HR: 09:20h
AN: T41G-06    [Abstracts]
TI: The Transition Zone Beneath the South Pacific Superswell From Receiver Functions
AU: * Barruol, G
EM: barruol@upf.pf
AF: Lab. Tectonophysique, CNRS, Univ. Montpellier II, Montpellier, 34095 France
AU: * Barruol, G
EM: barruol@upf.pf
AF: Lab. Terre Ocean, Univ. French Polynesia, Faaa, 98702 French Polynesia
AU: Legendre, C
EM: legendre.cedric@free.fr
AF: Lab. Tectonophysique, CNRS, Univ. Montpellier II, Montpellier, 34095 France
AU: Masson, F
EM: fmasson@dstu.univ-montp2.fr
AF: Lab Dynamique de la Lithosphere, CNRS, Univ. Montpellier II, Montpellier, 34095 France
AU: Vergne, J
EM: vergne@geologie.ens.fr
AF: Lab. Geologie, ENS, Paris, 75231 France
AU: Fontaine, F
EM: fontaine@dstu.univ-montp2.fr
AF: Lab. Tectonophysique, CNRS, Univ. Montpellier II, Montpellier, 34095 France
AU: Bokelmann, G
EM: Goetz.Bokelmann@dstu.univ-montp2.fr
AF: Lab. Tectonophysique, CNRS, Univ. Montpellier II, Montpellier, 34095 France
AB: The geometry and thickness of the transition zone (TZ) is a key question to discuss the upper or lower mantle origins of the mantle plumes and more generally the thermal state of the upper mantle. We investigate the TZ thickness in the south Pacific by detecting P to S converted waves at the 410 and 660 km discontinuities from receiver function techniques. The south Pacific and particularly the French Polynesia area are interesting for several reasons: i) The huge and low seismic velocity anomaly in the lower mantle beneath the south Pacific may indicate the presence of hot mantle rising toward the Earth's surface. ii) The presence of several volcanic alignments indicates the present or recent hotspots activity (e.g., the Society, Austral, Marquesas and Pitcairn alignments). iii) The presence of the large-scale bathymetry anomaly (the South Pacific Superswell) suggests a low-density anomaly at large depth. We used seismic data recorded during the last decades by the Geoscope, IRIS and LDG/CEA permanent stations running in the South Pacific but also recorded by a temporary network of 10 broad band seismic stations operating in French Polynesia for the Polynesian Lithosphere and Upper Mantle Experiment (PLUME) since the end of 2001. The P410S and P660S phases can be observed on high quality individual receiver functions but are more visible by stacking the individual receiver functions. Even if the results at the permanent stations are better constrained by a large number of individual receiver functions (between 50 and 200), the PLUME temporary stations generally provided 10 to 20 individual receiver functions and their stacking suggest coherent and homogeneous signatures throughout the studied area. The prominent result is that the P410S phases arrive generally late of 3 to 6 s after the expected arrival (44 s after the P wave in the IASPEI91 model), whereas the P660S phases arrive either on time (68 s after the P-arrival) or slightly later (1 to 3 s). The sum of the P660S and P410S travel times are anomalously high (between 2 and 10 s) suggesting an anomalously slow upper mantle, in the range 2-5 %, compatible with the available tomographic models. The P660S-P410S differential times are anomalous low of 1 to 5 s, suggesting a large-scale thinning of the TZ that could result from a temperature positive anomaly of 100 to 250 K at the base of the TZ.
UR: http://www.isteem.univ-montp2.fr/TECTONOPHY/polynesia/Polynesia\_home\_page.html
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting