ANISOTROPY IN THE NORTH AMERICAN CRATON

Berkeley North American regional seismic azimuthal anisotropy model reveals the presence of three anisotropic layers throughout the stable part of the North American cratonic upper mantle. While in the bottom asthenospheric layer the fast axis direction is parallel to the current plate motion direction, the top two lithospheric layers have distinct fast axis directions, with ancient suture zone trending directions in the top layer and a general north-south direction in the bottom layer, respectively. The boundary between the two lithospheric layers, as defined by systematic changes in the direction of azimuthal anisotropy, correlates well with the sharp mid-lithospheric negative velocity boundary found by available S-wave receiver functions measurements. This spatial correlation suggests that the two boundaries, found by the surface wave inversion and receiver functions, may share a common origin possibly during the lithosphere formation, which indicates that accretion by shallow subduction may have played an important role in the North American craton formation.