Growth and collapse of the Reunion Island volcanoes

Transcription

1 Bull Volcanol (2008) 70: DOI /s RESEARCH ARTICLE Growth and collapse of the Reunion Island volcanoes Jean-François Oehler & Jean-François Lénat & Philippe Labazuy Received: 3 January 2007 /Accepted: 7 June 2007 / Published online: 16 August 2007 # Springer-Verlag 2007 Abstract This work presents the first exhaustive study of the entire surface of the Reunion Island volcanic system. The focus is on the submarine part, for which a compilation of all multibeam data collected during the last 20 years has been made. Different types of submarine features have been identified: a coastal shelf, debris avalanches and sedimentary deposits, erosion canyons, volcanic constructions near the coast, and seamounts offshore. Criteria have been defined to differentiate the types of surfaces and to establish their relative chronology where possible. Debris avalanche deposits are by far the most extensive and voluminous formations in the submarine domain. They have built four huge Submarine Bulges to the east, north, west, and south of the island. They form fans km wide at the coastline and km wide at their ends, km offshore. They were built gradually by the superimposition and/or juxtaposition of products moved during landslide episodes, involving up to several hundred cubic kilometers of material. About 50 individual events deposits can be recognized at the surface. The landslides have recurrently dismantled Piton des Neiges, Les Alizés, and Piton de La Fournaise volcanoes since 2 Ma. About one third are interpreted as secondary landslides, affecting previously emplaced debris avalanche deposits. On land, landslide deposits are observed in the Editorial responsibility: J. White Electronic supplementary material The online version of this article (doi: /s ) contains supplementary material, which is available to authorized users. J.-F. Oehler (*) : J.-F. Lénat : P. Labazuy Laboratoire Magmas et Volcans, UMR 6524 CNRS, OPGC, Université Blaise Pascal, 5 rue Kessler,, Clermont-Ferrand Cedex, France J.F.Oehler@opgc.univ-bpclermont.fr extensively eroded central area of Piton des Neiges and in its coastal areas. Analysis of the present-day topography and of geology allows us to identify presumed faults and scars of previous large landslides. The Submarine Bulges are dissected and bound by canyons up to 200 m deep and 40 km long, filled with coarse-grained sediments, and generally connected to streams onshore. A large zone of sedimentary accumulation exists to the north east of the island. It covers a zone 20 km in width, extending up to 15 km offshore. Volcanic constructions are observed near the coast on both Piton des Neiges and Piton de la Fournaise volcanoes and are continuations of subaerial structures. Individual seamounts are present on the submarine flanks and the surrounding ocean floor. A few seem to be young volcanoes, but the majority are probably old, eroded seamounts. This study suggests a larger scale and frequency of mass-wasting events on Reunion Island compared to similar islands. The virtual absence of downward flexure of the lithosphere beneath the island probably contributes to this feature. The increased number of known flank failure events has to be taken into consideration when assessing hazards from future landslides, in particular, the probability of landslide-generated tsunamis. Keywords Reunion Island. Landslides. Multi-beam bathymetry. Sonar backscatter. Digital terrain model. Oceanic islands Introduction The building of a large volcano generally involves a succession of phases of volcanic construction (accumulation of lava flows and pyroclastic products) and phases of destruction by mass-wasting processes. Flank landslides

2 718 Bull Volcanol (2008) 70: Fig. 1 New land and sea numerical documents of high-resolution bathymetry and acoustic mapping for La Réunion volcanic edifice. a Shaded image (apparent illumination from the north west) calculated from the 100-m-gridded DTM. Contour interval is 1,000 m. Inset shows the location of Reunion Island in the Indian Ocean. M W FZ Mahanoro Whilshaw Fracture Zone, MFZMauritius Fracture Zone. b Fifty-meter gridded composite image, associating the acoustic mosaic and the slope map where reflectivity data are not available. Black is high backscatter; white is low backscatter. The thin white stripes are the boundaries of different datasets that could not be merged accurately (probably because of inhomogeneous water column correction)

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4 720 Bull Volcanol (2008) 70: Fig. 2 Slope map showing the coastal submarine and subaerial parts of La Réunion. The main topographical features on land are shown. Bathymetry contour interval is 500 m. The island sensu stricto is surrounded by a shallow submarine shelf with an approximate depth of 100 m. Submarine volcanic constructions in continuation with onland structures are interpreted as the prolongation of ancient (La Montagne massif, Etang Salé ridge) or active (Piton de La Fournaise north east and south east rift zones) rift zones are recognized as the most important and efficient masswasting process on volcanoes. They contribute to the building of the edifices by widening their base. They are observed on volcanoes in all geodynamical settings, from continental and subduction strato-volcanoes to intraplate oceanic shield volcanoes that have gentler slopes. In fact, the biggest landslides on Earth have been observed on volcanic oceanic islands, with some events involving more than 1,000 km 3 of products. Single events are known to have removed half the subaerial portion and 10 to 20% of the total volume of volcanoes (Holcomb and Searle 1991). The best-documented examples are those of the Hawaiian Ridge and of the Canary archipelago volcanoes. Sixty-eight major landslides, with the largest deposits attaining 200 km in length and 5,000 km 3 in volume, have been described in the Hawaiian Islands (Moore et al. 1989, 1994), and 18, involving up to 1,000 km 3 of material, in the Canary Islands (Ablay and Hürlimann 2000; Krastel et al. 2001a, b; Masson et al. 2002). In an oceanic context, the study of these phenomena requires extensive analysis of the submarine flanks where most of the associated deposits are accumulated. These observations have to be linked to the sub-aerial and internal structures to assess the source areas of the landslides and the mechanisms leading to destabilization. We have followed this approach in this study of Reunion Island. It is based on the Ph.D. thesis of the first author (Oehler 2005) where interested readers can find the details that could not be included in this paper ( fr/index.php?halsid=7468eeeb21a717fe9f1334ac4d670bae& view_this_doc=tel &version=1). High-resolution Fig. 3 Synthetic section across La Réunion. The internal structure is mostly based on geophysical works by Malengreau et al. (1999), Charvis et al. (1999), de Voogd et al. (1999), Gallart et al. (1999), and Lénat et al. (2001). Les Alizés volcano has been destroyed and cannot be observed on land; its inferred previous shape is shown. Piton des Neiges and Les Alizés volcanoes have large, dense hypovolcanic complexes, whereas the young Piton de la Fournaise volcano has not yet developed a comparable complex. Most of the submarine flanks have been constructed by the accumulation of mass-wasting products. The lithosphere does not show a downward flexure beneath La Réunion

