Widespread Small-Scale Anisotropic Structure in the Lowermost Mantle beneath the North American Continent and Northeastern Pacific

We constrain D″ anisotropy beneath the North American continent and northeastern Pacific using two approaches: (1) joint splitting analysis of SKS and SKKS phase pair for a common event, in which we obtain 158 pairs exhibiting discrepant splitting results and 791 pairs nondiscrepant splitting results; and (2) group splitting analysis of SKS (or SKKS) phase from neighboring events recorded at a common station, in which we observe 109 2°×2° grids with consistent splitting parameters, and 164 grids with abrupt changes from splitting to no splitting within 30–100 km. The seismic data from both analyses indicate that small-scale variations of D″ anisotropy are widespread beneath the studied regions, with a lateral scale up to tens of kilometers. For portion of the data recorded at the stations of simple upper-mantle anisotropy, we correct for the effects of upper-mantle anisotropy and obtain the splitting parameters of D″ anisotropy. The inferred D″ anisotropy exhibits a changing geographic pattern and lateral transition of anisotropy to a lateral scale of tens of kilometers. Such a length scale of changing anisotropy is also confirmed by synthetics modeling of the seismic data. We suggest that the inferred small-scale anisotropies could be best explained by the shape preferred orientation of widespread small-scale partial melt pockets derived by a composition change produced early in the Earth’s history, a similar compositional origin that was invoked to explain the African anomaly in the lower mantle.