The evolution of the hydration state of the Juan de Fuca plate from Ridge to Trench offshore Washington State

Constraining water content in oceanic crust is important for quantifying the supply of both hydrous minerals and free phase water in the downgoing plate, factors known to strongly affect subduction zone processes. We conduct a two-dimensional Vp travel time tomography study of a cross-plate, 250-km long, ocean bottom seismometer transect collected as part of the Juan de Fuca (JdF) Ridge to Trench study to investigate the structure, evolution and state of hydration of the JdF plate from Endeavor segment to subduction at the Cascadia margin offshore Washington. Results are compared to predicted velocities for crustal and upper mantle lithologies at temperatures estimated from a plate-cooling model and are used to provide constraints on water contents in these layers. Co-location of this transect with a multi-channel seismic (MCS) reflection image (Han et al., 2016), allows for comparison of the relationship between velocities and crustal structure. Comparisons with a transect across the southern JdF plate from Axial Volcano to offshore Oregon (Horning et al., 2016) facilitate a regional assessment of the velocity structure in the crust and shallow mantle across the JdF plate for crustal ages of 1 to 8 Ma. Results show upper crustal velocities consistently less than predicted at all ages, indicating a fractured and hydrated upper crust. Lower crust and uppermost mantle velocities are similar to predicted values for plate age, suggesting a lack of significant lower crust and upper mantle fracturing or hydration. The Washington Transect shows a modest decrease in velocities approaching the deformation front, where crustal faults are few and confined to the upper crust, indicating little fracturing and hydration due to subduction bend faulting near deformation front, in contrast to observations on the Oregon Transect. The Washington and Oregon Transects show distinct crustal structure and velocity regimes at comparable plate ages. Both show increased crustal velocities at 6-8 Ma, decreased velocities coincident with a zone of rough basement topography of 4-6 Ma, and a return to relatively high velocities and smooth basement crust at 1-4 Ma. The distribution of crustal types across the JdF plate and associated velocity anomalies suggest differing potential for hydration within the plate inherited from ridge processes.