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

Abstract

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.

Imaging the Northeastern Crustal Boundary of the Tibetan Plateau With Radial Anisotropy

“…Whereas, the negative radial anisotropy is commonly explained by the steeply dipping to the sub-vertical foliation plane of crustal anisotropic materials (Erdman et al, 2013). Continuous lateral crustal shortening under compressional stress could lead to the change in orientation of the foliation plane from sub-horizontal to steeply dipping, which may further cause strong folding and thrusting activities, similar to those widely reported in different orogenic belts (e.g., Chen et al, 2009;J. Wang, Gu, & Chen, 2020;W.…”

The Structure and Dynamics of the Uppermost Mantle of Southwestern Canada From a Joint Analysis of Geophysical Observations

“…If there is significant radial anisotropy, the isotropic shear wave velocity will be underestimated (Garber et al., 2018). To date, there have been no detailed studies of the anisotropic V S values in our study area, although anisotropy in the shallow craton has been observed (e.g., Bao et al., 2016; Currie et al., 2004; Wang et al., 2020). The global anisotropy model SUMUCB_WM1 (French & Romanowicz, 2014) shows radial anisotropy in our study area, with horizontal shear wave velocities (V SH ) higher than V SV (Figure 9c).…”

Rayleigh Wave Dispersion Spectrum Inversion Across Scales

“…Rayleigh waves recorded in land seismic surveys, are usually taken as noise and removed in reflection seismic processing. However, these strong coherent arrivals provide high-quality signals for S-wave velocity estimation in the upper tens of meters to tens of kilometers of the Earth (Xia et al, 1999;Shapiro et al, 2005;Gu et al, 2005;Lin et al, 2008;Tape et al, 2010;Strobbia et al, 2011;Nakata et al, 2011;Saygin & Kennett, 2012;Fang et al, 2015;Smith et al, 2019;J. Wang et al, 2020;Tromp, 2020).…”