5 Bull Volcanol (2008) 70: multi-beam bathymetry and acoustic imagery data collected over the last 20 years (between 1984 and 2003) off La Réunion have been compiled in this study, which is the first to analyze the entire surface of the Reunion Island volcanic system. Geological background Geodynamic context La Réunion is an active volcanic system located in the Mascarene Basin (southwestern Indian Ocean), 750 km east of Madagascar (Fig. 1). It is a flattened edifice, 220 to 240 km in diameter at the level of the surrounding seafloor and about 7 to 8 km in height. It was built on Upper Cretaceous to Paleocene oceanic lithosphere created by a system of NW SE paleo-rift segments shifted by NE SW fracture zones (e.g., Mahanoro Wilshaw or Mauritius Fracture Zones; Fig. 1a). The origin of La Réunion is attributed to a mantle hot spot. According to Duncan et al. (1989), its track extends from the Deccan Traps ( 65 Ma) to the Chagos Laccadive Ridge ( 50 Ma), the Mascarene Plateau ( 40 Ma), Mauritius ( 8 Ma), and finally, Reunion Island ( 5 Ma). An alternative hypothesis considers that the Mascarene Plateau and the younger edifices are an independent volcanic system that has developed on the African Plate during the last 30 Ma (Burke 1996). Geology The island of La Réunion sensu stricto is elliptical in shape (50 70 km) with a NW SE orientation and rises to more than 3,000 m a.s.l. It a priori consists of two juxtaposed volcanic massifs: Piton des Neiges and Piton de La Fournaise (Fig. 2). Piton des Neiges occupies the northwestern part of the island. It is a dormant volcano whose youngest eruptive products are dated at 12 ka (Deniel et al. 1992). Piton des Neiges emerged from the sea before 2.1 Ma (Billard and Vincent 1974). Its subaerial history started with the building of a basaltic shield volcano from before 2.08 Ma (age of the oldest outcropping lava flows; McDougall 1971) to after Ma (Oceanic Series). Piton des Neiges then erupted alkaline-differentiated lavas between 330 and 12 ka (Differentiated Series; McDougall 1971; Gillot and Nativel 1982; Deniel et al. 1992). The Piton des Neiges massif is dissected by deep valleys and three central major depressions: the cirques of Mafate, Salazie, and Cilaos. A fourth cirque to the east of Salazie, the cirque of Marsouins, has been partially filled by lava flows (Kieffer 1990a; Deniel et al. 1992). Piton de La Fournaise occupies the southeastern part of the island. It is a highly active basaltic shield volcano, whose recent activity is concentrated on the central cone and along its north east and south east rift zones. Piton de La Fournaise has grown since at least Ma (age of the oldest outcropping lava flows; Gillot and Nativel 1989). Bachèlery and Mairine (1990) distinguish two phases of building: the ancient shield (>0.15 Ma) and the recent shield (<0.15 Ma), punctuated by several volcano tectonic events. The present morphology of La Fournaise is characterized by a series of caldera-like rims and arcuate rims of valleys. For some workers, these structures result from classic caldera collapses (Chevallier and Bachèlery 1981; Bachèlery and Mairine 1990; Bachèlery 1995). For others, they could be the headwalls of successive eastward moving landslides (Duffield et al. 1982; Lénat and Labazuy 1990; Lénat et al. 1990, 2001; Labazuy 1991, 1996; Gillot et al. 1994). Merle and Lénat (2003) have developed a hybrid model where a vertical collapse could be generated in response of a flank landslide. All the authors, however, generally agree that the easternmost structure, the Grand Brûlé depression, is the scar of a 5-ka landslide (Bachèlery and Mairine 1990; Labazuy 1991, 1996; Mohamed-Abchir 1996). Internal structure As is generally the case for oceanic islands, the emergent part of La Réunion represents a small fraction of the system (only 3% according to de Voogd et al. 1999). It is, therefore, necessary to rely on indirect methods to infer the evolution and internal structure of the overall volcanic system. The main large-scale features of the structure of La Réunion can be determined from works based on gravity, seismic, and magnetic data (Fig. 3). Gravity The most recent published work on the gravity structure of the island of La Réunion is Malengreau et al. (1999), but new data has since been collected (Lambert, unpublished data, 2003; Lénat et al., unpublished data, 2003; Levieux unpublished data, 2004), allowing us to strengthen the interpretation. The Bouguer anomaly map of La Réunion is dominated by two large positive anomalies: one centered on Piton des Neiges volcano and the other on the eastern coast. Both can be associated with hypovolcanic complexes of gabbros and ultrabasic cumulates. Gabbro outcrops have been observed at the bottom of the Cilaos and Salazie cirques of Piton des Neiges (Bussière 1967; Upton and Wadworth 1972; Rançon 1982; Maillot 1999) and by drilling in the Salazie cirque (Demange et al. 1989). Gabbros and cumulates are present from a depth of about 1,000 m in a drill hole made over the large anomaly on the east coast (Rançon et al. 1989). The eastern anomaly is not connected to Piton de la Fournaise volcano and had been inferred to represent the hypovolcanic complex of an old, now concealed volcano named Les Alizés

