HR: 16:15h
AN: T34A-02 INVITED     [Abstracts]
TI: The seismic structure of the asthenosphere as seen by surface waves
AU: * Debayle, E
EM: Eric.Debayle@eost.u-strasbg.fr
AF: EOST, CNRS and Universite Louis pasteur, 5 Rue Rene Descartes, Strasbourg, 67084 France
AU: Maggi, A
EM: alessia@gps.caltech.edu
AF: Seismology laboratory, California Institute of Technology, 1200 E California Blvd, MS 252-21, Pasadena, CA 91125 United States
AU: Sieminski, A
EM: anne@geo.uu.nl
AF: Department of Earth Sciences Utrecht University, P.O. box 8002, Utrecht, TA 3508 Netherlands
AB: The global distribution of shear-wave azimuthal anisotropy and seismic heterogeneities in the upper mantle has been constrained with an unprecedent lateral resolution (~500-1500 km) from the inversion of over 100,000 fundamental and higher mode Rayleigh waveforms. In continental regions, the high velocity lid shows significant variation in thickness but does not in general extend deeper than 250 km. The Australian continent, on the fast-moving Australian plate, appears to be the only continent for which basal drag on the lithosphere is sufficient to cause azimuthal anisotropy in the asthenosphere aligned with plate motion. Beneath other continents, azimuthal anisotropy vanishes near 150 km depth and supports a frozen-in origin within the lithosphere with no evidence for a deeper layer. This lithospheric azimuthal anisotropy is compatible with a delay time of about 1 s, as typically observed in SKS studies. The weak azimuthal anisotropy observed at depths greater than 150 km for continents other than Australia is compatible with simple shear induced by slower moving plates leading to anisotropy with a plunging axis of symmetry. In oceanic regions, a well pronounced low velocity zone is in general observed beneath a high velocity lid associated with the oceanic lithosphere. Beneath the Pacific plate, this high velocity lid thickens progressively with age, approximately following the trend predicted by purely diffusive cooling. We are currently investigating whether a more detailed interpretation of the shear wave versus age trends can be reliably undertaken considering the current resolution of tomographic models. We also observe some well resolved deep low-velocity anomalies that extend from the asthenosphere down to the transition zone. Fast anisotropic directions and present-day plate motion are clearly correlated at asthenospheric depths, for the fast-moving oceanic plates, as is especially clear beneath the Pacific. Upwelling seems able to locally disturb this simple long wavelength pattern. We observe that azimuthal anisotropy does not correlate with plate driven flow over small anomalous regions of the northern Pacific that are systematically located westward of the hotspots associated with mantle plumes by Montelli et al. (2004).
UR: http://eost.u-strasbg.fr/recherche/ERIC/eric.html
DE: 7218 Lithosphere (1236)
DE: 7255 Surface waves and free oscillations
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005