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Abstract: We analyzed seismic waveforms recorded by the broadband ocean bottom seismographs deployed in the South Pacific superswell to determine the depths of the mantle discontinuities using a receiver function method. We estimated the thickness of the mantle transition zone (MTZ) to be 245km on average beneath the superswell region, which is close to the global average. The MTZ is found to be thinned locally beneath the Society hot spot by 30km. Temperature anomalies computed from the thinned transition zone and the Clapeyron slope of the olivine phase transformations are 150–200K beneath the Society hot spot. Previous studies, using land-based data, suggested the presence of a hot MTZ beneath the Pitcairn hot spot. The locally hot transition zone beneath the hot spots is attributed to narrow mantle plumes probably rising from the lower mantle. The normal average thickness obtained from the present study indicates that there is no broad upwelling across the transition zone beneath the entire superswell region at present. [Copyright &y& Elsevier]

Abstract: A shear-wave splitting analysis was performed in the Mariana trench on data recorded by an array of semi broad-band Ocean Bottom Seismometers (OBS), with the aim of studying the Mariana back-arc spreading system and clarifying the nature of the back-arc process. We look for possible mechanisms relating anisotropy and mantle flow. Direct sampling of ray paths through the slab suggests that anisotropy in the slab is relatively small compared with that of the mantle wedge. Fast polarizations at most OBS are consistent with the direction of relative motion between the subducting Pacific and overriding Philippine plates. Exceptions are seismic stations located on the ridge spreading axis which show fast polarizations perpendicular to the ridge curvature. From simple synthetic modeling we estimate an upper limit for the depth of the layer of mantle flow due to the spreading ridge, but as the data show, its influence wanes quickly with distance from the axis. These results suggest that the “sea-anchor” or “extrusion” models are the most favorable scenarios of back-arc opening among other geodynamic models. [Copyright &y& Elsevier]

Abstract: Three-dimensional P-wave velocity structure of the mantle beneath the South Pacific Superswell is determined through passive broadband seismic experiments on the ocean floor and islands between 2003 and 2005. We collected approximately 1500 relative times of long-period teleseismic P-waves by using a waveform cross-correlation. We analyzed this data set with relative time tomography to depths of 2000km. The resultant structure shows lateral heterogeneity of approximately ±2%, in which a distinct low velocity region is found beneath the center of the Superswell at a depth of 1600km. At 1200km depth, an elongated low velocity region is found beneath the Society to Pitcairn hotspots. At 800km depth, two linear low velocity regions are located beneath Tuamotu and Austral islands. Isolated low velocity regions are identified beneath the Society, Marquesas, and Macdonald hotspots at 400km depth. Our new tomographic images reveal that the large low velocity region rooted in the deep lower mantle is split into two sheets at 1200km depth and these terminate at approximately 800km depth. This feature appears to be consistent with the characteristics of a thermo-chemical pile or dome. [Copyright &y& Elsevier]

During the summer of 1996, five profiles with a total length of 892.5 km and 30 recovered Ocean Bottom Seismographs (OBSs) were acquired along the southern part of the Vøring Margin, mid-Norway. These profiles are oriented in both strike and dip direction, mapping the crustal structure along the western-most part of this area. By performing both forward and inverse velocity travel-time modelling of the vertical components of the OBSs (P-waves) regional models have been established. The thickness of the Cenozoic sediments ranges from 0.5 km at the structural highs (Møre Marginal High) to more than 3 km in the Vigrid Syncline. The Cretaceous layers vary considerably in thickness, from 1.7 km close to the Jan Mayen Fracture Zone, increasing to approximately 8 km below the Helland Hansen Arch. Several intrusions are found in the Cretaceous layers and they individually range in thickness from 0.1 to 0.6 km. These intrusions can be followed through crossing profiles, in both in the models presented in this paper, and in the models inferred from earlier OBS surveys along this margin. The pre-Cretaceous sediments are modelled as a layer with P-wave velocity of more than 5 km/s. The thickness of this layer is generally less than 5 km, but locally a thickness of 6.5 km is modelled. The crystalline basement is modelled as a 6+ km/s refractor with a general trend shallowing towards south- and northwest. Along a strike-profile, the transition from continental to oceanic crust is modelled by increased P-wave velocity from 6.2 to 7.3 km/s, as the profile crosses the Jan Mayen Lineament towards the Møre Marginal High. The thickness of the continental crystalline basement is largest landward towards the Helland Hansen Arch where it exceeds 10 km. Towards the SW and NW, where the crystalline basement is elevated, the thickness decreases to less than 5 km. In the lower crust, a high velocity 7+ km/s body, interpreted as magmatic underplating, is found along three of the profiles in the present work. The SE boundary of this magmatic underplated layer is located beneath the Helland Hansen Arch and towards the Møre Marginal High. The southwestward termination is less certain, but the Jan Mayen Fracture Zone seems to control the emplacement of intrusions in the southern Vøring Basin. The depth to Moho varies from 12 to 28 km, deepening landwards. [Copyright &y& Elsevier]