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.
Bibliographical noteFunding Information:
We thank Z. Eilon and an anonymous reviewer for their helpful comments, which greatly improved both the content and presentation of this manuscript. Data used in this research were provided by instruments from the Ocean Bottom Seismograph Instrument Pool (http://www.obsip. org) which is funded by the National Science Foundation. OBSIP data are archived at the IRIS Data Management Center (http://www.iris.edu). The seismic data used in this paper are available from the Data Management Center under the network codes 7D and X9 for the Cascadia Initiative and the Blanco transform experiment, respectively. This research was supported by the National Science Foundation under grants OCE- 1139701, OCE-1333196, and EAR- 1520694 to the University of Oregon, and OCE-1031858 and OCE-1131767 to Oregon State University.
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- Juan de Fuca
- dynamic upwelling
- mid-ocean ridges