Submarine lava flow emplacement and faulting in the axial valley of two morphologically distinct spreading segments of the Mariana back-arc basin from Wadatsumi side-scan sonar images

International audienceHigh-resolution, deep-tow side-scan sonar data were collected over two distinct spreading segments in the central part of the Mariana back-arc basin. These data allow mapping of small fissures and faults and the distinguishing of hummocky from smooth lava flows. Using these data, we observe spatial variations in seafloor deformation and volcanic activity within each segment, and also significant differences in the degree of tectonic deformation between the two segments. One segment, characterized by an hourglass shape suggestive of magmatic processes dominating over tectonic processes, is in fact currently dominated by intense deformation rather than volcanism. The other segment, which exhibits morphology (wide and deep linear axial valley) typical of magma-starved segments, is subjected to very limited deformation and is covered by mostly unfaulted, recent flows. Each segment also displays along-axis variations in the degree of tectonic deformation and in lava flows freshness. We observe a decrease of lava effusion rate from segment centers toward their ends. We also investigated the apparent asymmetry of the Mariana basin. On the southern segment of the study area, azimuths of tectonic structures are divided into two groups, one segment-parallel and one ∼15° oblique to strike of the segment. These two trends of linear features developed synchronously with volcanic activity. Currently oblique structures seem to be the most active ones. The tectonic structures that are parallel to the overall valley trend are distributed over the entire valley, whereas the oblique structures are only located proximal to the eastern valley wall. They are likely related to changes in the local stress field related to the obliquely trending eastern axial valley wall. Asymmetric character, such as nonuniform spacing and throw of faults, was not observed. These observations suggest that the asymmetry of the basin is not due to asymmetric spreading, but rather to eastward ridge jumps of several kilometers. Such small-amplitude ridge jumps likely occurred frequently during basin development because the asymmetric character of the basin is strongly pronounced and no abandoned rift valley is recognizable within the entire basi

Mean magnitude variations of earthquakes as a function of depth: Different crustal stress distribution depending on tectonic setting

International audienceThe mean magnitude of earthquakes in the Gulf of Corinth is found to increase strongly with depth (b-value decreases), whereas the dip of fault planes decreases. The b-value difference of 0.25, between shallow and deep earthquake distributions, is based on about 7,000 events and therefore is statistically highly significant. The same is true in California, but opposite patterns are observed in southern Iceland and in western Nagano, Japan. Because large mean magnitudes (low b-values) are indicative of relatively high stress levels, we propose that in the detachment layer at about 9 ± 2 km depth, earthquakes are generated at higher stresses than in the shallower parts of the crust. The correlation of low b-values with low faulting dips can be taken as line of evidence that low b-values map high stress regimes

Waning magmatic activity along the Southern Explorer Ridge revealed through fault restoration of rift topography

International audienceWe combine high-resolution bathymetry acquired using the Autonomous Underwater Vehicle ABE with digital seafloor imagery collected using the remotely operated vehicle ROPOS across the axial valley of the Southern Explorer Ridge (SER) to infer the recent volcanic and tectonic processes. The SER is an intermediate spreading ridge located in the northeast Pacific. It hosts the Magic Mountain hydrothermal vent. We reconstruct the unfaulted seafloor terrain at SER based on calculations of the vertical displacement field and fault parameters. The vertical changes between the initial and the restored topographies reflect the integrated effects of volcanism and tectonism on relief-forming processes over the last 11,000-14,000 years. The restored topography indicates that the axial morphology evolved from a smooth constructional dome >500 m in diameter, to a fault-bounded graben, ~500 m wide and 30-70 m deep. This evolution has been accompanied by changes in eruptive rate, with deposition of voluminous lobate and sheet flows when the SER had a domed morphology, and limited-extent low-effusion rate pillow eruptions during graben development. Most of the faults shaping the present axial valley postdate the construction of the dome. Our study supports a model of cyclic volcanism at the SER with periods of effusive eruptions flooding the axial rift, centered on the broad plateau at the summit of the ridge, followed by a decrease in eruptive activity and a subsequent dominance of tectonic processes, with minor low-effusion rate eruptions confined to the axial graben. The asymmetric shape of the axial graben supports an increasing role of extensional processes, with a component of simple shear in the spreading processes

Deep crustal earthquakes in North Tanzania, East Africa: Interplay between tectonic and magmatic processes in an incipient rift

