Structure and serpentinization of the northern Cascadia mantle wedge corner from receiver function modelling

The composition and structure of the mantle wedge corner (MWC) exert a primary control on seismogenic and fluid processes in subduction zone forearcs. Seismic evidence suggests widespread serpentinization of the MWC in Cascadia, but constraints on its internal structure and fabrics remain sparse. Here, we compile receiver function data for a line of seismograph stations in northern Cascadia to characterize the layered structure and seismic anisotropy in the MWC. We first invert for flat-layer shear-wave velocity (Vs) models and identify the presence of a dipping, ∼4 km thick seismic low-velocity layer (LVL) marked by low Vs (∼3 km/s) and high compressional-to-shear velocity ratio (Vp/Vs > 2). Above the LVL, the continental Moho is taken as the base of the low Vp/Vs (∼1.7) zone, previously interpreted as the silica-rich lower continental crust. Second, we solve for anisotropic parameters in the MWC, including layer thickness, Vp/Vs, percent anisotropy, and symmetry-axis orientation. We infer an ∼5 km thick layer at the base of the MWC with Vs of 3.6 km/s and Vp/Vs of ∼1.9. Anisotropy is characterized by a slow axis of hexagonal symmetry with 10% anisotropy and an orientation corresponding with sub-horizontal foliation fabrics. Based on these results, we infer 40%–100% serpentinization of the MWC localized within a thin layer above the plate interface shear zone, potentially hosting high pore-fluid pressure. Shear-induced foliated fabrics in serpentinites above the plate interface may play an essential role in controlling seismogenic processes by channelling fluids up dip and aiding in generating fluid overpressures.