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doi:10.2204/iodp.sp.340.2011

Background

Geodynamic setting

The Lesser Antilles arc results from the subduction of the Atlantic plate beneath the Caribbean plate (Fig. F1). Current convergence rates of the plates are relatively slow (2–4 cm/y; Feuillet et al., 2002), and magma productivity has been low relative to other arcs; estimations of magma productivity vary between 3 and 5 km3/m.y./km (Sigurdsson et al., 1980; Wadge, 1984; MacDonald et al., 2000). Volcanism along the arc started ~40 m.y. ago (Martin-Kaye, 1969; Bouysse et al., 1990). North of Dominica, the arc is divided into two island chains, sitting on top of a Cretaceous ocean island arc (Bouysse and Guennoc, 1983; Wadge, 1986). The eastern chain corresponds to an older extinct arc, and its basement is largely covered by thick carbonate platforms (Fig. F1). The western chain is the site of active volcanism since 20 Ma (Briden et al., 1979). South of Dominica, the older and recent arcs are superimposed, forming one chain of islands bordered to the west by the 2900 m deep backarc Grenada Basin. The Grenada Basin (Fig. F1) serves as a major depocenter for large debris avalanches, volcanic turbidites, large pyroclastic flows, and hemipelagic sediment (Sigurdsson et al., 1980; Deplus et al., 2001; Picard et al., 2006; Boudon et al., 2007). Generally, background hemipelagic sedimentation rates vary from 1–2 cm/1000 y west of the northern islands of the arc to 10–20 cm/1000 y in the Grenada Basin (Reid et al., 1996; Duchoiselle, 2003).

Eruptive history and long-term magmatic evolution of the volcanic arc

Volcanism varies markedly along the Lesser Antilles arc (MacDonald et al., 2000; Lindsay et al., 2005a). For example, magmas become enriched in incompatible elements, notably K, and radiogenic isotopes southward. This enrichment has been interpreted to reflect the increasing influence of subducted Orinoco sediment supplied from the South American continent into the forearc, as well as variations in subduction rate and geometry normal to the arc. The southern part of the arc (St. Vincent, Grenada, and the Grenadine Islands) is dominated by basaltic to basaltic andesitic magmas. The central part of the arc (St. Lucia, Dominica, and Martinique) is dominated by silicic magmas and characterized by relatively high eruption rates (e.g., 2.6 eruptions/1000 y for Montagne Pelée, Martinique; Duchoiselle, 2003), as inferred from marine tephra layers around these islands (Sigurdsson et al., 1980) and large volcanic centers. Volcanism in this area has also produced several large Plinian-style explosive eruptions with associated ignimbrites in the recent geological past. The northern part of the arc is characterized by andesitic and basaltic volcanism. The volcanoes of Soufrière Hills on Montserrat and Soufrière on Guadeloupe have produced dominantly andesitic magmas, whereas Nevis, St. Kitts, St. Eustatius, and Saba have erupted substantial volumes of basaltic as well as andesitic magmas. Magma production rates and eruption frequency are comparatively low in this area (e.g., 0.5 eruptions/1000 y, Soufrière on Guadeloupe; Komorowski et al., 2005). In addition, volcanoes in this area are also characterized by magmatism alternating on a timescale of ~1 m.y. from long production periods of almost constant magmatic composition to short periods of large compositional variations (from basalts to dacites; see, e.g., Mt. Pelée and adjacent centers; Boudon et al., 2005; Annen et al., 2006, 2008).

Much of the compositional variability in arc volcanoes can be related to the processes in the crust where magmas are generated and transported. Annen et al. (2006, 2008) provide a conceptual framework for understanding the dynamics of magma generation, magma differentiation, and transport, including the formation of differentiated magmas in deep hot zones, partial melting of the preexisting crust, transport, and formation of shallow magma chambers in which phenomena such as degassing, crystallization, and magma mixing can take place to control the characteristics of the erupted magmas and, ultimately, styles of volcanic activity. Magma flux rates are considered to be the major control on the formation of shallow magma chambers containing eruptible magma and on their compositions. Understanding the igneous processes of volcanic arcs and the subduction zone engine is fundamental because they provide a viable mechanism to generate continental crust and are a key component of global-scale geochemical cycling.

