Déformation asymétrique de houppiers pour la génération de représentations paysagères réalistes

National audienceLa génération de paysage naturel virtuel réaliste a beaucoup dapplications, aussi bien en infographie quen éco- logie par exemple. Ce problème est généralement traité en deux étapes. Une distribution des plantes sur le terrain est dabord calculée en utilisant des modèles simplifiés de plantes. Puis ces modèles sont utilisés pour générer des représentations géométriques détaillées des plantes. En suivant cette méthodologie, nous présentons dans ce travail une réalisation dun tel processus sur une forêt de hêtres. Pour cela, nos proposons dutiliser une méthode de distribution stochastique darbres basée sur les processus de Gibbs. Une contribution importante de ce travail est la prise en compte la plasticité des houppiers lors de la distribution en considérant notamment des formes asymétriques de houppier

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Consultable en ligne sur Revue.org, URL : http://perspective.revues.org/1193International audienc

A volcaniclastic deep-sea fan off La Réunion Island (Indian Ocean): Gradualism versus catastrophism

International audienceA new geophysical data set off La Réunion Island (western Indian Ocean) reveals a large volcaniclastic submarine fan developing in an open-ocean setting. The fan is connected to a torrential river that floods during tropical cyclones. Sediment storage at the coast is limited, suggesting that the sediments are carried directly to the basin. The fan morphology and turbidites in cores lead us to classify it as a sand-rich system mainly fed by hyperpycnal flows. In the ancient geological record, there are many examples of thick volcaniclastic successions, but studies of modern analogues have emphasized mechanisms such as debris avalanches or direct pyroclastic flow into the sea. Because the Cilaos deep-sea fan is isolated from any continental source, it provides information on architecture and noncatastrophic processes in a volcaniclastic deep-sea fan

Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure

Landslides are common features in the vicinity of volcanic islands. In this contribution, we investigate landslides emplacement and dynamics around the volcanic island of Martinique based on the first scientific drilling of such deposits. The evolution of the active Montagne Pelée volcano on this island has been marked by three major flank-collapses that removed much of the western flank of the volcano. Subaerial collapse volumes vary from 2 to 25 km3 and debris avalanches flowed into the Grenada Basin. High-resolution seismic data (AGUADOMAR – 1999, CARAVAL – 2002 and GWADASEIS – 2009) is combined with new drill cores that penetrate up to 430 m through the three submarine landslide deposits previously associated to the aerial flank-collapses (Site U1399, Site U1400, Site U1401, IODP Expedition 340, Joides Resolution, March-April 2012). This combined geophysical and core data provide an improved understanding of landslide processes offshore a volcanic island. The integrated analysis shows a large submarine landslide deposit, without debris avalanche deposits coming from the volcano, comprising up to 300 km3 of remobilized seafloor sediment that extends for 70 km away from the coast and covers an area of 2100 km2. Our new data suggest that the aerial debris avalanche deposit enter the sea but stop at the base of submarine flank. We propose a new model dealing with seafloor sediment failures and landslide propagation mechanisms, triggered by volcanic flank-collapse events affecting Montagne Pelée volcano. Newly recognized landslide deposits occur deeper in the stratigraphy, suggesting the recurrence of large-scale mass-wasting processes offshore the island and thus, the necessity to better assess the associated tsunami hazards in the region. This article is protected by copyright. All rights reserved

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Les 8 et 9 juillet 2010 à Munich, Willibald Sauerländer a accordé deux longs entretiens à Perspective, chez lui, dans sa bibliothèque, et au Zentralinstitut für Kunstgeschichte, dont il a été le directeur pendant vingt ans (1970-1989). Afin de cerner les différentes facettes de ce spécialiste de l’art médiéval français, et notamment de la sculpture, mais aussi grand connaisseur de Poussin et de Houdon, Perspective a demandé à Pierre-Yves Le Pogam, conservateur en chef au département des sculp..

