High seismic attenuation at a mid-ocean ridge reveals the distribution of deep melt

Geochem Geophys Geosyst

Hooft

et al. 2017

We use the delay times of teleseismic S phases recorded by ocean bottom seismometers during the plate‐scale Cascadia Initiative community experiment to constrain the heterogeneity of seismic velocity structure beneath young oceanic lithosphere. Our study area covers the entire Juan de Fuca (JdF) and Gorda plates, from their creation at the JdF and Gorda Ridges to their subduction beneath the North American continent, and the entire length of the Blanco transform fault. The range of the observed Vs anomalies requires variations in the melt fraction of the asthenosphere. The data require that low Vs anomalies extend to depths of at least 200 km, which is within the carbonatite melting regime. In the upper 200 km of the mantle, Vs increases rapidly to the east of the JdF Ridge, while there is no clear relationship with the age of the lithosphere in the Gorda region. The distribution of melt is asymmetric about both the JdF and Gorda Ridges. Dynamic upwelling – due to the buoyancy of the mantle – and accompanying downwelling can explain the rapid decrease in melt fraction to the east of the JdF Ridge, the asymmetry about the JdF Ridge, and the sinuous pattern of upwelling near the Blanco transform fault. Finally, mantle flow beneath the diffuse Gorda and Explorer plate boundaries is distinct from that beneath the discrete plate boundary of the JdF Ridge. In particular, shear between the Pacific and JdF plates appears to dominate mantle deformation over seafloor spreading beneath the Gorda Ridge.

Section: Discussionsupporting

confidence: 93%

Section: Discussionmentioning

confidence: 82%

Section: Discussionmentioning

confidence: 99%

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Mantle dynamics beneath the discrete and diffuse plate boundaries of the Juan de Fuca plate: Results from Cascadia Initiative body wave tomography

Byrnes

Geochem Geophys Geosyst

Hooft

et al. 2017

“…Additionally, our Vp age trend is qualitatively consistent with a number of other seismic observations offshore Cascadia. The abrupt increase in Pn velocities at ~4 Ma coincides with a similarly abrupt increase in upper mantle Vs and decrease in teleseismic body wave attenuation (Byrnes et al, ; Eilon & Abers, ). Ambient noise (Tian et al, ) and Rayleigh wave (Bell et al, ; Ruan et al, ) tomography reveals a thin JdF lithosphere at young plate ages that thickens rapidly beyond ~2.5 Myr.…”

Section: Interpretation and Discussion Of Tomographic Resultsmentioning

confidence: 74%

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

VanderBeek

2019

JGR Solid Earth

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.

“…Next, temperature variations are computed using

\frac{\partial ln V_{p}}{\partial T} ((), \times 10^{- 4} K^{- 1}) = - 0.88

from Karato et al (), assuming a Q p value of 300. Relatively high Q is observed beneath the JdF in regions far from the ridge (Eilon & Abers, ).…”

Section: Discussionmentioning

confidence: 99%

Buoyant Asthenosphere Beneath Cascadia Influences Megathrust Segmentation

Bodmer

Geophysical Research Letters

Hooft

et al. 2018

Great megathrust earthquakes (magnitude 8+) do not typically rupture an entire convergent margin but rather are limited to one or more along‐strike segments. A fundamental question of subduction zone dynamics and hazard assessment is what physical properties or dynamic processes govern megathrust segmentation. Here we use onshore‐offshore teleseismic delay time data to tomographically image the upper mantle seismic structure of the Cascadia subduction zone. Our results reveal along‐strike segmentation in the oceanic asthenosphere beneath the subducting plate with pronounced low‐velocity anomalies below regions of increased plate locking and greater occurrence of episodic tremor and slip. We attribute the anomalous asthenospheric velocities to independent mantle upwellings associated with hot spot‐derived material in northern Cascadia and fragmentation processes at a diffuse plate boundary in southern Cascadia. Based on these relations, we hypothesize that subslab buoyancy modulates the plate coupling force at the thrust interface, thereby contributing to the localization of subduction zone segmentation.