Structure and Evolution of Northern Juan de Fuca Crust and Uppermost Mantle Over the Last 8 Ma From an Active‐Source Seismic Tomography Study

We present results from a two‐dimensional wide‐angle controlled source seismic transect designed to characterize the velocity structure of the oceanic crust and uppermost mantle spanning the northern Juan de Fuca (JdF) plate from near Endeavor ridge to the Cascadia margin. Reflection and refraction travel time inversion is used to derive a tomographic Vp model of sediments, crust, and upper mantle. Velocity model results are compared to baseline reference Vp values for unaltered crustal and upper mantle rocks at temperatures assuming plate cooling. Effective medium theory is used to infer the degree of hydration of the crust and mantle. Results indicate a somewhat fractured and hydrated upper crust (≤2.5 wt% water), a near dry lower crust (≤0.7 wt%) and upper mantle (≤0.5 wt%) west of the deformation front, and an ∼75 km wide region of modestly lower velocity in the mid‐plate. Comparison with prior results from a complementary transect offshore Oregon indicates significant differences in Vp of the upper crust, with lower Vp‐inferred porosity along most of the Washington transects that may reflect different extents of fault‐related alteration and sediment burial histories. Approaching the deformation front, Washington transect Vp structure indicates dryer conditions than offshore Oregon, consistent with differences in extent of subduction bend faulting found in reflection imaging studies. On both transects, quasi‐abrupt changes in plate properties at ages of 8, 6, and 3.4/4 Ma are found. Distinct crustal accretion modes are recognized, aligning with changes in JdF plate motion and recent history of Cobb‐Eickelberg hotspot influence on crustal accretion at the JdF ridge. Plain Language Summary In this study, we use seismic data in order to characterize the velocity structure of the Juan de Fuca plate offshore Washington state, and as a result we are able to estimate the amount of water in the crust and upper mantle. Results of this analysis suggest a wet upper crust and dry lower crust and upper mantle in this region. We compare these results with previous studies in the area and provide suggestions of geologic motivation for differences and similarities. Results of these comparisons suggest that the structure of oceanic crust may be partially inherited from dynamics of the mid‐ocean ridge at the time of formation. Key Points Analysis of seismic data crossing the Juan de Fuca plate offshore Washington indicates a hydrous upper crust and dry lower crust/mantl