On an individual island scale, the islands of the Lesser Antilles arc are characterized by the development of a number of discrete volcanic centers, many of them overlapping in space and time (Lindsay et al., 2005a). The Volcanic Hazards Atlas of the Lesser Antilles by Lindsay et al. (2005a) presents a good synthesis of the knowledge and available references on these volcanoes. Volcanic activity along these centers is interpreted to be episodic, migrating either from north to south (Montserrat, Harford et al., 2002) or vice versa (Martinique, Boudon et al., 2005), or clustered in several contemporaneous centers (Dominica, Lindsay et al., 2005b). In each case, these centers appear to remain active for 5 × 105 to 5 × 106 y, although these inferred timescales are not well constrained. However, deciphering a complete eruption record from onshore geology is commonly problematic because of burial by deposits from younger events, dense vegetation, erosion, and catastrophic removal of deposit by flank-collapse events. Marine sediments preserve a much more complete record of volcanism (Sigurdsson et al., 1980; Le Friant et al., 2008). Recent marine tephrochronological studies have been undertaken for several volcanoes of the Lesser Antilles arc from piston cores (Duchoiselle, 2003; Vennat, 2004; Le Friant et al., 2008; fig. 5 in Machault, 2008). Correlations have been made between tephra sampled in a core off one island and on-shore deposits from different volcanoes of different islands. In the case of the Montagne Pelée volcano in Martinique, 25 eruptive events have been identified in marine deposits between 5,000 and 15,000 y before present versus only 10 magmatic events previously recognized onshore. For Montserrat (SHV), the marine core records several Plinian explosive eruptions, which have yet to be identified on land (Le Friant et al., 2008). Integration of marine tephrochronology and onshore geologic studies are thus the principal way to investigate the complete history of volcanoes. However, conventional coring only samples the recent activity (tens to hundreds of thousands of years), which is not sufficient to characterize the complete evolution of volcanic systems that can extend to a few million years.

Flank-collapse events (with large debris avalanche emplacement) and sedimentation processes along the arc

Volcano flank collapses are increasingly recognized as a normal process in the construction and destruction of volcanic edifices (Ida and Voight, 1995; McGuire, 1996; Voight, 2000). Thus, they play a significant role in the evolution of volcanic edifices and on the dynamics of subsequent eruptions and are a significant potential geohazard. The recognition of flank-collapse events is based on mapping debris avalanche deposits that can be traced to a generally horseshoe-shaped collapse depression (Voight, 1981). The most voluminous events (volumes from tens to hundreds or even thousands of cubic kilometers) have been recognized on oceanic islands: Hawaii (Lipman et al., 1988; Moore et al., 1989), La Réunion (Labazuy, 1996; Oehler et al., 2004, 2008), and in the Canary archipelago (Holcomb and Searle, 1991; Watts and Masson, 1995; Urgeles et al., 1997; Krastel et al., 2001).

On volcanoes of the Lesser Antilles arc, at least 52 flank-collapse events have been identified (Deplus et al., 2001; Le Friant, 2001; Le Friant et al., 2002, 2003a, 2003b, 2004; Lebas et al., 2011; Boudon et al., 2007). In the northern part of the arc, flank collapses are repetitive, do not exceed 1 km3 in volume, can occur in all directions, and are promoted by intense hydrothermal alteration and well-developed fracturing of the summit part of the edifices. For example, several prehistoric flank collapses have been recognized on the Soufrière Hills volcano, Montserrat (Fig. F2) (Le Friant et al., 2004; Lebas et al., 2011). The English’s crater event occurred ~2000 y ago, producing debris avalanche Deposit 1 (volume = ~1.5 km3). Debris avalanche Deposit 2 probably resulted from a combined submarine and subaerial flank collapse of the eastern flank of the volcano dated between 130 and 112 ka (Le Friant et al., 2004). In the southern part of the arc, flank collapses are larger (with volumes up to tens of km3), always directed to the west, and related to the higher overall slopes of the leeward side of the islands. For example, the evolution of the active Montagne Pelée volcano, Martinique, has been marked by three major flank collapses (~0.1 Ma, ~25,000 y ago, and ~9000 y ago) that systematically destroyed the western flank of the volcano (Le Friant et al., 2003a, 2003b; Boudon et al., 2005, 2007). Collapse volumes varied from 2 to 25 km3, and debris avalanches flowed down to the Grenada Basin (Fig. F3). In addition, marine and terrestrial evidence indicate a succession of at least three flank collapses on Dominica (Fig. F4) (Le Friant et al., 2002). Dominica was also the site of the generation of the most voluminous debris avalanches in this area, with submarine deposits that cover 3500 km2. The proximal debris avalanche deposit consists of megablocks (as long as 2.8 km and as high as 240 m) that reflect the predominance of lava flows and lava domes as observed in the source Plat Pays volcanic complex and in terrestrial relict debris avalanche material.

