Depth Dependent Azimuthal Anisotropy in the Western US Upper Mantle

Huaiyu Yuan and Barbara Romanowicz


We present the results of a joint inversion of long period seismic waveforms and SKS splitting measurements for 3D lateral variations of anisotropy in the upper mantle beneath the western US, incorporating recent datasets generated by the USArray deployment as well as other temporary stations in the region (Yuan and Romanowicz, 2010). We find that shallow azimuthal anisotropy closely reflects plate motion generated shear in the asthenosphere in the shallow upper mantle (70-150 km depth), whereas at depths greater than 150 km, it is dominated by northward and upward flow associated with the extension of the East-Pacific Rise under the continent, constrained to the east by the western edge of the North-American craton, and to the north by the presence of the east-west trending subduction zone. In particular, the depth integrated effects of this anisotropy explain the apparent circular pattern of SKS splitting measurements observed in Nevada without the need to invoke any local anomalous structures.

Figure 2.25: Azimuthal anisotropy variations with depth. Black bars indicate the fast axis direction and the bar length is proportional to the anisotropy strength. Blue, green and red arrows show the absolute plate motion (APM) directions of the North American, Juan de Fuca, and Pacific plates respectively, computed at each location using the HS3-NUVEL 1A model (Gripp and Gordon, 2002).
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Circular pattern of the SKS splitting: Depth Integrated Effects

The recent deployment of the Transportable Array (TA) of EarthScope, as well as several other temporary broadband networks in the western US, have provided the opportunity to measure SKS splitting at a significantly larger number of locations in the region than was previously possible. Combined with previously available SKS splitting data, these measurements have revealed an intriguing apparent ``circular'' pattern in the distribution of fast axis directions and amplitude of anisotropy, centered in south-central Nevada, with vanishing strength in the center of the pattern (Savage and Sheehan, 2000; Liu, 2009; West et al., 2009). Interestingly, some recent regional body wave tomographic studies also show the presence of a fast velocity anomaly extending into the transition zone, beneath the Cascades and High Lava Plains (e.g. van der Lee and Nolet, 1997; Xue and Allen, 2007; Sigloch et al., 2008; Burdick et al., 2009). Various geodynamic models have been proposed to address the mantle flow associated with these features, including: 1) initial impinging of the Yellowstone Plume into the lithosphere in the Basin and Range province (Savage and Sheehan, 2000; Walker et al., 2004); 2) toroidal flow around the southern edge of the sinking Gorda-Juan de Fuca plate, associated with its retreating and the creation of a slab (Zandt and Humphreys, 2008); and 3) asthenospheric flow associated with a sinking lithospheric instability (or ``drip'') in the center of the Basin and Range (West et al., 2009).

Our 3-D azimuthal anisotropy model (Figure 2.25; Yuan and Romanowicz, 2010) shows a strongly depth dependent azimuthal anisotropy pattern in the western US, with orientation of the fast axis controlled by plate motion related lithosphere-asthenosphere coupling at depths shallower than 150 km, and other processes at greater depths, likely representing the channeling of deep flow from the East Pacific Rise constrained by the presence of the craton margin to the west and subducted slabs to the north. We infer that all of these features combined significantly contribute to the circular pattern and large splitting times of the SKS splitting observations. The strong lateral and vertical variations throughout the western US revealed by our azimuthal anisotropy model reflect complex past and present tectonic processes. In particular, the depth integrated effects of this anisotropy (Figure 2.26) explain the apparent circular pattern of SKS splitting measurements observed in Nevada without the need to invoke any local anomalous structures (e.g. ascending plumes or sinking lithospheric instabilities; Savage and Sheehan, 2000; West et al., 2009): the circular pattern results from the depth-integrated effects of the lithosphere-asthenosphere coupling to the North American, Pacific and Juan de Fuca plates at shallow depths, and in the depth range 200-400 km, northward flow from the East Pacific Rise channeled along the craton edge and deflected by the Juan de Fuca slab, and, more generally, slab-related anisotropy.

Figure 2.26: Comparison of observed and predicted station averaged SKS splitting direction and time. Red bars indicate observations and are shown in the left panels only, for clarity. Black bars indicate the model predictions. Predicted splitting is shown for integration of the models over, (a) the full depth range of the azimuthal anisotropy models, (b) the top 150 km of the models, and (c) the portion of the model between 150 and 500 km, respectively.
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