PLUME investigates South Pacific Superswell (original) (raw)

Mapping the lowermost mantle using core-reflected shear waves

Journal of Geophysical Research, 1994

A map of laterally varying D-double prime velocities is obtained for the region from 50 deg S to 50 deg N in latitude and 70 deg E to 190 deg E in longitude. Velocities are found using an analysis of the differential travel time residuals from 481 ScS-S and 266 sScS-sS phase pairs. The long-period data are taken from the Global Digital Seismograph Network digital waveform catalog for the time period of January 1980 to March 1987. Each differential travel time is found by a cross correlation of the S phase ground displacement, corrected to simulate differential attenuation, with all following phases. Travel times are corrected for ellipticity and mantle heterogeneity outside of their D-double prime paths, and the remaining residuals are interpreted as the result of D-double prime heterogeneity. Ray-tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies. The resulting map reveals significant long-wavelength D-double prime structure including a 3% low-velocity region beneath northeastern Indonesia, surrounded by three identified high-velocity zones beneath northwestern Pacifica (+4%), Southeast Asia (+3%), and Australia (+3-5%). This structure is of continent/ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle. The low-velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature D-double prime rock which has been heated by the core has become gravitationally unstable and begun to rise. A chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site, much the way continents are dragged toward subduction zones. The high-velocity zones possibly result from the downward convection of cold lower mantle plumes, which pond at the core-mantle boundary. These seismic anomalies may also contain a chemical signature from faster iron-poor materials brought down through the lower mantle or the additional presence of SiO2 stishovite, perhaps in its higher-pressure polymorph.

Upper mantle structure of the Tonga-Lau-Fiji region from Rayleigh wave tomography

We investigate the upper mantle seismic structure in the Tonga-Lau-Fiji region by jointly fitting the phase velocities of Rayleigh waves from ambient-noise and two-plane-wave tomography. The results suggest a wide low-velocity zone beneath the Lau Basin, with a minimum SV-velocity of about 3.7 +-0.1 km/s, indicating upwelling hot asthenosphere with extensive partial melting. The variations of velocity anomalies along the Central and Eastern Lau Spreading Centers suggest varying mantle porosity filled with melt. In the north where the spreading centers are distant from the Tonga slab, the inferred melting commences at about 70 km depth, and forms an inclined zone in the mantle, dipping to the west away from the arc. This pattern suggests a passive decompression melting process supplied by the Australian plate mantle from the west. In the south, as the supply from the Australian mantle is impeded by the Lau Ridge lithosphere, flux melting controlled by water from the nearby slab dominates in the back-arc. This source change results in the rapid transition in geochemistry and axial morphology along the spreading centers. The remnant Lau Ridge and the Fiji Plateau are characterized by a 60–80 km thick lithosphere underlain by a low-velocity asthenosphere. Our results suggest the removal of the lithosphere of the northeastern Fiji Plateau-Lau Ridge beneath the active Taveuni Volcano. Azimuthal anisotropy shows that the mantle flow direction rotates from trench-perpendicular beneath Fiji to spreading-perpendicular beneath the Lau Basin, which provides evidence for the southward flow of the mantle wedge and the Samoan plume.

Upper mantle Anisotropy from Surface Wave studies

2008

Major advances in Structural Seismology during the last twenty years, are related to the emergence and development of more and more sophisticated 3D imaging techniques, usually named seismic tomography, at different scales from local to global. Progress has been made possible by the rapid developments in seismic instrumentation and by the extensive use of massive computation facilities. The scope of this chapter is limited to the tomographic elastic structure of the upper mantle. In order to obtain a good spatial coverage of this part of the Earth, it is necessary to make use of dispersive properties of surface waves. Most global tomographic models are still suffering severe limitations in lateral resolution, due to the imperfect data coverage, and to crude theoretical approximations. It is usually assumed that the propagating elastic medium is isotropic, which is a poor approximation. It is shown in this chapter how to take account of anisotropy of Earth’s materials and a complete ...