6 722 Bull Volcanol (2008) 70: (Rançon et al. 1989; Malengreau et al. 1999; Lénat et al 2001). The young Piton de la Fournaise volcano has not yet developed a large, dense hypovolcanic complex, although dense rocks beneath the oldest part of the volcano are suggested by gravity anomalies. The Piton de la Fournaise positive anomaly coincides with the location of the volcanic center during the earliest stage of the volcano s evolution (Bachèlery and Lénat 1993) and also with an area where xenoliths of gabbro and ultrabasic rocks are frequently found in tephra and lava flows. The modeling of the Bouguer anomalies suggests (1) that the two main positive anomalies of Piton des Neiges and Les Alizés volcanoes are created by deeply rooted, large hypovolcanic complexes and (2) that a smaller, less deeply rooted hypovolcanic complex exists beneath Piton de la Fournaise to the west of the present center of activity. Seismics A general view of the seismic structure of La Réunion was provided by the 1993 project Reusis (Charvis et al. 1999; de Voogd et al. 1999; Gallart et al. 1999). The main characteristics may be summarized as follows: (1) The core of the island is made of more massive rocks than the flanks; (2) we do not observe a large flexure of the lithosphere as found on most other oceanic islands; and (3) beneath the south-west part of the island, a high velocity layer at the base of the crust is interpreted as magmatic underplating. Magnetics The island of La Réunion has been studied using data from airborne and shipborne magnetic surveys by Lénat et al. (2001). The subaerial volcanism of La Réunion spans the last 2.1 Ma and, therefore, the Brunhes Matuyama geomagnetic reversal. This enables volcanic rocks older and younger than 0.78 Ma to be distinguished by studying the positive and negative magnetic anomalies. The lower submarine flanks appear as poorly magnetized, which is consistent with their interpretation as being mostly landslide deposits. Long wavelength anomalies are related to the magnetic anomalies of the oceanic crust. The core of the island is composed of highly magnetized rocks. Piton des Neiges volcano is composed mainly of rocks older than 0.78 Ma, and only its western flank and central area include thick piles of younger Brunhes rocks. Piton de la Fournaise volcano is a strongly and normally magnetized edifice, but its northern and eastern flanks are underlain at shallow depth by reversely magnetized formations. The latter are regarded as remnants of Les Alizés volcano, associated with the Grand Brûlé hypovolcanic complex. Therefore, at the Matuyama Brunhes transition, the island was composed of two main volcanoes: Piton des Neiges and Les Alizés. Previous submarine studies The first two detailed studies of the submarine flanks of La Réunion focused on the offshore eastern continuation of the active Piton de la Fournaise volcano. In 1984, Fournaise 1 project provided the first multi-beam coverage of the area (Lénat et al. 1989, 1990). During Fournaise 2 project in 1988, a high definition sonar image of the area was acquired using the deep-tow sonar SAR (Cochonat et al. 1990; Lénat and Labazuy 1990; Labazuy 1991, 1996; Ollier et al. 1998). In addition, bottom pictures were taken at selected sites, and rock samples were collected by dredging and coring. These data provided an image of the nature and distribution of the submarine features to the east of the active volcano. In particular, they suggested that, with the exception of a few features near the coast, the material on the submarine flank was entirely derived from mass wasting and sedimentation phenomena. The interpretative map of Labazuy (1991, 1996) delineates slide events deposits emplaced either as debris avalanches (forming a characteristic hummocky zone of accumulation, the Ralé-Poussé), or as slumps (forming a prominent submarine plateau resulting from the accretion of successive slide blocks). Labazuy (1991, 1996), thus, stated that at least 550 km 3 of slide deposits are found off Piton de la Fournaise. The bathymetry of the other flanks of La Réunion has long remained poorly known. A first general digital terrain model (DTM) of the edifice, with a resolution of about 1 km, was compiled by Lénat and Labazuy (1990). The authors noted that the submarine morphology of La Réunion departs significantly from that of a regular volcanic cone by the presence of four large bulges at the north east, the east, the south west, and the west. Because of similarities with the well-studied eastern bulge, they suggested that the three other bulges were also created by accumulation of mass wasting products. Detailed bathymetry for a large area of the southern and southwestern submarine flanks of the island was acquired in 1993 using the Hydrosweep multi-beam echo-sounder (Fretzdorff et al. 1998). Hummocky topographies were observed in these areas, suggesting that they are covered with debris avalanche deposits. Also in 1993, multichannel seismic reflection profiles, allowed de Voogd et al. (1999) to identify both superficial and deep slumps and debris avalanches to the south and the east of the island. Some reflectors are interpreted as slide or décollement surfaces, the deepest being the top of the oceanic sediments. More recently, Oehler et al. (2004) undertook a detailed study of the northern flank of the island using Simrad EM12D swath bathymetry and backscatter data collected in Although the coverage was only partial, the quality of the data allowed them to carry out a detailed analysis of the features of the area. They concluded that the flank was

7 Bull Volcanol (2008) 70: covered mostly with debris avalanche deposits and by sediments channeled within submarine valleys or canyons. At least 15 major flank landslides formed since 2 Ma were identified. Oehler et al. (2004) tried to link the submarine landslide deposits with the volcano tectonic evolution of the subaerial part of the island. Data compilation and processing We present in this paper a new compilation of all highresolution swath bathymetry and backscatter data collected over the last 20 years around La Réunion. The database includes the previously published data described above and unpublished multi-beam Simrad EM12D, Thomson TSM 5265B, and Thales Sea Falcon 11 data collected since 2000 (see Fig. 1 and ESM Fig. 1). The data have been processed using Caraibes software ( Ifremer) to construct (1) a 100-m gridded DTM of the submarine flanks and of part of the surrounding ocean seafloor (Fig. 1a) and (2) a corresponding 50-m gridded sonar image (Fig. 1b). These grids have been merged with conventional detailed bathymetry of the seashore supplied by the Service Hydrographique et Océanographique de la Marine (SHOM) and with the subaerial topography from the French Institut Géographique National (IGN). These maps are currently the best available for La Réunion volcanic edifice, although they remain incomplete and heterogeneous. General characteristics of the submarine features of La Réunion We divide the submarine environment of La Réunion into three main, concentric zones: (1) the coastal shelf (from sea level) to a depth of about 100 m, (2) the island flanks at depths of between 100 and 4,000 m, and (3) the surrounding oceanic plate with an average depth of more than 4,000 m. The coastal shelf The transition between the subaerial and submarine environments is characterized by a shallow submarine shelf, bounded at about 100 m by a marked break in slope (Fig. 2). This platform with a smooth surface is well developed in the Piton des Neiges area, with an average width of about 2 3 km and a maximum size of 7 km in the western part of the massif. In the Piton de la Fournaise area, it is generally much narrower, barely 0.5 to 1 km width on average to the north and south of the volcano. On the eastward facing Enclos Fouqué structure, the shelf is missing entirely. Coastal shelves are common features of volcanic islands and can be found in various geodynamical settings, for instance, around the Hawaiian Islands (Clague and Moore 2002), the easternmost Canary Islands (Ablay and Hürlimann 2000), or the Caribbean Islands (Le Friant et al. 2004). They are generally explained by either subsidence or eustatic sea-level changes (Moore and Clague 1992; Carracedo 1999; Le Friant et al. 2004). In La Réunion, the depth of the coastal shelf coincides well with sea level in the south-western part of the Indian Ocean during early deglaciation (Colonna 1994; Camoin et al. 2004). We, therefore, propose that the break in slope bounding the shelf corresponds to a paleo-coastline, related to eustatic sea-level variations. Accordingly, the shelf cannot be used as an indicator of the subsidence of La Réunion as suggested earlier by Lénat and Labazuy (1990). The submarine flanks of the island The surface morphology and the distribution of features on the submarine flanks are complex. The bathymetric and acoustic data allow us to identify at least three main types of terrains: (1) rough and chaotic terrains characterized by a speckled acoustic pattern, (2) smooth terrains with high sonar backscatter values, and (3) rugged steep formations with variable acoustic signatures. They are identified as representing landslide deposits, coarse sediments, and volcanic constructions, respectively. Landslide deposits Rough and chaotic terrains are mainly found draping the four large fan-shaped northern, eastern, southern and western Submarine Bulges (respectively NSB, ESB, SSB, and WSB; Fig. 1a and ESM Fig. 2a) off La Réunion (Lénat and Labazuy 1990; Oehler et al. 2004; Oehler 2005). These topographic highs are 20 to 30 km wide at the coastline and 100 to 150 km wide where they meet the seafloor, 70 to 80 km offshore. Their surface generally presents low sonar backscatter values, indicating a near-specular reflection of sonar waves and, therefore, a smooth surface, probably due to a coating of fine-grained sediments (Fig. 1a and ESM Fig. 2b). However, spots of high sonar backscatter at their surface create a characteristic speckled acoustic pattern indicating the presence of blocks protruding from a smoother surface. The largest blocks, several hundreds of meters to 1 km or so wide, are obvious features in the bathymetry (Fig. 1a and ESM Fig. 2a). The Submarine Bulges, thus, appear to be huge bodies of hummocky terrain, comparable to debris avalanche deposits described at the foot of other oceanic islands such as Hawaii (Moore et al. 1994), Tenerife (Watts and Masson 1995), El Hierro (Urgeles et al. 1997), or La Palma (Urgeles et al. 1999).

