Subsections

Slab-Plume Interaction Beneath the Pacific Northwest

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

Introduction

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. Globally, these geologic components are commonly separated into distinct provinces. The Juan de Fuca (JdF) plate that continues to subduct today is a remnant corner of the Farallon plate and is terminated to the south by the Mendocino Triple Junction (MTJ). Subduction beneath the Pacific Northwest has been continuous for more than 150 Ma. The westernmost US exibits several major Neogene to Quaternary volcanic provinces. The Columbia River Basalts (CRB) is the product of a phase of massive volcanic outpouring that occurred 17 Ma. The Yellowstone Snake River Plain (YSRP) hosts a bimodal volcanic trend that exhibits a time progressive sequence of volcanic centers. Two groups of hypotheses have been proposed to explain this surface geology: a stationary deep-seated whole mantle plume, or various lithospheric-driven processes of fracture and volcanism. Nevertheless, seismic imaging efforts to constrain the geometry of any Yellowstone plume anomaly through the mantle have been inconclusive. Here we take advantage of the Yellowstone region being now well covered by the dense USAarray deployment to provide constraints on the source of the hotspot, the process of subduction, and the inevitable interaction between the two in the mantle beneath the Pacific Northwest.

Figure 2.16: Tomographic 3D view of the DNA09 P-wave velocity structure of the mantle beneath the Pacific Northwest. a) is an oblique section through the currently subducting Gorda-Juan de Fuca slab (JdF) that clearly shows the southern edge of the slab beneath the Mendocino Triple Junction (MTJ). b) is a constant depth slice at 800km that illustrates the contrast between dominantly high velocity mantle to the north and slow velocities to the south where the Farallon slab is no longer present. c) is an E-W vertical slice at 46.5$^{\circ}$N. 3D blue isosurfaces show strong fast anomalies linked to the Gorda-JdF slab and to possible Farallon fragments (F1-F2). The red isosurface depicts the 3D geometry of a large slow anomaly that extends from beneath the Yellowstone-Snake River Plain (YSRP) hotspot track to the bottom of our model at 1000km depth and that we interpret as a mantle plume.
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Data and Method

To image the Earth's interior beneath the Pacific Northwest, we use all of the available Earthscope-USArray data recorded from January 2006 to July 2009. The station coverage extends from the west coast to 100$^{\circ}$W and from the Mexican border to the Canadian border. We also processed the data from two Earthscope temporary arrays (FlexArray Along Cascadia Experiment for Segmentation [FACES] and the Mendocino Experiment) deployed along the Cascadia trench and permanent seismic networks in the western US. The velocity structure of the mantle is retrieved through body wave finite frequency tomographic inversion. The dataset of our multi-frequency compressional model DNA09-P is derived from 58,670 traveltimes of direct P from 127 earthquakes measured in four frequency bands. The dataset used for our shear model DNA09-S includes 38,750 travel-time measurements, 34,850 S-wave observations from 142 events and 3,900 SKS observations from 24 events.

Result and Interpretation

We interpret the low velocity anomaly beneath the YSRP as a mantle plume with a lower mantle origin. Our interpretation, based on geometrical observations of our P- and S-wave models, is also supported by the high He3/He4 isotopic ratio typical of the YSRP volcanism (Graham et al., 2004), which is often interpreted as indicative of a lower mantle source. The low velocities are consistent with high temperatures and low density. A hot plume with a large volume of low density material, as observed in our models, accounts for the high heat flow, the broad topographic swell, the geoid high, and the large free air gravity anomaly observed in the YSRP area (Smith et al., 2009 and references therein), and also the 410km mantle discontinuity that deepens by 10km in this region (Fee and Dueker, 2004). The geometry and structure of the elongated slow anomaly observed in the upper 200km beneath the YSRP are consistent with the predictions of numerical models for the deflection of a plume head by the motion of an overlying lithospheric plate (Steinberger et al., 2004). It is elongated in the SW-NE direction parallel to the motion of the North American plate, the amplitude of the slow anomalies decreases to the southwest with increasing age of the calderas, and the plume conduit today coincides with active volcanism in the Yellowstone Caldera.

The geometry of the Cascadia subduction zone, namely the length and amplitude of the slab anomaly, displays north-south variations (Figure 2.16). In particular, the slab is virtually absent deeper than 300 km beneath Oregon, and is thus too short to act as a mechanical barrier to upper-mantle flow. This gap in the trench may allow the mantle underlying the JdF plate to flow eastward beneath the plate margin as the North American plate moves southwestward above it. This provides a possible explanation for the trench-normal fast direction of anisotropy retrieved from SKS splitting analysis in central and northern Cascadia (Eakin et al., 2010). The orientation of the fast direction in central and northern Cascadia differs from most other subduction zones where the fast direction is trench-parallel (Long and Silver, 2009). The Gorda-Juan de Fuca slab is thought to be in trench rollback, and it has been suggested that the Gorda slab plays a significant role. This is consistent with our model, where the Gorda slab dives deeper into the mantle and exhibits a faster anomaly, potentially indicative of cooler and denser material. Finally, the Cascadia subduction zone is also unusual due to the near-absence of deep seismicity. The fragmentation of the slab may play a role. There is no recorded seismicity deeper than 35km beneath Oregon, where the depth extent of the slab is only 300 km, thereby reducing the slab pull force usually responsible for intermediate depth down-dip-tension earthquakes. There is some sub-crustal seismicity beneath Northern California and beneath northern Washington, where the slab is imaged deeper into the mantle.

The analysis of the geometry of our tomographic models suggests that the arrival and emplacement of the large Yellowstone plume had a substantial impact on the nearby Cascadia subduction zone, promoting the tearing and weakening of the JdF slab. The existence of a whole-mantle plume and an active subduction zone within 1000km of one another as imaged in our models makes the tectonic setting of the Pacific Northwest unique. Also striking is the substantial fragmentation of the slab. The latitude where the slab is absent coincides with that of the Yellowstone plume (Figure 2.16). Around 19 Ma, there was a substantial change in the spreading rate at the Pacific-JdF ridge and also in the convergence rate of the Cascadia trench (Wilson, 1988). This change could result from a reduction in slab pull. The change also shortly predates the massive magma outpouring of the Columbia River Basalts and the onset of volcanism along the YSRP, which have been interpreted as the manifestation of Yellowstone plume head emplacement (Smith et al., 2009) around 17 Ma. We thus propose that the ascent of the Yellowstone plume, and its necessary encounter with the JdF slab, contributed to a rupture of the slab (Xue and Allen, 2007) (Figure 2.16) and the subsequent reduction of slab pull in the Cascadia trench. The composition of the CRB requires the presence of oceanic crust in the source (Takahahshi et al., 1998), which supports the hypothesis that the Yellowstone plume interacted with the JdF slab and carried fragments of oceanic crust back up to the melting zone. This interpretation also explains several intriguing geophysical properties of the Cascadia trench that contrast with most other subduction zones, such as the absence of deep seismicity and the trench-normal fast direction of mantle anisotropy.

Acknowledgements

We thank USArray TA for data collection and the IRIS DMC for data distribution. This work was supported by the National Science Foundation and a UC-National Laboratory Research program grant.

References

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