The tectonic history of Cascadia is well characterized from plate reconstructions and tomography, but how subduction has affected mantle flow is relatively unknown. Such knowledge about the mantle flow field is primarily obtained from studies of mantle anisotropy using observations such as shear wave splitting. Cascadia and the Mendocino Triple Junction (MTJ) are generally lacking in such observations compared to other subduction zones worldwide, despite showing patterns that are unique to the global data set, i.e. trench normal fast directions beneath the slab (Long and Silver, 2008).
Here we present shear wave splitting observations made using the datasets from FAME (Flexible Array Mendocino Experiment) and FACES (Flexible array Along Cascadia Experiment for Segmentation) networks (Figure 2.42). Data was available over the time period of Oct 2007-Sept 2008 for FAME and Nov 2007-July 2009 for FACES. Events of magnitude greater than 6.3, occurring within the given time period and and in the epicentral distance range 85 to 130 were selected. Fifty suitable events were identified from which SKS and SKKS phases were analyzed. Calculations of shear wave splitting were performed using SplitLab (Wüstfeld et al., 2007). Splitting measurements at each station where then stacked following a quality assessment.
A stacked result was obtained at 63 stations (Figure 2.42). The splitting pattern is highly uniform throughout Cascadia with a mean fast direction of N67E. This direction is normal to the trench and is comparable to the absolute motion of North American plate, the absolute motion of the Gorda-Juan de Fuca (G-JdF) plate (as shown by motion vectors on Figure 2.42), and also the subduction direction. The average delay time is 1.25 seconds.
On the west coast at the latitude of the MTJ, the observed fast direction dramatically rotates by almost 90 degrees. The mean splitting direction for stations south of the MTJ is N71W with an average splitting time of 1.48 seconds. There is also a gradual rotation from the NW-SE orientation immediately south of the triple junction back to a NE-SW orientation at stations to the east across the southern half of the FAME network (Figure 2.42).
Figure 2.43 shows our results, previous splitting observations in the region (Wang et al., 2008; West et al., 2009; Zandt and Humphreys, 2008 and references therein), and a vertical average (100-400 km) of tomographically imaged upper mantle velocities from the DNA09 P-wave model. The rotation of the splits south of the triple junction corresponds to the low velocity region that wraps around the southern end of the slab.
After considering the multiple possible source regions of anisotropy in a subduction zone, the sub-slab mantle is considered to be the most likely candidate. It is the only region that is large enough (200km) to produce the size of the observed delay times and is capable of generating the consistent splitting orientation throughout the subduction zone. As the fast direction is parallel to subduction of the G-JdF plate, this is consistent with entrained mantle flow beneath the slab as the source of the anisotropy.
The rotation of splitting south of the MTJ (Figure 2.43) suggests that the anisotropy is due to flow around the southern edge of the slab. During rollback, as the trench migrates in the direction of the oceanic plate, mantle material from below is forced around the edge of the slab into the mantle wedge which is under lower pressure. Such a mechanism has been inferred to account for trench-parallel splitting underneath subducting slabs, as the mantle tries to move around the slab which is undergoing rollback (Long and Silver, 2008). This study has produced evidence for flow around the slab edge from rollback but without trench parallel flow beneath the subducting plate. This is unique to Cascadia where the effect of rollback on the mantle flow field only appears to be at the slab edge.
Fig.2.43 also shows the regional pattern of splitting for the entire western US. The large scale circular pattern centered upon Nevada has been previously modeled as toroidal flow around the G-JdF slab (Zandt and Humphreys, 2008). More recently West et al. (2009) have interpreted a high velocity anomaly beneath central Nevada (Fig.2.43) as a lithospheric drip that could also explain the same circular pattern of splits. In our study we interpret the anisotropy as flow around the southern edge of the Gorda slab but on a smaller scale than previously proposed by Zandt and Humphreys (2008). The improved level of detail provided by our results allows us to distinguish that flow around the slab edge is a separate feature from anisotropy associated with the Nevada anomaly.
This work was funded by NSF awards EAR-0643392, EAR-0745934 and EAR-0643077. We extend our thanks to Andreas Wüstfeld for providing guidance concerning SplitLab, and to Gene Humphreys, Maureen Long, John West and George Zandt for sharing their shear wave splitting data sets.
Long, M.D., and P.G. Silver, The Subduction Zone Flow Field from Seismic Anisotropy: A Global View, Science, 319, 315-318, 2008.
Wang, X., J.F. Ni, R. Aster, E. Sandvol, D. Wilson, C. Sine, S.P. Grand, and W.S. Baldridge, Shear-wave splitting and mantle flow beneath the Colorado Plateau and its boundary with the Great Basin, Bull. Seis. Soc. Am., 98, 2526-2532, 2008.
West, J.D., M.J. Fouch, J.B. Roth and L.T. Elkins-Tanton, Vertical mantle flow associated with a lithospheric drip beneath the Great Basin, Nature Geoscience, 2, 439-444, 2009.
Wüstfeld, A., G. Bokelmann, C. Zaroli, and G. Barroul, SplitLab: A shear-wave splitting environment in Matlab, Computers and Geosciences, 34, 515-528, 2007.
Zandt, G. and E. Humphreys, Toroidal mantle flow through the western US slab window, Geology, 36, 295-298, 2008.
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