Lesser Antilles Arc Research Articles

Origin of Basalts by Hybridization in Andesite-dominated Arcs

Mafic magmas are common in subduction zone settings, yet their high density restricts their ascent to the surface. Once stalled in the crust, these magmas may differentiate, and assimilate crust and other melts and crystal mushes to produce hybridized intermediate magmas. The Soufriere Hills Volcano on Montserrat is a ‘type locality’ for such hybridization processes and yet, just 3?km south of the crater, voluminous basalts have erupted from the South Soufriere Hills volcano within the same time period as the Soufriere Hills Volcano was erupting hybrid andesites (131–128?ka). Basaltic South Soufriere Hills magmas have 48–53?wt % SiO2 and 4–6?wt % MgO. They were hot (970–1160°C), volatile-rich (melt inclusions contain up to 6·2?wt % H2O) and were stored at 8–13?km depth prior to eruption (based on olivine- and pyroxene-hosted melt inclusion volatile geochemistry). Melt inclusions do not preserve basaltic liquids: they are andesitic to rhyolitic in composition, related to one another by a line of descent controlled by simple closed-system fractionation. Whole-rock compositions, however, are best described by a hybridization model involving ‘back-mixing’ of andesitic to rhyolitic melts with mafic crystal phases such as magnetite, olivine, orthopyroxene and clinopyroxene. Phenocryst zoning illustrates repeated mixing events between evolved melts and mafic phenocrysts; this feature, when coupled with the heterogeneity of crystal compositions, strongly suggests that although the bulk compositions are basaltic (containing Fo80 olivine), they were assembled from disparate ingredients, probably derived from mafic crystal mushes and more evolved melt lenses of variable composition. The mixing events occur days to weeks prior to eruption. We propose that the South Soufriere Hills basaltic magmas, with their higher bulk density relative to andesites from neighbouring volcanoes, ultimately may have been eruptible owing to both the transtensional tectonics imposed by offshore grabens (related to oblique subduction in the Lesser Antilles arc) and surface unloading caused by large-scale edifice collapse. Our observations support the idea that compositional changes in arcs might reflect not only changes in source compositions, but also effects caused by variations in crustal strain and tectonics.

Time scales of intra‐oceanic arc magmatism from combined U‐Th and (U‐Th)/He zircon geochronology of Dominica, Lesser Antilles

AbstractThe island of Dominica, located in the intra‐oceanic Lesser Antilles arc, has produced a series of intermediate (mostly andesitic) lava domes and ignimbrites since the early Pleistocene. (U‐Th)/He eruption ages from centers across the island range from ∼3 to ∼770 ka, with at least 10 eruptions occurring in the last 80 ka. Three eruptions occurred near the southern tip of Dominica (Plat Pays Volcanic Complex) in the past 15 ka alone. Zircon U‐Th ages from individual centers range from near‐eruption to secular equilibrium implicating protracted storage and recycling of zircons within the crust. Overlapping zircon crystallization peaks within deposits from geographically separated vents (up to 40 km apart) indicate that magma associated with separate volcanic edifices crystallized zircon contemporaneously. Two lava domes from the southern sector of the island display exclusively young zircon rim ages (<50 ka) with narrow crystallization peaks consistent with the construction of a new magma reservoir. The younging of eruption and crystallization ages implies that the magmatic foci leading to the construction of this reservoir have migrated southward, arc‐parallel over time. Overall, our data support geochemical models for the ongoing construction of a silicic intrusive complex, consisting of varying amounts of crystal mush, beneath the island. U‐Pb zircon ages <1–2 Ma indicate that accumulation of this complex is entirely Quaternary in age. Together zircon U‐Th and U‐Pb ages for Dominica suggest that the magmatic processes and time scales operating in intra‐oceanic arcs are similar to those documented for continental arcs.

