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Abstract: If upper mantle anisotropy beneath fast-moving oceanic plates is expected to align the fast azimuths close to the plate motion directions, the upper mantle flow pattern beneath slow-moving oceanic plates will reflect the relative motion between the moving plate and the underlying large-scale convecting mantle. In addition to the non-correlation of the fast azimuths with the plate motion direction, the flow and anisotropy pattern may be locally perturbed by other factors such as the upwelling and the sublithospheric spreading of mantle plumes. Investigating such plume–lithosphere interaction is strongly dependent on the available seismological data, which are generally sparse in oceanic environment. In this study, we take the opportunity of recent temporary deployments of 15 seismic stations and 5 permanent stations on the Piton de la Fournaise volcano, the active locus of La Réunion hotspot and of 6 permanent stations installed along or close to its fossil track of about 3700km in length, to analyze azimuthal anisotropy detected by SKS wave splitting and to decipher the various possible origins of anisotropy beneath the Western Indian Ocean. From about 150 good and fair splitting measurements and more than 1000 null splitting measurements, we attempt to distinguish between the influence of a local plume signature and large-scale mantle flow. The large-scale anisotropy pattern obtained at the SW-Indian Ocean island stations is well explained by plate motion relative to the deep mantle circulation. By contrast, stations on La Réunion Island show a complex signature characterized by numerous “nulls” and by fast split shear wave polarizations trending normal to the plate motion direction and obtained within a small backazimuthal window, that cannot be explained by either a single or two anisotropic layers. Despite the sparse spatial coverage which precludes a unique answer, we show that such pattern may be compatible with a simple model of sublithospheric spreading of La Réunion plume characterized by a conduit located at 100–200km north of La Réunion Island. Anisotropy beneath the new GEOSCOPE station in Rodrigues Island does not appear to be influenced by La Réunion plume-spreading signature but is fully compatible with either a model of large-scale deep mantle convection pattern and/or with a channeled asthenospheric flow beneath the Rodrigues ridge. [Copyright &y& Elsevier]

Upper-mantle flow beneath the South Pacific is investigated by analysing shear wave splitting parameters at eight permanent long-period and broad-band seismic stations and 10 broad-band stations deployed in French Polynesia from 2001 to 2005 in the framework of the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). Despite the small number of events and the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity, upper-mantle seismic anisotropy has been detected at all stations except at Tahiti where two permanent stations with 15 yr of data show an apparent isotropy. The median value of fast polarization azimuths (N67.5°W) is parallel to the present Pacific absolute plate motion direction in French Polynesia (APM: N67°W). This suggests that the observed SKS fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving Pacific Plate and preserved in the lithosphere as the plate cools. However, analysis of individual measurements highlights variations of splitting parameters with event backazimuth that imply an actual upper-mantle structure more complex than a single anisotropic layer with horizontal fast axis. A forward approach shows that a two-layer structure of anisotropy beneath French Polynesia better explains the splitting observations than a single anisotropic layer. Second-order variations in the measurements may also indicate the presence of small-scale lateral heterogeneities. The influence of plumes or fracture zones within the studied area does not appear to dominate the large-scale anisotropy pattern but may explain these second-order splitting variations across the network. [ABSTRACT FROM AUTHOR]

Torsion oscillatory deformation experiments have been performed at high temperatures (600–1170°C) and over a wide range of low frequencies (20–2.10−3 Hz) on fine-grained gabbronorite samples from the Oman ophiolite in order to determine the shear wave attenuation as a function of temperature and melt fraction. The specimens have a small and uniform grain size (0.25–0.3 mm) and do not contain secondary, hydrated minerals. Measurements of internal friction were performed using a forced oscillatory torsion apparatus at small strains , and with increasing small temperature steps to reduce thermal microcracking. The general dependence of to frequency is , where ω is the angular velocity of forced oscillations and α is an empirical exponent. Below the melting temperature (∼1050°C), α has average values of ∼0.15 at low frequency (≤0.5 Hz) and 0.06 at higher frequency. Above the melting temperature, α has average values of ∼0.22 at low frequency and −0.02 at higher frequency. This frequency dependence of is attributed to a viscoelastic behaviour due to the diffusion controlled grain boundary sliding, and partially to the squirt flow of the melt-phase wetting grain boundaries. The onset of melting is associated with a markedly higher and a stronger dependence of on temperature. The melt-related mechanical dissipation process could be a melt squirt flow. The characteristic frequency for the melt squirt flow is when the melt pocket aspect ratio is . Around the melting temperature the internal friction can be approximated by an experimental power law with and . [ABSTRACT FROM AUTHOR]

Teleseismic receiver-functions and Rayleigh-wave dispersion curves are jointly inverted for quantifying S-wave velocity profiles beneath the active volcanic zone off Mayotte. We show that the lithosphere in the east-northeast quadrant is composed of four main layers, interpreted as the volcanic edifice, the crust with underplating, the lithospheric mantle, and the asthenosphere, the latter two presenting a main low-velocity zone. The depths of the old (10-11 km) and new Moho (28-31 km) coincide with the two magma reservoirs evidenced by recent seismological and petrological methods. We propose that the main magma reservoir composed of mush with an increasing amount of liquid extends down to 54 km depth. This magma storage develops from a rheological contrast between the ductile lower and brittle upper lithospheric mantle and accounts for most of the volcanic eruption-related seismicity. Finally, the abnormally small thickness of the lithosphericmantle (33 km) is likely a result of a thermal thinning since the onset of Cenozoic magmatism. [ABSTRACT FROM AUTHOR]