Shear Velocity and Radial Anisotropy beneath Southwestern Canada: Evidence for Crustal Extension and Thick-Skinned Tectonics

摘要

Deformation-associated craton assembly and Cordilleran orogenesis played major roles during the crustal formation beneath the western margin of North America. To improve the understanding of the deformation history in this region, we investigate the crustal shear velocity and anisotropy by analyzing fundamental mode Rayleigh and Love waves from ambient seismic noise. Continuous recordings from 118 regional broadband stations reveal lateral variations in both velocity and anisotropy with strong spatial affinities to major geological domains. Strong variations in radial anisotropy, which dips westward and extends to at least midcrustal depths, suggest a "thick-skinned" foreland thrust-and-fold belt beneath the Canadian Rocky Mountains. Our regional data also suggest increased horizontal shear velocities beneath the southern Canadian Cordillera, particularly near the Omineca Belt, which may have resulted from strong zonal deformation within the Cordilleran crust. This anisotropic anomaly migrates southward, and its spatial extent agrees well with the normal fault distribution to the west of the Rocky Mountain Trench. Our observations offer new evidence for the Eocene extension of the orogenic hinterland during the trans tensional motion of the Cordillera to the North American craton.