8 724 Bull Volcanol (2008) 70: Fig. 4 a Morphology and topography of the Eliane cone and its neighbors to the south (see location on Fig. 5a). b Morphology and topography of the Sonne cone Canyons and sedimentary accumulation zones Channels of smoother terrain are observed in many places between the rough and chaotic terrains described above (Fig. 1a and ESM Fig. 2a). They generally correspond to areas characterized by high sonar backscatter and can be interpreted as surfaces covered by coarse-grained sediments at the scale of the sonar wavelength (around 10 cm; Fig. 1b and ESM Fig. 2b). The Submarine Bulges, thus, appear to be dissected and bordered by these channels, which are as much as 200 m deep and 40 km long. The channels follow the steepest slopes and are generally connected to the coast. They are unambiguously erosion canyons, probably filled with material derived from land erosion. According to Krastel et al. (2001a, b), they primarily form by downslopeeroding mass flows and become deepened by further erosion and failures of the canyon walls and/or floor. The remarkably smooth north east proximal flank of the island, also called the north east sedimentation zone by Oehler et al. (2004), can be similarly regarded as a large zone of sedimentary accumulation (Figs. 1, 2, and ESM Fig. 2). It covers a zone 20 km in width, extending up to 15 km offshore, with a

9 Bull Volcanol (2008) 70: surface area of about 300 km 2. Around the island, several large alluvial deltas present at the mouths of subaerial rivers can be observed in bathymetry and sonar images. Volcanic constructions Some local features are clearly different from the types of feature described above. They are observed either near to the coast, as a continuation of subaerial structures, or as isolated seamounts on the flanks of La Réunion edifice, or on the surrounding seafloor. They are interpreted as volcanic constructions. Submarine volcanic constructions in continuation with land structures Around the coast of the island, four zones depart noticeably in morphology from the other submarine terrains: the continuation of La Montagne massif (also named Cap Bernard structure by Lénat and Labazuy 1990) to the north, those to the east and to the south of Piton de la Fournaise, and a ridge off Etang Salé City to the west (Fig. 2). They have rugged topography and steep slopes. They are also associated with negative magnetic anomalies (see Lénat et al. 2001, plate 1c) that are the most intense magnetic features on the submarine slopes of La Réunion. Modeling (Lénat et al. 2001) shows that the anomalies are created by shallow bodies with a large reverse magnetization, indicating that the zones are made up mainly of rocks older than the Brunhes Matuyama magnetic reversal. Therefore, the magnetic and morphological properties of these four zones strongly suggest that they are undisturbed volcanic constructions. On land, La Montagne massif, a pile of lava flows intersected by an abundance of dikes trending N20 W and interpreted as an ancient rift zone by Chevallier and Vatin- Pérignon (1982) is the oldest emergent part of the island, with ages up to 2.08 Ma (McDougall 1971). Offshore, beyond the coastal shelf, a submarine promontory and ridges mark the continuation of La Montagne massif (Fig. 2). The on-land formations also have a reversed magnetization (Lénat et al. 2001) that fits with their age. The presence of rocks older than the Brunhes Matuyama reversal was judged improbable in the eastern part of the island because the oldest rocks of Piton de la Fournaise volcano have ages of only 0.5 Ma (Gillot and Nativel 1989), but reverse magnetic anomalies have been identified and interpreted as indicating the presence of shallow reversely magnetized rocks (Lénat et al. 2001). The anomalies coincide with bathymetric highs beneath the continuation of the presentday north east and south east rift zones of Piton de la Fournaise (Fig. 2). From analysis of their detailed bathymetry, we follow Labazuy (1991) and Lénat et al. (1989, 1990) in interpreting them as remnants of old volcanic constructions, unrelated to the growth of the recent (<0.5 Ma) Piton de la Fournaise. However, to the west of the south east rift zone (i.e., near the south coast of the volcano; see also Fig. 9b and ESM Fig. 3b), we identify some areas as younger volcanic constructions. They have gentle slopes, small, kilometer-sized cones, bulges and ridges, and are, thus, similar to relatively young constructional features described on the submarine rift zones in Hawaii (Smith et al. 2002) and in El Hierro (Gee et al. 2001). Accordingly, we propose that this zone was built by relatively recent volcanic activity. The Etang Salé submarine ridge (Lénat and Labazuy 1990) is about 15 km long and a few kilometers wide (Fig. 2). Its detailed geology cannot be established with available information. Its elongate shape, radial to the island and comparable with hawaiian rift zones such as the Puna ridge (Smith et al. 2002), and the interpretation of the magnetic anomalies (Lénat et al. 2001), suggest, however, that it could be an ancient rift zone of Piton des Neiges, which has been partially eroded by mass wasting events. Isolated seamounts Eliane cone and its neighbors to the south The Eliane cone (Lénat et al. 1989), at least 600 m high and 3 km in diameter, is a very regular volcanic cone on the south flank of Piton de la Fournaise, 7 km from the coast (Fig. 4a). Its base lies at a depth of about 1,600 m. The pristine geometry of the seamount suggests that it has not been disturbed by flank landslides. Dredged samples from Fretzdorff et al. (1998) and Boivin et al. (unpublished data, 1989) indicate that the Eliane cone is basaltic. The rocks are possibly vesicular pillow lavas coated with Mn-oxides and Fe-oxyhydroxides. An age of ,000 years was found by Deniel (personal communication, 1989). The Eliane cone is the nearest to the coast of three seamounts aligned in a north south orientation (Fig. 4a). The middle one (seamount 1), although smaller than the Eliane cone (possibly because it is partly flooded by subsequent landslide products), has a similar conical shape. The southernmost (seamount 2), however, shows a more complex shape, nearer to that of the seamounts found on the plate around La Réunion (see below; Connection of La Réunion with the ridges at the south/south west ). Sonne cone The Sonne cone (so called because the first detailed cartography of the seamount was made aboard the German R/V Sonne; Fretzdorff et al. 1998) is a noteworthy 1,200-m-high, 8-km-long, 5-km-wide, 120 N-elongated seamount, located 40 km off the west coast of La Réunion (Fig. 4b). Dredged samples recovered from its north-western and northern flanks consist of lithified sediment and strongly altered basalt with thick Fe Mn crusts (Fretzdorff et al. 1998), suggesting an old volcanic structure. The Sonne

