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

Abstract

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.

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

“…Seismic velocities appear asymmetric about the ridge with slower velocities extending beneath the Pacific plate. This asymmetry may in part be due to limited resolution west of the ridge, but it is consistent with the asymmetric distribution of seamounts in the region (Davis & Karsten, 1986) and prior teleseismic studies that find slower and more attenuating mantle (Byrnes et al, 2017;Eilon & Abers, 2017) and possibly a thinner plate (Rychert et al, 2018) west of the Gorda ridge. Excluding the MTJ, the lowest seismic velocities (<7.6 km/s) are located beneath the Gorda ridge and are found occupying two irregularly shaped regions beneath the southern and northern ridge segments.…”

“…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.…”

Buoyant Asthenosphere Beneath Cascadia Influences Megathrust Segmentation

“…In northern Cascadia, we attribute the subslab low‐velocity anomaly (Figure c) to thermally buoyant mantle derived from nearby oceanic hot spots. Figures c, S8c, and S8d show that the subslab velocity anomaly extends back toward the northern JdF Ridge and intersects the ridge north of Axial Seamount, in a region where ridge mantle structure is most asymmetric and geochemical anomalies are observed (see Figure 15 of Byrnes et al, ). Several oceanic hot spots have been identified in a northwest‐southeast trend in the northeastern Pacific, including the Cobb, Bowie, and Anahim hot spots (Mercier et al, ; Zhao, ).…”

“…Low‐velocity anomalies are absent beneath the slab in the forearc of central Cascadia (Figure d), indicating that subslab heterogeneities are local rather than margin‐wide features, in contrast to previous work (Hawley et al, ). In southern Cascadia, deep low‐velocity anomalies (100–250 km, Δ V p ~−1.5%) are present beneath the slab and at the southern slab edge boundary; these low‐velocity anomalies extend northward to ~43°N and connect to shallower (<100 km) low‐velocity anomalies below the Gorda deformation zone (Figure e; Byrnes et al, ). The lateral extent of the shallow anomalies correlates well with the boundaries of the Gorda deformation zone (Chaytor et al, ).…”

“…(a) Tectonic features of the Cascadia subduction zone and Juan de Fuca plate system (modified fromByrnes et al, 2017). Arrows represent absolute plate motions(DeMets et al, 2010).…”