Seismic anisotropy beneath stable continental interiors

A robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory, as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle. This review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data, and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy. To this end, we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years: eastern North America, the Canadian Shield, Australia, and southern Africa. Based on this full suite of results, we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle, usually to depths of at least 200 km and perhaps more. An implication of these results is that tectonic plates are, at most, only partially coupled to the underlying mantle. The results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years, it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy. We suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics, magnetotellurics, gravity, and geodesy.