Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study analyses the interactions between deformation and reactive fluid and melt percolation, and their effects on sub-arc mantle seismic properties based on microstructural observations on mantle xenoliths extracted by the Tubaf and Edison seamounts close to the Lihir Island, in the Papua New Guinea archipelago. These xenoliths sample an oceanic lithosphere, which has experienced high-temperature deformation in the presence of fluids or melts. This was followed by metasomatism under static conditions. Syn-kinematic percolation of reactive Si-rich melts or fluids in peridotites has produced pyroxene-enrichment, grain size reduction, and dispersion of olivine crystal preferred orientation (CPO). Fourier transform infrared spectroscopy analyses show that olivine has very low water contents (1-4 wt. ppm H2O), similar to spinel peridotites from other subduction zones. These low values may record both low water solubility in olivine at low pressure and dehydration during transport and exhumation. Water contents in pyroxenes are highly variable and likely result from spatially heterogeneous melt or fluid percolation. Analysis of olivine CPO indicates dominant activation of both (010)[100] and (001)[100] slip systems, which are characteristic of deformation under high temperature, low stress, low pressure and low to moderate hydrous conditions. Fast S-wave polarization and P- and Rayleigh propagation directions are thus parallel to the mantle flow direction. The pyroxene enrichment by melt-rock reactions is accompanied by dispersion of olivine CPO and induces a significant decrease of the maximum S-wave and P-wave anisotropy in the peridotites. The calculated seismic properties also show that the lowest Vp/Vs ratios (< 1.7) mapped in fore-arc mantle may only be explained by taking in consideration the CPO-induced elastic anisotropy of the peridotites.
ISSN: 0040-1951

hal-00950058
https://hal.archives-ouvertes.fr/hal-00950058
DOI : 10.1016/j.tecto.2013.09.024

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We performed torsional deformation experiments on pre-hydrated fine-grained olivine aggregates using an innovative experimental assembly to investigate water weakening in mantle rocks at high shear strains. San Carlos olivine powder was cold-pressed and then hot-pressed under hydrous conditions, producing aggregates with average grain sizes of 7 or 15 μm. Deformation experiments were performed in a high-resolution gas-medium apparatus equipped with a torsional actuator, under a confining pressure of 300 MPa, a temperature of 1200 °C, and constant shear strain rates ranging from 8 × 10−5 to 1.4 × 10−4 s−1. Maximum shear stresses range from 150 to 195 MPa. These values are 30% lower relative to those determined in previous torsion experiments on dry, fined-grained dunites under similar conditions. Textures and microstructures of the starting and deformed specimens were characterized by scanning and transmission electron microscopy. All deformed aggregates exhibit a shape-preferred orientation marking a foliation and lineation, as well as a reduction in mean grain size from 15 μm down to 3-4 μm due to dynamic recrystallization. Olivine crystallographic fabrics developed rapidly (γ < 0.1), but their strength, characterized by the J-index, is low compared to naturally deformed peridotites or to polycrystalline olivine deformed at similar finite shear strains under dry conditions. The crystallographic fabrics are consistent with deformation by a dislocation accommodated creep mechanism with activation of multiple {0 k l}[1 0 0] systems, among which the (0 1 0)[1 0 0] slip system is dominant, and minor participation of the (0 1 0)[0 0 1] slip system. Transmission electron microscopy confirmed the occurrence of dislocations with [1 0 0] and [0 0 1] Burgers vectors in most grains. Analysis of unpolarized infrared spectra indicates that hydrogen concentration in the olivine lattice is below the saturation level of 18 ppm wt H2O, which is similar to those typically observed in spinel-bearing peridotite xenoliths, and also provide evidence for water-rich inter-granular material trapped in pores and grain boundaries. Seismic properties computed from the CPO observations correspond to those most commonly observed in naturally deformed mantle peridotites with fast P-wave propagation and S-wave polarization subparallel to the shear direction. These torsion experiments on fine-grained olivine polycrystals under hydrous conditions indicate that water weakening under lithospheric conditions is linked to various defects with hydrogen in the olivine structure, as well as with water-derived species in grain boundaries or pores.
ISSN: 0031-9201

hal-00745170
https://hal.archives-ouvertes.fr/hal-00745170
DOI : 10.1016/j.pepi.2012.05.001

