The most likely cause of seismic anisotropy in the Earth's upper mantle is lattice preferred orientation (LPO) of anisotropic minerals such as olivine. Its presence reflects dynamic processes related to the formation of the lithosphere as well as to present-day tectonic motions. A powerful tool to detect and characterize upper mantle anisotropy is the analysis of shear wave (SKS) splitting measurements. Because of the poor vertical resolution afforded by this type of data, a long lasting controversy has opposed proponents of a lithospheric origin for the observed splitting under stable continents, "frozen-in" at the time of the formation of the craton, to those4 who argue that the anisotropy originates primarily in the asthenosphere, and is induced by shear due to present-day absolute plate motions (APM). In addition, predictions from surface wave derived models are largely incompatible with shear wave splitting observations. Here we show that this disagreement can be resolved by simultaneously inverting surface waveforms and SKS splitting data. We present evidence for the presence of two layers of anisotropy with different fast axis orientations in the cratonic part of the North American upper mantle. At asthenospheric depths (200-400 km) the fast axis is sub-parallel to the absolute plate motion (APM), confirming the presence of shear related to current tectonic processes, whereas in the lithosphere (80-200 km), the orientation is significantly more northerly. In the western, tectonically active, part of North America, the fast axis direction is consistent with APM throughout the depth range considered, in agreement with a much thinner lithosphere.
See also related papers: Federica Marone, Yuancheng Gung and Barbara Romanowicz "High resolution 3D radial anisotropic structure of the North American upper mantle from inversion of surface waveform data, ", Geophys. J. Int. 171, 206-222, doi: 10.1111/j.1365-246X.2007.03465.x
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Caption: Models of isotropic structure (left), radial anisotropy (middle), and azimuthal anisotropy (right) at depths of 100 km (top) and 300 km) bottom in the mantle. Left and middle figure are from Marone, Gung and Romanowicz (2007, Geophys. J. Int). In the lower left, we reproduce the cartoon from Gung et al. (Nature, 2003) describing our conceptual model for the undulations of the lithosphere- asthenosphere boundary. In the left panels, faster than average velocities are indicated in blue and slower than average in red. Saturation is at +/- 2%. In the middle panel, blue color indicates Vsh faster than Vsv. At 100 km, the anisotropic part of the reference PREM model has been removed. Saturation is at +/- 2%. In the right panel, the red arrows indicate the direction of absolute plate motion, whereas the black lines indicate the direction and amplitude of fast velocity axis obtained by inversion. Variations found in upper mantle anisotropy beneath North America suggest the presence of two layers of varying thickness separated by the lithosphere-asthenosphere boundary (LAB), similarly to what has been found under oceans.