HR: 1340h
AN: T13A-0443    [Abstracts]
TI: Azimuthal and Radial Seismic Anisotropy Beneath the Baltic Shield
AU: * Pedersen, H A
EM: Helle.Pedersen@obs.ujf-grenoble.fr
AF: GeoForschungsZentrum Potsdam, Section 2.4 Telegrafenberg, Potsdam, D-14473 Germany
AU: * Pedersen, H A
EM: Helle.Pedersen@obs.ujf-grenoble.fr
AF: University of Potsdam, Institut fur Geowissenschaften Postfach 60 15 53, Potsdam, D-14415 Germany
AU: * Pedersen, H A
EM: Helle.Pedersen@obs.ujf-grenoble.fr
AF: University of Grenoble, LGIT BP 53, Grenoble, F-38045 France
AU: Bruneton, M
EM: mbruneto@nrcan.gc.ca
AF: Geological Survey of Canada, 615 Booth Street, Ottawa, ON K1A 0E9 Canada
AU: Maupin, V
EM: valerie.maupin@geo.uio.no
AF: University of Oslo, Dept of Geosciences POB 1047 Blindern, Oslo, N-0316 Norway
AB: The SVEKALAPKO passive seismic array in Finland provides us with an exceptional opportunity to study seismic anisotropy in and below the lithosphere in a shield. The array was composed of almost 150 sensors - out of which 46 were broadband - in a regular 2D grid which facilitated high-quality array analysis. We analyse phase velocities of both Love and Rayleigh waves to constrain radial and azimuthal anisotropy. We invert for the anisotropic parameters ξ and G_c on the one hand, and for the percentage of aligned olivine on the other. This latter parametrization of the inverse problem makes it straightforward to quantitatively compare the radial and the azimuthal anisotropies, under the assumption that aligned olivine dominates the anisotropy. The radial anisotropy, for which we have resolution in the lithosphere only, is strong, and can be explained by 40%-60% of the rock being olivine with the a-axis in the horizontal plane, equivalent to values of ξ between 1.09 and 1.14. This radial anisotropy is stronger than observed in shield areas in global models (e.g. Beghein and Trampert, 2004). The azimuthal anisotropy is on the contrary very small in the lithosphere. This indicates that the orientation of the olivine minerals is random within the horizontal plane or that the overall effect across the area is negligible due to different orientations in different domains. Results from body-waves (Plomerová et al., 2005, Vecsey et al., in prep.) would support the latter interpretation. The azimuthal anisotropy as estimated by Rayleigh wave analysis is on the contrary significant below 200-250km depth, and corresponds to approximately 15%-20% of the rock being olivine with the a-axis aligned in direction N20. Xenolith analysis in the area shows that the rheologic lithosphere is at most 250km thick, so we suggest that this observed anisotropy is sub-lithospheric. Interestingly, the fast direction is significantly different from the absolute plate motion of the Baltic Shield, indicating that the lithosphere is not simply coupled to the underlying convecting mantle.
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
DE: 8103 Continental cratons
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: Fall Meeting 2005