In many regions of the mantle, analyzing the anisotropy of seismic velocities can give us another constraint on mantle dynamics. Random orientations of the anisotropic minerals which make up the mantle tend to cancel out 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.
In the uppermost mantle, we confirm previous observations of regions with starting at 80 km under oceanic regions and 200 km under stable continental lithosphere (Gung et al., 2003), suggesting horizontal flow beneath the lithosphere (figure 33.1). We also observe a signature at 150-300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast-spreading segments. In the transition zone (400-700 km depth), regions of subducted slab material are associated with (figure 33.2), while the ridge signal decreases. We also confirm the observation of radially symmetric in the lowermost 300 km (Panning and Romanowicz, 2004). The 3D deviations from this signature (figure 33.3) are associated with the large-scale low-velocity 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.
This research was supported by NSF grant EAR-0308750.
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