Convection-driven motion of the north American craton:: evidence from P-wave anisotropy

We study the effect of anisotropy on P-wave traveltimes and generalize Backus (1965) well-known expression of azimuthal velocity dependence to the sphere. The variation of P-wave traveltimes on the lower hemisphere due to anisotropy is described by a set of 15 parameters. For single-crystal olivine a set of six parameters describes about 98 per cent of the variation and for practical purposes of mantle seismology this set of six parameters should be sufficient. We find a coherent pattern of anisotropy throughout the North American craton. The anisotropy is characterized by fast directions dipping toward the southwest, roughly in the direction of absolute plate motion, and shallow dip angles (<45°). This is consistent with a simple-shear deformation of the deep lithospheric roots, due to the relative motion between plates and deeper mantle. If this is correct, it implies that the deeper mantle convects faster than the plate velocity and that the mantle helps to drive the motion of the North American plate.