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1 Geomorphology 36 (0) Contents lists available at ScienceDirect Geomorphology journal homepage: DEM-based reconstruction of southern Basse-Terre volcanoes (Guadeloupe archipelago, FWI): Contribution to the Lesser Antilles Arc construction rates and magma production Pierre Lahitte, Agnès Samper, Xavier Quidelleur Univ Paris-Sud, Laboratoire IDES, UMR848, Orsay, F-9405, France CNRS, Orsay, -9405, France article info abstract Article history: Received 3 March 00 Received in revised form 6 March 0 Accepted 5 April 0 Available online April 0 Keywords: Surface modeling Volume quantification Magma production Volcanic arc K Ar geochronology Lesser Antilles Arc We combine radiometric ages and geomorphologic investigations to quantify the relief creation of well preserved volcanic surfaces, applied to the volcanoes of southern Basse-Terre (Guadeloupe, Lesser Antilles Arc). The last 650 ka volcanic evolution of this island has been modeled using ten main stages constrained by K Ar ages previously obtained on lava flows and domes from the volcanic massifs of the Axial Chain ( ka) and from the Grande Découverte Volcanic Complex (50 ka present). Based on the construction of a 50,000-point database inferred from the analysis of the Guadeloupe Digital Elevation Model, 3D reconstructions of the successive volcanic stage landforms were calculated and the correlated geochronological maps drawn using ArcGIS software. Volumes and rates of construction were computed for each time span separating these ten stages. The average construction rates calculated here are.4 ± km 3 /yr for the last million years, 0.9 ± km 3 /yr for the last 00 ka, and 0.9± km 3 /yr for the last 5 ka. Although Basse-Terre volcanism is characterized by a marked dominance of effusive products, our estimates should be considered as minimum values because the material that went into the sea during explosive events was not taken into account. However, we note that a relatively high construction rate of 4.5± km 3 /yr has been obtained for the Icaques volcano, which was emplaced during the ka time interval, within the depression formed by the first large-scale flank collapse having affected southern Basse-Terre. The sudden release of the lithostatic load induced by this mass-wasting event could explain this value, which is significantly higher than the average value of 0.8± km 3 /yr obtained for the last 650 ka. Finally, the comparison with other analogous volcanic massifs from islands and continental arcs points to a relatively low magmatic production for southern Basse-Terre, which could be tentatively related to the relatively slow subduction rate of the Atlantic plate. 0 Elsevier B.V. All rights reserved.. Introduction Volumes of magma emitted by volcanoes through time can be estimated by the reconstruction of a volcano's morphology and time sequences. However, composite volcanoes often have complex morphologies resulting from the alternation of short effusive and explosive construction phases with long pauses in volcanic activity when erosion and catastrophic landslides become the dominant processes. In this paper we present 3D reconstructions of a range of composite volcanoes, the overall activity of which spanned the last millions years. Classical approaches for quantifying erupted volumes Corresponding author at: Univ Paris-Sud, Laboratoire IDES, UMR848, Orsay, F-9405, France. Tel.: address: pierre.lahitte@u-psud.fr (P. Lahitte). GEOTOP and Département des Sciences de la Terre et de l'atmosphère, Université du Québec à Montréal, Canada. on active volcanoes are based on the difference between pre- and post-eruption digital elevation models (DEM) when pre-eruptive surfaces are available (Stevens et al., 999; Murray and Stevens, 000). When these surfaces are poorly constrained, which is often the case for older and eroded volcanoes, other approaches using primary volcanic landforms are required to estimate the amount of the both erupted and eroded material (Hildreth et al., 003; Szekely and Karatson, 004), or, alternatively, to locate eruptive centers (Hampton and Cole, 009). We propose to combine here the two previous approaches in order to constrain volcanic production with a relatively high time-resolution covering three orders of magnitude, from 0 ka to Ma. Furthermore, our determinations are provided with a confidence interval, which is often ignored in many quantitative geomorphological studies. Based on the reconstructed primary volcanic landforms and a large amount of K Ar ages (Blanc, 983; Carlut et al., 000; Samper et al., 007, 009), the edifices of southern Basse-Terre (Guadeloupe, Lesser Antilles Arc) are good candidates to apply a high-resolution temporal X/$ see front matter 0 Elsevier B.V. All rights reserved. doi:0.06/j.geomorph

2 P. Lahitte et al. / Geomorphology 36 (0) reconstruction of their successive building and destruction phases over the last million years. Although erosion has played a significant part in the current morphology of Basse-Terre, the successive composite volcanoes are spatially separated enough to create a model according to ten construction stages. These topographic models enable us to propose reliable on-land volume estimations and related construction rates. Note that the term construction includes the volcanic material that remains on-land during explosive or effusive eruptions, which has therefore contributed effectively to the creation of the relief of the island.. Regional setting A 0 N 5 N Basse Terre Caribbean Sea GUADELOUPE Grande-Terre La Desirade Atlantic Ocean NAM 0 mm/yr Located in the central part of the western Lesser Antilles Volcanic Arc, the island of Basse-Terre is the most recent island of the Guadeloupe archipelago. The 0 km-wide volcanic front of the arc has been active since the Pliocene (Wadge and Shepherd, 