Lithosphere and Asthenosphere Structure of the Juan de Fuca and Gorda Plates

Based on a new high-resolution Rayleigh wave phase speed dataset in Cascadia (Zhang et al. 2021), we produce a 3D oceanic Vs model west of the subduction trench using a novel thermo-seismic hybrid parameterization. In the lithosphere, the seismic structure, which is controlled principally in our parameterization by the apparent thermal age, agrees with conductive cooling across most of the region with several exceptions (Figures a-c). Several discrepancies can be explained by the fracturing of the lithosphere and lateral heat transfer. In the underlying asthenosphere, a west to east slow-fast-slow Vs pattern is observed beneath the Juan de Fuca plate (Figures d-e). The middle fast Vs anomaly fits with prediction of Vs near the solidus but devoid of melt, while the other two anomalies may be caused by partial melting. The reason behind the eastern slow Vs anomaly and the possible partial melting is not clear but may be related to the heating simply due to the subduction, or from the south which initially comes from buoyant mantle originating near northwestern California.