The effect of a hydrous phase on P‐wave velocity anisotropy within a detachment shear zone in the slow‐spreading oceanic crust: A case study from the Godzilla Megamullion,Philippine Sea

We studied the contributions of plagioclase, clinopyroxene, and amphibole to the P‐wave velocity properties of gabbroic mylonites of the Godzilla Megamullion (site KH07‐02‐D18) in the Parece Vela Rift of the central Parece Vela Basin, Philippine Sea, based on their crystal‐preferred orientations (CPOs), mineral modes, and elastic constants and densities of single crystals. The gabbroic mylonites have been classified into three types based on their microstructures and temperature conditions: HT1, HT2 and medium‐temperature (MT) mylonites. The P‐wave velocity properties of the HT1 mylonite are dominantly influenced by plagioclase CPOs. Secondary amphibole occurred after deformation in the HT1 mylonite, so that its effect on P‐wave velocity anisotropy is minimal due to weak CPOs. Although the HT2 mylonite developed deformation microstructures in the three minerals, the P‐wave velocity properties of the HT2 mylonite are essentially isotropic, resulting from the destructive interference of different P‐wave velocity anisotropy patterns produced by the distinct CPOs of the three constituent minerals (i.e., plagioclase, clinopyroxene, and amphibole). The P‐wave velocity properties of the MT mylonite are influenced mainly by amphibole CPOs, whereas the effect of plagioclase CPOs on P‐wave velocity anisotropy becomes very small with a decrease in the intensity of plagioclase CPOs. As a result, the gabbroic mylonites tend to have weak P‐wave velocity anisotropy in seismic velocity, although their constituent minerals show distinct CPOs. Such weakness in the whole‐rock P‐wave velocity anisotropy could result from the destructive contributions of the different mineral CPOs with respect to the structural framework (foliation and lineation). These results show that amphibole has a high potential for P‐wave velocity anisotropy by aligning both crystallographically and dimensionally during deformation in the hydrous oceanic crust. The results also suggest that the effect of a hydrous phase on P‐wave velocity anisotropy within the detachment shear zone in a slow‐spreading oceanic crust varies depending on the degree of deformation and on the timing of hydrothermal activity.     

Subduction related antigorite CPO patterns from forearc mantle in the Sanbagawa belt,southwest Japan

Antigorite (Atg) is stable throughout large parts of the wedge mantle of most subduction zones. Atg shows strong acoustic anisotropy and crystallographic preferred orientation (CPO) patterns of this mineral may contribute significantly to seismic anisotropy in convergent margins. Atg CPO patterns from the Higashi-Akaishi (HA) forearc mantle body of southwest Japan adds to the data set suggesting the most common Atg CPO pattern has a c-axis perpendicular to the foliation and a b-axis parallel to the stretching lineation. Statistical analysis using the eigenvector method of Atg CPO from two mutually perpendicular directions in the same sample (YZ-section and XZ-section) shows no significant differences implying sample preparation has no significant affect on the resulting Atg CPO. Reuss (uniform stress) averages of anisotropy for the Higashi-Akaishi samples are approximately treble the values for Voigt (uniform strain) averages. When comparing calculated anisotropy of hydrated mantle peridotite samples—such as the Higashi-Akaishi unit—with observed S-wave delay times in convergent margins, the appropriate averaging method needs to be considered.     

Shear sense inversion in the Hilti mantle section (Oman ophiolite) and active mantle uprise

Structural analyses in the well-exposed Hilti mantle section in the Oman ophiolite suggest a model of forceful horizontal flow in the uppermost mantle at the edge of a diapir below a oceanic spreading center. Detailed structural mapping, focussed on high-T deformation (i.e., asthenospheric flow), revealed a gently undulated flat structure with a uniform east-west flow direction. When it is related to the N–S to NNW–SSE trending, vertical sheeted dike complex located to the east, this mantle flow is parallel to the spreading direction. Because the Moho is so flat lying, a large dunite occurrence at the south-western region is possibly ascribed to the Moho Transition Zone. Kinematic analysis shows that the shear direction generally changes from top-to-the west in the upper level, to top-to-the east in the lower level with respect to the Moho. This shear sense inversion is explained by a model of forceful flow due to an active mantle uprise and it is not compatible with a passive mantle uprise. In the plan section, the boundary of the shear sense inversion is subparallel to the flow direction and subperpendicular to the spreading axis. In cross section, the boundary appears to occur at various depths in the range of 200 m to 500 m. It shows that the active mantle uprise in the diapir center resulted in a channelled horizontal flow.     

Effect of grain growth on cation exchange between dunite and fluid: implications for chemical homogenization in the upper mantle

The effect of grain growth on the cation exchange between synthesized forsterite aggregates (i.e., dunite) and nickel-rich aqueous fluid was evaluated experimentally at 1.2 GPa and 1,200°C. The grain boundary (GB) migration caused nickel enrichment in the area swept by the GBs in a fashion similar to that reported for stable isotope exchange in the quartz aggregates. The progress of the grain growth resulted in an increase in the average nickel concentration in the dunites of up to ~80 times that was calculated for a system having stationary GBs. The overall diffusivity of the nickel along the wet GBs and interconnected fluid networks was found to be 6.5 × 10⁻¹⁹–6.7 × 10⁻¹⁸ m³/s, which is 4–5 orders of magnitude higher than the grain boundary diffusivity in the dry dunite. These results show that the grain growth rate is a fundamental factor in the evaluation of the time scale of chemical homogenization in the upper mantle.     

Rheological contrast between garnet and clinopyroxene in the mantle wedge: An example from Higashi-akaishi peridotite mass,SW Japan

Garnet clinopyroxenites occur within foliated dunite in the Higashi-akaishi peridotite mass, located within the subduction-type high-pressure/low-temperature Sanbagawa metamorphic belt. The garnet clinopyroxenites contain 3–80% garnet, and garnet and clinopyroxene are homogeneously distributed. Garnet crystals contain extensive, regular dislocation arrays and dislocation networks, suggesting that dislocation creep was the dominant deformation mechanism. Analyses of crystallographic orientation maps indicate similar grain sizes and aspect ratios for garnet and clinopyroxene, regardless of modal composition, indicating that these minerals deformed with similar degree of plasticity. Moreover, indexes of crystallographic fabric intensity (i.e., J-index and M-index) for both garnet and clinopyroxene tend to increase with increasing modal composition of garnet. Fourier-transform infrared spectroscopy analysis revealed that water content in garnet is ～60 ppm, whereas that in clinopyroxene is ～70 ppm. Olivine crystal-preferred orientations in the Higashi-akaishi peridotite mass, characterized by [0 0 1] (0 1 0), are thought to have developed during deformation under wet conditions. Consequently, we argue that the presence of water could act to enhance garnet plasticity during deformation. The results reveal contrasting influences of water on the deformation of garnet and diopside: under wet conditions compared with dry, the strain rate increases by two orders of magnitude for garnet but by an order of magnitude for diopside. Given the influence of water on the creep strength of garnet, garnet within the Higashi-akaishi mass may have become significantly as weak as clinopyroxene during deformation.     

Amphibolitization within the lower crust in the termination area of the Godzilla Megamullion,an oceanic core complex in the Parece Vela Basin

Gabbroic rocks and amphibolites were collected from the KR03‐01‐D10 dredge site located on the West Arm Rise of the Godzilla Megamullion, close to the Parece Vela Rift which appears to correspond to the termination area of a detachment fault, the Philippine Sea. The gabbroic rocks and amphibolites reveal the occurrence of a high hydrothermal activity in the lower crust close to a paleo‐ridge. In the gabbroic rocks, plagioclase compositions of both porphyroclasts and matrix were transformed into sodium‐rich compositions close to albite. Amphiboles are of secondary rather than igneous origin based on their microstructural occurrences. In the amphibolites, anorthite contents of porphyroclasts and matrix plagioclase are relatively lower than those of the gabbroic rocks, whereas the chemical compositions of amphibole within the amphibolites are similar to those of amphibole within the gabbroic rocks. Amphibolites represent the product of retrograde metamorphism associated with hydrothermal alteration of the gabbroic body by the reaction: clinopyroxene + calcic plagioclase + fluid → amphibole + sodic plagioclase. The estimated temperatures of the amphibolites derived from the amphibole thermobarometer and the gabbroic rocks derived from the hornblende–plagioclase geothermometer show ～700–950°C and 650–840°C, respectively. The hydrothermal alteration recorded in the gabbroic rocks possibly occurred under high‐T conditions; the rocks were then metamorphosed to the amphibolites during a retrogressive stage. Our study indicates that amphibolitization took place with various degrees of deformation. It may imply that the hydrothermal activity increased as the Godzilla Megamullion developed as an oceanic core complex in the paleo‐ridge.     

Application of scanning X-ray analytical microscope to the petrographic characterization of a ductile shear zone: an alternative method to image microstructures

A ductile shear zone within a metasomatic biotite band in the Ryoke granite, Teshima, SW Japan, has been studied using the scanning X-ray analytical microscope (SXAM). This enabled the quantitative distributions of major elements, such as Si, K, Fe, Al and Ca, to be determined within the shear zone. These element maps were processed to transform them into images showing the distribution of minerals such as quartz, biotite, plagioclase and K-feldspar, which form the major minerals within the biotite band and the granite protolith. Mineral profiles based on these mineral maps compared with the simple shear strain profile reveal that the shear zone is most intense where quartz and biotite have been substituted for the primary mineral assemblage of the granite protolith, suggesting that the stresses imposed on the granite caused the shear strain to localize along the biotite band to produce the observed shear zone. It appears that the rheological behavior changed around 50–60% of quartz modal composition.     

Propagation of seismic slip from brittle to ductile crust: Evidence from pseudotachylyte of the Woodroffe thrust, central Australia

The Woodroffe thrust, central Australia, is a > 1.5-km-wide mylonitized shear zone marked by large volumes of mm- to cm-scale pseudotachylyte veins. The pseudotachylytes display typical melt-origin features, including rounded and embayed clasts, spherulitic and dentritic microlites, and flow structures within a fine-grained matrix. Three types of pseudotachylyte are identified on the basis of deformation texture, vein morphology, and host-rock lithology: cataclasite-related (C-Pt), mylonite-related (M-Pt), and ultramylonite-related (Um-Pt). The M-Pt and Um-Pt veins intrude into mylonite and ultramylonite and are themselves overprinted by subsequent mylonitization. These pseudotachylytes contain an internal foliation defined by flattened porphyroclasts and layering of the fine-grained vein matrix, and the foliation is generally oriented parallel to foliation in the surrounding mylonite and ultramylonite. These observations constrain the timing and environment of M-Pt and Um-Pt pseudotachylyte formation to a protracted period of deformation and mylonitization within the ductile regime of the crust. The M-Pt and Um-Pt veins, as well as the host mylonite, are overprinted by cataclasis and multiple generations of late-stage C-Pt veins that were generated in the brittle-dominated regime of the upper crust during uplift and exhumation of the shear zone.The coexistence of multiple generations of voluminous C-Pt, M-Pt, and Um-Pt veins indicates that the pseudotachylyte veins represent a large number of large earthquakes and accompanying seismic slip over an extended period of seismicity on the Woodroffe thrust. The timing and distribution of pseudotachylyte indicate that the earthquakes nucleated at the base of the brittle-dominated seismogenic zone and propagated down through the brittle–ductile transition into the ductile-dominated regime of the crust.     

Variable microstructure of peridotite samples from the southern Mariana Trench: Evidence of a complex tectonic evolution

We retrieved samples of peridotite from a dredge haul (KH92-1-D2) collected during Cruise KH92-1 undertaken by the research vessel (R/V) Hakuho in 1992 at the landward trench slope of the southern Mariana Trench (11°41.16′N, 143°29.62′E; depth 6594–7431 m), which is the deepest ocean in the world. Ten of 30 retrieved samples possessed both a foliation and lineation, as assessed from 46 thin sections of various orientations and observations of hand samples. The samples showed marked variation in microstructure, ranging from coarse (> 5 mm) equigranular and intensely elongated textures to finer (< 1 mm) porphyroclastic and fine-grained equigranular textures. Olivine fabrics also varied among the different samples, with (010)[100] and (010)[001] patterns (termed A- and B-type, respectively) observed in samples with coarse textures and no clear patterns observed in samples with fine textures. Even though the peridotite samples were retrieved from a single dredge site, some contain primary tectonic microstructures and some contain secondary microstructures. Recent bathymetric and topographic analyses indicate that the lithosphere in this region is as thin as 20 km. Such a thin lithosphere may have been intensely deformed, even perhaps in the ductile regime, during fore-arc extension; consequently, the observed variations in microstructure within the peridotite samples probably reflect the complex tectonic evolution of the southern Mariana region.