10 726 Bull Volcanol (2008) 70: Fig. 5 Organization, morphology, and topography of the seamounts and of the sedimentary ridges to the north/north east of La Réunion (a shaded relief, b detailed 3D view of the sedimentary ridges, and c topographical profiles). Among the eight small seamounts identified in the area, at least four are flat-topped. Seamounts 2, 3, and 4 seem to have their base on the seafloor. Seamount 1 and the seamount to the north seem to have theirs at significantly higher altitudes. Sedimentary ridges are principally located at the front of the Submarine Bulges. They are generally organized in fields of 30- to 40 m-high ridges cone seems to protrude from the flank formations of La Réunion, and its shape suggests that it might be one of the seamounts present on the oceanic plate around the island (see below; Connection of La Réunion with the ridges at the south/south west ). Flat-topped seamounts to the north and north east Aseries of seven small seamounts are observed at the base of the north east submarine slope of La Réunion and one at midslope to the north (Fig. 5). Among them, at least four are flattopped. Flat-topped seamounts are commonly found in various oceanic environments including the oceanic seafloor and the submarine flanks of volcanoes (e.g., Smith 1996; Bridges 1997;Clagueetal.2000). Most have a diameter of a few kilometers and are interpreted as monogenetic volcanoes. They can have a perfectly flat top, a central pit-crater or a rim. Their formation has been discussed by several authors (e.g., Clague et al. 2000; Zhu et al. 2002). La

11 Bull Volcanol (2008) 70: Fig. 6 Map showing the connection of La Réunion with the seafloor ridges at the south west. The ridges are shown by discontinuous heavy lines. The contours show the estimated seafloor topography from Smith and Sandwell (1997). Shaded bathymetry is shown where high-resolution data are available. The Sonne cone (labeled S) could be a seamount along one of the ridges. The cone labeled M is partly overwhelmed by debris avalanches from La Réunion Réunion flat-topped seamounts have dimensions comparable to previously described flat-topped seamounts in other places. Whereas seamounts 2, 3, and 4 on Fig. 5 seem to have their base on the seafloor (around 4,100 m), seamount 1 and the seamount to the north seem to have theirs at significantly shallower depths (ca. 3,800 and 2,750 m, respectively). If this is true, it would mean that these latter seamounts formed during the growth of La Réunion edifice and must be relatively young. The rather pristine shape of these flat-topped volcanoes also tends to indicate that they are younger than the other seamounts of the zone. The surrounding oceanic plate Although the high-resolution coverage beyond the slopes of La Réunion is restricted, in some areas, we are able to observe the transition between the volcanic edifice and the oceanic structures. Where no high-resolution data exists, the estimated seafloor topography from Smith and Sandwell (1997) can be used to visualize the main bathymetric features. The surrounding ocean floor generally exhibits a low sonar backscatter indicating a smooth surface covered by finegrained materials, probably hemi-pelagic and turbidite sediments. This homogeneous signature is disrupted in some places by (1) curvilinear lineaments of relatively higher backscatter, corresponding to small sedimentary ridges, and (2) significant bathymetric reliefs with generally high sonar response. Sedimentary ridges Sedimentary ridges identified on the proximal part of the seafloor off La Réunion are 30- to 40-m high structures, principally located in front of the Submarine Bulges (Fig. 5). They are generally organized in fields of ridges, each structure being parallel to the others with an average wavelength of 2 km. These fields can cover large surfaces, as for example, to the east of the island where they are observed over at least 400 km 2. La Réunion sedimentary ridges are comparable to features observed along the Hawaiian Ridge, but only on volcanoes west of Kauai and older than 5 Ma (Moore et al. 1994), El Hierro (Masson et al. 1998) or La Palma (Urgeles et al. 1999). They are interpreted as mud or sediment waves. For Moore et al. (1994), the absence of these structures on the younger slides in Hawaii suggests that they form slowly by subsequent slippage of blocks following a sufficient accumulation of sediment on

12 728 Bull Volcanol (2008) 70: Wilshaw transform faults (the transforms faults that bound the La Réunion compartment in the Mascarene Basin), as well as to most of the observed magnetic lineaments (Fretzdorff et al. 1998; Dyment 1991; Lénat and Merle, submitted). The ridges appear to be more or less continuous and are punctuated by seamounts of variable dimension and height. Fretzdorff et al. (1998) suggest that they could be cross-grain ridges associated with magmatism along tension cracks on the oceanic plate during the rifting, whereas Lénat and Merle (submitted) propose that they could be related to intrusion of hot spot plume material into the lithosphere. Where detailed swath bathymetry exists (Fretzdorff et al. 1998; Oehler et al. 2004; Oehler 2005), the Smith and Sandwell (1997) seafloor topography agrees well with the observed detailed topography (Fig. 6). Using the detailed data, we observe individual seamounts with different morphology and orientation along the ridges of the general bathymetry. They protrude from a flat seafloor covered by a Fig. 7 Top Map showing a seamount to the north of La Réunion. The contours show the estimated seafloor topography from Smith and Sandwell (1997). Shaded bathymetry is shown where high-resolution data is available. The high-resolution bathymetry swath covers only a portion of the seamount whose boundary is shown by a heavy dotted line. Bottom Topographic profile along the dashed line shown on the map. Note the elevation difference of the seafloor between the north and the south the slope. According to Masson et al. (1998) and Urgeles et al. (1999), they consist of small scarps resulting from destabilization of the sedimentary pile during the deposition of debris avalanches. We propose a different interpretation. These structures clearly appear to be linked with masswasting events. They could correspond to the folding of ductile pelagic sediments in response to compressive strains created at the front of advancing debris avalanches. The folds are, thus, perpendicular to the flow direction of the debris avalanche, and they can be used to delimit the extension of the deposits and to estimate their direction of propagation. Connection of La Réunion with the ridges at the south/south west Using the Smith and Sandwell (1997) seafloor topography, 50 N trending ridges are observed to the south of La Réunion (Fretzdorff et al. 1998; Lénat and Merle, submitted). They are oblique to the Mauritius and Mahanoro Fig. 8 Top Map showing the south east lower slope of La Réunion and the Mauritius Fracture Zone. The contours show the estimated seafloor topography from Smith and Sandwell (1997). Shaded bathymetry is shown where high-resolution data is available. Bottom Topographic profile along the dashed line shown on the map. Note that the slope of La Réunion extends practically up to the fracture zone