984), while Basse-Terre subaerial activity began about at.79±0.04 Ma (Samper et al., 007). A simple observation of Basse-Terre morphology indicates that volcanic activity followed an overall north-to-south migration trend through time, as the northern massifs are much more eroded than the southern ones, the active lava dome of La Soufrière (highest Lesser Antilles point: 467 m) being located within the southern massif of Grande Découverte Volcanic Complex (GDVC). Structurally, the volcanic morphology of Basse Terre (Fig. A) is controlled by competition between the NW SE Basse-Terre-Montserrat en-échelon normal fault system and both E W rift systems of La Désirade to the north and of Marie Galante to the south (Feuillet et al., 00, 004). These two distinct fault systems, which characterize the northern part of the Lesser Antilles Arc, result from the tectonic interaction of the Caribbean plate with the North American plate, from late Pliocene to late Pleistocene (Feuillet et al., 00). Subaerial volcanic activity in Basse-Terre was demonstrated to have followed a north to south migration at a rate of 8 km/ma between at least.8 Ma and present time (Samper et al., 007). In the northern half of the island, sub-aerial activity occurred from.8 to.5 Ma along a NNW SSE trending lineament within the Basal Complex (BC) between.79 and.69 Ma, and then between.8 and.5 Ma within the so-called Septentrional Chain. South of the Septentrional Chain (SC) and since at least Ma, volcanism migrated southeastward. The Axial Chain (AC) was active over about 0.6 Myr, from.0 to Ma (Blanc, 983; Carlut et al., 000; Samper et al., 007, 009). The AC defines the northern limit of the Grande Découverte Volcanic Complex (GDVC), which occupies the southern one-third of Basse-Terre since at least 00 ka (Fig. B). The Monts Caraïbes massif, located at the southernmost end of Basse-Terre, is coeval with the end of Axial Chain emplacement ( Ma; Blanc, 983). Southern Basse-Terre is formed by a NW SE striking alignment of composite volcanoes throughout the Axial Chain to the GDVC. Like many other islands of the Lesser Antilles, Basse-Terre has been affected by major cubic-kilometer flank collapses (e.g. Boudon et al., 007). Since 650 ka, two of such events have affected the area of southern Basse-Terre that is studied here (Samper et al., 007). A large scar, culminating at Matéliane point (98 m a.s.l. present-day AC highest point), cuts the Axial Chain from east to west (Figs. B, ). It has been related to major destructive events that marked the evolution of Southern Basse-Terre and of the Axial Chain in particular. Samper et al. (007) and Boudon et al. (007) proposed the scar to result from a two-phase collapse sequence (Komorowski et al., 005; Samper et al., 007; Boudon et al., 007). Boudon et al. (007) proposed on the basis of debris avalanche deposits identified on-land on the western coast, that the two collapses were located to the west of the Axial Chain and were directed westwards, whereas Samper et al. (007) proposed on the basis of morphological and geochronological data that one collapse was located to the west and directed to the southwest, and that the second collapse was directed eastwards Marie- 0 N Les Saintes Galante 70 W 65 W 60 W 55 W B SEPTENTRIONAL CHAIN Ma BASAL COMPLEX Ma AXIAL CHAIN Ma GRANDE DECOUVERTE VOLCANIC COMPLEX < 0. Ma studied MONTS CARAIBES area Ma km Fig.. A) Geodynamic setting of the Lesser Antilles Arc. Main faults from Feuillet et al. (00). Inset shows Basse-Terre in the Guadeloupe Archipelago. B) Basse-Terre Island shaded digital elevation model (light from NE). Large numbers are successive volcanic massifs separated by heavy lines. White star: active volcano of La Soufriere. The box represents the studied area (Fig. ). Coordinates are in UTM 0 N. and situated beneath the eastern branch of the scar where the Capesterre volcanoes lies today. Their occurrences during the last 650 ka have been associated with the on-land termination of the Marie Galante rift system (Feuillet et al., 00). Based on a detailed chronology, the Grande Découverte Volcanic Complex effusive volcanism appears as a succession of seven different constructional episodes since 400 ka, with spatially distinct eruptive centers (Blanc, 983; Carlut et al., 000; Samper et al., 007, 009).

3 50 P. Lahitte et al. / Geomorphology 36 (0) Stage I ( ka) Stage IV-a ( ka) Stage VI (50-50 ka) Stage IX (70-45 ka) Stage II ( ka) Stage IV-b ( ka) Stage VII (50-00 ka) Stage X-a (45-5 ka) Stage III (< 550 ka) Stage V (30-50 ka) Stage VIII (00-70 ka) Stage X-b (5-0 ka) Beaugendre river Vieux-Habitant river river Sans Toucher Volcano (IV-b) Icaques Volcano (II) Saint Louis river Classriver Class Grand Carbet river Perou river GDS- (VII) C Holocene activity (X-b) GDS-3 (VIII) Lava domes and MPA (TRMF-) (IX) ML (V) scoria cones PM (V) cited in text C: La Citerne Holocene Monts E: L'Echelle activity S: La Soufriere Caraibes (IV-a) TRMF- (X-b) ML: Morne Lafitte (VIII) PM: Petite Montagne GCr: Grosse Corde river GDS : Grande Decouverte-Soufriere MPA : Madeleine - Palmiste Alignment TRMF : Trois Rivieres - Madeleine Field 3 Galion river pre 650 ka Axial Chain (I) GDS- (VI) S Mateliane Capesterre E r. Capesterre Volcano (III) GDS-3 (VIII) (X-a) GCr Fig.. Shaded DEM and map showing the present-day extent of the volcanic stages including major volcanoes, collapses and river features of the southern Basse-Terre Island, as discussed in the text. Coordinates are in UTM 0 N. Units are labeled by stage number (roman numerals in brackets). Upper case letters refer to lava domes cited in text. ) Rivers; ) Post 650 ka flank collapse scarps; 3) Buried part of the flank collapse scarps. 