Pressure and temperature dependence of P- and S-wave velocities, seismic anisotropy and density of sheared rocks from the Sierra Alpujata massif (Ronda peridotites, Southern Spain)

Abstract

Seismic wave velocities and densities, and their pressure and temperature dependence, of rock samples recovered from surface exposures at the Sierra Alpujata were determined in order to better understand the petrological controls on velocity and density. The rock samples studied are representative of the major lithologies of the Ronda ultramafic body and its metamorphic aureole. The suite of ultramafic rocks includes dunite, harzburgite and lherzolite exhibiting different degrees of deformation and serpentinization. The rocks representing the metamorphic aureole comprise coarse-grained migmatites to ultramylonitic migmatites, gneisses and eclogites. The experimental data include measurements of elastic wave velocities ( V P, V S) and densities at confining pressures up to 600 MPa and temperatures up 600°C (at 600 MPa), the determination of the pressure and temperature derivatives of velocities and densities, and the determination of velocity anisotropy. Average P- and S-wave velocities (at 600 MPa) for rocks of the ultramafic body vary from 6.47 to 8.22 km s −1 and 3.27 to 4.75 km s −1, respectively, mainly due to different degrees of serpentinization. Maximum V P and V S anisotropy is about 8.0% and 9.5%, respectively, and is primarily related to the preferred orientation of olivine. In rocks of the metamorphic aureole, average P-velocities are in the range 5.61–7.23 km s −1 and S-velocities vary from 3.48 to 4.04 km s −1. P- and S-velocity anisotropy is highest (about 6.4% and 4.6%) in sheared rocks (mylonites) and is mainly due to the preferred orientation of biotite and sillimanite. The geophysical significance of the experimental data is briefly discussed.