[1] Antigorite, the high-temperature (HT) form of serpentinite, is believed to play a critical role in various geological processes of subduction zones. We have measured P-and Swave velocities (V p and V s ), anisotropy and shear-wave splitting of 17 serpentinite samples containing >90% antigorite at pressures up to 650 MPa. The new results, combined with data for low-temperature (LT) lizardite and/or chrysolite, reveal distinct effects of LT and HT serpentinization on the seismic properties of mantle rocks. At 600 MPa, V p = 5.10 and 6.68 km/s, V s = 2.32 and 3.67 km/s, and V p /V s = 2.15 and 1.81 for pure LT and HT serpentinites, respectively. Above the crack-closure pressure (~150 MPa), the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature. Serpentine contents within subduction zones and forearc mantle wedges where temperature is >300 C should be at least twice that of previous estimates based on LT serpentinization. The presence of seismic anisotropy, high-pressure fluids, or partial melt is also needed to interpret HT serpentinized mantle with V p < 6.68 km/s, V s < 3.67 km/s, and V p /V s > 1.81. The intrinsic anisotropy of the serpentinites (3.8-16.9% with an average value of 10.5% for V p , and 3.6-18.3% with an average value of 10.4% for V s ) is caused by dislocation creepinduced lattice-preferred orientation of antigorite. Three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics (S-, L-, and LS-tectonites) formed by three categories of strain geometry (i.e., coaxial flattening, coaxial constriction, and simple shear), respectively. Our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide.Citation: Ji, S., A. Li, Q. Wang, C. Long, H. Wang, D. Marcotte, and M. Salisbury (2013), Seismic velocities, anisotropy, and shear-wave splitting of antigorite serpentinites and tectonic implications for subduction zones,

Current stress state data are important in underground mine construction, analysis the of tunnel stability, the forecasting of coal and gas rock bursts in underground mines. At present, the hollow inclusion gauge is primarily used for in situ stress measurement in underground mines. However, the sensors of these gauges often fail to fully adhere to the borehole wall, which can reduce the reliability of the measurement results. In this study, the anelastic strain recovery (ASR) in situ stress measurement method based on oriented cores is introduced. The effectiveness of this technique is analyzed by comparing the test results with those obtained by the hydraulic fracturing method. In addition, the reliability of the ASR method is evaluated by repeatability testing. The ASR results indicate a horizontal minimum principal stress error of less than 10% compared with those obtained by the hydraulic fracturing method. The repeatability test results show good consistency in the in situ stress measurement results of two back-to-back test samples. The ASR method is a safe and highly efficient technique that is not limited by the depth and measurement environment. Therefore, it is expected to have broad application for underground mine in situ stress measurement.

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.

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The in situ stress state is a key parameter for shale gas exploration and geodynamics. To obtain the ultra-depth stress state of the Xuefengshan area, the hydraulic fracturing in situ stress method was employed in this study to determine the stress state at a depth range of 170–2021 m. The test results, which are the first reported for borehole depth greater than 2000 m in China, show that the magnitude of the in situ stress increases with the depth of the borehole. At a depth of 2021 m, the measured maximum and minimum horizontal principal stresses were 66.31 MPa and 43.33 MPa, respectively. The linear fitting relationships between the maximum and minimum horizontal principal stresses with the depth of the test borehole were SH = 0.033999D + 5.9996 and Sh = 0.020729D + 4.8058, respectively. In the borehole depth range of 170–800 m, the relationships of the three principal stresses were SH > Sh > SV, which is favorable for reverse faulting. At borehole depths of 1000–2021 m, the relationships changed to SH > SV > Sh, which implies that the deep stress regime of this area is strike-slip faulting.