Chlorine isotopes of thermal springs in arc volcanoes for tracing shallow magmatic activity

The evaluation of the status of shallow magma body (i.e., from the final intrusion stage, to quiescence, and back to activity), one of the key parameters that trigger and sustain volcanic eruptions, has been challenging in modern volcanology. Among volatile tracers, chlorine (Cl) uniquely exsolves at shallow depths and is highly hydrophilic. Consequently, Cl enrichment in volcanic gases and thermal springs has been proposed as a sign for shallow magmatic activities. However, such enrichment could also result from numerous other processes (e.g., water evaporation, dissolution of old chloride mineral deposits, seawater contamination) that are unrelated to magmatic activity. Here, based on stable isotope compositions of chloride and dissolved inorganic carbon, as well as previous published 3He/4He data obtained in thermal springs from two recently erupted volcanoes (La Soufrière in Guadeloupe and Montagne Pelée in Martinique) in the Lesser Antilles Arc, we show that the magmatic Cl efficiently trapped in thermal springs displays negative δ37Cl values (≤−0.65‰), consistent with a slab-derived origin but distinct from the isotope compositions of chloride in surface reservoirs (e.g. seawater, local meteoric waters, rivers and cold springs) displaying common δ37Cl values of around 0‰. Using this δ37Cl difference as an index of magmatic Cl, we further examined thermal spring samples including a 30-year archive from two thermal springs in Guadeloupe covering samples from its last eruption in 1976–1977 to 2008 and an island-wide sampling event in Martinique in 2008 to trace the evolution of magmatic Cl in the volcanic hydrothermal systems over time. The results show that magmatic Cl can be rapidly flushed out of the hydrothermal systems within <30 to 80 years after the eruption, much quicker than other volatile tracers such as CO2 and noble gases, which can exsolve at greater depths and constantly migrate to the surface. Because arc volcanoes often have well developed hydrothermal systems where magmatic Cl is easily transferred to the surface following its exsolution from shallow magma body, we suggest that δ37Cl has great potential to be a unique proxy to monitor the cessation and revival of infrequent arc volcanoes, particularly at centennial time scales.

Relative Motion between St. Croix and the Puerto Rico-Northern Virgin Islands Block Derived from Continuous GPS Observations (1995–2014)

St. Croix is located inside the sweep of the Lesser Antilles arc and near the southeastern edge of the Greater Antillean ridge. It is separated from the Puerto Rico and the Northern Virgin Islands (PRNVI) block by the Virgin Islands basin. Recent seismic activities demonstrate that the Virgin Islands basin is tectonically active. A better understanding of fault activities in the basin would improve seismic hazard assessment in this region. This study illustrates out a detailed way of deriving relative motion between St. Croix and the PRNVI block using current GPS geodesy infrastructure in the PRVI region. The local geodesy infrastructure includes over 20 continuous GPS stations and a Stable PRNVI Reference Frame (SPRNVIRF). Twenty-year continuous GPS observations (1995–2014) on St. Croix indicate that the island is presently moving away from the PRNVI block toward the southeast (S55°E) at a steady rate of 1.7 mm/year. The velocity vector can be decomposed into two 1.2 mm/year components along the long-axis and short-axis directions of the rhomboidal Virgin Islands basin. Quantitative results indicate that the Virgin Islands basin presently experiences left-lateral motion in a nearly east-west direction and extension in a nearly north-south direction.

Lower crustal assimilation in oceanic arcs: Insights from an osmium isotopic study of the Lesser Antilles

We present whole rock 187Os/188Os data for the most mafic lavas along the Lesser Antilles arc (MgO=5–17wt.%) and for the subducting basalt and sediments. 187Os/188Os ratios vary from 0.127 to 0.202 in the arc lavas. Inverse correlations between 187Os/188Os and Os concentrations and between 187Os/188Os and indices of differentiation such as MgO suggests that assimilation, rather than source variation, is responsible for the range of Os isotopic variation observed. 87Sr/86Sr, La/Sm and Sr/Th are also modified by assimilation since they all correlate with 187Os/188Os. The assimilant is inferred to have a MORB-like 87Sr/86Sr with high Sr (>700ppm), low light on middle and heavy rare earth elements (L/M-HREE; La/Sm ∼2.5) and 187Os/188Os>0.2. Such compositional features are likely to correspond to a plagioclase-rich early-arc cumulate. Given that assimilation affects lavas that were last stored at more than 5kbar, assimilation must occur in the middle-lower crust.Only a high MgO picrite from Grenada escaped obvious assimilation (MgO=17%wt.%) and could reflect mantle source composition. It has a very radiogenic 87Sr/86Sr (0.705) but a 187Os/188Os ratio that overlaps the mantle range (0.127). 187Os/188Os and 87Sr/88Sr ratios of the sediments and an altered basalt from the subducting slab vary from 0.18 to 3.52 and 0.708 to 0.714. We therefore suggest that, unlike Sr, no Os from the slab was transferred to the parental magmas. Os may be either retained in the mantle wedge or even returned to the deep mantle in the subducting slab.

