Global Azimuthal Seismic Anisotropy and the Unique Plate-Motion Deformation of Australia

Global Azimuthal Seismic Anisotropy and the Unique Plate-Motion Deformation of Australia SV-wave heterogeneity and azimuthal anisotropy (black bars oriented along the axis of fast propagation) at 100 and 200 km depth obtained from the inversion of 100,779 Rayleigh waveforms. Hotspot locations are indicated by green circles. The length of the black bars is proportional to the maximum amplitude of azimuthal anisotropy (bar length for 2% peak to peak anisotropy shown at top). SV-wave perturbations (in per cent relative to PREM) are represented with the colour scale.
Differences in the thickness of the high-velocity lid underlying continents as imaged by seismic tomography, have fuelled a long debate on the origin of the ‘roots’ of continents. Some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth, with horizontally polarized shear waves travelling faster than vertically polarized ones. This azimuthally averaged anisotropy could arise from present-day deformation at the base of the plate, as has been found for shallower depths beneath ocean basins. Such deformation would also produce significant azimuthal variation, owing to the preferred alignment of highly anisotropic minerals. Here we report global observations of surface-wave azimuthal anisotropy, which indicate that only the continental portion of the Australian plate displays significant azimuthal anisotropy and strong correlation with present-day plate motion in the depth range 175–300 km. Beneath other continents, azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans. We infer that the fast-moving Australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy. Simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents.
</p><p>E. Debayle, B. Kennett and K. Priestley, Global azimuthal seismic anisotropy and the unique plate-motion deformation of Australia, Nature, 433, 509-512, doi:10.1038/nature03247, 2005.
</p><p>Acknowledgements: This work was supported by programme DyETI conducted by the French Institut National des Sciences de l’Univers (INSU). The data used in this work were obtained from the GEOSCOPE, GDSN, IDA, MEDNET and GTSN permanent seismograph networks, and completed with data collected after the PASSCAL broadband experiments, the SKIPPY and subsequent broadband deploy- ments in Australia, and the INSU deployments in the Horn of Africa and the Pacific (PLUME experiment). Supercomputer facilities were provided by the IDRIS and CINES national centres in France. Special thanks to J. M. Brendle at EOST for technical support, S. Fishwick for providing broadband data from the Western Australian craton field deployment, the staff of the Research School of Earth Science who collected the SKIPPY data in the field, and A. Maggi for suggestions that improved the manuscript.</p>


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