Analysis of Seismic Anisotropy of the Typical Slate from the Gaoligong Mountains, Yunnan Province, China

Abstract

Slate, as one of the low-degree metamorphic rocks, is widely distributed in China. Research on the seismic velocity of slate can help distinguish this kind of transitional rocks as well as understand anisotropy of the upper crust. This paper presents laboratory research on the seismic velocity of slate samples collected from the Bingzhongluo district of Yunnan Province, and part of the experiment was conducted at the Dalhousie High Pressure Laboratory, Canada. The laboratory testing yielded the seismic velocities in different structural directions as a function of pressures. The seismic velocities in the three directions (X, Y and Z) are 6.58, 6.46 and 5.91 km/s at the confining pressure of 600 MPa, respectively, with an average velocity of 6.30 km/s. S wave has an average velocity of 3.62 mm/s and a VP/VS ratio of 1.74. The preliminary analysis shows the changing law of seismic velocities and shear wave splitting of slate, revealing that the seismic anisotropy of slate decreases significantly with pressurization at low confining pressures (<150 MPa). This is primarily attributed to the orientation alignment of microcracks within slate. With progressive increase of confining pressure (>150 MPa), all the microcracks are largely closed. Thus, the orientation alignment of flaky minerals such as biotite and actinolite is a leading factor for seismic anisotropy. With the pressure up to 600 MPa, the anisotropy of VP and VS stabilizes at 13% and 16%. Therefore, laboratory data and seismic property of the slate documented in this paper will provide basis for determination of preferential orientations of microcracks in the upper crust, anisotropy analysis in the shallow crust, and geophysical model constraints.

Radial anisotropy and S-wave velocity depict the internal to external zones transition within the Variscan orogen (NW Iberia)

“…In crystalline low porosity rocks, the complete closure of microcracks at grain boundaries is observed in laboratory experiments at about ~200 MPa, which is equivalent to depths around 8 km (e.g. Christensen, 1985;Guo et al, 2014). In the case of slates from the WALZ, recent laboratory data have shown that the magnitude of the anisotropy increases with the decrease of confining pressure, as the orientation of the microcracks is strongly controlled by the shape fabric in very well foliated slates (Cárdenes et al, 2021).…”