3. Methods Our aim is to model realistic three-dimensional successive landforms showing the spatio-temporal evolution of southern Basse-Terre over the last million years, including constructional and destructive events, such as large composite cone formations and sector collapses. At higher spatial resolution, we have made the distinction between constructional (lava flows, scoria cones and pyroclastic layers) and erosional volcanic features such as radial valleys and ridge patterns that are controlled by the initial volcanic landforms (Hampton and Cole, 009). Because lava flows are often erupted at a central vent, volcanoes develop, to a first order, a radial cone-like shape with an axial symmetry. Since volcanic edifices do not show necessarily a perfect conical geometry, we have selected the most representative flank profiles on which our model could be based. Fortunately, southern Basse-Terre, and in particular the Axial Chain (AC), is characterized by the spatial displacement of centrals vents, such that successive cones have radiated around different distinct summits that are easily identifiable. For each of the ten selected key stages, we have been able to identify the resultant remnant surfaces that altogether represent the morphological evolution of the island. We label each main morphological step as stage whereas other shorter geological events such as eruption episode or period of erosion are labeled as phase. Using the GIS software ArcGIS, we analyzed the ten selected surfaces from the IGN (Institut Géographique National) DEM of

4 P. Lahitte et al. / Geomorphology 36 (0) Guadeloupe Archipelago. The DEM horizontal resolution is 50 m and it uses the 0th north zone of Universal Transverse Mercator (UTM) projection. Our original morphometric approach allows the reconstruction of most of Quaternary age old surfaces on arc volcanoes as well as on basaltic shield volcanoes (e.g. Salvany et al., 0). Because uppermost volcanic stage surfaces resulted from different settings and periods of construction and/or denudation, our aim was to define for each of them the optimal parameters that best model these surfaces. We then correlate the ten successive modeled surfaces with the entire massif history by taking into account the erosion processes in order to propose a set of ten realistic subsequent topographic and geochronological unit maps. For each stage, we performed the following six-step process: ) extraction, from the present-day topography, of the points whose altitudes are significant to the uppermost surface of the considered volcanic stage; ) modeling of the uppermost volcanic surface of the considered stage; 3) independently, modeling of the erosional evolution of the landforms of the previous stages; 4) modeling of flank-collapse scarp evolution; 5) comparison of the modeled surfaces of second and third steps in order to define the actual extent and geometry of the modeled surface of the new volcanic stage and to create a new geochronological unit map; 6) calculation of the volume of the considered volcanic stage, and estimation of both construction and height increase rates. 3.. Extraction of points and creation of a database Firstly, we created a database of 50,000 points, each one corresponding to a cell of the original DEM. For further convenience of identification and selection, the following characteristics were attributed to each DEM cell: geological stage, altitude (ALT_DEM), XY coordinates, local slope, local aspect, profile curvature, and plan curvature. Profile curvature represents the rate of change of the slope in the vertical plane oriented as the aspect direction. It is negative if the shape is concave, positive if the shape is convex and equal to zero if the surface is planar. Plan curvature is the shape of the surface viewed as if a horizontal plane was cutting through the surface at the target point. It is calculated as the second derivative of the contour line. Some points of the present DEM are located on ridges belonging to the uppermost volcanic surfaces of earlier volcanic edifices. We interpret them as representative enough of their original stage surface (large and small black dots in Fig. 3), which, when identified, receive a specific attribute (UPPER_SURF) corresponding to the time at the end of the considered stage (nd parameters in Fig. 4). Alternative surfaces on which part of our reconstructions can be based are planèzes. They correspond to the dissection of a sub-mature volcano, where sectors of the former constructional surfaces only survive on the interfluves between deeply eroded substantial valleys (Hildenbrand et al., 008; Germa et al., 00). In both approaches (ridge or planèze landforms), selected points are the ones where the integrated amount of erosion through time is minimal or even null. Finally, each point received two more attributes (third and forth parameters in Fig. 4): the stage age of the present-day outcropping surface (AGE_OUTCROP) and the uppermost modeled altitude reached during the growth of the volcanic stage that presently outcrops (ALT_OUTCROP, see details in Section 3.). The youngest stages (i.e. Grande Découverte Volcano, 50 0 ka, stages V to X) still significantly display large areas on their uppermost GDS- GDS-3 GDS-3 CV GDS-3 MC GDS- GDS- STV IcV AC- Fig. 3. Method of surface reconstruction. 3D shaded DEM of southern Basse-Terre (looking southward) showing in the foreground the pre-collapse Axial Chain stage (AC-, dark surfaces) and background, the post AC- volcanic stage (bright surfaces). Dots: points of the DEM considered as representative to the uppermost surface and significant for the models (large black dots: AC-; small black dots: IcV, CV, MC, STV; white dots: GDS-, GDS- and GDS-3 stages). Black grid: modeled surface of AC- pre-erosion uppermost stage. White vertical lines: height difference (i.e. amount of erosion) between AC- modeled surface and present topography. For clarity, only difference for 400, 600, 800, and 000 m initial modeled altitudes are shown here.

