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Article AbstractsPresentation AbstractsB. Tauzin, E. Debayle and G. Wittlinger, The Mantle Transition Zone as Seen by Global Pds Phases : no Clear Evidence for a Thin Transition Zone Beneath Hotspots, Oral Presentation, American Geophysical Union Fall Meeting, 2007.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. B. Tauzin, E. Debayle and G. Wittlinger, Global Imaging of Upper Mantle Seismic Heterogeneities and Discontinuities From Observations of Body-Wave Travel-Times and Rayleigh Wave Dispersion, Poster, American Geophysical Union, 2007.The tomographic inversion of 100779 Rayleigh waveforms including higher modes has recently allowed us to constrain the SV-wave heterogeneities in the upper mantle with a lateral resolution of a few hundred kilometers and a vertical resolution of a few tens of kilometers. We are currently working to improve the resolution of our model in the transition zone by increasing the number and the accuracy of the higher modes measurements. In addition to the large-scale constraints provided by surface-waves on the 3D distribution of shear-wave velocities, we have accumulated global observations on the topography of the 410-km and 660-km discontinuities through the measurement of arrival times of converted and reflected body-wave phases. SS precursors phases can efficiently complete the analysis of P-to-s receiver functions to build a global image of the topography of the transition zone seismic discontinuities. We present results obtained from the separate analysis of each type of body-wave dataset. We are currently working on the simultaneous inversion of body waves and surface waves higher modes for the 3D distribution of SV-wave velocities and the depth of seismic discontinuities. We discuss the strategy for the inversion and present our preliminary results. [Poster] B. Tauzin, E. Debayle, G. Wittlinger and C. Zaroli, Seismic Imaging of Deep mantle Discontinuities, Poster, Statoil International Student Conference, 2007.The Earth reflectivity reveals deep seismic discontinuities at 410 and 660 km depth which are often interpretated as pressure-induced phase-transformations of the olivine mineral, a major constituent of the mantle. It has been shown that the depth of these discontinuities is temperature dependent and could give some insights on the nature of exchanges between lower and upper mantle. We process teleseismic records of earthquakes at 3-components broad-band seismological stations distributed all over the world. As it may be done in oil exploration, we derive the absolute depth of the discontinuities from the analysis of travel-times of reflected and converted waves at these discontinuities. P-to-S conversions give constraints on the structure under the receiver. To enhance conversion energy on seismograms, we apply a receiver-function technique which consists on deconvolving the P-wave energy from the Sv component. Underside reflections of shear-waves are more sensitive to the structure under bounce points between the seismic source and the receiver and enable a better coverage of oceanic regions. Conversion data are gathered by receivers and reflection data by common bounce points. Weak converted/reflected phases are then enhanced by stacking in the time-slowness domain the records having the highest signal to noise ratio. We finally use a radial velocity model to convert our travel-time measurements into absolute depth of discontinuities. We present our observations of perturbations of the thickness between both discontinuities. We show that these perturbations are partly correlated with expected locations of downwelling of cold subducted material from the surface. [Poster] B. Tauzin, E. Debayle, G. Wittlinger, A global study of transition zone thickness, Poster, American Geophysical Union Fall Meeting, 2006.We present a global study of transition zone thickness from PdS converted waves at the 410 and 660 km discontinuities. We collect several thousand three-component broad-band seismograms from GEOSCOPE, IRIS-USGS, IRIS-IDA and GT station networks. The data are low pass- filtered at 5s. The receiver functions are built by applying an iterative deconvolution of the SV trace by the P waveform. Weak 410 and 660 phases are then enhanced by stacking in the time-slowness domain the records having the highest signal to noise ratio under each station. The quality of the PdS measurements is checked by comparing our results with previous work at several seismic stations. The agreement between different studies strongly rely on the choices made in the data selection (the geographical distribution of stations, the selected range of back-azimuth at the station, the frequency range of analysis and the signal to noise threshold used in the data selection). The depth migration makes the transition zone thickness strongly dependant on the chosen velocity model. The average thickness of the transition zone is not inconsistent between different studies but the amplitude of the variations are more difficult to explain. Because PdS phases at the 410 and 660 km discontinuities have narrow Fresnel zone (few hundred of kilometres) compared to inter-station distances in oceanic areas, we do not built maps of the topography of the discontinuities. We prefer to take advantage of the recent improvement in worldwide station coverage to select data from three subsets of stations sampling equally the three major tectonic provinces that we have defined for the transition zone : «plume », « normal mantle » and « slabs». Taking into account the likely error measurements, we investigate in details whether there are significant differences between stations belonging to the different provinces, consistent with olivine structural transformation. [Poster] B. Tauzin, E. Debayle, G. Wittlinger, Global Topography of Transition Zone Discontinuities from SS Underside Reflection and P-to-s Converted Data Sets, Poster, IPGS, 2005.B. Tauzin, E. Debayle, G. Wittlinger, Structure of the Mantle Transition Zone from Surface-Waves and Body-Waves Constraints, Poster, SPICE R&T workshop 2, 2005.
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Contact Address:
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