Seismic anisotropy in the upper mantle 2. Predictions for current plate boundary flow models

Abstract

The anisotropic seismic structure due to flow‐induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary. The orientation distributions of olivine:enstatite aggregates evolve along streamlines of each flow model, based on each grains plastic response to the local stress/strain field. The effective elastic tensor for these textured aggregates provides predictions of P wave anisotropy and shear wave splitting throughout the model space. P and S travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model. Comparison of predicted fast P direction for our method versus a finite‐strain based estimate shows that agreement is acceptable for much of the model space, but notable differences occur in regions up to several tens of kilometers in size. Two‐dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle. The effect of flow in the third dimension is addressed in a few calculations. For one comparison of flow in the mantle wedge at a subduction zone, the introduction of trench parallel flow causes significant changes in the predicted patterns of P wave anisotropy (magnitude, more than orientation) and SKS splitting.