International audienceIn this study, we explore the origin of lower crustal seismicity and the factors controlling rift propagation using seismological data recorded within the youngest part of the East African Rift System, the North Tanzanian Divergence (NTD). Most earthquakes below Lake Manyara occur at depth ranging between 20 and 40 km and have a swarm-like distribution. Focal mechanisms of 26 events indicate a combination of strike-slip and normal faulting involving Archaean basement structures and forming a relay zone. The derived local stress regime is transtensive and the minimum principal stress is oriented N110°E. Crustal seismic tomography reveals low-velocity anomalies below the rifted basins in the NTD, interpreted as localized thermomechanical perturbations promoting fluid release and subsequent seismicity in the lower crust. SKS splitting analysis in the NTD indicates seismic anisotropy beneath 17 stations most likely due to aligned magma lenses and/or dikes beneath the rift and to the lithospheric fabrics. Our results favor a strain pattern intermediate between purely mechanical and purely magmatic. We suggest that melt products arising from a large asthenospheric thermal anomaly enhance lithospheric weakening and facilitate faulting and creeping on critically oriented inherited structures of the Precambrian lower crust. Although the crust is unlikely weakened at a point comparable to other parts of the East African Rift System, this deep-seated thermomechanical process is efficient enough to allow slow rift propagation within the eastern Tanzanian cratonic edge

Étude de la réponse structurelle d’un bâtiment de grande hauteur à partir d’enregistrements accéléromètriques et de la modélisation par éléments finis

International audienceThe Nice prefecture is equipped by 24 accelerometric sensors, located at different levels, which continuously record the building response. A finite element model of the building is created based in design plans and information about materials provided during the conception project. Such model is used to obtain the natural frequencies of the structure, the modal shapes and the seismic response acceleration time histories. Modal frequencies of the building obtained from the data analysis are used to calibrate the numerical model. The response of the building to earthquakes, in the form of acceleration time histories at different storeys, is evaluated using the finite element model. Signals recorded during the recent Barcelonnette earthquake (April 2014, Mw = 4.9) are used as excitation at the base of the building. The imposition of a single or multiple signals at the base of the structure shows the importance of taking into account the rocking effects in the building response modelling. The numerical results are in agreement, in amplitude and phase, with accelerometric records filtered at different frequency bands.La préfecture de Nice est équipée d’un ensemble de 24 capteurs accélérométriques, répartis sur différents étages, qui enregistrent en continu les vibrations du bâtiment. Un modèle par éléments finis du bâtiment est créé à partir des plans de construction et des données sur les matériaux, issues du projet de dimensionnement d’origine. Ce modèle est utilisé pour obtenir les fréquences propres de vibration de la structure, les déformées modales et la réponse sismique dans le temps. Les fréquences modales du bâtiment obtenues par l’analyse des enregistrements sont utilisées pour calibrer le modèle numérique. La réponse du bâtiment aux séismes, en terme de séries temporelles d’accélération aux étages, est évaluée par le modèle d’éléments finis. Les signaux enregistrés lors du récent séisme de Barcelonnette (avril 2014, Mw = 4.9) sont utilisés comme sollicitations appliquées à la base du bâtiment. L’imposition d’une seule ou plusieurs excitations à la base de la structure mon- tre l’importance de la prise en compte des effets de «rocking» du bâtiment pour mieux comprendre sa réponse aux séismes. Les signaux numériques sont en accord dans différentes bandes de fréquence, en amplitude et en phase, avec les enregistrements

No evidence of association between NOD2/CARD15 gene polymorphism and atherosclerotic events after renal transplantation.

International audienceStable renal transplant recipients (RTR) display high rates of atherosclerotic events (AE). Innate immunity and especially vascular inflammation play a role in the pathogenesis of atherosclerosis. It is illustrated both by an increased occurrence of postrenal transplant cardiovascular events in patients with elevated levels of C-reactive protein and by a correlation between posttransplant AE and Toll-like receptor-4 Asp299Gly polymorphism. Here, we analyze the influence NOD2/CARD15 gene polymorphism since NOD2 can modulate macrophage pro-inflammatory activity and macrophage is present in early atherosclerotic lesions. The incidence of single nucleotide polymorphism (SNP) in the three major polymorphic region of NOD2 gene (SNP8, SNP12 and SNP13) was assessed in 182 RTR and the correlation between such polymorphism and the development of AE was analyzed. No correlation was observed between NOD2 gene polymorphism and the occurrence of AE after renal transplantation. NOD2 gene polymorphism thus does not appear to influence cardiovascular complications in RTR

