Our 3D radial anisotropic model (Figure 13.46) shares the large scale features of previous regional tomographic studies for North America (e.g. Van der Lee and Nolet, 1997; Grand, 2001). We confirm the pronounced difference in the isotropic velocity structure between the western active tectonic region and the central/eastern stable shield, with the boundary being sharp and almost perfectly coincident with the Rocky Mountain Front. At transition zone depths, we document the presence of subducted material (Juan de Fuca and Farallon plate). Concerning the anisotropic signature, we observe a positive anomaly in correspondence of the cratonic areas between 200 and 300 km depth.
Comparison of models obtained with different crustal correction approaches (linear vs. linear/non-linear perturbation approximation) shows that shallow-layer features can significantly bias the mantle structure down to 400 km depth and therefore confirms that accurate crustal corrections are an essential step in high resolution regional tomographic studies. Both shape and amplitude of the imaged anomalies can be affected by inaccurate removal of the crustal signature.
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