The effect of plagioclase textures on velocity anisotropy and shear wave splitting at deeper crustal levels

Average elastic data for lower crustal rocks have been calculated on the basis of crystallographic plagioclase orientations from greenschist to granulite facies rocks and the respective single crystal elastic constants. The anisotropy of the P-wave velocity caused by different plagioclase textures is between 2% and 8%, whereas the highest differences in the two perpendicular polarized shear-wave velocities vary between 0.08 and 0.18 km/s. These relatively low values, compared with those of the single crystal, result from generally weak preferred crystallographic plagioclase orientations as well as from the opposing influence of the preferred orientation of different crystallographic directions. Model rocks of identical composition and density but with different types of plagioclase textures show Poisson's ratios in the range of 0.16-0.23. The impedance contrasts between these rocks are generally low. Only the contrast between, strong plagioclase texture and random orientation would produce reflection coefficients large enough to result in seismic reflection. Thus, the influence of plagioclase textures upon the seismic anisotropy of rocks with strongly oriented minerals such as amphiboles and pyroxenes should be fairly low.