche into a debris flow. The strong reflectors at the base of deposit 2 are interpreted as being due to basal shear. The strong internal reflectors within deposit 2 may indicate two stages of emplacement.
The new seismic data analysis modified the extent of deposit 4 and results in an increased estimate of its volume (∼3.4 km3). The absence of clear structures on land or on the submarine slope prevents us from determining its origin. Thus, deposit 4 could either (1) result from an on-land flank collapse that affected both the old flank of the South Soufrière Hills-Soufrière Hills volcanic complex and its submarine part, later buried by subsequent volcanic activity, or (2) originate from a submarine slope failure. The thinning toward the southwest on the GWADASEIS seismic profiles (Figure 4c) indicates a southwesterly direction of deposit propagation (Figure 8b). A sediment core located above deposit 4 revealed the presence of a dark basaltic scoria deposit at its base [Le Friant et al., 2004]. This deposit was related to the South Soufrière Hills Volcano, suggesting an age of, at least, 130 kyr [Le Friant et al., 2004]. The location of deposit 4 below this deposit induces an age of >130 kyr. As proposed by Le Friant et al. , deposit 4 could have been originated from an older flank collapse event, with the South Soufrière Hills Volcano infilling the collapse depression.
The shape and the location of deposit 12, as well as the presence of associated flank scarps, suggest an origin in a flank failure of seamount A of the Kahouanne Volcanoes (Figure 8b).
5.3.3. Deeply Buried Deposits (8, 9, 10, and 11)
The voluminous deposit 8 has been offset by faults of the MHFS and BMFS. Small offsets were identified between the lower limits of the two lobes on the seismic profiles (Figure 6, line gwa44–40). These offsets are interpreted as resulting from the deposition of syntectonic sediments [Feuillet et al., 2010]. Using a maximum sedimentation rate of 15 cm kyr−1 deposit 8 was emplaced ∼900 kyr ago (section 5.2). At that time the active volcano of Montserrat was the Centre Hills Volcano (∼950–550 kyr [Harford et al., 2002]), and the South Soufrière Hills-Soufrière Hills volcano complex had not yet formed. Thus, an origin from the current submarine flanks of the South Soufrière Hills-Soufrière Hills volcanic complex (<170 kyr [Harford et al., 2002]) is excluded. The deposit volume is ∼20 km3 and is of a similar order of magnitude to mass flow deposits recognized off Dominica, St Lucia and Martinique [Deplus et al., 2001; Le Friant et al., 2002; Boudon et al., 2007]. The origin of deposit 8 (on-land flank collapse and/or submarine slope failure) cannot be determined due to subsequent volcanic activity of the island and/or erosion of related scars. However, the volume and extent of deposit 8 would suggest more a submarine slope failure as origin, possibly in concomitance with a flank collapse event. The shape, as well as the magnitude of the deposit is of interest, given its unusual form of two lobes in the proximal region, merging into a single unit distally. The present-day form of the island suggests sources both east and south of Soufrière Hills, but it is clear from the dates of subaerial volcanic rocks that Soufrière Hills had not yet formed, and the sources areas of the lobes of deposit 8 are thus likely to be buried beneath younger volcanic rocks. However, the form of the deposit still requires an explanation, and it may be that the deposit, resulting from a failure off the Centre Hills Volcano, was forced around the proto-Soufrière Hills seamount, which may have been forming at this time (Figure 8c). In such a case, the early construction of the Soufrière Hills volcanic complex would have induced a deflection of the deposit, following the lowest bathymetric path, resulting in the formation of two lobes (Figure 6, line gwa44–40) which subsequently merged. However, deposit 8 can also have been formed before the construction of the proto-Soufrière Hills seamount and subsequently buried by its formation. Strong reflectors at the base of the deposit are likely to have a similar origin to those in deposit 2.
Deposit 9 is located east of Centre Hills Volcano. A debris avalanche deposit identified on-land near the coastal cliff of Statue Rock on the northeast coast (Figure 8c), has been interpreted by Boudon et al.  as the result of a subaerial flank collapse that affected the east flank of Centre Hills Volcano. It was associated to the scarp Sa described by Le Friant et al.  (Figure 8c) and located on the northwest bank of Bottomless Ghaut (Figure 1, inset). The continuity between the scarp Sa and the submarine deposit 9 suggests that both result from the same instability event on Centre Hills.
The debris avalanche deposits 10 and 11, located near the Kahouanne Volcanoes, are probably related to one of the seamounts A or B. We propose that deposit 10 resulted from a flank collapse on the east side of the seamount B since it thins out roughly northeastward, suggesting a north-northeast direction of propagation (Figure 8c). The scar located at the north of seamount B seems too insubstantial to be the source, but no other scar is visible.
5.4. Distribution of the Deposits Resulting From Instabilities
Mapping of the ten submarine deposits identified offshore Montserrat shows that most of the seafloor surrounding the island to the southeast quarter is covered by debris avalanche and related debris flow deposits (∼520 km2; Figure 9). Submarine landslide deposits are absent elsewhere (Figure 4a) as documented by the extensive grid of AGUADOMAR and CARAVAL seismic profiles located to the north and west of the island (Figure 2). Most of the debris avalanche deposits to the southeast of Montserrat (deposits 1, 2, 8, 11 and 12) are located within the Bouillante-Montserrat half-graben underlining a strong influence of the half-graben slopes on bathymetry, and hence on the distribution and propagation of deposits (Figures 7, 8, and 9). The other landslide deposits (deposits 3, 4, 5, 9 and 10) are not directly located within the half-graben but their sources are probably associated with the major fault systems bordering it (BMFS, BMAF). This concentration of deposits within and around the half-graben suggests a major link between the formation of debris avalanche and other mass flow deposits offshore Montserrat and local fault systems.
This study again increases the number, frequency and total volume of submarine landslide deposits recognized around volcanic islands in the Lesser Antilles Arc [e.g., Boudon et al., 2007] to 52 examples. Montserrat is now the first recognized island in the northern part of the Lesser Antilles Arc to have some submarine debris avalanche deposits with volumes up to ∼10 to 20 km3. Such large volume deposits were previously observed only for volcanic islands further south in the arc (Martinique, St Lucia, Dominica). The large-scale structure of the islands (very steep slopes on the west contrasting with gentle slopes in the east) and the presence of the Grenada Basin controlled the occurrence of mass-wasting events in the southern part of the arc [e.g., Deplus et al., 2001; Boudon et al., 2007]. In this study, we find that most of the landslides off Montserrat are concentrated near major fault systems around and within the Bouillante-Montserrat half-graben suggesting, that the half-graben may play a similar role to the Grenada Basin further south in controlling the location of instabilities that result in mass-wasting events. Relatively small volcanic islands such as Montserrat can therefore be affected by repeated large-volume mass-wasting processes that deposit very large amounts of sediments into the surrounding basins. The ten landslide deposits recognized around Montserrat transported ∼40 km3 of material, which is equivalent to four times the annual sediment supply from all of the world's present-day rivers [Talling et al., 2007].
We thank the captains, officers, crews, and scientific teams of the N/O L'Atalante, N/O Le Suroît and RRS James Cook. We thank Yujin Choi for helpful technical support and Frédérique Leclerc for exchanges in Paris regarding the data. We acknowledge the MVO staff for logistic aspects during the marine surveys. We thank Barry Voight, Claudia Romagnoli, and Geoff Wadge for their fruitful reviews, which improved the paper. We used the GMT software developed by P. Wessel and W. Smith (available at http://www.soest.hawaii.edu/gmt/) for Figures 1, 2, 3, 7, and 8. IPGP contribution 3151.