Measurements of wave velocity and electrical conductivity of an amphibolite from southwestern margin of the Tarim Basin at pressures to 1.0 GPa and temperatures to 700 °C: comparison with field observations

Abstract

SUMMARY In situ measurements of elastic wave velocities and electrical conductivities in the three structural directions (normal to foliation Z, perpendicular to lineation in foliation Y and parallel to lineation X) for an amphibolite collected from southwestern margin of the Tarim Basin, northwest China, were carried out in the laboratory. The elastic wave velocity was measured with thecombinedtransmission–reflectionmethodatpressuresupto1.0GPa(atroomtemperature) and temperatures up to 700 ◦ C (at 1.0 GPa) and the electrical conductivity was measured with the impedance spectroscopy from 250 to 700 ◦ C at 1.0 GPa. The experimentally determined data included compressional (Vp) and shear wave velocities (Vs), velocity anisotropy (Av), intrinsic pressure and temperature derivatives of Vp and Vs, electrical conductivity (σ), electrical conductivity anisotropy (Aσ) and the parameters of the Arrhenius relationship. Elastic wave velocities increase in the structural directions Z, Y, X, with Vp of 6.63, 6.78 and 6.95 km s −1 and Vs of 3.75, 3.82 and 3.96 km s −1 for Z, Y and X, respectively, at pressure of 1.0 GPa. Elastic wave velocities increase linearly with pressure at room temperature and pressures between 0.25 and 1.0 GPa and decrease linearly with increasing temperature at 1.0 GPa. The pressure coefficients of the sample are in the range of 0.1883–0.2308 km s −1 GPa −1 for Vp and 0.1149–0.1678 km s −1 GPa −1 for Vs. The temperature coefficients are in the range of 2.09–2.35 × 10 −4 km s −1 GPa −1 for Vp and 1.28–1.68 × 10 −4 km s −1 GPa −1 for Vs .T he electrical conductivity increases with increasing temperature, consistent with the Arrhenius relationship. Activation energies for the three structural directions of the amphibolite are in the range of 0.71–0.75 eV. The amphibolite shows velocity anisotropy (4.15–4.86 per cent for Vp and 5.29–5.84 per cent for Vs at 0.25–1.0 GPa) and electrical conductivity anisotropy (11.1–25.2 per cent). Based on the regional crust model and geothermal gradient, velocity and electrical conductivity-depth profiles were calculated for the sample. These profiles were then compared with those derived from seismic reflection/refraction data and from electromagnetic data. Our results showed that the amphibolite sample has Vp and Vs in agreement with those of the middle and lower crust obtained from seismic reflection/refraction data, and σ in accord with that of the lower crust deduced from electromagnetic data. The lower crust of the electromagnetic crust model is roughly equivalent to the middle and lower crust layers of the seismic crust model. Therefore, it is suggest that the amphibolite may be one of the constituents of the present middle and lower crust in the Tarim Basin.