Steam and gas emission rate from La Soufriere volcano, Guadeloupe (Lesser Antilles): Implications for the magmatic supply during degassing unrest

Since its last magmatic eruption in 1530AD, La Soufrière andesitic volcano in Guadeloupe has displayed intense hydrothermal activity and six phreatic eruptive crises. Here we report on the first direct quantification of gas plume emissions from its summit vents, which gradually intensified during the past 20years. Gas fluxes were determined in March 2006 and March 2012 by measuring the horizontal and vertical distributions of volcanic gas concentrations in the air-diluted plume and scaling to the speed of plume transport. Fluxes in 2006 combine real-time measurements of volcanic H2S concentrations and plume parameters with the composition of the hot (108.5°C) fumarolic fluid at exit. Fluxes in 2012 result from MultiGAS analysis of H2S, H2O, CO2, SO2 and H2 concentrations, combined with thermal imaging of the plume geometry and dynamics. Measurements were not only focused on the most active South crater (SC) vent, but also targeted Tarissan crater and other reactivating vents. We first demonstrate that all vents are fed by a common H2O-rich (97–98mol%) fluid end-member, emitted almost unmodified at SC but affected by secondary shallow alterations at other vents. Daily fluxes in 2012 averaged 200tons of H2O, 15tons of CO2, ~4tons of H2S and 1ton of HCl, increased by a factor ~3 compared to 2006. Even though modest, such fluxes make La Soufrière the second most important volcanic gas emitter in the Lesser Antilles arc, after Soufriere Hills of Montserrat. Taking account of other hydrothermal manifestations (hot springs and diffuse soil degassing), the summit fumarolic activity is shown to contribute most of the bulk volatile and heat budget of the volcano. The hydrothermal heat output (8MW) exceeds by orders of magnitude the contemporaneous seismic energy release. Isotopic evidences support that La Soufrière hydrothermal emissions are sustained by a variable but continuous heat and gas supply from a magma reservoir confined at 6–7km depth. By using petro-geochemical data for La Soufrière magma(s) and their dissolved volatile content, and assuming a magmatic derivation of sulfur, we estimate that the volcanic gas fluxes measured in 2012 could result from the underground release of magmatic gas exsolved from ~1400m3d−1 of basaltic melt feeding the system at depth. We recommend that fumarolic gas flux at La Soufrière becomes regularly measured in the future in order to carefully monitor the temporal evolution of that magmatic supply.

Main issues of an evacuation in case of volcanic crisis: social stakes in Guadeloupe (Lesser Antilles Arc)

The last major eruption of La Soufriere volcano in Guadeloupe (Lesser Antilles) in 1976–1977 caused the mass evacuation of part of the population, whereby a total of 76,000 people were displaced for a period of 3–6 months. This evacuation has left a bitter memory among the inhabitants who believed that the political authorities of the time had not anticipated the possibility of an eruption crisis and that decisions were taken in haste. La Soufriere remains active, and future eruptions could once again lead to partial or even total evacuation of the population if there were a major Plinian eruption. This article offers an investigation of future evacuation procedures, questioning different aspects of Guadeloupe’s current territorial and social challenges (the multi-risk context, the reporting to the scientists and to the authorities, the importance of local solidarity). In order to do so, we used the Focus Group Discussion method, making it possible to identify resources and gaps in crisis management on the basis of previous event history.

Assimilation of sediments embedded in the oceanic arc crust: myth or reality?