Despite the importance of flank collapses and associated debris avalanches in the lifetime of arc volcanoes, mainly areas of intraplate volcanism associated with large submarine volcanic landslides, such as Hawaii (ODP Leg 136: e.g., Garcia, 1993, Garcia and Meyerhoff Hull, 1994; ODP Leg 200: e.g., Garcia et al., 2006) and the Canary Islands (ODP Leg 157: e.g., Schneider et al., 1997; Goldstrand, 1998) have been drilled. However, all these drill sites were located in distal turbidites, far away from proximal debris avalanche deposits. Flank-collapse deposits in those locations (e.g., Canary or Hawaiian Islands and Izu Bonin arc) extend to deeper (>4000 m) water depths and are relatively thick, which limits the resolution and penetration of seismic surveys. Despite considerable efforts, avalanche deposits around the Canary and Hawaiian Islands or the Izu Bonin arc have not been successfully penetrated and seismically imaged in much detail. In contrast, previous two-dimensional seismic surveys show that debris avalanche deposits and their basal contacts can be imaged successfully in the Lesser Antilles (Deplus et al., 2001; Le Friant et al., 2004). This can be largely explained by the fact that collapse deposits around Montserrat occur in shallower water (<1200 m) and that offshore Dominica and Martinique, the debris avalanche flew into the Grenada Basin and was emplaced on top of sedimentary units that provide good seismic reflectors. Seismic images in the Lesser Antilles thus provide the essential background information to facilitate effective drilling into debris avalanche deposits.

Different morphologies and deposit geometries of debris avalanche deposits have been observed along the Lesser Antilles. Large hummocks (as large as 2 km) characterize the debris avalanches off Dominica, whereas the morphology of the debris avalanches off Martinique is smooth. These differences are probably related to contrasted lithologies of the volcanic products (dominantly pyroclastic deposits versus massive lavas) (Boudon et al., 2007). Northern island volcanoes collapsed repeatedly and contrast with southern island volcanoes where collapses are infrequent. Such size and frequency variations along a volcanic arc have not been documented previously for any other arc.

The majority of detrital material resulting from the erosion of the islands of the arc is transported into the surrounding ocean (e.g., Sigurdsson et al., 1980; Le Friant et al., 2004; Picard et al., 2006). Studies of offshore deposits from the 1902 eruption of St. Vincent (Carey and Sigurdsson, 1982), from the recent eruption of SHV (Le Friant et al., 2004; Hart et al., 2004; Trofimovs et al., 2006, 2008) and from prehistoric eruptions on Dominica (Sigurdsson et al., 1980; Whitham, 1989) demonstrate that most of the erupted material reaches the ocean. Volcanogenic sediments are channeled by debris flows, turbidity currents, and persistent ocean currents through deep submarine canyons located west of the volcanoes and which, for Guadeloupe and Dominica, lead into the northern part of the Grenada Basin (Fig. F1).

Around Montserrat there are examples of single or multiple stacked carbonate turbidites that contain reworked shallow-water sediment and fauna. These are likely sourced from large carbonate platforms associated with islands such as Antigua and Redonda. Understanding the origin of these bioclastic turbidites is important because, for example, the volume exceeds that of volcaniclastic deposits associated with the more recent (<100 ka) eruptions of the Soufrière Hills volcano. Shallow vibrocores have only recorded bioclastic turbidites associated with the late glacial period. One possibility is that they are caused by instability of carbonate platforms during rapid sea level rise at the end of major glaciations. Alternatively, major regional earthquakes may trigger them, in which case the occurrence of such events may be unrelated to climatic cycles.

