Seismic anisotropy and mantle flow beneath the northern Great Plains of North America

Abstract

A diverse set of tectonic features and the recent availability of high-quality broadband seismic data from the USArray and other stations on the northern Great Plains of North America provide a distinct opportunity to test different anisotropy-forming mechanisms. A total of 4138 pairs of well-defined splitting parameters observed at 445 stations show systematic spatial variations of anisotropic characteristics. Azimuthally invariant fast orientations subparallel to the absolute plate motion (APM) direction are observed at most of the stations on the Superior Craton and the southern Yavapai province, indicating that a single layer of anisotropy with a horizontal axis of symmetry is sufficient to explain the anisotropic structure. For areas with simple anisotropy, the application of a procedure for estimating the depth of anisotropy using spatial coherency of splitting parameters results in a depth of 200-250 km, suggesting that the observed anisotropy mostly resides in the upper asthenosphere. In the vicinity of the northern boundary of the Yavapai province and the Wyoming Craton, the splitting parameters can be adequately explained by a two-horizontal layer model. The lower layer has an APM-parallel fast orientation, and the upper layer has a fast orientation that is mostly consistent with the regional strike of the boundary. Based on the splitting measurements and previous results from seismic tomography and geodynamic modeling, we propose a model involving deflecting of asthenosphere flow by the bottom of the lithosphere and channeling of flow by a zone of thinned lithosphere approximately along the northern boundary of the Yavapai province.

Seismic azimuthal anisotropy beneath the eastern United States and its geodynamic implications

“…The resulting depth estimates, when compared with the thickness of the lithosphere proposed by previous seismic tomographic studies [ van der Lee , ; Li et al , ; Yuan and Romanowicz , ], suggest that the observed anisotropy is mostly from the upper asthenosphere. This conclusion of mainly asthenospheric origin is consistent with that obtained in the central U.S. based on the same spatial coherency procedure [ Refayee et al , ; Yang et al , ].…”

“…On the horizontal plane, the strike of the keel makes a sharp turn in the northern part of the study area and becomes mostly E‐W, as exemplified by one of the recent tomographic images (Figure ) [ Burdick et al , ]. The SWS observations can be well explained by a simple model involving both APM‐induced fabric and a mantle flow system deflected horizontally around the lateral edges of the keel of the North American Craton [ Fouch et al , ; Refayee et al , ; Yang et al , ]. We propose that the APM‐induced flow system ( V c ) is mainly developed in the transitional layer between the partially coupled lithosphere and the asthenosphere and is present beneath the entire area, while the horizontally deflected flow system exists only beneath the cratonic edges and goes around the keel (Figure ).…”

“…As a nearly ubiquitous characteristic of the Earth's upper mantle, the azimuthal dependence of seismic velocities, i.e., azimuthal anisotropy, has been investigated for decades by numerous studies, mostly using shear wave splitting (SWS) analyses [e.g., Francis , ; Leven et al , ; Mainprice and Nicolas , ; Silver and Chan , ; Gao et al , ; Savage , ; Yang et al , , ]. In particular, splitting of P ‐to‐ S converted phases ( S K S , S K K S , and P K S , hereafter collectively called X K S ) from the core‐mantle boundary is among the most frequently employed structural seismological techniques to determine mantle anisotropy beneath the stations.…”

Azimuthal anisotropy beneath north central Africa from shear wave splitting analyses

“…Under the assumption that this zone represents the approximate edge of the African continental root, E‐W oriented asthenospheric flow locally deflected by the root can explain the E‐W fast orientations. Such a modulation of mantle flow by a continental root has been used to explain edge‐parallel fast orientations around the western and southern edges of the North American continental root [ Fouch et al ., ; Refayee et al ., ; Yang et al ., ]. It has also been suggested beneath central America [ Miller and Becker , ] and NW Africa [ Miller et al ., ].…”

“…Numerous geodynamic modeling and seismic anisotropy studies suggest that the rheologically different lithosphere and asthenosphere interact through partial coupling [ Becker and O'Connell , ; Marone and Romanowicz , ; Doglioni et al ., ; Refayee et al ., ; Yang et al ., ]. The direction of the simple shear in the boundary layer between the partially coupled lithosphere and asthenosphere is determined by the relative velocities between the two layers.…”