Arc magmas are commonly assumed to form by melting of sub-arc mantle that has been variably enriched by a component from the subducted slab. Although most magmas that reach the surface are not primitive, the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle. This jeopardises the integrity of mass balance recycling calculations. Here we use Sr and O isotope data in minerals from a suite of volcanic rocks from St Lucia, Lesser Antilles arc, to show that assimilation of oceanic arc basement can be significant. Analysis of 87Sr/86Sr in single plagioclase phenocrysts from four Soufrière Volcanic Complex (SVC; St Lucia) hand samples with similar composition (87Sr/86Sr=0.7089–0.7091) reveals crystal isotopic heterogeneity among hand samples ranging from 0.7083 to 0.7094 with up to 0.0008 difference within a single hand sample. δO18 measurements in the SVC crystals show extreme variation beyond the mantle range with +7.5 to +11.1‰ for plagioclase (n=19), +10.6 to +11.8‰ for quartz (n=10), +9.4 to +9.8‰ for amphibole (n=2) and +9 to +9.5‰ for pyroxene (n=3) while older lavas (Pre-Soufriere Volcanic Complex), with less radiogenic whole rock Sr composition (87Sr/86Sr=0.7041–0.7062) display values closer to mantle range: +6.4 to +7.9‰ for plagioclase (n=4) and +6 to +6.8‰ for pyroxene (n=5). We argue that the 87Sr/86Sr isotope disequilibrium and extreme δO18 values provide compelling evidence for assimilation of material located within the arc crust. Positive correlations between mineral δO18 and whole rock 87Sr/86Sr, 143Nd/144Nd and 206,207,208Pb/204Pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in St Lucia but also in the whole Lesser Antilles since St Lucia encompasses almost the whole-arc range of isotopic compositions. This highlights the need for detailed mineral-scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes.

Carbon and helium isotopes in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles): Implications for volcanological monitoring

La Soufrière (Guadeloupe, Lesser Antilles) is one of the most active and dangerous volcanoes of the Lesser Antilles arc. The renewal of the volcanic activity at La Soufrière requires replenishment of the volatile-poor andesitic stored magma with more primitive, volatile-rich basaltic magma. This event will trigger early release of large quantities of highly volatile species, like CO2 and helium, in the hydrothermal system. To use these precursors, first their baseline abundances and their behaviour in the hydrothermal system must be known during the current phase of dormancy. In this study, we have combined a previous systematic investigation of noble gases with carbon data of hydrothermal fluids flowing out of La Soufrière.The results for dissolved gases reveal that an intense interaction between rising magmatic volatiles and groundwaters currently takes place at La Soufrière. We show that the pristine magmatic composition, with a δ13CCO2 value of −3.19±0.03‰, a 3He/4He ratio of 8.1±0.2 Ra and a CO2/3He ratio of 1.03±0.18×1010, may vary during the ascent of magmatic fluids due to either fractionation or mixing processes with meteoric water and/or an organic component. Here, we provide a baseline for CO2 and helium isotopes at each monitored spring and describe the superficial processes affecting both elemental and isotopic ratios.

Erosive effects of the storm Helena (1963) on Basse Terre Island (Guadeloupe — Lesser Antilles Arc)

Erosive effects of the tropical storm Helena that hit the volcanic island of Basse Terre (Guadeloupe — Lesser Antilles Arc) on 24 October 1963 has been measured using 80 aerial images acquired by the French geographic institute (IGN — Institut Géographique National) at the approximate scale of 1/8000 less than three months after the storm Helena. On these images, 253 landslides triggered during the storm were identified and mapped. These landslides were located in the central region of the island where catchments exhibit the highest relief. Even though the average thickness of the landslides was only 1m, i.e., less than the thickness of the weathered layer, the total volume of displaced sediments corresponded to an average denudation of 2800tkm−2, i.e., 1.4mm, on the watersheds affected by landsliding. To assess the erosional significance of this single climatic event, we compare the volume of sediment mobilized by the storm Helena to the long-term denudation rate. The latter, estimated from a calculation of the total volume of material eroded since the emplacement of lavas using a digital elevation model, is found to be 0.14mm/y. Assuming that Helena is representative of the storms that hit Basse-Terre Island during the Quaternary, we find that a return period of about 10 to 15years is enough to account for the long term denudation rate recorded for this Island. Such a period is comparable with the actual return period of the tropical cyclones of the order of 4 to 5years, suggesting that the erosion of Basse-Terre Island is entirely controlled by tropical hurricanes.