Site survey data

Previous work has involved on-land geological, geochemical, petrological, geochronological, and geophysical studies and offshore marine studies:

  1. The Endeavour cruise of 1979 gathered a regional collection of piston cores allowing assessments of rates of volcanism and sedimentation, dating of major explosive eruptions, recognition of submarine pyroclastic flow deposits, and establishment of a biostratigraphic framework for the eastern Caribbean (Sigurdsson et al., 1980; Sparks et al., 1980a, 1980b; Reid et al., 1996). Westbrook and McCann (1986) analyzed large-scale seismic experimental data on the overall arc crust and showed that the subduction history has been episodic (e.g., shift of the axis of volcanism in the Lesser Antilles at the beginning of the Pliocene).

  2. Site 30 of Deep Sea Drilling Project (DSDP) Leg 4 in 1969 was drilled to investigate the geologic history of the Aves Ridge (west of the Grenada Basin). Site 48 of DSDP Leg 15 was located north of Site 30, also investigating the Aves Ridge. However, no DSDP, ODP, or IODP sites have been studied in the Grenada Basin or close to the Lesser Antilles islands up to now.

  3. During the Aguadomar (December 1998–January 1999) and Caraval (March 2002) cruises of the RV L’Atalante (principal investigators (PIs): C. Deplus and G. Boudon), Simrad EM12D swath bathymetry and backscatter data, 3.5 kHz echosounder, gravity, magnetic, and six-channel seismic reflection data were collected from Montserrat to St. Vincent (Deplus et al., 2001). During Caraval, seismic profiles using a 24-channel streamer (Deplus et al., 2002), sediment piston cores, and dredge samples were also collected (see overview Table T1).

  4. During the JCR123 cruise of the RRS James Clark Ross in May 2005 (PI: R.S.J. Sparks), sediment cores were collected from around Montserrat to study the submarine pyroclastic deposits from the recent eruption (Trofimovs et al., 2006).

  5. Two Natural Environment Research Council (NERC)-funded cruises took place in December 2007. The first cruise (PI: M. Palmer) collected box cores and shallow gravity cores at ~34 sites around Montserrat to constrain how diagenesis of tephra from the recent eruptions has influenced seawater geochemistry. The second cruise, a component of the SEA-CALIPSO seismic experiment sponsored by the US National Science Foundation (NSF), NERC, and collaborating agencies (PIs: Profs. S. Sparks and B. Voight), aimed at imaging the interior of Montserrat and the Soufrière Hills volcano, using source seismic techniques in combination with ~240 onshore seismometers and offshore ocean borehole seismometers (OBS) (Voight et al., 2008; Sparks et al., 2008).

  6. During the Gwadaseis cruise of the RV Suroit (February–March 2009), high-resolution seismic data were collected as well as piston cores (see overview Table T1).

  7. During the JC45/46 cruise of the RRS James Cook in April–May 2010 (PI: P. Talling), high resolution 2D and 3D seismic data were collected around Montserrat (see overview Table T1).

The swath bathymetry data were processed using the Caraïbes software developed by the Institut francais de recherche pour l’exploitation de la mer (IFREMER). The multichannel seismic reflection data were filtered, stacked, and migrated using the Seismic Unix software for the 1999, 2002, and 2009 data, with Landmark’s ProMAX software for the 2010 data. The Aguadomar and Caraval cruise data were migrated at a seawater velocity of 1450 m/s, a normal move-out (NMO) correction was applied for the Gwadaseis cruise data, and the JC45/46 data were migrated at a linearly increasing velocity of 1450–2500 m/s.

To estimate the average thickness of debris avalanche deposits, we selected a seismic velocity of 2150 m/s, derived from JC45/46 seismic data analysis (common-reflection point NMO velocity picking). This value is slightly higher than some assumed velocity values used previously in similar deposits (e.g., 1800 m/s: Urgeles et al., 1997; Collot et al., 2001; Le Friant et al., 2004; 2000 m/s: Bull et al., 2009) and in the proposal 681-Full2. Consequently, to account for uncertainties related to the seismic velocity used, drilling depths have been recalculated using a maximum seismic velocity of 2200 m/s.

The supporting site survey data for Expedition 340 are archived at the IODP Site Survey Data Bank.

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