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Melt generation in the continental crust is thought to significantly influence seismic bulk velocities and anisotropy, although existing laboratory data provide limited constraints on such seismic attributes. In this study we measured ultrasonic compressional wave speeds (Vp) and anisotropy during sample compaction, mica-breakdown and melt-generation in synthetic, foliated quartz–muscovite aggregates. Measurements were performed at peak conditions of 300 MPa hydrostatic confining pressure and 750 °C, over a six hour time period, for three separate samples where wave propagation directions were aligned at 0°, 45° and 90° with respect to the foliation plane. The experiments are initially marked by sample compaction and rapid reduction in porosity (from 25% to <2 vol%), with a corresponding increase in Vp. During initial stages of the experiment Vp ranges from 4.65 to 5.45 km/s, depending on the direction of the P wave propagation, resulting in up to ∼16% anisotropy, which originates mainly from the preferred orientation of muscovite and possible incipient melt lenses oriented parallel to foliation. As the experiment progresses the bulk Vp decreases gradually and Vp anisotropy drops from ∼16% to ∼2% after five hours into the experiment. The drop in anisotropy is caused by muscovite-breakdown and increasing amount of partial melting of the aggregate. The bulk velocities are considerably lower than predicted for a mixture of muscovite and β-quartz (stable at 300 MPa and 750 °C). Thermodynamic modeling indicates that mica breakdown and production of melt with ∼2 wt% dissolved water in the aggregate can explain the low Vp, even though β-quartz is stable; the modeled Vp is ∼4.75 km/s at 300 MPa and 750 °C. These ultrasonic attributes signify movement of the melt to fill vacant pores and coating grain boundaries, effectively connecting melt films and reducing seismic anisotropy. The results may apply to regions where crustal melting is on-going, in particular regions of elevated temperature that are only weakly deforming, with coupled anomalously low bulk velocities and seismic anisotropy.
ISSN: 0012-821X

hal-01243569
https://hal.archives-ouvertes.fr/hal-01243569
DOI : 10.1016/j.epsl.2015.05.039

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Energy dissipation is observed on seismic data when a wave propagates through a porous medium, involving different frequency regimes depending on the nature of rock and fluid types. We focus here on the role of partial fluid saturation in unconsolidated porous media, looking in particular at P-wave phase velocity and attenuation. The study consists in running an experiment in a sand-filled tank partially saturated with water. Seismic propagation in the tank is generated in the kHz range by hitting a steel ball on a granite plate. Seismic data are recorded by buried accelerometers and injecting or extracting water controls the partial saturation. Several imbibition/drainage cycles were performed between the water and gas residual saturations. A Continuous Wavelet Transform applied on seismic records allowed us to extract the direct P-wave at each receiver.We observe an hysteresis in phase velocities and inverse quality factors between imbibition and drainage. Phase velocities and inverse quality factors are then jointly inverted to get a final poro-visco-elastic model of the partially saturated sand that satisfactorily reproduces the data. The model formulation consists in generalizing the Biot theory to effective properties of the fluid and medium (permeability and bulk modulus) in order to properly explain the phase velocity variation as a function of the saturation. The strong level of attenuation measured experimentally is further explained by an anelastic effect due to grain to grain sliding, adding to Biot's losses. This study shows that fluid distribution at microscopic scale has strong influence on the attenuation of direct P-waves at macroscopic scale and confirms that seismic prospection may be a powerful tool for the characterization of transport phenomena in porous media.
ISSN: 0956-540X

