Subduction transforms azimuthal anisotropy in the Juan de Fuca plate

Abstract

To clarify the internal structure of the oceanic lithosphere-asthenosphere system, here we determine a new 3-D model of azimuthal anisotropic shear-wave velocity (Vs) down to ∼200 km depth from the Juan de Fuca (JdF) mid-ocean ridge to the Cascadia subduction zone, by inverting newly measured teleseismic fundamental mode Rayleigh-wave phase and amplitude data at periods of 25–100 s. The JdF lithosphere is clearly imaged as a high-velocity anomaly from the ridge to the subduction zone. Distinct azimuthal anisotropies are revealed in the JdF lithosphere before and after its subduction. Obvious trench-normal fast-velocity directions (FVDs) exist in the JdF plate beneath the Pacific Ocean, whereas the subducting JdF slab beneath the Cascadia margin generally exhibits trench-parallel FVDs. We propose that the subduction-induced structure and mineral alignment in the JdF slab transform the fossil fabrics in the JdF plate formed at the mid-ocean ridge and altered during the plate motion and cooling.