5 5 P. Lahitte et al. / Geomorphology 36 (0) Present topography Initial modelled surfaces Modelled surfaces lower than previous stages topography successive eroded topographies 00 ka 70 ka ka A Attributes Attribute codes A B C D E ALT_DEM (m a.s.l.) P UPPER_SURF (age in ka) P AGE_OUTCROP (age in ka) P ALT_OUTCROP (m a.s.l.) P ALT_PRE_00 (m a.s.l.) P σ ALT_PRE_00 (m) P MODEL_00 (m a.s.l.) σ MODEL_00 (m) P7 P GEOL_POST_00 (unit code) P0>P5 -->00, P0<P5-->50 P ALT_POST_00 (ma.s.l.) P σ ALT_POST_00 (m) P height increase (m) (=P7 - P5)) P σ height increase (m) (=sqrt(p8 +P6 )) P ALT_PRE_070 (ma.s.l.) P σ ALT_PRE_070 (m) P MODEL_070 (ma.s.l.) P σ MODEL_070 (m) P GEOL_POST_070(unit code) P6>P4 -->70, P5<P8<=P9-->00, P8<=P5-->50 P ALT_POST_070 (ma.s.l.) P σ ALT_POST_070 (m) P height increase (m) (=P4 - P) P σ height increase (m) (sqrt(p3 +P5 )) P B C 00 ka 70 ka 70 ka 0 ka 70 ka 0 ka D E 50 ka 00 ka 0 ka 70 ka Fig. 4. Idealized example of calculation of database attributes. From the present topography (heavy lines), and definitions of units, the uppermost points are extracted and the next stage modeled uppermost surface calculated, and then compared to the previous stage topography in order to define the next stage altimetry with its correlated geochronological map. surfaces, thus all the points except the ones located along gullies, valleys or dismantled areas are considered as representative of their respective uppermost volcanic surface (white dots in Fig. 3). However, as the oldest stages generally show no evidence of large uppermost volcanic surfaces (i.e. Axial Chain, ka, black dots in Fig. 3), we only extract the points corresponding to the local highest altitudes as the best approximation of their uppermost paleo-topographies. Other valid points, with the highest values of longitudinal curvature (large and small black dots in Fig. 3), are also extracted along the present ridges and along the limits of the main watershed outlines. A detailed examination is then carried out in order to remove abnormal points, located along the upstream or the downstream limits of river subbasins or showing high values of profile curvature, typical of eroded surfaces (Fig. 3). As we proceed, for each of the ten modeled stages, four new attributes are calculated for each point of the database (Fig. 4): ) the altitude reached prior to the construction of the next modeled volcanic surface of the considered stage (ALT_PRE_XXX attributes, e.g. ALT_PRE_00); ) the uppermost surface altitude modeled for the newly considered volcanic stage (MODEL_XXX attributes, e.g. MODEL_00); 3) the final altitude reached when the construction was completed (ALT_POST_XXX attributes, e.g. ALT_POST_00); 4) the geochronological stage code corresponding to the stage age of the volcanic surface that outcropped at that moment (GEOL_POST_XXX attributes; e.g. GEOL_POST_00). In Fig. 4, examples of calculations of these database attributes are displayed for the 00 ka and 70 ka stages. Note that this figure is a theoretical case study that attempts to display all possible situations. 3.. Modeling of the uppermost volcanic surface of a stage For each stage, we first carried out the modeling of the uppermost volcanic surface of a stage (MODEL_XXX). For this, we used two different methods depending on the nature of the available preserved surface, based on the spatial extent and distribution of the points selected as significant for the corresponding uppermost surface. Where these points are widespread and well distributed all around the edifice summit, we created the uppermost surface by an ordinary

6 P. Lahitte et al. / Geomorphology 36 (0) kriging interpolation. The regular black grid in Fig. 3 displays an example of this type of surface for the Axial Chain first phase (MODEL_650). Where selected points only represented a small sector of the whole edifice surface (e.g. for the 50 50, and ka stages in Fig. 3), we needed to calculate the geometric surface of revolution that best fits the selected points. The generatrix of this surface (chosen as coplanar to the z-axis) can generally be either a straight line (e.g. 3D conical shape), or an exponential (e.g. classical andesitic volcano shape) or a Gaussian curve (e.g. smaller lava flow fields). This method allowed the prediction of uncertainty values (σ ALT_POST_XXX ) for the altitudes of each modeled surfaces through the use of standard error maps Evolution of the uppermost stage surfaces: model of erosion When an uppermost stage surface is modeled, we seek to determine its actual initial extent. In order to draw it, we needed to model how the surface of the previous stage had evolved up to the end of the considered stage. Two different methods were followed, depending on two different cases. a) Where for a considered stage, its volcanic units have not been covered by more recent ones, we used a linear model of evolution through time of the altitude, from the uppermost modeled surface to the present day topography: ALT PRE T = ALT DEM + ðalt OUTCROP ALT DEMÞ T = AGE OUTCROP where T is the moment for which we want to model the topography. For instance, the altitude of point C in Fig. 4, has evolved between 00 ðþ and 70 ka from 430 to 388 m (a.s.l.) as ALT_PRE_070=90+(430 90).70/00=388 m. b) Where a stage is today covered by younger material, we estimate the change in altitude through time of the buried area, based on the evolution of adjacent areas that still crop out (Fig. 5). In order to illustrate this approach we now present the case of the Axial Chain (AC) emplaced between and 0.65 Ma, which also corresponds to the basal surface of most of the GDVC. AC is largely covered by younger volcanic products that therefore prevent the use of any extrapolation method. In Fig. 3, the vertical lines connect the points of the modeled uppermost AC surface (ALT_POST_650) whose altitudes are equal to 400±5, 600±5, 800±5 or 000±5 m (a.s.l.) with the equivalent present-day DEM points. The height of each line mimics the amount of erosion that has occurred here since 650 ka, and the dispersion of heights illustrates the range of intensity of erosion for the same initial altitude. The altitude of each buried stage surface can then be estimated by the following method: for every altitude i of the uppermost modeled surface (ALT_POST_650), we calculate the average altitude of the DEM (ALT_DEM) for the points verifying: i 5b= ALT_POST_650b=i+5(black curve, Fig. 5A). Fig. 5A shows the range of altitudes of the present-time northern Axial Chain for each modeled altitude (gray curves, Fig. 5A). The difference between the modeled altitudes i and the averaged present-time altitude provides an estimate for the amount of erosion that is a function of altitude: dz 650 ðþ= i j =;n ALT POST 650 j = n j =;n ALT DEM j = n ðþ where ALT_POST_650 j and ALT_DEM j are the altitudes of the uppermost stage surface and of the modern surface, respectively, for the n points verifying i 5b=ALT_POST_650b=i+5 (Fig. 5B). The (m a.s.l.) A alt max min range Uppermost Axial Chain surface altitude (m a.s.l.) (m) 00 B dz dz±σ Uppermost Axial Chain surface altitude (m a.s.l.) Fig. 5. Extraction of erosion parameters from the pre-collapse Axial Chain surface (AC-) still cropping out. A) Average (black curve), minimal, maximal (heavy gray curves) and range of altitudes (light gray curve) of the present-day AC- surface (Y-axis) compared to AC- pre-erosion modeled altitudes (X-axis). B) Average erosion (±σ) as a function of AC- pre-erosion modeled altitudes. Vertical gray lines illustrate similar altitude examples as in Fig. 3.