Slab tears and intermediate‐depth seismicity

Active tectonic regions where plate boundaries transition from subduction to strike slip can take several forms, such as triple junctions, acute, and obtuse corners. Well‐documented slab tears that are associated with high rates of intermediate‐depth seismicity are considered here: Gibraltar arc, the southern and northern ends of the Lesser Antilles arc, and the northern end of Tonga trench. Seismicity at each of these locations occurs, at times, in the form of swarms or clusters, and various authors have proposed that each marks an active locus of tear propagation. The swarms and clusters start at the top of the slab below the asthenospheric wedge and extend 30–60 km vertically downward within the slab. We propose that these swarms and clusters are generated by fluid‐related embrittlement of mantle rocks. Focal mechanisms of these swarms generally fit the shear motion that is thought to be associated with the tearing process.

Multiscale Mapping of Completeness Magnitude of Earthquake Catalogs

We propose a multiscale method to map the spatial variations of the com- pleteness magnitude Mc of earthquake catalogs. The Gutenberg-Richter law describ- ing the earthquake frequency-magnitude distribution (FMD) might not hold over the entire magnitude range, and small areas may exhibit a specific type of seismicity, especially in volcanotectonic contexts. For these reasons, any scaling relation should be obtained by adapting the dimension of the studied zone to the range of the event magnitude. Here, we associate ranges of larger magnitudes with increasing areas for data selection based on empirical relations in seismotectonics. Then, for each point in space, we document the earthquake FMD at all length scales within the corresponding earthquake magnitude ranges. High resolution of the Mc-value is achieved through the determination of the smallest space-magnitude scale in which the Gutenberg-Richter law is verified. The multiscale procedure isolates the magnitude range that meets the best local seismicity and local record capacity. Using artificial catalogs and earthquake catalogs of the Lesser Antilles arc, this Mc-mapping method is shown to be efficient in regions with mixed types of seismicity, a variable density of epicenters, and various levels of registration.

The Albian–Turonian Island Arc Rocks of Tobago, West Indies: Geochemistry, Petrogenesis, and Caribbean Plate Tectonics

An elemental and radiogenic isotope study of Cretaceous island arc rocks on Tobago, West Indies, reveals the magmatic processes taking place at the eastern edge of the Pacific-derived Caribbean Plate during development of the Greater Antilles Arc. The ∼110–103 Ma Volcano-Plutonic Suite comprises the ultramafic–intermediate Tobago Pluton and genetically related Tobago Volcanic Group. The volcanic rocks (breccias, tuffs, and mafic–intermediate lavas) have undergone shallow-level fractional crystallization involving plagioclase, clinopyroxene, olivine, and Fe–Ti oxides, but also preserve trace element evidence for ‘cryptic’ amphibole fractionation. The suite is inferred to have formed from a spinel lherzolite mantle wedge source fluxed largely by slab- and recycled volcanogenic sediment-derived fluids. A tonalitic mega-dyke intruding the pluton resembles high-silica adakites, and geochemical constraints indicate a likely origin by partial melting of the arc crust. A mafic dyke swarm (∼103–91 Ma) is partly coeval with the volcanic rocks, but some, perhaps the youngest dykes, are derived from isotopically distinct arc mantle sources compared with the volcanic rocks. Rare Nb-enriched and high-Nb dykes may relate to melting of a high field strength element-enriched source. Current Caribbean tectonic models involve the continuation of east-dipping Farallon Plate subduction beneath the proto-Caribbean seaway either until an Early Cretaceous initiation of proto-Caribbean subduction, or collision of the Caribbean Oceanic Plateau with the Greater Antilles Arc at ∼90–80 Ma. Both models may be compatible with the tectono-magmatic history of Tobago, wherein Tobago is thought to have detached from the fore-arc of the Caribbean arc system during Eocene intra-arc extension, the growth of the Grenada Basin, and inception of the Lesser Antilles Arc. Tobago- or La Desirade-like Mesozoic arc crust underlies much of the present-day Lesser Antilles Arc and not, as has recently been proposed, portions of the plume-derived Caribbean Oceanic Plateau.

