Search

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. New shear wave splitting results disclose layered anisotropy beneath the western LaurentiaDifferential splitting times from two‐layer anisotropic modeling are correlated with Moho depths, suggesting crustal impact on the shear wave splitting observations across AlbertaThe upper‐layer fossil anisotropic fabrics reveal two collision‐related convergent events that formed the western North America

Understanding the present‐day fast seismic velocity anomalies in the mantle requires an accurate kinematic reconstruction of past convergent tectonics. Using the paleomagnetism‐based absolute reconstruction method from Wu and Kravchinsky (2014), we present here the restoration of the closing of the Mongol‐Okhotsk Ocean (MOO) that existed between Siberia and North China‐Amuria (NCA) during the Mesozoic. Three stages, i.e., 250–200 Ma, 200–150 Ma, and 150–120 Ma, are identified from the time‐varying convergence rates of Siberia and NCA. The spherical distance between the suture margins was reduced by approximately 66.7% at an average convergence rate of 8.8 ±0.6 cm/yr during the first stage at 250–200 Ma, when approximately 62.5–76.1% of the slabs associated with the MOO lithosphere were formed primarily through intraoceanic convergence. In the second stage at 200–150 Ma, the spherical distance was reduced by another 21.1% with a convergence rate of 3.6 ±0.3 cm/yr. During this stage, approximately 14.2–30.9% of the MOO slabs were formed and continental‐oceanic convergence outpaced intraoceanic subduction. In the last stage at 150–120 Ma, the convergence rate dropped to approximately 0.4–0.6 cm/yr with the formation of approximately 4.6–9.8% slabs associated with the MOO lithosphere. The final closure of the remnant MOO basin could have been accomplished by 130–120 Ma, which explains the origin of the fast‐velocity anomalies inside the restored continents at 120 Ma near the suture margins. Absolute motions of stable Europe, Siberia and North China‐Amuria since 260 Ma are reconstructed using paleomagnetic dataTime‐varying convergence rates disclose three stages during the closing of the Mongol‐Okhotsk Ocean in the MesozoicThe genesis for the seismologically defined slab morphology underneath Eurasia is investigated from plate reconstructions