In many regions of the mantle, analyzing the anisotropy of seismic velocities can give us another type of constraint on mantle dynamics. Nearly all the constituent minerals of the mantle have strongly anisotropic elastic properties on the microscopic scale. Random orientations of these crystals, though, tend to cancel out this anisotropy on the macroscopic scale observable by seismic waves, unless crystals or materials with strongly contrasting elastic properties are aligned through deformation processes. While in the relatively cold regions of the lithosphere these anisotropic signatures can remain frozen in over geologic time-scales (Silver, 1996), observed anisotropy at greater depths likely requires dynamic support (Vinnik et al., 1992). Thus, the anisotropy observed at sub-lithospheric depths is most likely a function of the current mantle strain field, and these observations can help us map out mantle flow.
The common features of S tomographic models are present in the isotropic model. The uppermost 200 km is dominated by tectonic features, with fast continents and slower oceans that show an age-dependent increase in velocity away from the slow velocities near ridges. Regions of active tectonic processes are, in general, slower, such as western North America, the major circum-Pacific subduction zones, and the East African rifting. In the transition zone depth range, the most prominent features are the fast velocities of subducted slabs, while the slow ridges are no longer present. Mid-mantle velocity anomalies are low in amplitude, and more white in spectrum. Finally, in the lowermost 500 km, the amplitudes of heterogeneity increase again, and become dominated by a degree 2 pattern with rings of higher velocities surrounding two lower velocity regions under the central Pacific and Africa, commonly referred to as superplumes.
In the model of the upper mantle (Figure 28.1), we confirm observations of regions with positive anomalies () starting at 80 km under oceanic regions and 250 km under old continental lithosphere, suggesting horizontal flow beneath the lithosphere (Gung et al., 2003). We also observe a signature at 200-300 km depth beneath major ridge systems with amplitude correlated with spreading rate. In the transition zone (400-700 km depth), regions of subducted slab material are associated with negative anomalies () (Figure 28.1), while the ridge signal decreases except under the East Pacific Rise.
We also confirm the observation of strong radially symmetric in the lowermost 300 km (Figure 28.2) (Panning and Romanowicz, 2004). The 3D deviations from this degree 0 signature are associated with the transition to the large-scale superplumes under the central Pacific and Africa, suggesting that is generated in the predominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes.
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