[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,

[1] The Chinese Continental Drilling Project (CCSD) has drilled to a depth of 5100 m at Maobei (N34.40, E118.67), Donghai County, Jiangsu Province in the eastern segment of the Dabie-Sulu ultrahigh pressure (UHP) metamorphic terrane. The borehole, which penetrated through all of the high velocity layers and seismic reflectors observed within the uppermost crust on seismic refraction and reflection profiles, reveals the main lithologies to be coesite-bearing felsic gneisses, metabasic rocks (i.e., amphibolite, retrogressed, and non-retrogressed eclogites) and ultramafic rocks (i.e., garnet peridotite and serpentinite). P wave velocities, anisotropy, and hysteresis of 31 typical CCSD core samples and 35 representative surface samples collected from the Sulu UHP belt were measured at hydrostatic confining pressures up to 800 MPa. The velocity-pressure curves can be well described by a four-parameter exponential equation derived from theory: V(P) = V 0 + DP À B 0 exp(ÀkP), where V 0 is the projected velocity at zero pressure if pores/cracks were absent; D is the intrinsic pressure derivative of velocity in the linear elastic regime; B 0 is the initial velocity drop caused by the presence of pores/cracks at zero pressure; and k is the decay constant of the velocity drop in the nonlinear poro-elastic regime. The seismic hysteresis is caused by irreversible changes in grain contact, increases in microcrack aspect ratios and reduction of void space during the pressurizationdepressurization cycle. The statistical properties of P wave velocities in the UHP rocks provide an important set of basic information for the interpretation of field seismic data from the root zones of continental convergent orogenic belts and modern and ancient subduction zones.

A large portion of the middle to lower crust beneath the continents and oceanic island arcs consists of amphibolites dominated by hornblende and plagioclase. We have measured P and S wave velocities (Vp and Vs) and anisotropy of 17 amphibole‐rich rock samples containing 34–80 vol % amphibole at hydrostatic pressures (P) up to 650 MPa. Combined petrophysical and geochemical analyses provide a new calibration for mean density, average major element contents, mean Vp‐P and Vs‐P coefficients, intrinsic Vp and Vs anisotropy, Poisson's ratios, the logarithmic ratio Rs/p, and elastic moduli of amphibole‐rich rocks. The Vp values decrease with increasing SiO2 and Na2O + K2O contents but increase with increasing MgO and CaO contents. The maximum (≤0.38–0.40 km/s) and minimum S wave birefringence values occur generally in the propagation direction parallel to Y and normal to foliation, respectively. Amphibole plays a critical role in the formation of seismic anisotropy, whereas the presence of plagioclase, quartz, pyroxene, and garnet diminishes the anisotropy induced by amphibole crystallographic preferred orientations (CPOs). The CPO variations cause different anisotropy patterns illustrated in the Flinn diagram of Vp(X)/Vp(Y)‐Vp(Y)/Vp(Z) plots. The results make it possible to distinguish, in terms of seismic properties, the amphibolites from other categories of lithology such as granite‐granodiorite, diorite, gabbro‐diabase, felsic gneiss, mafic gneiss, eclogite, and peridotite within the Earth's crust. Hence, amphibole, aligned by dislocation creep, anisotropic growth, or rigid‐body rotation, is the most important contributor to the seismic anisotropy of the deep crust beneath the continents and oceanic island arcs, which contains rather little phyllosilicates such as mica or chlorite.

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We calibrated the magnitude and symmetry of seismic anisotropy for 132 mica‐ or amphibole‐bearing metamorphic rocks to constrain their departures from transverse isotropy (TI) which is usually assumed in the interpretation of seismic data. The average bulk Vp anisotropy at 600 MPa for the chlorite schists, mica schists, phyllites, sillimanite‐mica schists, and amphibole schists examined is 12.0%, 12.8%, 12.8%, 17.0%, and 12.9%, respectively. Most of the schists show Vp anisotropy in the foliation plane which averages 2.4% for phyllites, 3.3% for mica schists, 4.1% for chlorite schists, 6.8% for sillimanite‐mica schists, and 5.2% for amphibole schists. This departure from TI is due to the presence of amphibole, sillimanite, and quartz. Amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation, forming orthorhombic anisotropy with Vp(X) > Vp(Y) > Vp(Z). Effects of quartz are complicated, depending on its volume fraction and prevailing slip system. Most of the mica‐ or amphibole‐bearing schists and mylonites are approximately transversely isotropic in terms of S wave velocities and splitting although their P wave properties may display orthorhombic symmetry. The results provide insight for the interpretation of seismic data from the southeast Tibetan Plateau. The N‐S to NW‐SE polarized crustal anisotropy in the Sibumasu and Indochina blocks is caused by subvertically foliated mica‐ and amphibole‐bearing rocks deformed by predominantly compressional folding and subordinate strike‐slip shear. These blocks have been rotated clockwise 70–90° around the east Himalayan Syntaxis, without finite eastward or southeastward extrusion, in responding to progressive indentation of India into Asia.

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[1] The hydrostatic pressure (P) dependence of dynamic Poisson's ratios (u) has been investigated for 54 samples of the crystalline rocks from the Sulu-Dabie orogenic belt (China) using pulse transmission techniques. The Poisson's ratio of each sample was calculated from its mean P and S wave velocities from three orthogonal directions corresponding to the tectonic framework (X-Y-Z) defined by foliation and lineation. The experimental results display two typical categories of u -P relationship in the range of 40-800 MPa: (1) with increasing pressure, u increases rapidly below $200 MPa and then becomes quasi-constant at higher pressures, and (2) u shows little variation with P. Types 1 and 2 are observed in 32 and 22 samples, respectively. The origin of type 1 can be reasonably interpreted by a small volume fraction (0.1-0.5%) of randomly distributed and randomly oriented thin disk-shaped microcracks that are progressively closed during pressurization. Type 2 is originated from the combined effects of microcrack orientation, crystallographic preferred orientations, and compositional layering. The present study confirms that the crystalline rocks at pressures above $200 MPa show no significant changes in Poisson's ratio with increasing pressure. Below 200 MPa, however, both modal composition and confining pressure play a critical role in influencing the Poisson's ratio.Citation: Wang, Q., and S. Ji (2009), Poisson's ratios of crystalline rocks as a function of hydrostatic confining pressure, J. Geophys.