Evidence for the presence of two layers of anisotropy with different orientations in the North American upper mantle is presented. At asthenospheric depths, the fast axis is sub-parallel to the plate motion, confirming the presence of shear related to current tectonic processes, whereas within the lithosphere, the orientation is significantly different, indicating that anisotropy at these shallower depths was 'frozen-in' long ago.
- Federica Marone
- Barbara Romanowicz