hal-00779920
https://hal.archives-ouvertes.fr/hal-00779920
https://hal.archives-ouvertes.fr/hal-00779920/document
https://hal.archives-ouvertes.fr/hal-00779920/file/Barriere_et_al_HAL.pdf
DOI : 10.1111/j.1365-246X.2012.05691.x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The paper presents coastal structures design in the presence of waves by the minimization of a cost function. It aims to show that shape optimization can be efficiently applied to ocean engineering. This is an underlying guiding principle for the design of harbors or offshore breakwaters. We compute the solution of a specific simplified boundary value problem describing the short wave propagation toward a vertical sea cliff or vertical wall and modify accordingly the shape of defense structures in order to minimize a pre-defined cost function taking into account the strength (energy) of the water waves. The optimization procedure relies on a global semi-deterministic search algorithm able to escape from local minima.
ISSN: 0378-3839

hal-00411905
https://hal.archives-ouvertes.fr/hal-00411905
DOI : 10.1016/j.coastaleng.2007.06.006

Éditeur(s) : HAL CCSD American Institute of Mathematical Sciences
Résumé : International audience
In this paper, shape optimization tools are applied to coastal engineering. A barred beach protection study is described, using a new type of water wave attenuator device, namely geotextile tube, to reduce the mobilization of sediments. A global optimization algorithm, able to pursue beyond local minima, is used to search for the optimum properties of the geotextile tube. The optimal configuration is then post-validated for generated currents.
ISSN: 1531-3492

hal-00475636
https://hal.archives-ouvertes.fr/hal-00475636
DOI : 10.3934/dcdsb.2010.13.759

Éditeur(s) : HAL CCSD Springer-Verlag
Résumé : The seismology is the most effective method to explore the Structure of subduction zones to great depth. The distinguishing feature of the mantle in the subduction regions is the presence of hydrated phases, which transport water into the Earth's interior and release it with dramatic local consequences, triggering earthquakes and melting. The seismological detection of these hydrous phases and geo-dynamic interpretation of flow in the hydrous mantle depend on knowledge of the anisotropic elastic properties and the characteristics of the wave propagation in anisotropic media. In this paper we briefly recall the distinguishing features of anisotropic wave propagation and the observable parameters. We suggest the Vp/Vs ratio is a physically sound parameter than can be observed by seismology in anisotropic regions of the Earth, whereas the Poisson's ratio, which is often quoted, is not directly observable and does not correspond to the characteristics of wave propagation in an anisotropic or isotropic medium. We report for the first time the ratios of Vp/Vs1 and Vp/Vs2, where Vs1 is the fastest and Vs2 slowest S-wave velocities of an anisotropic media. We present the current knowledge of the anisotropic seismic properties of hydrous minerals in the upper mantle, transition zone, and lower mantle that are stable along low temperature geotherms associated with subduction, and identify which minerals are likely to influence seismological observations because they have very high volume fractions, or very high anisotropies, or both of these.;In the upper mantle antigorite and talc are exceptionally anisotropic (Vp 71% Vs 68% and Vp 65%, Vs 68%, respectively) and chlorite is also very anisotropic for S-waves (Vp 35%, Vs 76%). Comparatively less anisotropic are hornblende (Vp 27%, Vs 31%), used as proxy for tremolite in subduction zones, and clinohumite (21.8%, Vs 15.9%). Brucite at 4 GPa (Vp 26.5%, Vs 30.9%) and the dense hydrogen magnesium silicate (DHMS) phase A at 9GPa (Vp 9.3%, Vs 17.6%) are the only hydrous minerals stable in the upper mantle that have had their elastic properties measured at in situ mantle pressures. Except for the phase A, all these minerals are more anisotropic than olivine for at least one parameter (Vp, Vs, Vp/Vs1, or Vp/Vs2). In the transition zone the major phases hydrous wadsleyite and ringwoodite C have moderate to weak anisotropies (Vp 16.3%, Vs 16.5%, and Vp 1.9%, Vs 4.4%, respectively). The DHMS Superhydrous B has a moderate anisotropy (Vp 6.9%, Vs 11.6%). At greater depth the DHMS phase D Lit 24 GPa (Vp 10.8%, Vs 18.0%) is the only hydrous phase that can transport hydrogen from the transition zone into the lower mantle. The phase D is more anisotropic for S than P waves like many of the hydrous phases. From these data it is clear that hydrous phases are in general very anisotropic. However, pressure can play a strong role in reducing anisotropy. It is the case for brucite and talc, in which increasing pressure from ambient to 4 GPa reduces the anisotropy by about 50% for both P and S waves. In contrast, the anisotropy of the DHMS phase A does not change significantly with pressure.;Our picture of the seismic anisotropy of hydrated minerals remains incomplete; the elastic properties of many have not been measured even at ambient conditions (e.g., 10 angstrom phase and phase E) or not measured in their true elastic symmetry (e.g., clinochlore). The majority of hydrous minerals have not been measured at high pressure. and none related to hydrated mantle have been measured at elevated temperature. We have shown in the few cases where hydrous minerals have been measured as a function of pressure, that this variable has an important effect oil the velocity distribution and in most cases reduces the degree of anisotropy, hence we Would expect seismic anisotropy to play a key role in the determination of the shallow structure of subduction zones in the upper mantle.
Subduction zone geodynamics

