Proposed tectonic model

Using the relative plate motion between the Juan de Fuca plate and the North American Plate (HS3-NUVEL1A), the estimated total length of the slab subducted to the east in the last 17 Ma is $\sim{480}$ km. This is less than the total imaged slab length of $\sim{660}$ km from the present trench in an east-west direction. Therefore, the bottom edge of the slab we observe today was east of the trench around 17 Ma. Assuming a similar slab geometry to today, the $\sim{180}$ km length of slab would reach a depth of $\sim{60}$ km, comparable to the likely thickness of the continental lithosphere. We propose that the absence of the slab below 400 km depth today is due to the arrival of the Yellowstone plume head around 17 Ma, which destroyed the Juan de Fuca slab at depths greater than the thickness of the continental lithosphere. As the plume head material would spread westward beyond the trench, possibly as far as the Juan de Fuca Ridge, traction with the subducting plate would then pull some plume head material down into the mantle. We image this material as the low velocity layer beneath the slab in our Vs model. The observed low Vs anomaly of up to 3$\%$ is comparable with what is expected for plume head material 100-300$^{\circ}$C hotter than the surrounding asthenosphere. Finally, this hot remnant plume head material beneath the slab may be partly responsible for the absence of a Wadati-Benioff zone associated with the subduction of the Juan de Fuca plate (Xue and Allen, 2007).

Figure 2.47: Tectonic map for the study region. Plate motions from HS3-NUVEL 1A are shown as black arrows (Gripp and Gordon, 2002). Blue dotted lines show the depth contours of the Juan de Fuca slab surface (McCrory et al., 2006). The thick black lines delineate the Cascade Range. Age contours of initial rhyolitic volcanism along the Newberry track are shown in 1 Ma increments extending to the Newberry Caldera (NC) (Jordan et al., 2004). Major rhyolitic caldera centers along the Yellowstone track are shown with age in Ma extending to the Yellowstone Caldera (YC) (Pierce and Morgan, 1992). Both tracks initiate in the region of the McDermitt Caldera (MC), which is shown as a red circle. The Columbia River Basalt is shown in blue (12 to 17 Ma)(Christiansen et al., 2002). Dike swarms associated with the 17 Ma basaltic outpourings are shown in gold (Camp and Ross, 2004); Christiansen and Yeats, 1992): Chief Joseph Dike Swarm (CJDS), Monument Swarm (MS), Steens Basalt (SB), and Northern Nevada Rift Zone (NNRZ). The seismic stations used in this study are shown as triangles and squares with a total number of 52. Inset shows the distribution of the 95 events used in the inversion for the S-wave velocity model. The red square outlines the study region. The thick black line across OATS array indicates the location of the cross section shown in Figure 2.48
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Figure 2.48: Vertical slices through the (a) Vs and (b) Vp models along the OATS line indicated in Figure 1. The envelope of the synthetic slab ending at 400 km depth is shown by the blue outline. The contour interval is 0.25$\%$ indicated by vertical lines in the color bar of the velocity scale. Zero contours are not shown. The locations of the Cascades and the Newberry Volcano are shown as blue and red triangles respectively.
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Figure 2.49: Proposed tectonic model for the interaction between the subducting Juan de Fuca Plate (blue) and the Yellowstone plume head (pink). Snapshots in time are shown as: (a) 25 Ma, the Yellowstone Plume is approaching the subducted Juan de Fuca plate; (b) 20 Ma, the plume head has intersected the Juan de Fuca plate and preferentially flowed westward along the base of the slab; (c) at 17 Ma, the plume head punched through the Juan de Fuca plate, destroyed a larger portion of the slab and caused the volcanism at the surface; (d) 15 Ma, the plume head material spreads out in a larger region at the base of the lithosphere; (e) 8 Ma, the subducting slab drags the remnant plume head material down into the mantle; (f) at present, the hot material from the remnant plume head has been brought to greater depth by the ongoing subducting slab. The vertical dashed line indicates the progression of the current Yellowstone caldera to the west. The red plume stem represents a hypothetical Yellowstone Plume since the arrival of the plume head (pink). Note: this model builds on the tectonic models proposed by Geist and Richards, 1993; Pierce, et al., 2000.
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