7 54 P. Lahitte et al. / Geomorphology 36 (0) altitudes of the buried stage surface are thus estimated by the following final function: ALT PRE TðALT POST 650; TÞ = ALT POST 650 dz 650 ðalt POST 650Þ ðstage AGE TÞ= STAGE AGE ð3þ For instance, altitude of an Axial Chain surface that is modeled as reaching 800±30 m (a.s.l.) at 650 ka is modeled at 435 ka as: ALT_ PRE_435(800,435)=800 dz 650 (800) ( )/650=767± 58 m (a.s.l.). In this particular case, integrated erosion is modeled as dz 650 (800)=00±50 m (Fig. 5). By following a similar method, three other erosion models are created in order to estimate the evolution of the subsequent buried areas, for Icaques (IcV) and Capesterre (CV) volcanoes, and for the Monts Caraïbes massif (MC) Evolution of the flank-collapse scarps The two-stage collapse sequence that affected the Axial Chain was a major destructive event in the story of southern Basse-Terre. After these events, volcanic activity resumed either inside or south of the resulting horseshoe-shaped scarps, starting within each of the two collapsed structures (IcV and CV, stages II and III). Regressive erosion is proposed in this paper as the main mechanism that connected the two collapse scarps (Samper et al., 007). Their original rims have now largely disappeared by erosion or lie underneath younger lava flow units (Fig. 6). In order to properly model what the initial shape of collapse scars may have been, their lateral limits are defined by the use of the maximal extent of the subsequent lava flows that cover the floor of the depression, since it has been shown that the accumulation rates of the lava flows following a volcano flank collapse can increase significantly (Hildenbrand et al., 004). Due to flank collapse events, the material deposited below the scarps and the subsequent lava flows quickly covering the floor of the depression contribute to change the morphology of the area around the volcano, and to trigger significant changes in the whole drainage system. River shifts are common, as well as valley incisions on the outskirts of stage landforms (Branca, 003), and because flank collapse walls are very high and steep, their shapes evolve principally through regressive erosion. To determine the spatial expanse of the landforms after each flank collapse event, the geometrical evolution of the two sector collapse scarps has to be estimated through time. As no access is possible to what the initial slopes of the flank-collapse walls may have been, we need to refer to analogs. Recent studies modeled the slopes of Mount Saint Helens sector collapse walls (Obanawa and Matsukura, 008) and showed that the cliffs of the collapse scar quickly evolved from about 55 to 45 during the first two decades after the collapse event. We use in our modeling the value of 45 for the initial geometry of the two Basse- Terre escarpments, which also helps us to define a plausible location of their intersection with the Axial Chain surface (ALT_POST_650). We then model independently the Axial Chain surface still outcropping (surface in Fig. 6, i.e. to the north of the current watershed boundary in 3D images of Fig. 6), as well as the surfaces of IcV, CV and STV (surface in Fig. 6, i.e. south of the current watershed boundary in 3D images of Fig. 6) using Eq.. As these surfaces are connected to points corresponding to the top of the flank-collapse scarps, we assume a wall retreat through time defined by the evolutions of both the location and the altitude of the summit of the scarps (large black dots in Fig. 6). We then model both the scarp surface ( in Fig. 6) and the disappeared Axial Chain surface (3b in Fig. 6) by interpolating the edges of the surfaces of the still outcropping units (surfaces and in Fig. 6) and the summit of the scarp. The movement of wall retreat through time from its initial to present-time locations can be followed on 3D images of Fig. 6 (dashed and heavy black lines, respectively) Topographic modeled surfaces and geochronological units map We seek to define as precisely as possible the location of each new stage by comparing the modeled altitude of the surface of the eroded previous volcanic stage with the altitude of the newly modeled younger volcanic stage. Where the latter surface is higher than the former, the corresponding points are considered as a significant extension of the stage. They are thus selected as part of the new modeled younger stage surface and a new stage code is affected to the attribute GEOL_ POST_XXX. The following algorithm illustrates the attribute determination for an arbitrary stage YYY: if the MODEL_YYYNALT_PRE_YYY condition is verified then GEOL_POST_YYY=YYY and ALT_POST_ YYY=MODEL_YYY (see for instance points A and B at 70 ka in Fig. 4). Elsewhere, i.e. where the modeled surface of new volcanic stage is lower than previous eroded surface (MODEL_YYYb=ALT_PRE_YYY), we consider that the new stage did not reached this area. We then search for which of the previous volcanic stages outcropped at that time, i.e. had the given altitude at that moment: if ALT_PRE_ XXXbALT_POST_YYYb=ALT_POST_XXX then GEOL_POST_YYY=XXX. For instance at 00 ka, points for which the Sans Toucher Volcano (435 ka) was still outcropping (GEOL_POST_00=435) verify ALT_PRE_ 435bALT_POST_00b=ALT_POST_435 and GEOL_POST_435=435. They correspond, from the points where STV was outcropping at 435 ka, to points with altitudes at 00 ka comprised between the ones of the Sans Toucher Volcano's basal and uppermost stage surfaces (see for instance STV evolution in Fig. 6). Point E (Fig. 4) illustrates comparable theoretical evolution at 70 ka. Finally three-d reconstructions have been made. They illustrate for each successive stages, both their topography distribution and their geochronological map (Fig. 7) Volumes, construction and height increase rates calculations Volumes (V XXX ) are obtained for each stage XXX by integrating the difference between the points of the uppermost stage surfaces with those of the previous surfaces: Vxxx= i =,n dx dy dz i where dx=dy =50 m is the cell DEM resolution and where dz i = ALT_POST_XXX ALT_PRE_XXX is the variation of altitude for one point between two successively modeled surfaces, restricted only to the n points belonging to the considered stages (GEOL_POST_XXX= XXX). Then, the calculation of the volume uncertainty consists of integrating both uncertainties in the basal and uppermost stage surface altitudes for each cell (σ dzi ) value: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi σ VXXX = σ dzi = σ ALT PRE XXXi + σalt POST XXXi i =;n i =;n The construction rate (C.R. in km 3 /yr) of each stage is then determined by dividing its calculated volume V XXX by its duration (ΔT). In the same fashion, we estimated either the eroded or collapsed volume of some well-constrained events by integrating the difference between an upper older surface and a lower younger one. The height increase rate (H.I.R., measured in m/ka) is derived by dividing the construction rate by the total area of the volcanic units that were emplaced during the considered erupted stage. 