VARIATIONS IN SKS SPLITTING ACROSS WESTERN CANADA

Measurements of SKS splitting parameters have been made at stations of the Canadian National Seismograph Network for events over the period 1993-mid 1994. Here we report on results obtained from western station s. In the northern Cordillera the alignment of polarization directions with major transcurrent fault systems suggests that the crust and upp er mantle are being deformed coherently. Splitting parameters in the s outhern Cordillera, however, are oriented approximately perpendicular to the strike of major tectonic features, notably the Cascadia subduct ion Bone. This contrast in the character of fast polarization directio ns between the southern and northern Cordillera is thought to reflect the change in the tectonic style of the adjacent plate margin from con vergent along the southern B.C. coast to transform north of the triple junction. One plausible explanation is that upper mantle anisotropy i n the southern region is due primarily to mantle flow perpendicular to the margin involving upwelling at the Juan de Fuca ridge, subduction along the coast and associated back-are asthenospheric flow beneath th e B.C. interior.