Volcanic stratigraphy and geochemistry of the Soufrière Volcanic Centre, Saint Lucia with implications for volcanic hazards

The Soufrière Volcanic Complex (SVC), Saint Lucia, represents one of the largest silicic centres in the Lesser Antilles arc. It comprises extensive pumiceous pyroclastic flow deposits, lava flows as well as Peléan-style domes and dome collapse block-and-ash-flow deposits. These deposits occur within and around the Qualibou Depression, a ~10-km diameter wide sector collapse structure. To date, vent locations for SVC pyroclastic deposits and their relationship to the sector collapse have been unclear because of limited stratigraphic correlation and few radiometric ages. In this study we reconstruct the geologic history of the SVC in light of new and recently published (U–Th)/He, U–Th and U–Pb zircon chronostratigraphic data, aided by mineralogical and geochemical correlation. Compositionally, SVC deposits are monotonous medium-K, calc-alkaline rocks with 61.6 to 67.7wt.% SiO2 and display similar trace element abundances. Combined U–Th and (U–Th)/He zircon dating together with 14C ages and mineral fingerprinting reveals significant explosive eruptions at 640, 515, 265, 104, 60 and 40ka (producing deposits previously grouped together as the “Choiseul” unit) and at 20ka (Belfond unit). The mineralogically and geochemically distinct Belfond unit is a large, valley-filling pumiceous pyroclastic flow deposit distributed to the north, northeast, south and southeast of the Qualibou Depression that was probably deposited during a single plinian eruption. The unit previously referred to as ‘Choiseul tuff’ is much less well defined. The typical Choiseul unit comprises a series of yellowish-white, crystal-poor, non-welded pumiceous pyroclastic deposits cropping out to the north and southeast of the Qualibou depression; however its age is poorly constrained. A number of other units previously mapped as Choiseul can be distinguished based on age, and in some cases mineral and whole rock chemistry. Pyroclastic deposits at Micoud (640±19ka), Bellevue (264±8ka), Anse John (104±4ka) and La Pointe (59.8±2.1ka), Anse Noir and Piaye were all previously grouped with or associated with the Choiseul tuff (all uncertainties 1σ). We suggest that these units represent individual periods of activity spanning a range of ages, whereas Choiseul pumice at the type locality has yielded a (U–Th)/He zircon age of 515±19ka. Their overall geochemical and mineralogical similarities with the Choiseul at the type locality suggest that they might have all originated from the same centre. Morne Tabac (532±21ka) is a dome truncated by the depression escarpment, whereas Morne Bonin (273±15ka), Gros Piton and Petit Piton (71±3ka and 109±4ka, resp.), Belfond (13.6±0.4ka) and Terre Blanche (15.3±0.4ka) are domes within the Qualibou Depression. Belfond and Terre Blanche have whole rock geochemistry and mineral assemblages similar to the Belfond pyroclastic flow deposit, thus possibly representing late-erupted degassed portions of the magma that produced the Belfond pyroclastics. The geochemical characteristics and similar zircon age distributions of the silicic lava domes and pyroclastics of the SVC suggest that these share a common magma source beneath the Qualibou depression. The distribution of the pyroclastic flows and the wide range in their eruption ages makes it unlikely that these were erupted during caldera-forming activity, and we instead invoke a series of smaller-volume explosive eruptions from the area of the current depression, the earliest of which occurred from a large proto-Qualibou edifice that subsequently underwent sector collapse. Activity from this proto-Qualibou centre may have ceased sometime between 38 and 59ka ago, it therefore seems unlikely given our present understanding that there will be another eruption from the southern central highland region. However, the young dome-forming activity in the Qualibou depression may have occurred in or close to the Holocene, and there have been dome collapse events and explosion craters formed since then. A new dome eruption or renewed activity at a dome within the depression, growing in the style of the ongoing Soufrière Hills lava dome on the nearby island of Montserrat, is possible; as is a future plinian eruption from this area. Such an eruption would not only have a devastating impact on Saint Lucia, but would also have significant regional and global impacts.

A review of volcanic island evolution and magma production rate: an example from a Cenozoic island arc in the Caribbean

Different stages of volcanic island evolution are evident in the Cenozoic Lesser Antilles arc developed near the eastern edge of the Caribbean Plate. The early stage of seamount evolution is represented by the Kick-em-Jenny submarine volcano near the southern end of the arc. The volcanically active islands known as the Volcanic Caribbées are composed of a series of composite volcanoes and are examples of an emergent volcanic island stage. Those islands that were formerly active, known as the Limestone Caribbées, provide evidence that an erosional to submergent stage occurred in the northern part of the island arc during the Oligocene and Miocene epochs. Growth of the islands and underlying crust is dependent upon the magma production rate along the arc, which has for the last 100 ka ranged from less than 1 km 3 to 40 km 3 . These differences in magma production and volcanic rock composition along the arc are attributed to the obliquity of plate convergence and the plate convergence rates.