4. Southern Basse-Terre landforms, volumes and construction rates evolution 4.. Reconstructions of the evolution through time of the axial chain and Grande Decouverte Volcanic Complex Each modeled stage is illustrated by two perspective views (from the south and the west, Fig. 7A D). Their respective volumes are compared in Fig. 8A. ð4þ

8 P. Lahitte et al. / Geomorphology 36 (0) ) Evolution through time of the altitude of the surface of the part of the Axial Chain (AC-, after 650 ka) that is still outcropping (i.e. eroded by surface runoff, not by the wall retreat) using Eq. in text. 3b 600 ka 435 ka 50 ka ) Evolution through time of the altitude of the surface of the volcanoes built inside the collapse depressions (Icaques, Sans Toucher and Capesterre volcanoes, ka) eroded by surface runoff using Eq. in text. 45 ka 3) Evolution through time of the scarp wall retreat () and the part of the Axial Chain (3b) that disappeared due to this wall retreat. First, we calculate locations of the scarp summit (large dots) that retreated linearly through time (its elevation is calculated using Eq. 3, in text). Then, and 3b surfaces are linearly interpolated from both the edges of surfaces and (small dots) and the scarp summit. 650 N b 3b 3b b 3b 3b b 3b Fig. 6. Sketch illustrations of the wall retreat of the Icaques scarp from 650 ka to present. In 3D views, the light black curves mark the initial position of the scarp scars, whereas the dashed and straight lines illustrate its retreat thought time. In order to highlight variation of elevation and scarp retreat, a 000 meter height transparent line follows the location and elevation of the current watershed boundary. Also shown are the evolution of the eroded surfaces and its implication for the definition of geological units through time. See for instance evolution of Icaques (550 ka) and Sans Toucher (435 ka) volcanoes. Stage I (Fig. 7A) corresponds to the Axial Chain (AC) morphology at 650 ka. The northernmost part of the Axial Chain was built over the southern products of the Septentrional Chain, and is composed of lava flows associated with composite and sometimes compound volcanoes. We define the basal level of the aerial part of the AC massif as the present sea level, and consider that most of the products erupted from past effusive activity are still preserved on-land, hence showing a shape similar to what it looked like 650 kyr ago when activity ceased. To proceed with the reconstruction, we extract from the points located along the present ridges and the main watershed boundaries (corresponding to the local highest altitudes reached by lava flows) those with the highest longitudinal curvature (large and small black

9 56 P. Lahitte et al. / Geomorphology 36 (0) Fig. 7. A to D: Models of the succession of reconstructed stage topographies displayed in perspective views. The heavy black line is the present-day coast. Pins mark the position of the present-day Axial Chain and Grande Découverte volcanic complex highest points (Matéliane, 98 m, and La Soufrière, 467 m, respectively).

10 P. Lahitte et al. / Geomorphology 36 (0) Fig. 7 (continued).

11 58 P. Lahitte et al. / Geomorphology 36 (0) Fig. 7 (continued).

12 P. Lahitte et al. / Geomorphology 36 (0) Fig. 7 (continued). dots in Fig. 3). A lava dome (Morne Boucanier) on the northern flank of the Monts Caraibes volcano displays similar composition and mineralogy to those of the main Axial Chain (G. Boudon, pers. comm.). This comparison supports the hypothesis that the extension of the southern flank of the Axial Chain could have reached the northern area of the Monts Caraibes. For the whole Axial Chain that was emplaced before the occurrence of the flank-collapse events, we have calculated a volume of 89±30 km 3 (AC- in Table ) with a summit culminating at 390 m a.s.l. about km to the south-east of the present location (98 m a.s.l.). The first flank collapse event led to the formation of an arcuate structure that, with the renewal of volcanic activity, hosted the subsequent emplacement of the Icaques volcano (IcV) (Samper et al., 007), as shown for the stage II at 600 ka (Fig. 7A). Following a similar approach to stage I, we model a volume of 7±5 km 3 for the remnant horseshoe-shaped structure that used to cover about 60 km. IcV's total volume reaches 3.6±4.0 km 3. We have calculated that at least.±. km 3 have been eroded since the collapse event. Before stage III (550 ka), a second flank collapse event directed towards the southeast affected the Axial Chain on its southeastern side (Fig. 7A), followed by the emplacement of the Capesterre volcano around 550 ka ago (Samper et al., 007). Following the same approach used for the previous collapse, we have computed a volume of ± 6km 3 for the collapse structure, and.4±5.3 km 3 for the Capesterre Volcano, for which we have calculated that a quarter of its volume has since been removed by erosion (.5±. km 3 ). Note that the relative uncertainty is high (88%) for this former volume because the absolute uncertainty on the uppermost surface altitude is almost equal to the amount of erosion. Stage IV (435 ka) shows the emplacement of the Monts Caraibes and Sans Toucher volcanoes (STV), respectively to the north and south of the Axial Chain (Fig. 7B). A modeling procedure similar to stages I to III was followed. Monts Caraibes was