Shear‐wave splitting beneath western United States in relation to plate tectonics

Abstract

We have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western United States. Fast polarization directions (ϕ) and delay times (δt) show spatial variations that are coherent within geologic provinces. Stations located near the San Andreas fault show clear evidence for fault‐parallel anisotropy in the crust and upper mantle (115–125 km thickness). This can be explained by the finite strain associated with the relative plate motion between the North American and Pacific plates. The lateral extent of this strain field is probably narrow to the west, because stations 55 km west of the San Andreas fault do not show fault‐parallel anisotropy in southern California. Station LAC located 80 km east of the San Andreas fault shows large fault‐parallel anisotropy. This suggests that the Pacific‐North American plate boundary in the mantle might be displaced to the east in southern California. A deeper E‐W oriented fast direction of anisotropy underlies the fault‐parallel anisotropic layer in the vicinity of the San Andreas fault. An E‐W fast feature is also present beneath the western Basin and Range and the foothills of the Sierra‐Nevada, although local variations are present. The magnitude of delay times suggests that this feature resides in the asthenosphere. We interpret this feature as the asthenospheric flow in the slabless window left behind the Farallon plate. The flow‐induced anisotropy may partially be frozen‐in at shallow depths. Station ORV is located near the southern edge of the Gorda slab where no anisotropy is detected. The absence of anisotropy at this location could therefore mark a boundary between Farallon associated flow and regions where E‐W oriented asthenospheric flow did not occur. The lack of evidence for NE‐SW fast orientation within the Walker Lane Shear Belt of western Nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the San Andreas fault. Stations located over the young subducting Gorda plate mark a change in the fast direction to nearly NE‐SW. This direction aligns well with the maximum compressive stress direction in the overlying North American plate and the NE‐SW directed internal shearing of the Gorda plate. The anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions. This implies that the anisotropic thickness may include some of the asthenosphere. Alternatively, using a higher anisotropy of 8% can bring thicknesses in line with other measures of lithospheric thicknesses. The correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method, and that asthenospheric flow may be a significant contributor to seismic anisotropy.