Letter

Nature 464, 885-889 (8 April 2010) | doi:10.1038/nature08951; Received 31 October 2009; Accepted 17 February 2010

Seismic evidence for widespread western-US deep-crustal deformation caused by extension

M. P. Moschetti1,2, M. H. Ritzwoller1, F. Lin1 & Y. Yang1

  1. Center for Imaging the Earth’s Interior, Department of Physics, University of Colorado at Boulder, Campus Box 390, Boulder, Colorado 80309, USA
  2. Present address: Geologic Hazards Science Center, US Geological Survey, Denver, Colorado 80225, USA.

Correspondence to: M. P. Moschetti1,2 Correspondence and requests for materials should be addressed to M.P.M. (Email: mmoschetti@usgs.gov).

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 ~65Myr 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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