partly built under water (Westercamp and Tazieff, 980; Boudon et al., 988) but its subaerial part reached 7± km 3. The nature of the Monts Caraibes volcanism suggests that this volume is underestimated. In this edifice pyroclastic material is dominant with hyaloclastites erupted during large hydromagmatic eruptions for the first edifice, and voluminous plinian fallout, scoria flow and block-and-ash flow deposits for the second edifice (Boudon et al. 988). With not enough lava flows able to resist the erosion, the initial uppermost surface has probably been uniformly lowered and no present topography can be recognized as equivalent to the uppermost surface. Consequently the modeled surface altitudes are too low, which leads to an underestimation of both the volume and the amount of eroded material for this stage IV. Sans Toucher volcano was partly built over the Icaques volcano, and our modeling shows that STV may have been the smallest volcano of the Axial Chain with only.±0.3 km 3, 0.5±0.3 km 3 having been eroded since emplacement. We also infer that it is mostly during this stage that the remnant ridge previously located between the two flank-collapse rims (Fig. 7B), disappeared by regressive erosion. Now, it is replaced by a large watershed that hosts the Class river (Fig. ). Stage V shows the southern Basse-Terre morphology at 50 ka (Fig. 7B), before the construction of the Grande Découverte Volcanic Complex (GDVC). If several edifices were emplaced during this stage, they are now either eroded or covered by the GDVC products, as only two lava domes still crop out to the southeast (Morne Laffite, 34± ka and Petite Montagne, 6±7 ka; Samper et al., 009; ML and PM, respectively, in Fig. ). Between 630 and 435 ka, volcanism most likely remained confined inside the two flank-collapse depressions (Fig. 6); as a result, the southern AC- surface was lowered by river erosion, from a north to south stream network, similar to the one still visible on northern AC-. As volcanic activity probably kept occurring from 435 to 50 ka, products are likely to have been buried by more recent phases that make us unable to constrain their extent. Then, as GDVC grew, the stream network has probably been modified towards a renewed radial distribution, as observed elsewhere around volcanic edifices (e.g., Branca, 003). A record of this complex history of capture and diversion must be partially registered in the present circum -GDVC rivers network. Stage VI (50 ka; Fig. 7B) marks the start of the GDS emplacement (here referred as GDS-). To constrain this modeling, we have used the remnant uppermost surface of two GDS- lava flows. One reached the upper area between Icaques and Sans Toucher volcanoes (GDS- slopes, Fig. 3) and the other one was channeled northeasterly between the sector collapse cliff and Capesterre volcano (Fig. 7B). The latter is now topographically above and isolated by the incision of the deviated Class and Capesterre rivers (Fig. ). Thus, its floor acts as a perfect paleo-altimeter that has been recording the rate of erosion since its emplacement. The volcanic cone we model for GDS- reaches 3.7±. km 3 (Table ) and is centered at 64,300,,775,600 (long/lat UTM coordinates) with an altitude of 335 m a.s.l. (the peak being located 00 m above and 400 m south of the current highest point).

13 60 P. Lahitte et al. / Geomorphology 36 (0) Fig. 8. A) On-land construction volume (in km 3 ). B) Construction rates ( 0 4 km 3 /yr). C) Height increase rates (in m/ka). Same acronyms as in Table and Fig.. Error bars are given at σ. Stage VII (00 ka) shows the second episode of growth of GDS (GDS- ), which occurred between 50 and 00 ka (Fig. 7C). We use the remnant uppermost surfaces (Fig. 3) to model an exponential profile centered at 64,600,,775,900 with a height of 5 ma.s.l. (peak located 00 m above and 600 m north-west to the presently highest point), yielding a total volume of 7.±3.3 km 3 (Table ). Due to the uncertainty of the modeled altitude at stage I of the underlying Axial Chain, and because of its poorly constrained evolution until 00 ka, GDS- could have extended further to the southeast (dark area in Fig. 7C) and thus the volume of the ka period may be overestimated. Also, the ka period is likely to have been explosive dominant, generating mostly pyroclastic deposits and only a few lava domes and lava flows (G. Boudon, pers. comm.) that mainly originated from the volcanic edifice GDS- emplaced during Stage VI. GDS- episode, the most voluminous phase of the GDVC (Table ), which largely covers the GDS- episode, most likely caused the filling of the southeastern parts of both flank-collapse depressions. As a consequence, the upper streams Saint Louis and Perou rivers that had previously developed along the southern inner border of the arcuate depressions were likely deviated, to the northwest and the northeast respectively (Fig. ), thereby eroding a more central part of both IcV and CV. Stage VIII (70 ka; Fig. 7C) shows the GDS, when its maximum growth was reached, with an additional 3.±. km 3 associated with GDS-3 episode. It also shows the large lava flows that were emplaced south of GDS and down to the coast (TRMF-,.6±.3 km 3 ; Table ), which marked the onset of the Trois Rivières Madeleine Field emplacement (TRMF; Samper et al., 009). For this reconstruction, we use the largely remnant outcropping uppermost surface (Fig. 3) to model an exponential profile curve centered at location 64,600,,775,900 that reaches 537 ma.s.l. (peak located 30 m above and 700 m north-east of the present highest point). This is the highest modeled altitude we have obtained in this study. At this stage, the main cone of GDS thus contributes to about 9±4 km 3 to the 0± 4km 3 of the total volume of GDVC (Table ). Stage IX shows the emplacement of the Madeleine Palmiste alignment (MPA also referred here as TRMF-) within the peripheral