Hydrothermal activity along the slow-spreading Lucky Strike ridge segment (Mid-Atlantic Ridge) : distribution, heatflux, and geological controls

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 431 (2015): 1730185, doi:10.1016/j.epsl.2015.09.025.We have reviewed available visual information from the seafloor, and recently acquired microbathymetry for several traverses across the Lucky Strike segment, to evaluate the distribution of hydrothermal activity. We have identified a new on-axis site with diffuse flow, Ewan, and anactive vent structure ~1.2 km from the axis, Capelinhos. These sites are minor relative to the Main field, and our total heatflux estimate for all active sites (200-1200 MW) is only slightly higher than previously published estimates. We also identify fossil sites W of the main Lucky Strike field. A circular feature ~200 m in diameter located on the flanks of a rifted off-axis central volcano, is likely a large and inactive hydrothermal edifice, named Grunnus. We find no indicator of focused hydrothermal activity elsewhere along the segment, suggesting that the enhanced melt supply and the associated melt lenses, required to form central volcanoes, also sustain hydrothermal circulation to form and maintain large and long-lived hydrothermal fields. Hydrothermal discharge to the seafloor occurs along fault traces, suggesting focusing of hydrothermal circulation in the shallow crust along permeable fault zones.This work has been partly financed by ANR (France) Mothseim Project NT05-3 42213 toJE, and by EU-RTN-MOMARNET to MC. The French Ministry of Research financed ship, ROV and AUV time (Graviluck’06, MOMAR’08, Bathyluck’09, MOMARSAT cruises in 2010-2015

DRELIO : Un drone hélicoptère pour le suivi des zones littorales

International audienceLa connaissance du littoral, sa protection, son aménagement nécessitent un suivi des changements qui s'y produisent. Toutefois, en France, devant l'étendue des façades maritimes à couvrir, les techniques de mesures in situ ne peuvent être appliquées de façon systématique. Dans ce cas, les techniques de télédétection spatiale ou aérienne sont des approches complémentaires. A partir de ces plates-formes, des images stéréoscopiques multi-temporelles sont, après une série de traitements photogrammétriques appropriés, directement exploitables sous formes d'orthophotographies et de Modèles Numériques de Terrain (MNT). Ce type de produits permet notamment la quantification des changements morphosédimentaires à l'interface Terre-Mer (transport transversal et longitudinal de sédiments, érosion,...). Actuellement, la résolution spatiale des MNT générés à partir d'images aériennes ou satellitaires (<50 cm) est insuffisante pour la plupart des applications sur la frange littorale. En outre, dans le cas d'interventions consécutives à un événement extrême (tempête, raz de marée, pollution...), ces systèmes manquent de souplesse (trajectoires prédéfinies, contraintes d'altitude, de vitesse, coût de mise en oeuvre...) L'utilisation de drones constitue donc une alternative intéressante pour des suivis de précision ou des interventions rapides

Les quotas de genre dans le monde académique

Quel peut être l’effet de l’introduction de quotas face aux inégalités de genre dans l’enseignement supérieur et la recherche ? Dans une étude récente, Pierre Deschamps s’est intéressé à l’impact de la mise en place depuis 2015 d’un quota de 40% de femmes dans les comités de sélection à l’université en France. Il s’est appuyé sur des données administratives sur 455 comités académiques et 1548 candidates appartenant à 3 universités publiques françaises. Publiés dans le Working paper n°82 du LIEPP « Gender Quotas in Hiring Committees : a Boon or a Bane for Women », les résultats de cette étude interrogent la conception et les effets des politiques d’égalité dans l’enseignement supérieur et la recherche. En effet, si le quota est effectivement respecté, la progression de la proportion de femmes dans les comités semble s’être accompagnée d’une diminution des chances de recrutement des femmes candidates. Ces conclusions ont fait l’objet d’une discussion interdisciplinaire lors d’un « Débat du LIEPP » organisé le 5 avril 2019. Prenant appui sur des travaux de droit et de science politique, Anne Revillard est revenue sur l’origine et les objectifs des réformes instituant le quota. Marie Sautier a mis en perspective les résultats de cette étude à partir d’une analyse sociologique des mécanismes de production des inégalités de genre. Représentant la Mission pour la place des femmes au CNRS, Mathieu Arbogast a pointé les obstacles spécifiques à la mise en œuvre des quotas dans le monde de la recherche, et présenté plusieurs pistes de réformes dans le prolongement de ces travaux