Interpreting seismic tomography: Controls of microstructure and pore pressure on the elastic wave speeds of a metapelite

In subduction zones, fluids are thought to contribute to deformation, including different modes of fault slip, and metasomatism. In some active subduction zones, low P-wave (Vp) and S-wave (Vs) velocities and high Vp/Vs ratios are documented near and along the plate boundary at depths between 25-40 km, and are interpreted to reflect high pore fluid pressures that may promote fault slip. However, there is limited data that allow for quantitative comparisons between tomography and elevated pore pressure in metasedimentary rocks thought to be abundant in these locations. We measured the ultrasonic velocities of the Orocopia schist, a greenschist facies metapelite, in the laboratory to determine whether the velocities of metasedimentary rocks are consistent with low wave speeds and high Vp/Vs.Velocities were measured under dry and saturated conditions, at effective pressures between 1 and 136 MPa, parallel and normal to foliation, and at room temperature. We find that, under fluid saturated conditions, the velocities of the Orocopia schist and Vp/Vs are consistent with anomalous values in subduction zones (Vp < 6.5 km/s, Vs < 3.2 km/s, and Vp/Vs > 2) at effective pressures less than 10 MPa and normal to foliation. We combined our experiments with differential effective medium models and microscopy to quantify the effects of mineralogy, mineral fabric, porosity, and pore geometry on Vp, Vs, and their anisotropies. Our results show that, mineralogy and mineral anisotropy are important to control on the wave velocities, but are not enough to explain the in-situ velocities. We find that aligned, small aspect ratio pores (0.005 to 0.03) are necessary to obtain the measured velocities and high Vp/Vs. The low Vp and Vs and high Vp/Vs in subduction zones can be explained by phyllosilicate-rich metapelitic rocks under near-lithostatic pore pressure.