MODELS OF SEISMIC ANISOTROPY IN THE DEEP CONTINENTAL LITHOSPHERE

Seismological observations (SKS-wave polarizations, systematic P-resid ual variations, azimuthal dependence of P(n)- and surface-wave velocit ies or a dispersion of surface waves) are not consistent with isotropi c, if laterally heterogeneous, upper-mantle structure. Therefore, an a nisotropy should be considered as an a priori aspect of future large-s cale studies of mantle structure. Most studies of anisotropy, however, have assumed horizontal or vertical axes of symmetry, but such orient ations cannot explain bipolar patterns of spatial variations of P resi duals, which we have observed at many seismological stations. On the b asis of the petrophysical properties of real upper-mantle rocks we con sider anisotropy formed either by hexagonal or by orthorhombic aggrega tes composed of olivine, orthopyroxene, and clinopyroxene. Rotations o f the aggregates about vertical and horizontal axes allow us to find t he three-dimensional orientations of symmetry axes that fit combinatio ns of both P and S seismological observations in Central Europe an in western North America. The orientations with plunging symmetry axes (v elocity extremes) seem to be consistent across large, spatially unifor m tectonic units and change abruptly at important suture zones.