Pn Tomography of the Juan de Fuca and Gorda Plates: Implications for Mantle Deformation and Hydration in the Oceanic Lithosphere

Abstract

Tomographic analysis of Pn arrivals—the guided P wave propagating within the lithospheric mantle—is ideal for studying uppermost mantle structure. While plate‐scale seismic images of Pn velocities are common beneath the continents, similar scale studies have not been possible within ocean basins due to the sparse distribution of seismic stations. The Cascadia Initiative (CI) data set provides the first opportunity to image spatial variations in lithospheric structure from accretion to subduction across an entire oceanic plate. We invert Pn travel times from local earthquakes for 3‐D variations in isotropic and anisotropic P wave velocity and event hypocentral parameters. Despite surficial evidence of extensive faulting, we find that the velocity structure of the Gorda uppermost mantle is remarkably consistent with predictions from a conductive cooling model. Limited brittle deformation at mantle depths is supported by seismic anisotropy measurements, which show the fast direction of P wave propagation rotates in concert with the magnetic anomaly lineations. This rotation may be explained by local plate kinematics without internal deformation and hydration of the shallow mantle. In contrast to Gorda, the seismic velocity structure of the Juan de Fuca (JdF) plate does not exhibit a clear age dependence. Anomalously slow mantle velocities are found along the southern edge of the JdF plate and are spatially associated with the termini of pseudofaults. We attribute these velocity reductions to mantle alteration by seawater. Our results highlight the heterogeneous nature of the oceanic mantle lithosphere and show that patterns of mantle alteration are not readily discerned from surficial indicators of seafloor deformation.

Depth‐Dependent Azimuthal Anisotropy Beneath the Juan de Fuca Plate System

“…We observe uneven consistency between Pn ‐derived fast axes and the fast directions in the shallowest layer of our inversions. In the Gorda region, anisotropic fast directions in our lithospheric lid are indistinguishable from measured Pn phase data given reported uncertainties; we observe φ ~113° and VanderBeek and Toomey (2019) report an average value of φ = 116 ± 6°. On the other hand, within the interior of the JdF plate, our inversion suggests φ = ~40° in the uppermost layer, which is at a high angle to Pn measurements of φ = 87 ± 3° in the JdF interior or φ = 122 ± 1° on the (Endeavour) ridge axis (VanderBeek et al, 2016; VanderBeek & Toomey, 2017).…”

“…Our shorter‐period data can be explained by a model involving a Gorda plate with strong lithospheric anisotropy presumably generated by MOR processes and internal strain (Gaherty, 2004; VanderBeek & Toomey, 2019) and weak intermediate depth anisotropy due to diminished or recently extinct shear over underlying mantle. The JdF region has weak lithospheric anisotropy, but a strong asthenospheric fabric caused by motion of the plate in combination with local mantle flow fields.…”

“…As for Figure 5, but showing results from the Gorda region. The Pn result is from VanderBeek and Toomey (2019). …”

“…The Juan de Fuca profile is in blue and Gorda in pink, as in Figure 1. Stars denote average Pn measurements for the two plate interiors (VanderBeek & Toomey, 2017, 2019). Lines with diamonds at the bottom show mean SKS splitting observations in these regions (Bodmer et al, 2015).…”

“…These observed values are straddled by kinematically relevant azimuths that might have been expected to dominate internal fabric: absolute plate motion (APM) is further counterclockwise, at ~55 ± 5° according to no‐net‐rotation (Kreemer et al, 2014) or absolute (Wang et al, 2018) reference frames, while ridge‐perpendicular (fossil spreading parallel) fabric should trend ~110°. In the Gorda region, Pn anisotropy tomography shows a clockwise rotation of fast axes from spreading parallel (~90°) at the Gorda ridge to ~130° within the plate, commensurate with internal shear of the Gorda lithosphere (VanderBeek & Toomey, 2019), which has strongly internally deformed since 2–3 Ma (Wilson, 1989).…”