The dynamic elastic properties of anisotropic metamorphic rocks collected near the Alpine Fault, New Zealand

We measured the dynamic elastic properties, densities, porosity, and mineral modes on a suite of anisotropic metamorphic rocks collected at outcrops extending outwards from the Alpine Fault, S. Island, New Zealand. The samples displayed increasing levels of deformation dependent on proximity to the fault and progressively characterized as proto-mylonite, mylontie, and ultramylontie. On the basis of thin section analysis, we presumed the materials were transversely isotropic and as such required a minimum of five measurements of wave speed in differing directions in order to allow the full set of elastic stiffnesses to be determined. The 10 cm dimension hand samples were machined into trapezoidal prisms to allow for the measurement of P- and S-wave speeds in directions perpendicular, parallel and oblique to the foliation plane. Ultrasonic wave forms were obtained to hydrostatic confining pressures of 200 MPa. One difficulty in the determination of dynamic elastic properties of anisotropic rocks is the measurement of 45^o P-wave phase velocity that suffers from effects of beam skew, diffraction, and confining pressure. We modified the experimental configuration improving the accuracy of the measured dynamic elastic properties, especially the anisotropic Poisson's ratios and the bulk modulus. With analysis of the experimental results, we concluded: (1) the anisotropy of the samples is caused by the preferred orientation of micas and micro-fractures; (2) the decrease in the fractional anisotropy of P-wave better indicates the closing fracture compared with the increase in P-wave velocity; (3) care must be taken when try to interpret the lithology basing on the ratio of P-wave group velocity over fast S-wave group velocity because the ratio changes significantly with the direction of observation and the confining pressure.