hal-00420893
https://hal.archives-ouvertes.fr/hal-00420893

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The seismic anisotropy of the deep Earth is reviewed as a profile from the upper mantle to the solid inner core at the centre of the Earth. The upper mantle is by far the most anisotropic region of the Earth, followed the D” layer above the core mantle boundary. In contrast it is shown that many minerals that are present in the upper mantle, transition zone, lower mantle, D” layer and solid inner core are elastically anisotropic to different degrees. The anisotropy of hydrous phases present in subduction zones is briefly introduced. The basic concepts of single crystal and polycrystalline elasticity and the extrapolation of elastic properties to high temperature and pressure are presented. The specific features of elastic wave propagation in an anisotropic medium of any arbitrary symmetry are illustrated using mantle and inner core phases. The roles of crystal preferred orientation, water and melt in producing seismic anisotropy in the upper mantle are discussed.
Treatese of Geophysics, vol.2

hal-00408321
https://hal.archives-ouvertes.fr/hal-00408321
DOI : 10.1016/B978-044452748-6.00045-6

Éditeur(s) : HAL CCSD Editions Paralia CFL
Résumé : Des techniques d'optimisation de formes sont utilisées dans des problèmes du Génie Côtier pour la protection des plages. En collaboration avec le bureau d'étude Bas-Rhône Languedoc ingénierie, un projet d'aménagement concret de la plage du Lido de Sète, utilisant des ouvrages immergés en géotextile, a été étudié. Des solutions optimales, visant à réduire la mise en suspension des sédiments, pour les structures de défenses ont été obtenues à partir d'un modèle hydrodynamique de propagation de houle. Une étude complémentaire à partir d'un modèle de circulation et des travaux expérimentaux en canal à houle permettent ensuite de valider les résultats optimaux numériques obtenus. Shape optimization techniques, focusing on coastal structures design, are used to solve coastal engineering problem. In collaboration with the Bas-Rhône Languedoc engineering Company, a actual development plan concerning the beach of the Lido of Sète has been studied. Optimal solutions, aimed at reducing sediments suspended, for defense structures have been obtained with a hydrodynamic water wave model. An additional study using a circulation model and experimental works in flume tend to validate optimal numerical results.
ISSN: 1760-8716

hal-00854031
https://hal.archives-ouvertes.fr/hal-00854031
DOI : 10.5150/jngcgc.2008.025-I

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
This paper describes how to tackle new challenging coastal engineering problems related to beach erosion with a shape optimization approach. The method modifies the shape of the sea bottom in order to reduce beach erosion effects. Global optimization is shown to be necessary as the related functionals have several local minima. We describe the physical model used, the proposed protection devices against beach erosion and real case applications.
ISSN: 0029-5981

hal-00411912
https://hal.archives-ouvertes.fr/hal-00411912
DOI : 10.1002/nme.2209