Shallow Mantle Anisotropy Beneath the Juan de Fuca Plate

The anisotropic fabric of the oceanic mantle lithosphere is often assumed to parallel paleo‐relative plate motion (RPM). However, we find evidence that this assumption is invalid beneath the Juan de Fuca (JdF) plate. Using travel times of seismic energy propagating through the topmost mantle, we find that the fast direction of P wave propagation is rotated 18° ± 3° counterclockwise to the paleo‐spreading direction and strikes between Pacific‐JdF relative and JdF absolute plate motion (APM). The mean mantle velocity is 7.85 ± 0.02 km/s with 4.6% ± 0.4% anisotropy. Synthesis of the plate‐averaged Pn anisotropy signal with measurements of Pn anisotropy beneath the JdF Ridge and SKS splits across the JdF plate suggests that the anisotropic structure of the topmost mantle continues to evolve away from the spreading center to more closely align with APM. We infer that the oceanic mantle lithosphere may record the influence of both paleo‐RPM and paleo‐APM. Key Points Seismic anisotropy within the topmost ~10 km of the mantle is rotated 15°–21° from the spreading direction beneath the Juan de Fuca plate Pn and SKS anisotropy measured near the spreading center and across the plate constrain evolution of seismic anisotropy Shallow mantle anisotropy beneath an oceanic plate may record both relative and absolute plate motion