Crustal attenuation beneath western North America: Implications for slab subduction, terrane accretion and arc magmatism of the Cascades

Abstract The western margin of the North American continent is a composite of diverse crustal domains featuring ancestral cratons, the Cordilleran terranes and the subducting Cascadia slab. Their existence and spatial extent have been largely inferred from seismic imaging, which is highly sensitive to variations in both crustal seismic velocity and attenuation structures. In this study, we improve upon existing, yet relatively limited, knowledge of crustal attenuation by developing a new Lg-wave attenuation ( Q 0 ) model. Benefiting from a comprehensive data set from dense seismic networks in Canada and the United States, our model reveals a clear transition from the low-attenuation ( Q 0 > 450 ) Canadian Shield to the highly-attenuated ( Q 0 150 ) crust near the Cascade arc. In the former region, the low-attenuation structures agree well with the distribution of thick, high-velocity crust in the Cordilleran foreland, suggesting dominant effects of elastic properties such as scattering and geometry of the crustal waveguide on the attenuation characteristics of stable cratons. On the other hand, we observe strong crustal attenuations in magmatically active areas including the Cascade arc and the Yellowstone-Snake River Plain volcanic province. The excellent spatial correlation between the high-attenuation Cascades and the surface projection of the subducting slabs further suggests a possible role of slab dehydration in increasing the fluid content of the overlying continental crust and modifying seismic attenuation. The sharp termination of the highly-attenuated crust forms a structural boundary in the Cascade back-arc that likely marks the eastern subsurface limit of the Siletzia terrane. The close spatial affinity of this boundary to the isotopic ratio lineament of 87Sr/86Sr = 0.706, which is the suggested ancestral continental margin, highlights the distinctive crustal types and/or heterogeneous structures between young accreted terranes and ancient cratons.