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Global upper mantle azimuthal anisotropy and the peculiar behavior of the Australian plate

Global upper mantle azimuthal anisotropy and the peculiar behavior of the Australian plate. Eric Debayle 1 , Brian Kennett 2 and Keith Priestley 3 1 Ecole et Observatoire des Sciences de la Terre, CNRS and Université Louis Pasteur, Strasbourg

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Global upper mantle azimuthal anisotropy and the peculiar behavior of the Australian plate

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  1. Global upper mantle azimuthal anisotropy and the peculiar behavior of the Australian plate Eric Debayle1, Brian Kennett 2 and Keith Priestley3 1 Ecole et Observatoire des Sciences de la Terre, CNRS and Université Louis Pasteur, Strasbourg 2 Research School of Earth Sciences, The Australian National University, Canberra, Australia. 2 Bullard Laboratories, University of Cambridge, Cambridge, United Kingdom. This work is subject to press embargo

  2. Our global SV-wave heterogeneities + azimuthal anisotropy model • 100779 Rayleigh waveforms (fundamental plus 4 higher modes) matched between 50 s and 160 s. • Short paths (1200-6000 km) : • improve the lateral resolution (~ 800 km -1500 km) Number of paths Paths length (km)

  3. Debayle and Kennett, JGR, EPSL (2000) 100 km Vs ref= 4.41 km/s 200 km Vs ref= 4.44 km/s

  4. Australia Continents except Australia Oceans Depth-distribution of azimuthal anisotropy

  5. Inversion output Synthetic experiment Input model

  6. Azimuthal distribution of rays Optimized Voronoi diagram (Debayle and Sambridge, JGR 2004) : each geographical point belongs to the smallest cell for which the azimuthal variation of Sv waves can be resolved.

  7. Correlation between fast anisotropic direction and absolute plate motion good correlation bad correlation |FastSV||APM|cos(2f)

  8. Averaged correlation between fast anisotropic direction and absolute plate motion Australia Continents except Australia Oceans

  9. Debayke, Kennett and Prietsley (Nature, 2005) Australia Other continents (VSH/VSV)2 SKS SKS (VSH/VSV)2 1.1 1 >1 0 1 crust crust Moho Moho 50 Frozen in anisotropy (east-west) Frozen in anisotropy ? Debayle and Kennett (EPSL 2000) Gaherty and Jordan (Science 1995) 100 150 MBL MBL Plate-motion azimuthal anisotropy (north-south) Weak or null azimuthal anisotropy 200 HVL HVL 250 Depth (km) HVL : Base of the High Velocity Lid MBL : Mechanical Boundary Layer

  10. Conclusions • The fast-moving Australian plate would contain the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy. • For continents other than Australia, weak influence of basal drag on the lithosphere may explain why azimuthal anisotropy is observed only in a layer located in the uppermost 100 km of the mantle. This layer shows a complex organisation of azimuthal anisotropy suggesting a frozen in origin of deformation, compatible with SKS observations. • NB : we have performed various tests on our model that will be available on line on the Nature website in early 2005 as supplementary information on the paper.

  11. Effect of the non-inverted 4q azimuthal terms a) C0 + 2q terms inversion (only 2q terms shown) b) C0 + 2q + 4q terms inversion (only C0 and 2q shown)

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