HR: 09:48h
AN: DI51B-10    [Abstracts]
TI: The Mantle Transition Zone as Seen by Global Pds Phases : no Clear Evidence for a Thin Transition Zone Beneath Hotspots
AU: * Tauzin, B
EM: benoit.tauzin@eost.u-strasbg.fr
AF: CNRS and EOST, Universite Louis Pasteur, 5 rue Rene Descartes, Strasbourg, 67084, France
AU: Debayle, E
EM: eric.debayle@eost.u-strasbg.fr
AF: CNRS and EOST, Universite Louis Pasteur, 5 rue Rene Descartes, Strasbourg, 67084, France
AU: Wittlinger, G
EM: gerard.wittlinger@eost.u-strasbg.fr
AF: CNRS and EOST, Universite Louis Pasteur, 5 rue Rene Descartes, Strasbourg, 67084, France
AB: We present a new global study of the transition zone from Pds converted waves at the 410 and 660 km discontinuities. Our observations complete previous global Pds studies with a larger dataset, especially in oceanic regions where we have been able to measure Pds travel-times sampling the mantle transition zone (MTZ) beneath 26 hotspot locations. We find significant lateral variations of the MTZ thickness. Both the maximum variations (± 35 km) and the long wavelength pattern are in overall agreement with previous SS precursors studies. We observe a negative correlation between the apparent depth of the 410-km discontinuity and the MTZ thickness. This negative correlation is enhanced, if we use recently published S-wave tomographic models to correct our Pds travel-times for the 3D velocity variations above the MTZ. This supports the idea that a significant part of the 410-km and 660-km topography variations are due to the effect of temperature on the olivine phase transformations. The MTZ is generally thick beneath subduction zones and the observed MTZ variations are consistent with thermal anomalies ranging between -100° K and -300° K. In central and North America, we observe a NW-SE pattern of thick MTZ which can be associated with the fossil Farallon subduction. We do not find clear evidence for a thin MTZ beneath hotspots. However, the 410-km discontinuity remains generally depressed after our corrections for 3D heterogeneities above the MTZ, and its topography variations can be explained by thermal anomalies between +100° K and +300° K. The depth of the 660-km discontinuity may therefore be less temperature sensitive in hot regions of the mantle, which is consistent with the effect of a phase transition from majorite-garnet to perovskite at a depth of 660 km.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting