Any petrologic model of the lower oceanic crust must be consistent with three sets of data: (1) the seismic structure of the oceanic crust, (2) the petrology of oceanic dredge samples, and (3) laboratory measurements of seismic velocity through such samples. A review of these data indicates that within the framework of earlier three‐layer models of oceanic seismic structure the crust is internally complex and varies markedly with age, azimuth, and tectonic province. Mantle compressional wave velocities Vp are anomalously low under the ridge (7.2–7.7 km/s) but increase to 8.0–8.3 km/s beyond 15 m.y.; layer 3 thickens by 2 km within 40 m.y. of formation and decreases in Vp from 6.8 to 6.5 km/s within 80 m.y.; both the mantle and layer 3 are statistically anisotropic. Dredge lithologies consist predominantly of serpentinized ultramafics and mafic igneous rocks ranging from basalt to gabbro, the gabbro often showing evidence of fractionation. Metamorphism of mafic rocks from zeolite to amphibolite facies grade is common. Velocities in oceanic serpentinites and basalts are generally lower than layer 3 refraction velocities. Unaltered gabbros have compressional wave velocities of approximately 7.0 km/s, which is high for layer 3, together with shear wave velocities Vs of 3.8 km/s and values of Poisson's ratio σ of 0.30. Metabasites containing hornblende and plagioclase have values of Vp = 6.8 km/s, Vs = 3.8 km/s, and σ = 0.28, in good agreement with those of layer 3. On the basis of petrology and velocity it is suggested that layer 3 is composed of hornblende metagabbro underlain by normal gabbro. In a model consistent with geophysical observations of heat flow, seismicity, gravity, and seismic structure at the ridge it is proposed that layer 2 and the upper levels of layer 3 form near the median valley but that deeper levels of layer 3 thicken for 40 m.y. by intermittent offridge intrusion fed from the underlying anomalous mantle. Ophiolites in such a model represent segments of thin immature ridge crest obducted onto continental margins during subduction of a spreading ridge.

Mineralogical, chemical, and isotopic results from seven drilling legs that visited DSDP/ODP Hole 504B over 14 years are compiled here to present an integrated view of hydrothermal alteration of oceanic crust at Site 504. Hole 504B reaches to 2111 mbsf, through 274.5 m sediment, 571.5 m of volcanic rocks, a 209 m transition zone, and 1050 m into a sheeted dike complex. The volcanic section was altered through a series of processes involving interaction with seawater at low temperatures, with the effects of cold, oxidizing seawater decreasing downward. These processes and their effects on the volcanic section are generally similar to those in other oceanic upper crustal sections.The transition zone and upper dikes were altered in a subsurface mixing zone, where hydrothermal fluids upwelling through the dikes mixed with cooler seawater circulating in the overlying more permeable volcanic rocks. Alteration of the transition zone and upper dikes (down to 1500 mbsf) occurred in a series of stages, reflecting the thermal and chemical evolution of the hydrothermal system from (1) early chlorite, actinolite, albite-oligoclase, and titanite, to (2) quartz, epidote and sulfides, to (3) anhydrite, and finally to (4) zeolites and local calcite. The maximum temperature estimated for the first two stages is 350°-380°C, and the inferred mineral assemblages for these early stages are typical of the greenschist facies.The lower dikes (1500-2111 mbsf) underwent an early, high-temperature (>400°C) alteration stage, resulting in the formation of hornblende and calcic secondary plagioclase, consistent with reactions inferred to occur in deep subsurface reaction zones, where hydrothermal vent fluids acquire their final compositions. Much of the subsequent reactions produced greenschist assemblages at ~300°-400°C. The lower dikes have lost metals and sulfur and are a source of these elements to hydrothermal vent fluids and seafloor sulfide deposits. The lower dikes underwent subsequent alteration stages similar to the upper dikes, with rare epidote + quartz veins recording the presence of upwelling hydrothermal fluids, and limited late off-axis effects (zeolites and prehnite). Anhydrites in the lower dikes indicate more reacted fluid compositions than in the upper dikes.Alteration of the sheeted dikes from Hole 504B is heterogeneous, with recrystallization controlled by fracturing and access of fluids. Defining the position of the seismic Layer 2/3 transition depends upon the scale of observation, but the change at Site 504 occurs within the sheeted dikes and is correlated with progressive changes in porosity and hydrothermal alteration. However, we still do not know the nature of the transition from sheeted dikes to gabbros in in situ ocean crust, or the nature of the inferred fault at the base of Hole 504B and its role in fluid flow and alteration.

Although the ophiolites are widely recognized as segments of oceanic crust emplaced on land, direct correlation between the ophiolites and the oceanic crust has proven difficult owing to the near absence of common criteria on which to base a comparison; the ophiolites are defined petrologically, while the oceanic crust is defined largely in terms of seismic structure. To bridge this gap the seismic velocity structure of a traverse through the Blow‐Me‐Down massif of the Bay of Islands ophiolite complex, Newfoundland, has been reconstructed in detail from values of compressional (Vp) and shear (Vs) wave velocity measured in the laboratory under conditions of hydrostatic confining pressure and water saturation thought to approximate conditions in the oceanic crust through oriented samples collected from 60 closely spaced sites of known stratigraphic level. The velocity structure thus determined is indistinguishable from that of normal oceanic crust: The uppermost velocity unit in the massif consists of 0.5 km of prehnite‐pumpellyite facies metabasalt with Vp ≤ 5.70 and Vs ≤ 3.10 km/s, underlain by 0.8 km of greenschist facies pillow basalts and brecciated dikes with Vp ≤ 6.20 and Vs ≤ 3.35 km/s. Between 1.3 and 6.4 km, in a thick unit composed of metadolerite sheeted dikes underlain by coarse‐grained metagabbro grading downward through pyroxene and troctolitic olivine gabbro, Vp and Vs increase from 6.75 and 3.75 km/s near the top to 7.40 and 3.90 km/s near the base, respectively. This increase is gradational, except at 5.3 km, where a step increase in Vp to 7.40 km/s marks an increase in olivine content. A sharp velocity inversion in Vp, caused by quartz‐rich late differentiates, is found in the upper levels of this unit between 2.8 and 3.3 km. The deepest level of the complex, composed of ultramafics, is characterized by values of Vp and Vs of 8.4 and 4.9 km/s, respectively. A comparison of the seismic velocity structure and petrology of the traverse across the Blow‐Me‐Down massif with oceanic seismic structure suggests that at many sites in the ocean basins, (1) layer 2 consists of prehnite‐pumpellyite and greenschist facies pillow basalts and brecciated dikes, (2) the layer 2–3 boundary separates greenschist facies metabasalts and brecciated dikes at the base of layer 2 from epidote‐amphibolite facies sheeted dikes at the top of layer 3, (3) layer 3 consists of metadolerite sheeted dikes underlain by metagabbro, pyroxene gabbro, and troctolitic olivine gabbro, (4) the 7.4‐km/s basal layer observed in sonobuoy studies consists of interlayered olivine gabbro, troctolite, and plagioclase peridotite, (5) the Mohorovičić discontinuity represents a relatively sharp transition from gabbro to dunite and peridotite, and (6) pronounced, but laterally discontinuous, velocity inversions may be present at the base of layer 2 below the relatively high velocity prehnite‐pumpellyite facies metabasalt level and at intermediate levels in layer 3 in association with late differentiates.

[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.

No abstract

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.

No abstract