Shear Wave Splitting Discloses Two Episodes of Collision‐Related Convergence in Western North America

Abstract

Seismic anisotropy imposes first‐order constraints on the strain history of crust and upper mantle rocks. In this study, we analyze the mantle seismic anisotropy of the Western Canada Sedimentary Basin using a new shear wave spitting data set consisting of 1,333 teleseismic arrivals from 82 seismic stations. The resulting 332 high‐quality measurements yield a regional mean apparent splitting time (i.e., the magnitude of anisotropy) of 1.1 ± 0.3 s and an average fast orientation (i.e., the direction of anisotropy) of 54.6° ± 17.2°, which favor a two‐layer anisotropic model based on the 90° back azimuthal periodicity in both parameters. The northeast trending fast orientations dominate the lower layer at lithospheric depths and are approximately parallel to the present‐day absolute plate motions (APMs; i.e., <35°) due to the active asthenospheric flow. On the other hand, deviations from the APMs along the Canadian Rocky Mountain foothills could reflect disrupted mantle flow surrounding a southwestward migrating cratonic lithosphere. Also revealed are two elongated upper‐layer anisotropic anomalies in the lithosphere that are spatially correlated with Moho depths. Their characteristics suggest frozen‐in anisotropy imprinted along two convergent boundaries: (1) the Paleoproterozoic Snowbird Tectonic Zone that separates northeast (north) from northwest (south) fast directions and (2) the foothills of the Rocky Mountains that exhibit northeast trending orientations consistent with those of the APMs, maximum crustal stress, and electromagnetic anisotropy. Compressions associated with the Cordilleran orogenesis could be responsible for the spatial changes in the shear wave anisotropy from the foothills to the cratonic interior.

Spatiotemporal Variations in Crustal Seismic Anisotropy Surrounding Induced Earthquakes Near Fox Creek, Alberta

“…One of the classic methods to detect seismic anisotropy is shear wave splitting, which measures the travel time and polarization differences between fast and slow S waves in an anisotropic medium. Previous studies conducted in Alberta consistently reported the existence of azimuthal anisotropy in western Canada at lithospheric depths: The upper‐mantle anisotropy is mostly aligned along an NE‐SW orientation according to SKS splitting measurements from teleseismic arrivals, which is roughly parallel to the direction of absolute plate motion (Courtier et al, ; Saruwatari et al, ; Shragge et al, ; Wu et al, ). Crustal anisotropy, on the other hand, is usually stress‐ (Crampin, ; Gavin & Lumley, ) or structure‐induced (Licciardi et al, ; Meadows & Winterstein, ), especially in the upper crust (Balfour et al, ; Tarayoun et al, ).…”

“…• Supporting Information S1 mostly aligned along an NE-SW orientation according to SKS splitting measurements from teleseismic arrivals, which is roughly parallel to the direction of absolute plate motion (Courtier et al, 2010;Saruwatari et al, 2001;Shragge et al, 2002;Wu et al, 2019). Crustal anisotropy, on the other hand, is usually stress- (Crampin, 1991;Gavin & Lumley, 2016) or structure-induced (Licciardi et al, 2018;Meadows & Winterstein, 1994), especially in the upper crust (Balfour et al, 2012;Tarayoun et al, 2017).…”

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

“…Despite relatively few published studies (Bao et al, ; Courtier et al, ; Dalton et al, ; Wu et al, ), western Canada offers a natural laboratory for analyzing continental deformation owing to its protracted tectonic history dating back to three billion years ago (Ross et al, ). The lithosphere beneath this region has sustained major lithospheric deformation, most notably Paleoproterozoic multiplate convergence surrounding the Archean Hearne Province and Phanerozoic orogenesis responsible for the Canadian Cordillera (Figure ; Hoffman, ; Ross, ).…”

“…Much of the tectonic imprints are currently buried under the thick Phanerozoic sediments in the Western Canada Sedimentary Basin (WCSB) east of the Rocky Mountain Foothills, where direct geological sampling is often untenable. Consequently, the understanding of regional seismic anisotropy relied heavily on occasional reports of shear wave splitting measurements (e.g., Courtier et al, ; Currie et al, ; Liu et al, ; Wu et al, ) and/or frequency‐dependent surface waves (e.g., Bao et al, ; Yuan & Romanowicz, ) from broadband passive seismic data. These studies consistently document a NE‐SW fast orientation approximately parallel to the direction of absolute plate motion (see Figure ), suggesting large‐scale deformation within a potentially coupled lithosphere‐asthenosphere system.…”

Strain Accumulation and Release Rate in Canada: Implications for Long‐Term Crustal Deformation and Earthquake Hazards

“…R. Peltier et al, 2015W. R. Peltier et al, , 2018Wu et al, 2019). Therefore, the nature of the mantle rheology within the Cordillera and the Pacific Coast may have allowed for a fast response of the lithosphere to PGR thereby limiting the present-day effect of GIA in western Canada (James et al, 2000).…”