Anisotropy of Vp and Vs in an amphibolite of the deeper crust and its relationship to the mineralogical, microstructural and textural characteristics of the rock

Abstract

Laboratory seismic measurements of compressional (Vp)- and shear-wave (Vs) velocities have been carried out on a deformed amphibolite from the Ivrea Zone at pressures and temperatures up to 600 MPa and 600°C, respectively. Amphibolite is considered to be an important rock of the deeper crust. From crystallographic orientations of the component minerals (hornblende, plagioclase and diopside) and the respective elastic constants, we calculated the average elastic constants (Voigt average) for the rock specimen. Compressional- and shear-wave velocities in three orthogonal directions (normal and parallel to foliation) measured at high confining pressure (600 MPa) are in good agreement with the calculated velocity surface. This suggests that seismic-wave velocity anisotropy is controlled by preferred orientations of minerals in rocks (texture) at high confining pressure, where the effect of microcracks on seismic wave velocities is eliminated. In the amphibolite sample, the velocity anisotropy is dominated by the texture of hornblende because of the strong preferred orientation and large volume fraction of this mineral species. A large S-wave splitting is documented in the directions parallel to the foliation (particularly in the lineation) which strongly argues against the microcrack origin of S-wave splitting, at least at conditions of high effective pressure. © 1989.