Gravity-driven sliding and associated deformations along complex submarine slopes: a laboratory modeling approach based on constraints observed offshore Martinique Island (Lesser Antilles)

Submarine gravity-driven sliding of sediments are common processes in the vicinity of volcanic islands. In the Lesser Antilles arc, the Montagne Pelée volcano on Martinique Island underwent several flank-collapse events during its long-term eruptive history, resulting in debris avalanches. When the debris avalanches entered into the seawater, they were emplaced over the unstable slope of the volcano, triggering a seafloor sediment failure and massive landslides downslope. Using a laboratory modeling approach, we simulated the gravity-driven sliding of a sand layer lying above a silicone layer. The experiments were performed using various slope geometries (slope lengths and number of slope breaks separating the slopes with different angles), under both dry and aqueous conditions, and while varying the amount of additional sand inputs upslope. The resulting deformations were characterized in each experiment in order to compare the obtained structures with those shown by the seismic lines offshore to the west of Martinique Island. During all the experiments, a compressional frontal deformation zone made of several reverse faults formed downslope, often near the slope breaks. Downslope, a portion of the sediments was mostly displaced and poorly deformed in a damping zone, while an extensional deformation zone formed upslope. The displacements of the surficial markers were measured through time to characterize the sliding dynamics. Our study demonstrates that the slope geometry and additional sand inputs primarily favor and increase the sliding deformation, whereas the hydrostatic pressure plays a secondary catalytic role over time. These results provide new constraints on the driving factors and their consequences on gravity-driven sliding in terms of deformations and runout distance over time. This may have a significant impact on the associated hazard assessment related to offshore infrastructures, in a region known for its seismic and volcanic risks

The relationship between eruptive activity, flank collapse, and sea level at volcanic islands: A long-term (>1 Ma) record offshore Montserrat, Lesser Antilles

Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass-wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (?930 ka to ?900 ka, ?810 ka to ?760 ka, and ?190 ka to ?120 ka) that coincide with periods of increased volcano instability and mass-wasting. The youngest of these periods marks the peak in activity at the Soufrière Hills volcano. The largest flank collapse of this volcano (?130 ka) occurred towards the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufrière Hills edifice, and their timing also coincides with periods of rapid sea-level rise (>5 m/ka). Available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea-level rise, but this is not observed for volcanoes in intra-plate ocean settings. We thus infer that rapid sea-level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean-island settings

How ethnic are African parties really? Evidence from Francophone Africa

Though African party systems are said to be ethnic, there is little evidence for this claim. The few empirical studies rarely rely on individual data and are biased in favour of Anglophone Africa. This paper looks at four Francophone countries, drawing on representative survey polls. Results reveal that ethnicity matters, but that its impact is generally rather weak and differs with regard to party systems and individual parties. ‘Ethnic parties’ in the strict sense are virtually absent. In particular, the voters’ location seems more important than ethnic affiliation. Other determinants such as regional ties, elite strategies, cross-cutting cleavages, and rational preferences deserve more attention in the future study of voting behaviour in Africa

Rapid onset of mafic magmatism facilitated by volcanic edifice collapse

Volcanic edifice collapses generate some of Earth's largest landslides. How such unloading affects the magma storage systems is important for both hazard assessment and for determining long-term controls on volcano growth and decay. Here we present a detailed stratigraphic and petrological analyses of volcanic landslide and eruption deposits offshore Montserrat, in a subduction zone setting, sampled during Integrated Ocean Drilling Program Expedition 340. A large (6–10 km3) collapse of the Soufrière Hills Volcano at ~130 ka was followed by explosive basaltic volcanism and the formation of a new basaltic volcanic center, the South Soufrière Hills, estimated to have initiated <100 years after collapse. This basaltic volcanism was a sharp departure from the andesitic volcanism that characterized Soufrière Hills' activity before the collapse. Mineral-melt thermobarometry demonstrates that the basaltic magma's transit through the crust was rapid and from midcrustal depths. We suggest that this rapid ascent was promoted by unloading following collapse