3-D shear wave radially and azimuthally anisotropic velocity model of the North American upper mantle

Abstract

Using a combination of long period seismic waveforms and SKS splitting measurements, we have developed a 3-D upper-mantle model (SAWum_NA2) of North America that includes isotropic shear velocity, with a lateral resolution of ∼250 km, as well as radial and azimuthal anisotropy, with a lateral resolution of ∼500 km. Combining these results, we infer several key features of lithosphere and asthenosphere structure. A rapid change from thin (∼70–80 km) lithosphere in the western United States (WUS) to thick lithosphere (∼200 km) in the central, cratonic part of the continent closely follows the Rocky Mountain Front (RMF). Changes with depth of the fast axis direction of azimuthal anisotropy reveal the presence of two layers in the cratonic lithosphere, corresponding to the fast-to-slow discontinuity found in receiver functions. Below the lithosphere, azimuthal anisotropy manifests a maximum, stronger in the WUS than under the craton, and the fast axis of anisotropy aligns with the absolute plate motion, as described in the hotspot reference frame (HS3-NUVEL 1A). In the WUS, this zone is confined between 70 and 150 km, decreasing in strength with depth from the top, from the RMF to the San Andreas Fault system and the Juan de Fuca/Gorda ridges. This result suggests that shear associated with lithosphere–asthenosphere coupling dominates mantle deformation down to this depth in the western part of the continent. The depth extent of the zone of increased azimuthal anisotropy below the cratonic lithosphere is not well resolved in our study, although it is peaked around 270 km, a robust result. Radial anisotropy is such that, predominantly, ξ > 1, where ξ= (Vsh/Vsv)2, under the continent and its borders down to ∼200 km, with stronger ξ in the bordering oceanic regions. Across the continent and below 200 km, alternating zones of weaker and stronger radial anisotropy, with predominantly ξ < 1, correlate with zones of small lateral changes in the fast axis direction of anisotropy, and faster than average Vs below the LAB, suggesting the presence of small scale convection with a wavelength of ∼2000 km. Finally, in the western United States, complex 3-D patterns of isotropic velocity and anisotropy reflect mantle dynamics associated with the rich tectonic history of the region.