Laboratory experiments have established that many of the materials comprising the Earth are strongly anisotropic in terms of seismic-wave speeds1. Observations of azimuthal2,3 and radial4,5 anisotropy in the upper mantle are attributed to the lattice-preferred orientation of olivine caused by the shear strains associated with deformation, and provide some of the most direct evidence for deformation and flow within the Earth’s interior. Although observations of crustal radial anisotropy would improve our understanding of crustal deformation and flow patterns resulting from tectonic processes, large-scale observations have been limited to regions of particularly thick crust6. Here we show that observations from ambient noise tomography in the western United States reveal strong deep (middle to lower)-crustal radial anisotropy that is confined mainly to the geological provinces that have undergone significant extension during the Cenozoic Era (since ∼65 Myr ago)7,8. The coincidence of crustal radial anisotropy with the extensional provinces of the western United States suggests that the radial anisotropy results from the lattice-preferred orientation of anisotropic crustal minerals caused by extensional deformation. These observations also provide support for the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension9,10,11.
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This manuscript benefited from discussions with K. Mahan, C. Jones and P. Molnar. Research was supported by the US National Science Foundation, Division of Earth Sciences. M.P.M. received support from an US National Defense Science and Engineering Graduate Fellowship from the American Society for Engineering Education. The facilities of the IRIS Data Management System, and specifically the IRIS Data Management Center, were used to access the waveform and metadata required in this study.
Author Contributions M.P.M. carried out ambient noise tomography for the Rayleigh-wave measurements, performed the three-dimensional inversion of surface-wave dispersion measurements and co-wrote the paper. M.H.R. guided the study design and co-wrote the paper. F.L. carried out ambient noise tomography for the Love-wave measurements. Y.Y. carried out the multiple-plane-wave earthquake tomography. All authors discussed the results and provided comments on the manuscript.
This file contains Supplementary Information: Inability of additional parameters to resolve the Rayleigh-Love discrepancy, Supplementary Table 1, Supplementary Figures 1-6 with legends and Supplementary References.
About this article
Surveys in Geophysics (2018)