Azimuthally anisotropic model of the oceanic upper mantle

Abstract

A mixed form of forward and backward modelings of azimuthally anisotropic oceanic upper mantle is attempted for a data set of surface-wave velocities of periods 30–100 s. One of the basic configurations of the anisotropy postulated in the LID and LVZ is axial symmetry of elastic properties with a horizontal axis parallel to the spreading direction as found in ultramafic rocks from ophiolite complexes of the Bay of Islands and Oman. Basic features of the resulting model are a thin lithosphere of ∼45 km, a decrease of S-wave velocities, and an increase of P-wave velocities and of partial melting/dislocation relaxation with depth in the LVZ. This single model explains the overall features of anisotropy observations in the Pacific ocean: P n velocity anisotropy, SH-SV anisotropy and azimuthal anisotropy of surface waves and ScS polarization anisotropy. This fact leads us to a conclusion that the ultramafic rocks from the ophiolite complexes can explain the seismic properties of the oceanic LID and LVZ.