14 P. Lahitte et al. / Geomorphology 36 (0) Table Area, volume, period of activity and construction and height increase rates of the main construction stages of southern Basse-Terre. Stage acronym Stage description Stage code Area (km ) V± σ (km 3 ) Begin (ka) End (ka) ΔT (kyr) C.R.± σ ( 0 4 km 3 /yr) H.I.R.± σ (m/ka) AC- Axial Chain before st flank collapse I ± ±0.9.±0. Last 650 ka II IX 47 54± ± ±0.05 AC- Axial Chain after st flank collapse II IV 59 33± ±0.4.0±0. IcV Icaques Volcano II 55 4± ±.3 8.3±.3 CV Capesterre Volcano III 74 ±6 un. un. un. un. un. MC Monts Caraibes massif IV ± ± ±0.4 STV Sans Toucher Volcano IV 0.± ± ±8. GDVC Grande Découverte Volcanic Complex VI X 0± ±0. 0.7±0. GDS Grande Découverte Soufrière VI X 00 5± ±0. 0.6±0. GDS- GDS st phase VI 3.7± ±0..8±0.5 GDS- GDS nd phase VII 84 7.± ±0.7.7±0.8 TRMF- Trois Rivières Madeleine Field st phase VIII 8.6± ±0.4 3.±.6 GDS-3 GDS 3 rd phase VIII 37 3.± ±0.7.9±.9 Stage VIII Sum of the two previous stages VIII ± ± ±.3 MPA (TRMF-) Madeleine Palmiste Alignment IX 3.3± ±0.4 7.±0.4 TRMF Trois Rivières Madeleine Field VIII IX 3 4.9± ±0..9±0.8 Last 5 ka X.3± ±0. 4.0±0.9 Last 00 ka VIII X ± ± ±0. Southern Basse-Terre (AC, GDVC, MC) Axial Chain, Grande Découverte Volcanic Complex, Monts Caraibes massif I X ± ± ±0.05 Basse-Terre (SC, AC, GDVC, MC) Septentrional Chain, Axial Chain, Grande Découverte Volcanic Complex, Monts Caraibes massif TRMF massif (Fig. 7C) before 45 ka. Due to its young age and to its location off-axis from the main volcanic centers, this phase can be considered as one of the less eroded and therefore, except for the gullies, most of its uppermost surface is used for the reconstruction. Furthermore, because of its dendritic pattern, MPA basal surface is accurately interpolated by kriging, which enables us to constrain a volume of.3±0. km 3 for TRMF- (Fig. 3). This eruptive phase induced an important reorganization of the southern GDVC river network, as most streams that used to run over the 70 ka lava flows from the top of GDV to the southern coasts must have been blocked by the MPA relief. As a response to this morphological change, they were likely diverted and captured by the southeastern and southwestern watersheds of GDS. For instance the downstream Galion River watercourse illustrates this phenomenon to the southwest (Fig. ). This is less evident to the southeast, mostly because of the emplacement of younger lava flows (Samper et al., 009). Note that the low development of rivers beyond the MPA southern boundary is also a consequence of its emplacement. Stage X shows the present state of southern Basse-Terre Island. It began with a major plinian pumiceous eruption (referred as the Pintade eruption, 4± ka; Komorowski et al., 005) that dismantled GDS- and resulted in the formation of the caldera of Grande Découverte (Boudon et al., 007). Associated deposits have been identified in several areas of the GDVC with an unknown amount of material that is likely to have also flowed into the sea. Nez Cassé ridge (45 5 ka unit in Fig. and Fig. 7D) dated at 34±4 ka (Samper et al., 009) is a remnant part of the Carmichaël Volcano emplaced after the Pintade plinian eruption. Due to the lack of significative upper remnant surface for this phase, we are unable to infer its geometry. In fact, the present morphology of GDVC is mainly controlled by the volcanic activity that has been occurring since 5 ka. The Holocene La Soufrière and L'Echelle lava domes, and La Citerne scoria cones (S, E, C in Fig. ) lying on top of the GDS volcano were emplaced after the last major GDS summit-collapse that led to the formation of the Amic crater depression at about 8500 yr BP (Komorowski et al., 005). This stage also takes into account the.5 ka and 3. ka partial collapses respectively linked to a Bandai-san and Bezymianny eruption types, which dismantled the western upper part of GDVC (Boudon et al., 987). The volumes of the collapses have been estimated as 500 and 300 million m 3, respectively. As for stage IX, most of the uppermost surface was available for the modeling. The total erupted volume with an age b5 ka was estimated to be.3±0.3 km 3 (Table ). Holocene volcanic activity also contributed to modify GDVC's stream network. As the GDVC massif mainly consists of domes, cones or lava flows isolated from each other, the network disruptions are not straightforward as it has affected the upstream rivers that go around the new volcanic features. For instance the emplacement of lava flows associated with the eruption L'Echelle lava dome induced a reorganization of the upstream Grand Carbet River watershed (Fig. ). These lava flows divided this single stream network and watershed into two smaller hydrological systems: a) a reduced upstream Grand Carbet River watershed and its stream network, b) a newly isolated Grosse Corde river watershed (GCr in Fig. ). Downstream, the Grosse Corde river was diverted towards the Grand Carbet River watershed by lava flows emplaced during the eruption of La Citerne scoria cone. 4.. Southern Basse-Terre construction rates Volume and construction rates calculated for the main eruptive stages are given in Table and shown in Fig. 8A and B, respectively. The total volume of the Axial Chain emplaced between and 0.6 Ma, before the two flank collapse events, is 89±30 km 3 over 460 km. This corresponds to a construction rate of 5.4± km 3 /yr, hence the highest rate of southern Basse-Terre massifs (Table ). The wellpreserved Icaques Volcano (IcV) was emplaced in a short time span of about 30 kyr, and hence shows a relatively high construction rate of 4.5± km 3 /yr. Due to the lack of K Ar ages, the construction rate of Capesterre Volcano (CV) cannot be constrained. For the Monts Caraibes (MC) and Sans Toucher (STV) volcanoes, relatively low construction rate values of 0.8± km 3 /yr and 0.9± km 3 /yr, respectively, have been calculated. The GDS- episode, which marks the onset of the GDVC volcanism, is characterized by a low construction rate of about 0.4± km 3 /yr. Construction rate increased during episodes GDS- and GDS-3, up to.4± and.± km 3 /yr, respectively. TRMF volcanism is characterized by a relatively constant construction rate of 0.9± km 3 /yr during its two episodes, which is similar to the overall construction rate calculated for both the last 00 ka and 5 ka volcanism (Table ). Finally, preserved volcanic materials of southern Basse-Terre edifices reach a total volume of 43 ±3 km 3, which yields a construction rate of.4± km 3 /yr for the last million years (Table and Fig. 8B).