Role of large flank-collapse events on magma evolution of volcanoes. Insights from the Lesser Antilles Arc

Flank-collapse events are now recognized as common processes of destruction of volcanoes. They may occur several times on a volcanic edifice pulling out varying volumes of material from km3 to thousands of km3. In the Lesser Antilles Arc, a large number of flank-collapse events were identified. Here, we show that some of the largest events are correlated to significant variations in erupted magma compositions and eruptive styles.On Montagne Pelée (Martinique), magma production rate has been sustained during several thousand years following a 32ka old flank-collapse event. Basic and dense magmas were emitted through open-vent eruptions that generated abundant scoria flows while significantly more acidic magmas were produced before the flank collapse. The rapid building of a new cone increased the load on magma bodies at depth and the density threshold. Magma production rate decreased and composition of the erupted products changed to more acidic compared to the preceding period of activity. These low density magma generated plinian and dome-forming eruptions up to the Present. In contrast at Soufrière Volcanic Centre of St. Lucia and at Pitons du Carbet in Martinique, the flank-collapses have an opposite effect: in both cases, the acidic magmas erupted immediately after the flank-collapses. These magmas are highly porphyritic (up to 60% phenocrysts) and much more viscous than the magmas erupted before the flank-collapses. They have been generally emplaced as voluminous and uptight lava domes (called “the Pitons”). Such magmas could not ascent without a significant decrease of the threshold effect produced by the volcanic edifice loading before the flank-collapse.

Curie point depth in Venezuela and the Eastern Caribbean

We estimate the Curie point depth (CPD) variations of Venezuela (continental crust, South American plate) and the Eastern Caribbean (oceanic crust, Caribbean Plate) by using spectral analysis of the magnetic anomalies, extracted from the 2010 Enhanced Magnetic Model (EMM2010), available at the National Oceanic and Atmospheric Administration (NOAA). To test the reliability of the spectral content of this model, for a small region, we compare he CPD derived from the EMM2010 against the one from aeromagnetic data. We also compile heat flow data from previous studies to correlate them with the CPD lateral variations. The estimations show that the CPD in Venezuela and the Eastern Caribbean ranges between 54 and 17km. The mean depth value within the continental crust is around 38km. On the Guayana Shield, it has a mean value of 42km and reaches a maximum of 54km. As the Moho depth is at most 50km, the upper mantle beneath the craton is magnetized. Continental lateral variations appear to be linked to the isostatic state and age of the different provinces, and mark the limit between the Precambrian and the Paleozoic provinces. The Maracaibo Basin is revealed as a thermally stable one with a constant CPD, while the Eastern Venezuela Basin is thermally affected. Most of the Eastern Caribbean seems stable, with a large non-perturbed area with a mean CPD value of 23km. As the crustal thickness is at most 20km, the isotherm is located within the upper mantle. A CPD minimum located on the Lesser Antilles arc is concentrated in its northern part, and can be related to the subduction zone that is most active. Finally, a shallow area within the Eastern Caribbean corresponds to the thin crust region in the Venezuela Basin, although it might be linked to mantle dynamics.

Crustal structure of Guadeloupe islands and the Lesser Antilles arc from a new gravity and magnetic synthesis

