Small-Scale Anisotropic Structure in the Lowermost Mantle Beneath the North American Continent and Northeastern Pacific
报告摘要：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 129 pairs exhibiting discrepant splitting results and 262 pairs non-discrepant splitting results; and 2) group splitting analysis of SKS (SKKS) phases in a common station, in which we observe 116 2ox 2o grids with consistent splitting parameters, 11 grids with inconsistent splitting parameters, and 163 grids with abrupt changes from splitting to NULL within 10–100 km. The seismic data from both analyses indicate that small-scale variation of D” anisotropy is widespread beneath the studied regions, with a lateral scale up to 10–100 km for the group splitting analysis. For portion of the data recorded at the stations of simply upper mantle anisotropy, we correct for the effects of upper mantle anisotropy and obtain the splitting parameters of D” anisotropy. The inferred D” anisotropy occurs in localized regions with splitting time delays of 0.8–1.7 s and lateral transition from no anisotropy within a very small scale (10–100 km). It also exhibits a changing geographic pattern with the fast splitting directions beneath the west coast and northwest of the United States approximately parallel with the western coast, and those beneath the northeastern Pacific near the coast of Canada and Alaska generally parallel or within an angle of 30o to the orientation of the coast of Canada and Alaska. Our results could be explained by either the shape preferred orientation caused by the localized partial melt pockets or the lattice preferred orientation of anisotropy minerals induced by small-scale mantle flow to a length scale of 10–100 km in the lowermost mantle.