13 Bull Volcanol (2008) 70: few hundred meters of sediments (Fretzdorff et al. 1998). It is clear that at least some of the lineaments intersect the edifice of La Réunion. On Fig. 6, we can observe debris avalanche products from La Réunion that have flowed among the oceanic seamounts. These positive reliefs have probably acted as topographic barriers during the evolution of the submarine slopes of La Réunion. Similarly, the Sonne cone (labeled S on Fig. 6) could be a seamount along one of the ridges. Interference of La Réunion construction with a seamount at the north To the north of La Réunion, a high resolution swath crosses an oceanic construction (Fig. 7). The Smith and Sandwell (1997) seafloor topography suggests that the high-resolution swath is located on the edge of a large seamount. The bathymetric profile shows a significant altitude difference between the southern and northern flanks of the seamount. This difference is probably due to the accumulation of products deriving from La Réunion. Transition between La Réunion slope and the Mauritius fracture zone The Mauritius transform fault (Fig. 8), characterized by a central deep flanked by two topographic highs, has been described by Fretzdorff et al. (1998). Although the slope of La Réunion virtually vanishes about 50 km from the transform fault, we note that the bathymetry between the foot of La Réunion and the transform fault is not flat but decreases gently toward the fault. This suggests that deposits from La Réunion extend down to this area, possibly as fine sediments derived from landslides, for example, as turbidity current deposits. Evidence of multiple landslide units off La Réunion Previous studies of the submarine domain of La Réunion have revealed the complex organization of the Submarine Bulges. They have formed by the accretion of multiple mass-wasting events deposits (Labazuy 1991, 1996; Bachèlery et al. 1996; Oehler et al. 2004; Oehler2005). We have carried out the first detailed submarine cartography of the landslide units on the basis of well-defined geological (structural and erosional) or geophysical (acoustic and magnetic) criteria and on the evidence of submarine secondary landslides (Fig. 9, ESM Figs. 3 and 4). This allows us to differentiate the different units and, in some cases, to determine a relative chronology of the events. As the final map is constructed using these criteria, we describe them below using selected examples. Structural criteria Superimposition structures Successive debris avalanches in the same area create a layered structure in which parts of the underlying units may be visible. The boundaries of the deposits from individual events are usually marked by a scarp. The relative chronology of the units is deduced directly from the stratigraphy. Figure 9a (see also ESM Fig. 3a) illustrates this kind of relationship between different units in the case of the WSB South Lobe. Basal SL1 unit clearly represents the older debris avalanche products. Upper SL2 and SL4 tongues, with apparent thicknesses of about 300 and 500 m, respectively, result from younger landslide events. Deviation of flows by obstacles Debris avalanches are extremely rapid flows, and depending on their speed and energy, they can overcome topographical highs (Voight et al. 1981; Moore et al. 1989, 1994). More generally, however, they go around obstacles, forming two lobes that can merge on the downhill side of the obstacle. Many such structures are identified on the submarine flanks of La Réunion. The topographic obstacles may be seamounts or previous debris avalanche deposits. The geometric relationship between the obstacle and the flow deposits allows us to differentiate the structures and to establish their relative chronology. Examples of deviated flow deposits are shown in Fig. 9b (see also ESM Fig. 3b). The SSB MC1 debris avalanche was deflected by a seamount. The two 175 N and 150 N ridges and the depression in the downslope shadow zone of the seamount clearly illustrate this deflection. Upstream, MC1 itself constitutes an obstacle to the MC3 debris avalanche. MC3 deviates (and have been partially eroded by the emplacement of) MC4. Thus, the relative chronology of the three debris avalanches can be established. Lineament directions Debris avalanche block (or hummock) alignments give the local flow direction. Although different lineament directions can occur within individual flow deposits, this information may also be used to discriminate coalescent debris avalanche deposits. Figure 9c (see also ESM Fig. 3c) illustrates such a case. The analysis of the lineaments of the acoustic image reveals 60 N 80 N block alignments underlain by high backscatter in a zone to the west and 45 N in a zone at the south east. This characterizes two different flow directions and allows us to delimit NSB formations from those of the ESB.

14 730 Bull Volcanol (2008) 70:

15 Bull Volcanol (2008) 70: Fig. 9 (continued) RFig. 9 Illustrations of the criteria used to differentiate the landslide units. Inset shows the location of the different examples. See text, ESM Figs. 3 and 4 for more details. a Superimposition criterion applied to the WSB South Lobe. b Deviation of flows by obstacles criterion applied to the SSB Median Complex. c Lineament directions criterion applied to the junction between NSB and ESB formations. d Magnetic signature criterion applied to the WSB East Block. e Evidence of a submarine secondary landslide on the eastern flank of Piton de La Fournaise Erosion-dissection structures Debris avalanches have the capacity to erode their substratum. Erosion-dissection structures may, thus, form during their emplacement. Figure 9b (see also ESM Fig. 3b) shows such an erosional feature between the SSB MC3 and MC4 units. Similarly, the northern rim of the NSB Eastern Channel is interpreted as the result of the erosion of the

16 732 Bull Volcanol (2008) 70: median formations during the emplacement of unit EF2 (see ESM Fig. 4a). Erosional criteria Erosion channels According to Urgeles et al. (1999), erosion channels form along topographical lows at the margins of debris avalanche lobes, and their presence can be used to delimit deposits of successive landslides. This criterion is applicable to some areas of our study. For instance, the Western and Eastern Channels separate topographically distinct lobes (western, median, and eastern formations) of the NSB (see Oehler et al. 2004, Fig. 2, and ESM Fig. 4a). These canyons are connected to the coast, and their surface characteristics (smooth bathymetry and high backscatter) indicate that they are possibly filled with coarse-grained sediments. Relative degree of erosion The degree of erosion of the flank formations may provide a criterion to differentiate formations of different ages. For example, Oehler et al. (2004) describe two superimposed units with contrasting degrees of erosion within the WSB North Lobe (see Oehler et al Fig. 2 and ESM Fig. 4b). The basal unit (NL1 on ESM Fig. 4b), characterized by a ridge and furrow morphology, is carved by several small Fig. 10 Interpretative maps of La Réunion volcanoes flank landslides. a Global structural scheme. b Detailed map on the subaerial and proximal submarine parts of the edifice. Offshore, landslides are assigned acronyms according to the Submarine Bulges (w west flank, n north flank, e east flank, and s south flank), the source volcano (A Les Alizés volcano, N Piton des Neiges volcano, and F Piton de la Fournaise volcano) and their relative chronology (1, 2, ). sf2 reads as south flank, Piton de la Fournaise, second landslide. a, b,andc are additional labels used when needed for distinct landslides presumed to have the same age. On land, outcrops of debris avalanche breccias and the main topographic features interpreted as the potential source zones of the landslides are overlaid on a shaded image topography

