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Shear wave anisotropy beneath the Cascadia subduction zone and western North American craton

2004; Oxford University Press; Volume: 157; Issue: 1 Linguagem: Inglês

10.1111/j.1365-246x.2004.02175.x

1365-246X

Claire A. Currie, J. F. Cassidy, R. D. Hyndman, M. G. Bostock,

earthquake and tectonic studies

SUMMARY We have examined shear wave splitting of SKS phases at 26 permanent broadband stations in western North America to constrain regional trends in anisotropy at the Cascadia subduction zone (CSZ) and adjacent regions. At forearc stations above the Juan de Fuca Plate, the fast directions are approximately parallel to the direction of absolute plate motion of the main Juan de Fuca Plate (∼N70 ◦ E). Delay times of 1.0 to 1.5 s indicate a mantle source for the anisotropy, most likely strain-induced lattice-preferred orientation of anisotropic mantle minerals. The anisotropy may be related to present-day subduction-induced deformation of the mantle beneath the subducting plate. The delay times show an increase with distance from the deformation front (trench), which may be indicative of 3‐5 per cent anisotropy within the forearc mantle wedge, with a fast direction parallel to the subduction direction. Above the Explorer Plate at the northern end of the CSZ, the fast directions are N25 ◦ E. This may reflect either the more northerly subduction direction of that plate, or a transition from subduction-related deformation to along-margin flow parallel to the transcurrent Queen Charlotte Fault to the north. At four stations in the central backarc of the CSZ, fast directions are parallel to the Juan de Fuca‐North America convergence direction, consistent with mantle deformation due to subduction-induced mantle wedge flow, as well as deformation of the uppermost backarc mantle associated with motion of the overriding plate. No clear splitting was observed at the two most northern backarc stations, indicating either little horizontal anisotropy or highly complex anisotropy beneath these stations, possibly associated with complex mantle flow around the northern edge of the subducted plate. The hot, thin backarc lithosphere of the Cascadia subduction zone extends to the Rocky Mountain Trench, the western boundary of the cold, stable North America craton. At two stations on the North America craton the shear wave splitting parameters show significant azimuthal variations with a 90 ◦ periodicity, characteristic of multiple layers of anisotropy. The observations were fitted with a two-layer model with an upper anisotropic layer with a fast direction of N12 ◦ E and delay time of 1.4 s, and a lower layer with a fast direction of N81 ◦ E and delay time of 2.0 s. The North America craton is characterized by a thick lithosphere. Thus, the two anisotropic layers may reflect an upper layer of fossil anisotropy within the cool (<900 ◦ C) lithosphere and an underlying anisotropic layer produced by present-day deformation.