Dehydration of the Subducting Juan de Fuca Plate and Fluid Pathways Revealed by Full Waveform Inversion of Teleseismic P and SH Waves in Central Oregon

Abstract

AbstractThe dehydration of subducting slabs expels a massive amount of water into the forearc and backarc mantle which is responsible for the serpentinization of the mantle wedge, as well as the production of melt and arc magmatism. These processes are expected to have characteristic signatures in density and seismic velocities models, which remain largely elusive to date due to the limited spatial resolution of classical passive tomographic approaches. Here we present a tomographic model of density, VP, VS, and VP/VS beneath central Oregon, obtained by inverting complete teleseismic P and SH waveforms recorded by the CASC93 temporary experiment. The final model shows an east‐dipping low‐velocity layer less than 10 km thick that can be associated with the fluid‐saturated Juan de Fuca oceanic crust. The distribution of tremors at the surface closely coincides with the horizontal extent of this low‐velocity layer. Below 40 km depth, seismic velocities and density increase progressively to the values of a typical mantle. This transitional domain corresponds to the eclogitization of the oceanic crust. Silica‐saturated fluids released by pore collapse migrate upward, producing serpentinization reactions in the forearc mantle that lower the density and seismic velocities. The very low VP/VS ratio documented in the Cascadia forearc crust is evidence that these silica‐saturated fluids reach the crust, where they produce extensive quartz mineralization. At greater depth, a low seismic velocity and high VP/VS ratio anomaly provides evidence for partial melting at around 75 km depth beneath the volcanic arc.