17 Bull Volcanol (2008) 70: Fig. 10 (continued)

18 734 Bull Volcanol (2008) 70: channels filled with coarse-grained, high-backscatter material. These channels do not connect to the coast but are interrupted by the upper unit (NL2), which has a smoother surface. Consequently, unit NL1 is older than unit NL2. Geophysical criteria Acoustic signature contrast The acoustic signature of a formation depends upon its surface acoustic properties and texture. Variations in acoustic signature can help delineate juxtaposed formations. This criterion allows Oehler et al. (2004) to distinguish three debris avalanche units within the NSB median formations (see Oehler et al. 2004, Fig. 2 and ESM Fig. 4a): The upper, most recent formation (MF3 on ESM Fig. 4a) exhibits a relatively high reflectivity (in fact, a dense speckled pattern) corresponding to the presence of a large density of blocks at the surface. The lower oldest landslide deposits of the zone (MF1 and MF2 formations) have lower backscatters interpretated as a dominant coating of finegrained sediments. The degree of erosion of the formations also corroborates the stratigraphy established for this zone. The surface of MF1 is characterized by a ridge and furrow morphology and is significantly rougher than that of MF2 and MF3. Magnetic signature It has been noted by Lénat et al. (2001) that, except for a few volcanic constructions near the coast (see above; Submarine volcanic constructions in continuation with land structures ), the submarine flanks of La Réunion do not show magnetic anomalies that would be created by contrasts in magnetization intensity or direction of volcanic rocks within the edifice. Accordingly, they proposed that this could be explained if the submarine flanks are constructed by breccias derived from mass-wasting events. In breccias, the usually dominant remanent magnetization of the volcanic rocks is statistically canceled by the random rotation of the blocks within the breccia. Thus, the presence of a significant magnetic anomaly will indicate that the underlying rocks have not been affected by landslides or have, at least, remained coherent, even if they have undergone rotation. Figure 9d (see also ESM Fig. 3d) shows an example of where the magnetic signature can be used to reinforce an interpretation based on a morphological analysis. The WSB east block (EB), 10 km long and 8 km wide, is a structure located to the north of the Etang Salé Ridge. The EB surface morphology does not correspond to the typical hummocky one of debris avalanche deposits. On the contrary, the EB appears to be a more massive unit, consisting of several more or less coherent blocks, some of them forming shelves several kilometers wide. The fact that a magnetic anomaly is associated with the EB structure indicates that the EB rocks have remained coherent. We consequently interpret the EB as a slump structure where the coherence of the rocks have been preserved during translation and, possibly rotation, of the blocks. Evidence of submarine secondary landslides Several submarine structures can be interpreted as landslides developed from submarine debris avalanche deposits. For this reason, we have designated these structures secondary submarine landslides. One example is a structure observed to the east of Piton de la Fournaise (Fig. 9e; see also ESM Fig. 3e). The ESB Northern Depression (ND) is about 30 km long, 2 km wide, 300 m deep, and has a difference in altitude of 1,500 m between its head and foot. In contrast to the erosion channels, the ND is not connected to the coast, but is bounded upstream by the ESB western block formations. Its surface exhibits a speckled acoustic signature, typical of debris avalanche deposits. Near its downhill termination, topographical profiles suggest that 100 to 150 m of products are accumulated, and 0 N block alignments are observed, in agreement with the depression axis direction. The source zone of this debris avalanche is obviously located within previous mass-wasting deposits accumulated within the ESB. The ESB western block formations are interpreted as being derived from the more recent subaerial flank landslide of Piton de la Fournaise volcano. If, as suggested by some authors (Bachèlery and Mairine 1990; Labazuy 1991, 1996; Mohamed-Abchir 1996), this landslide formed at about 5 ka, then the ND secondary debris avalanche is penecontemporaneous or younger. This, therefore, draws our attention to the possibility that this secondary slide, and possibly others, may be very young, and that such failure events may recur in the near future. Synthesis The above criteria have been applied in the analysis of the whole of our dataset. The result is the map presented in Fig. 10a. Because the data are heterogeneous (see above; Data compilation and processing ) and the coverage is incomplete (see ESM Fig. 1), the construction of this map clearly suffers from uncertainties: Some unit boundaries have been extrapolated where data was lacking, the termination of the more distant units may be imprecisely defined, and some small-scale features may have gone unnoticed. We are, however, confident that the main surface units have been correctly mapped and that this document may, therefore, be used to discuss the evolution of La Réunion volcanic edifice.

