TY - JOUR T1 - Pn Tomography of the Juan de Fuca and Gorda Plates: Implications for Mantle Deformation and Hydration in the Oceanic Lithosphere JF - Journal of Geophysical Research: Solid Earth Y1 - 2019 A1 - VanderBeek, Brandon P. A1 - Toomey, Douglas R. KW - Cascadia KW - mantle hydration KW - oceanic lithosphere KW - offshore seismicity KW - Pn tomography KW - seismic anisotropy AB - 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. VL - 124 UR - https://onlinelibrary.wiley.com/doi/abs/10.1029/2019JB017707 N1 - _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2019JB017707 ER - TY - JOUR T1 - Seismic evidence that black smoker heat flux is influenced by localized magma replenishment and associated increases in crustal permeability JF - Geophysical Research Letters Y1 - 2017 A1 - Arnoux, Gillean M. A1 - Toomey, Douglas R. A1 - Hooft, Emilie E. E. A1 - Wilcock, William S. D. A1 - Morgan, Joanna A1 - Warner, Mike A1 - VanderBeek, Brandon P. VL - 44 UR - http://doi.wiley.com/10.1002/2016GL071990 ER - TY - JOUR T1 - Shallow Mantle Anisotropy Beneath the Juan de Fuca Plate JF - Geophysical Research Letters Y1 - 2017 A1 - VanderBeek, Brandon P. A1 - Toomey, Douglas R. VL - 44 UR - https://onlinelibrary.wiley.com/doi/abs/10.1002/2017GL074769 ER - TY - JOUR T1 - Segmentation of mid-ocean ridges attributed to oblique mantle divergence JF - Nature Geoscience Y1 - 2016 A1 - VanderBeek, Brandon P. A1 - Toomey, Douglas R. A1 - Hooft, Emilie E. E. A1 - Wilcock, William S. D. VL - 9 UR - http://www.nature.com/articles/ngeo2745 ER -