Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The three-dimensional S-wave velocity structure beneath the South Pacific Superswell is obtained from joint broadband seismic experiments on the ocean floor and islands. We collected only approximately 800 relative times of long-period teleseismic SH-waves by using a waveform cross-correlation from 76 events occurring from January 2003 to May 2005. We conducted relative time tomography to obtain a 3D structure to depths of 1600 km. In the resultant image, we find a characteristic distribution of low- velocity regions. The most prominent features are a large doughnut-shaped low-velocity region at 800 km depth, and an elongated large low-velocity region beneath the Society to Pitcairn hotspots at 1200 km depth. Our model suggests that a large low shear velocity province rooted in the D00 extends upwards and culminates near the top of the lower mantle beneath the central part of the South Pacific Superswell although its perfect continuity is not still confirmed.
ISSN: 0094-8276

hal-01389114
http://hal.univ-reunion.fr/hal-01389114
http://hal.univ-reunion.fr/hal-01389114/document
http://hal.univ-reunion.fr/hal-01389114/file/Tanaka_PLUME_lower_mantle_GRL2009.pdf
DOI : 10.1029/2009GL037568

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The South Pacific region is characterized by a broadly elevated seafloor known as the South Pacific superswell. This region has a concentration of midplate volcanoes that experienced massive eruptions in the mid-Cretaceous period (90-120 Ma). These characteristics suggest the presence of a large-scale mantle plume beneath the South Pacific, called the South Pacific superplume. The geometry, origin depth, temperature, and composition of the superplume remain controversial, however, mainly due to the lack of seismological data that documents the mantle structure beneath the South Pacific. Seismic stations are sparse in the area due to its remote ocean environment. To obtain a better seismic image of the superplume, we deployed temporary broadband seismographs on oceanic islands and the seafloor in the South Pacific, which made possible the highest spatial resolution that has ever been achieved for the mantle structure beneath the region. The seismic image obtained from this new seismic data indicates that large-scale low-velocity anomalies (on the order of 1000 km in diameter), indicative of the superplume, are located from the bottom of the mantle to a depth of 1000 km, and small-scale low-velocity anomalies (on the order of 100 km in diameter) are present above it. A comparison of the seismic image with recent mantle convection studies based upon laboratory and numerical experiments suggests that the superplume may be a hot and chemically distinct mantle dome, and that the small-scale anomalies may be narrow plumes generated from the top of the dome. This model may explain various characteristics of hot spots in the South Pacific, such as the seafloor swell, short-lived hot spot chains, and the periodicity of massive eruptions.
ISSN: 1525-2027

hal-00443845
https://hal.archives-ouvertes.fr/hal-00443845
https://hal.archives-ouvertes.fr/hal-00443845/document
https://hal.archives-ouvertes.fr/hal-00443845/file/Suetsugu_PLUME_tomo_G32009_hal.pdf
DOI : 10.1029/2009GC002533

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
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 2000 km. 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 1600 km. At 1200km depth, an elongated low velocity region is found beneath the Society to Pitcairn hotspots. At 800 km 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 400 km depth. Our new tomographic images reveal that the large low velocity region rooted in the deep lower mantle is split into two sheets at 1200 km depth and these terminate at approximately 800 km depth. This feature appears to be consistent with the characteristics of a thermo-chemical pile or dome.
ISSN: 0031-9201

hal-00413036
https://hal.archives-ouvertes.fr/hal-00413036
https://hal.archives-ouvertes.fr/hal-00413036/document
https://hal.archives-ouvertes.fr/hal-00413036/file/Tanaka_tomo_PLUME_PEPI2009_hal.pdf
DOI : 10.1016/j.pepi.2008.10.016

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Upper mantle structures and flow related to the South Pacific Superswell and to the associated short-lived hotspots are investigated using seismic stations deployed in French Polynesia. Beside island stations, broadband ocean bottom seismometers ( BBOBS) have been deployed between the various Polynesian archipelagos to investigate the oceanic upper mantle "unperturbed'' by thickened crust or hotspot activity. We investigate seismic anisotropy measured at BBOBS stations and found it rather similar to that from island stations. This confirms the ability of OBS to be used for anisotropy purposes and suggests some homogeneity in the upper mantle pervasive deformation. The two OBS installed on the head of the Society hotspot provide different anisotropy signatures than most of the other French Polynesian stations: one is "isotropic'' to SKS waves and the other provides a fast split direction normal to the average value observed in this area, suggesting a more complex mantle flow induced by the plume upwelling.
ISSN: 0094-8276

hal-00420907
https://hal.archives-ouvertes.fr/hal-00420907
https://hal.archives-ouvertes.fr/hal-00420907/document
https://hal.archives-ouvertes.fr/hal-00420907/file/Barruol_SKS_PLUME_BBOBS_GRL2009.pdf
DOI : 10.1029/2009GL038139