19 Bull Volcanol (2008) 70: Thirty-seven landslides, essentially debris avalanches, accumulated on the Submarine Bulges are recognized. They attest to the occurrence and recurrence of destructive masswasting events of different scales on La Réunion volcanoes during their growth and evolution. The units generally have a triangular shape and radiate from the island. The geometry of most of them implies that their source area is subaerial. These may be qualified as primary mass-wasting events that have affected the constructed part of the volcanic edifice. In addition, purely submarine, as well as coastal, secondary landslides (Fig. 10) have remobilized the products of previous landslides. Geological and morphostructural study of Reunion Island: landslide deposits, source zones, ages, and relationships with sea structures As a large proportion of the submarine landslide deposits have their source on land, we have carried out a geological and morphostructural analysis of the island to find the evidence of landslide scars (products and faults). We describe several significant examples for which information on the age of the landslides is deduced from the available absolute radiometric dates. Piton des Neiges debris avalanche breccias Debris avalanche deposits identified on land in La Réunion (see Bachèlery et al and 2003; Maillot 1999; Bret et al. 2003; Fèvre et al. 2001, 2003, 2004; Arnaud et al. 2003, 2004; Arnaud 2005; Oehler 2005) show the same general characteristics as formations of similar origin on other volcanoes, for example, Mont St. Helens (Lipman and Mullineaux 1981), Mont Shasta (Crandell et al. 1984; Ui and Glicken 1986), Bezymianny (Gorshkov 1959; Siebert et al. 1987), Augustine (Siebert et al. 1995), and Cantal (Cantagrel 1995; Nehlig et al. 2001) volcanoes. They are essentially brecciated formations of bimodal composition, comprising a block with a matrix facies. The block facies corresponds to fragments of the dismantled volcanic cone, with block dimensions varying from a few meters to several hundred meters. Block texture and original stratification are often preserved, but the blocks are internally fractured and deformed. Characteristic jigsaw cracks (Ui 1983; Siebert 1984), identified on many scales in the deposits, testify to the pulverization and the progressive disintegration of the blocks within the avalanche during transport. They result from collisions between elements and from internal flow vibrations phenomena (Ui et al. 1986; Glicken 1996; Bachèlery et al. 2003). The matrix facies derives from the blocks disintegration. It consists of a non-classified, non-stratified mixture of centimeter- to decimeter-sized clasts and of fine, pulverized fragments, generally with the same composition as the surrounding blocks. Debris avalanche breccias are abundant and widespread within the Piton des Neiges massif (Fig. 10b). In contrast, they are not observed at Piton de La Fournaise, where most of the surface is covered by lava flows. The main outcrops are found (1) in the Piton des Neiges littoral zone, for example, in Saint-Gilles (outcrop 1 in Fig. 10b; see Bret et al. 2003; Bachèlery et al. 1996, 2003); or in the Sainte- Suzanne sector (outcrop 13; see Bret et al. 2003; Oehler 2005), where the deposits present a typical hummocky morphology; (2) at the mouths, or in the valleys, of the main rivers, for example, Rivière des Galets or Rivière des Pluies (outcrops 2 and 3, respectively; see Fèvre et al. 2001, 2003, 2004;Bretetal.2003;Oehler2005); and (3) within the three major inner depressions: the cirques of Mafate, Salazie, and Cilaos (outcrops 7 12; see Maillot 1999; Bret et al. 2003; Arnaud et al. 2003, 2004; Arnaud 2005; Oehler 2005). Most of the Piton des Neiges debris avalanche breccias can be interpreted in terms of subaerial proximal deposits of the landslide units mapped offshore. The most convincing example is certainly the case of the Sainte-Suzanne breccias (outcrop 13) located upstream of the nn5 submarine debris avalanche deposits (Fig. 10). These formations, therefore, probably constitute the land and sea evidence of a single mass-wasting event at least older than 0.23 Ma (Oehler et al. 2004). We also note that some on-land landslide breccias associated with small-size events do not continue offshore. For example, the Entre-Deux breccias (outcrop 4; see Oehler 2005) could correspond to the deposits of a moderate-sized, strictly subaerial landslide younger than Ma (the age of overlying lava flows; Kieffer 1990b). Similarly, an event younger than 0.23 Ma (McDougall 1971) could be responsible for part of the Rivière des Galets breccias (outcrop 2; Oehler 2005). Identification of the landslide source zones The source zone of a landslide is typically a horseshoeshaped amphitheatre. In La Réunion, the headwalls of some source areas should have an extension that can reach up to several tens of kilometers to be commensurate with the size of the largest submarine landslide units. However, the vestiges of ancient landslide scars are not obvious in the geology and in the morphology. The ancient structures are often filled and concealed by the products of subsequent volcanic activity, or dismantled by erosion. One exception is the Grand Brûlé depression (GB on Fig. 10b), interpreted as being the result of the most recent destabilization of the eastern flank of Piton de La Fournaise volcano (Chevallier and Bachèlery1981; Lénat and Labazuy 1990; Lénat et al. 1990; Bachèlery and Mairine 1990; Gillot et al. 1994; Bachèlery 1995; Labazuy 1991, 1996; Lénat et al. 2001;

20 736 Bull Volcanol (2008) 70: Fig. 11 Chronology diagram for the recognized mass-wasting events. We use Billard and Vincent (1974) for the Piton des Neiges series and Bachèlery and Mairine (1990) for the Piton de la Fournaise ones. Vertical axis is time in Ma. The horizontal axis is used to differentiate the Submarine Bulges. The vertical lines (continuous or discontinuous) represent the landslide units. Their thickness is proportional to the estimated volume. The solid and discontinuous parts of the lines Merle and Lénat 2003). Therefore, the search for ancient landslide scars relies on careful analysis of geological observations and morphological studies of the topography. This approach leads us to propose several possible source areas for the observed landslides. We should stress, however, that our results are based on a limited amount of geological and structural observations. Therefore, this part of the work should be considered only as exploratory. The identified structures are classified into three categories (Fig. 10b): 1. Arcuate coastal slope breaks or scarps, some on land, and some submarine, are interpreted as headwall faults of landslides draped by more recent products (class 1 on Fig. 10b). The age of the overlying formations (if available) allows the age of the landslides to be estimated. The associated deposits are often identified offshore in front of the faults. For example, the Saint-Paul structure (StP on Fig. 10b; see also Oehler et al. 2004; Fig. 6) is interpreted as the source of a secondary landslide older than the overlying 0.23 Ma lava flows draping these coastal cliffs. This structure is also younger than 0.34 Ma because it truncates the Saint-Gilles upper breccias (outcrop 1 on Fig. 10b; see Bachèlery et al. 2003), themselves show the estimated age of the units (solid reliable, discontinuous uncertain). Black lines correspond to primary landslides and gray ones to secondary events. BrI to BrIV Saint-Gilles debris avalanches, DA Dos d Ane landslide, MAPE Mare à Poule d Eau landslide, IV Ilet à Vidot landslide, RP Rivière des Pluies landslides, RPD Ravine des Patates à Durand landslides, PR Palmiste Rouge landslide, ED Entre-Deux landslide, BC Bras de Cilaos landslides. See ESM Fig. 5 for more details older than this age. The associated landslide products probably form the WSB wn6 unit (Fig. 10). 2. Topographic features on land delineate the boundaries of large amphitheatres that generally intersect the central zones of the volcanoes (class 2). They are interpreted as the remnants of old debris avalanche headwalls and are associated with La Réunion s most important masswasting events. Most of the traces of these faults have been disrupted or concealed by subsequent volcanic, volcano tectonic and erosional phenomena. However, some portions are obvious, and separate terrains of different ages, giving a constraint on the age of the landslides. For example, the northern flank of Piton des Neiges volcano is dissected by three interlocked class 2 structures (namely Mafate, Salazie, and Marsouins structures; Maf, Sa, and Mar on Fig. 10b, respectively; see also Oehler et al. 2004; Fig. 6). The Marsouins and Mafate structures cut the 2-Ma La Montagne massif and are filled by lava flows of about 1 Ma. These structures would, thus, correspond to the scar of two gigantic landslides that occurred between 1 and 2 Ma. Their deposits may be tentatively associated with NSB nn1 and nn2a-c units, respectively (Fig. 10a). The Salazie structure is interpreted as the scar of a more recent event, as it is filled by