Abstract Guadeloupe island (West French Indies) is one of the twenty islands that compose the Lesser Antilles arc, which results from the subduction of the Atlantic ocean plate beneath the Caribbean one. The island lies in a complex volcano-tectonic system and the need to understand its geological context has led to numerous on- and offshore geophysical investigations. This work presents a compilation and the processing of available, on-land, airborne and marine, gravity and magnetic data acquired during the last 40 years on Guadeloupe islands and at the scale of the Lesser Antilles arc. The overall dataset provides new Bouguer and reduced to the pole magnetic anomaly maps at the highest achievable resolution. Regionally, the main central negative gravity trend of the arc allows defining two subsident areas. The first one is parallel to the arc direction (~N160°E) to the north, whereas the second unexpected southern one is oriented parallel to oceanic ridges (N130°E). Along the Outer arc, the long wavelength positive anomaly is interpreted, at least along the Karukera spur, as an up-rise of the volcanic basement in agreement with the seismic studies. To the NE of Guadeloupe, the detailed analysis of the geophysical anomalies outlines a series of structural discontinuities consistent with the main bathymetric morphologies, and in continuity of the main fault systems already reported in this area. Based on geophysical evidences, this large scale deformation and faulting of the Outer arc presumably primarily affects the Atlantic subducting plate and secondarily deforms the upper Caribbean plate and the accretion prism, as evidenced in bathymetry as well as on the islands. At the scale of Guadeloupe island, combined gravity and magnetic modeling has been initiated based on existing interpretation of old seismic refraction profiles, with a general structure in three main layers. According to our geophysical anomalies, additional local structures are also modeled in agreement with geological observations: i) the gravity and magnetic signals confirm an up-rise of the volcanic basement below the limestone platforms outcropping on Grande-Terre island ; ii) the ancient volcanic complexes of Basse-Terre island are modeled with high density and reverse magnetized formations; iii) the recent volcanic centre is associated with formations consistent with the low measured density and the underlying hydrothermal system. The E-W models coherently image a NNW-SSE depression structure in half-graben beneath Basse-Terre island, its western scarp following the arc direction in agreement with bathymetric and seismic studies to the north of the island. The so-defined depressed area, and particularly its opening in half-graben toward the SW, is interpreted as the present-day front of deformation of the upper plate, associated with the recent volcanic activity on and around Guadeloupe. Based on this regional deformation model, perspectives are given for further integrated investigation of key targets to address the internal structure and evolution of the Lesser Antilles arc and Guadeloupe volcanic system.

How wide is the seismogenic zone of the Lesser Antilles forearc?

Abstract The Lesser Antilles subduction zone has produced no recent strong thrust earthquakes, making it difficult to quantify the seismic hazard from such events. The Lesser Antilles arc has a low subduction rate and an accretionary wedge that is very wide at its southern end. To investigate the effect of the wedge on seismogenesis, numerical models of forearc thermal structure were constructed along six transects perpendicular to the arc in order to determine the thermally predicted width of the seismogenic zone. The geometry of each section is constrained by published seismic profiles and crustal models derived from gravity and seismic data and by earthquake hypocenters at depth. A major constraint on the deep part of the model is that mantle temperature beneath the volcanic arc should achieve a temperature of 1,100°C to generate partial melts. Predicted surface heat flow is compared to the available heat flow observations. Thermal modeling results indicate a systematic southward increase in the width of the seismogenic zone, more than doubling in width from north to south and corresponding to a dramatic southward increase in forearc width (distance from the arc to the deformation front of the accretionary wedge). The minimum width of the seismogenic zone (distance between the intersections of the subduction interface with the 150°C and 350°C isotherms) increases from about 80 km, north of 16°N, to 230 km, at 13°N. The maximum width (between the 100°C and 450°C isotherms) ranges from about 150 km in the north to up to 320 km in the south. This large variation in the width of the seismogenic zone is a consequence of the increasing width of the accretionary wedge to the south, caused by the increased thickness of sediment on the subducting plate. There is good agreement between the thermally predicted seismogenic limits and the sparse distribution of recorded thrust earthquakes, which are observed only in the northern portion of the arc. Possible scenarios for mega-thrust earthquakes are discussed. Depending on the segment length (along-strike) of the rupture plane, the occurrence of an event of magnitude 8–9 cannot be excluded.

Statistical models in seismology: Lesser Antilles arc case

AbstractSeismic risk calculation is based on stochastic models describing at best the seismic activity. The focus is on calculating earthquakes occurrences probability and analyzing spatiotemporal evolution of these probabilities. The point process theory is the main tool for modeling seismic data and taking into account past history prior to a given date. Thus, probability and statistical tools were developed by scientists from research teams located in regions with very high seismicity regions: California (Turcotte), Japan (Ogata) and New Zealand (Vere-Jones). Statistical inference is based on likelihood techniques in order to optimize information processing and projection strategy. We consider the ETAS model and analyze the epicenter and magnitude distribution of earthquakes occurred in the Lesser Antilles arc from 1999 to 2004. A comparison of different probability density functions for the triggered event times is performed.