Pn Tomography of the Juan De Fuca and Gorda Plates: Constraints on Mantle Deformation and Hydration in Young Oceanic Lithosphere

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Tomographic analysis of Pn arrival times—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) dataset provides the first opportunity to image spatial variations in lithospheric structure from accretion to subduction across an entire oceanic plate. We measure 2,862 Pn arrivals from local earthquakes recorded by the CI array and the complimentary Blanco and Gorda arrays. We invert the measured arrival times for 3D variations in isotropic and anisotropic P-wave velocity and event hypocentral parameters. Despite surficial evidence of extensive active faulting, the velocity structure of the Gorda uppermost mantle is remarkably consistent with predictions from a conductive cooling model. Limited deformation at mantle depths is supported by seismic anisotropy measurements that 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 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. Beneath the central JdF plate, P-wave speeds remain relatively constant (~7.7 km/s) out to ~4-5 Myr before abruptly increasing to ~7.9 km/s at older plate ages. Curiously, this transition occurs near the onset of asthenospheric downwelling inferred from teleseismic body wave tomography [Byrnes et al., 2017] and attenuation [Eilon and Abers, 2017] suggesting that the underlying geometry of mantle flow may influence the physical properties of young oceanic lithosphere.

Presenting Author: Douglas Toomey

Authors