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TitleContrasting upper-mantle shear wave anisotropy across the transpressive Queen Charlotte margin
AuthorCao, L; Kao, H; Wang, K
SourceTectonophysics vol. 717, 2017 p. 311-320,
Alt SeriesNatural Resources Canada, Contribution Series 20170118
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia; Western offshore region
NTS103B; 103C; 103F; 103J/04; 103K/01; 103K/02; 103K/03; 103H; 103G
AreaHaida Gwaii; Queen Charlotte Islands
Lat/Long WENS-136.0000 -128.0000 58.0000 52.0000
Subjectstectonics; structural geology; structural interpretations; mantle; subduction zones; anisotropy; plate tectonics; earthquakes; teleseismic SKS-phase splitting
Illustrationslocation maps; tables; graphs; plots
ProgramAssessing Earthquake Geohazards, Public Safety Geoscience
AbstractIn order to investigate upper mantle and crustal anisotropy along the transpressive Queen Charlotte margin between the Pacific (PA) and North America (NA) plates, we conducted shear wave splitting analyses using 18 seismic stations in and around the island of Haida Gwaii, Canada. Despite the limited station coverage at present, our reconnaissance study does reveal a systematic pattern of mantle anisotropy in this region. Fast directions derived from teleseismic SKS-phase splitting are mostly margin-parallel (NNW¿SSE) near the plate boundary but transition to predominantly E¿W-trending farther away. We propose that the former is associated with the absolute motion of PA, and the latter reflects a transition from this direction to that of the absolute motion of NA. The broad width of the zone of transition from the PA to NA direction is probably caused by the very obliquely subducting PA slab that travels primarily in the margin-parallel direction. Anisotropy of Haida Gwaii based on local earthquakes features a fast direction that cannot be explained with regional stresses and is probably associated with structural fabric within the overriding crust. Our preliminary shear wave splitting measurements and working hypotheses based on them will serve to guide more refined future studies to unravel details of the geometry and kinematics of the subducted PA slab, as well as the viscous coupling between the slab and upper mantle in transpressive margins.
Summary(Plain Language Summary, not published)
Seismic anisotropy is referred to the phenomenon that seismic waves travelling along different azimuths have different speeds. When a seismic wave enters an anisotropic media, the wave train splits into two phases according to the fast and slow directions. In general, seismic anisotropy is caused by either the flow pattern of mantle materials or tectonic deformation associated with plate motions. In this study, we study the seismic wave splitting phenomenon at seismograph stations in the Haida Gwaii region. Our result indicates a clear change in the fast direction from stations on the Haida Gwaii island to those further inland, suggesting that the pattern of deformation is dominated by two different physical mechanisms. In the vicinity of the Queen Charlotte transform boundary, the relative motion between the Pacific Plate and the North American plate is the most important controlling factor, whereas the absolute motion of the North American plate becomes dominant in the continental interior.