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Global azimuthal seismic anisotropy and the unique plate-motion deformation of Australia

Abstract

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 continents1,2,3,4,5. 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 ones2. 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 basins6. Such deformation would also produce significant azimuthal variation, owing to the preferred alignment of highly anisotropic minerals7. 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.

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Figure 1: 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.
Figure 2: Azimuthal anisotropy amplitude and correlation with plate motion for different tectonic provinces.
Figure 3: Correlation between fast direction of SV waves and plate motion directions.

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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 deployments 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.

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Correspondence to Eric Debayle.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Figure 1

Ray path density per 400 × 400 km cell across the globe for the Rayleigh wave data set used to build our tomographic model. (PDF 346 kb)

Supplementary Figure 2

Distribution of epicentre-station lengths for the 100779 Rayleigh waveforms used in this study. (PDF 11 kb)

Supplementary Figure 3

Optimized Voronoi diagram based on the work of Debayle and Sambridge26. (PDF 374 kb)

Supplementary Figure 4

Influence of the non-inverted parameters: a, 2θ azimuthal anisotropy of the phase velocity predicted from integration of the G parameters. b, 2θ azimuthal anisotropy obtained from direct regionalization of the phase velocity curves. c, Same as b but the 4θ azimuthal terms are included in the inversion. (PDF 950 kb)

Supplementary Figure 5

Synthetic experiment to test vertical smearing. (PDF 27 kb)

Supplementary Figure 6

Predicted SKS splitting following Montagner et al.31 in the input model of Supplementary Fig. 5a (a);the inversion output of Supplementary Fig. 5b (b); and the model from the actual inversion (Fig. 1) (c). (PDF 64 kb)

Supplementary Figure Legends

Figure legends for Supplementary Figs 1-6. (PDF 7 kb)

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Debayle, E., Kennett, B. & Priestley, K. Global azimuthal seismic anisotropy and the unique plate-motion deformation of Australia. Nature 433, 509–512 (2005). https://doi.org/10.1038/nature03247

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