Plume vs. Plate: Convection Beneath the Pacific Northwest

Mathias Obrebski, Richard M Allen, Mei Xue (Tongji University, Shanghai, China) Shu-Huei Hung (National Taiwan University, Taipei, Taiwan)


The Pacific Northwest of western North America is unusual in that both a subducting slab and a hotspot occur within  1000 km of one another. The Juan de Fuca plate (JdF) that continues to subduct today (Figure 2.38) is a remnant corner of the Farallon plate. Subduction beneath the Pacific Northwest has been continuous for more than $\sim$150 Ma (Atwater, 1989) and we would expect, and previous work has imaged, several thousand kilometers of slab extending deep into the mantle (Sigloch et al., 2008). The westernmost US hosts major Neogene to Quaternary volcanic provinces. The Columbia River Basalts (CRB) are the product of a phase of massive volcanic outpouring that occurred  17 Ma. The Yellowstone Snake River Plain is a bimodal volcanic trend that exhibits a time progressive sequence of calderas. Two groups of hypotheses have been proposed to explain this surface geology: a stationary deep-seated whole mantle plume (Pierce et al., 2000; Camp and Ross, 2004; Smith et al., in review), or various lithospheric-driven processes of fracture and volcanism (Dickinson, 1997; Humphreys et al., 2000; Christiansen et al., 2002). Nevertheless, seismic imaging efforts to constrain the geometry of any Yellowstone plume anomaly through the mantle have been inconclusive.

Figure 2.38: Geologic-tectonic features of the Pacific Northwest of the United States overlaid on topography and bathymetry. The Juan de Fuca plate is subducting beneath the North American plate with an oblique convergence rate of 41 mm/yr. The estimated depth of the top of the subducting slab is shown with blue contours labeled in km. The location of all M$>$4 earthquakes with depth $>$ 35km since 1970 are shown as blue dots. Volcanoes are shown as orange triangles. The Yellowstone Hotspot Track exhibits a series of time-progressive calderas (red outline) from McDermitt ($\sim$16.1 Ma) to the currently active Yellowstone Caldera. The track is approximately parallel to the absolute plate motion of North America. The Columbia River Flood Basalt Province was a massive outpouring of basalt from $\sim$16.6 to $\sim$15.0 Ma and is shown in pink.
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Data and Method

Here we compile a seismic waveform dataset consisting of teleseismic body-waves, both direct and core phases, recorded at seismic stations across the western United States. We use data from the US from Earthscope's USArray, the regional seismic networks and temporary seismic deployments that together provide an array of more than 1000 seismometers with an unprecedented density and spatial extent. Relative body-wave traveltime delays are measured by cross-correlation in the 1.25 to 0.5 sec frequency band for compressional arrivals and 50 to 10 sec for shear arrivals. We use events at greater than 30$^{\circ}$epicentral distance with magnitude greater or equal to 5.5 from January 2006 to July 2009. The compressional-arrival dataset is derived from 30,670 traveltime observations of direct P from 127 earthquakes. The shear-arrival dataset includes 38,750 travel-time measurements, 34,850 S-wave observations from 142 events and 3,900 SKS observations from 24 events. In this study, we combine this regional dataset with a tomographic technique utilizing finite frequency sensitivity kernels. The banana-doughnut-shaped kernels account for the frequency- and depth-dependent width of the region to which teleseismic body-waves are sensitive and account for wave-front healing effects. Our tomographic method uses paraxial kernel theory to calculate the forward scattering sensitivity kernels for teleseismic arrival times (Hung et al. 2004).

Figure 2.39: Cross sections through our P model. (a) Constant depth slice at 200 km showing the continuous high-velocity Juan de Fuca slab and the strong low-velocities below the Yellowstone Caldera. (b) through (d) are E-W cross-sections at locations shown in (a). The currently subducting slab is visible in all sections though the amplitude of the slab anomaly reduces from south to north. The Yellowstone plume has a strong signature as deep as 900km in (b) and (c). The parallel fossil-slab anomaly is strong in (d). (e) is composed of an oblique section through the Juan de Fuca slab, a constant depth slice at 800km, the E-W vertical slice (b) and a 3D isosurface that illustrates the 3D geometry of the Yellowstone plume.
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Results and Discussion

The main features imaged in both our P and S models (Figure 2.39) include a whole mantle plume beneath Yellowstone emplaced between slab fragments, the surprisingly short and uneven Juan de Fuca plate, and the presence of a fossil slab fragment adjacent to the currently subducting slab. Interpreted together, all these features are strong indications that the Yellowstone plume broke through the Farallon slab to reach the surface, and that it thermally consumed part of the slab. We propose that as the plume head rose towards the surface the Farallon slab first stopped it. Due to continuous thermal erosion and flexure imposed by the increasing volume of hot and buoyant plume material, the portion of the slab located above the head of the plume weakened and was assimilated by it. Free from structural barrier, the resulting blend continued its ascent to the surface and fed the CRB. The geochemistry of the of the Grande Ronde basalts, representing the climax stage of the CRB and more than 80% of the volume of the CRB, has been interpreted as a heterogeneous source of plume material containing fragments of oceanic crust (Takahahshi et al., 1998), consistent with this interpretation. Farther from the plume conduit, the slab that was not thermally consumed eventually broke. Assuming a subduction rate of around 5cm/y to account for the higher Pacific-North America convergence velocity during the Neogene (Riddihough, 1984), the slab rupture evidenced by our model would have been achieved at the time of or slightly before the CBR outpouring.


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