Katsuyoshi Michibayashi

Trench-parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge.

Seismic anisotropy is a powerful tool for detecting the geometry and style of deformation in the Earth’s interior, as it primarily reflects the deformation-induced preferred orientation of anisotropic crystals. Although seismic anisotropy in the upper mantle is generally attributed to the crystal-preferred orientation of olivine, the strong trench-parallel anisotropy (delay time of one to two seconds) observed in several subduction systems is difficult to explain in terms of olivine anisotropy, even if the entire mantle wedge were to act as an anisotropic source. Here we show that the crystal-preferred orientation of serpentine, the main hydrous mineral in the upper mantle, can produce the strong trench-parallel seismic anisotropy observed in subduction systems. High-pressure deformation experiments reveal that the serpentine c-axis tends to rotate to an orientation normal to the shear plane during deformation; consequently, seismic velocity propagating normal to the shear plane (plate interface) is much slower than that in other directions. The seismic anisotropy estimated for deformed serpentine aggregates is an order of magnitude greater than that for olivine, and therefore the alignment of serpentine in the hydrated mantle wedge results in a strong trench-parallel seismic anisotropy in the case of a steeply subducting slab. This hypothesis is also consistent with the presence of a hydrous phase in the mantle wedge, as inferred from anomalously low seismic-wave velocities.

Infrared microspectroscopy analysis of water distribution in deformed and metamorphosed rocks

Infrared microspectroscopy has been applied to thin sections of various deformed and metamorphosed rocks in order to investigate water content of quartz in these rocks. The broad IR band absorbance around 3400 cm−1 probably due to fluid-inclusion molecular water (H2O) was used to calculate water contents. Deformed granitic rocks from the Yanazawa-Kamimura area near the Median Tectonic Line (MTL) showed an increase of water content in quartz from about 300 ppm to 2500 ppm toward the MTL with increasing degree of deformation. Metacherts from Sambagawa metamorphic rocks (Asemigawa route) showed a systematic decrease of water content in quartz from about 1000 ppm to 200 ppm with increasing metamorphic degree from chlorite, garnet, albite-biotite to oligoclase-biotite zones. An Archaean metachert from the Napier Complex (granulite facies) has only 40 ppm water. Comparing this with an Inuyama unmetamorphosed chert sample having water contents of 3500 to 7000 ppm (a starting point of metamorphism), the systematic decrease of water in quartz with increasing metamorphic grade may extend from unmetamorphosed cherts to the highest-grade metacherts. Water contents in high-pressure metamorphic rocks in Japan are mostly of the order of 500 ppm (300–700 ppm), except for those from Kurosegawa with a water content as high as 1700 ppm. These results represent an exploratory analysis of intragranular water contents in quartz in various deformed and metamorphosed rocks. Further micro FT-IR studies of the distribution of water in crustal rocks will provide a quantitative basis for examining the geochemical cycle of water in the earths crust.

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid‐Atlantic Ridge 30°N

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Shape preferred orientation of rigid particles in a viscous matrix: reevaluation to determine kinematic parameters of ductile deformation

The development of the shape preferred orientation of rigid elliptical bodies during non-coaxial deformation is theoretically simulated in a two-dimensional Newtonian matrix. The angular velocity of a rigid elliptical body (φ) can be expressed as φ = caseVR2 + 1 [R2 sin2 φ cos Θ + cos2 φ cos Θ − (R2 − 1) sin2 φ sin Θ] where φ is the angle between the shear plane and the longest axis of the ellipse, R is the aspect ratio of the ellipse, V is a constant, and Θ is the newly introduced index angle to describe degree of non-coaxiality between simple shear and pure shear defined as tan Θ = φg3φgg · φgg and φg3 are simple shear strain rate and pure shear strain rate, respectively. The initial distribution pattern of the elliptical bodies is assumed to be random in an R/φ diagram, and a series of the distribution patterns was calculated using the above equation with increasing deformation at varying Θ. When deformation is simple shear (i.e. Θ = 0 dg), all elliptical bodies rotate with various angular velocities, resulting in a skewed distribution in the R/φ diagram. In contrast, for pure shear (i.e. Θ = 90 °) all of them asymptotically settle their longest axes on a plane perpendicular to the compression axis, resulting in strongly concentrated and symmetric distribution patterns in the R/φ diagram. When deformation is general non-coaxial (0 ° < Θ < 90 °), distribution patterns in the R/φ diagram change systematically from the pattern similar to that of Θ = 0 ° to that of Θ = 90 ° with increasing Θ. These R/φ diagrams can be used for estimating the degree of non-coaxiality. We analyzed shape preferred orientation of porphyroclasts in two mylonites, and concluded that deformation within the mylonites contain a certain amount of pure shear component that superimposes on a simple shear component.

A serpentinite-hosted ecosystem in the Southern Mariana Forearc

Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures and vent communities occur in different tectonic settings. The discovery of the Lost City hydrothermal field in the Mid-Atlantic Ridge has stimulated interest in the role of serpentinization of peridotite in generating H2- and CH4-rich fluids and associated carbonate chimneys, as well as in the biological communities supported in highly reduced, alkaline environments. Abundant vesicomyid clam communities associated with a serpentinite-hosted hydrothermal vent system in the southern Mariana forearc were discovered during a DSV Shinkai 6500 dive in September 2010. We named this system the “Shinkai Seep Field (SSF).” The SSF appears to be a serpentinite-hosted ecosystem within a forearc (convergent margin) setting that is supported by fault-controlled fluid pathways connected to the decollement of the subducting slab. The discovery of the SSF supports the prediction that serpentinite-hosted vents may be widespread on the ocean floor. The discovery further indicates that these serpentinite-hosted low-temperature fluid vents can sustain high-biomass communities and has implications for the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.

Magnitude and symmetry of seismic anisotropy in mica- and amphibole-bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast Tibetan Plateau

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.

Syntectonic development of a strain-independent steady-state grain size during mylonitization

Abstract Michibayashi, K., 1993. Syntectonic development of a strain-independent steady-state grain size during mylonitization. Tectonophysics, 222: 151–164. For a distance of greater than 50 km along the Median Tectonic Line, Japan, Syntectonic recrystallization of quartz has resulted in the development of a strain-independent steady-state grain size within the core of the Kashio Shear Zone, that is also independent of rock-type. The mean grain size of quartz in recrystallized aggregates progressively decreases towards the shear zone core, becoming stable at around approximately 37 μm, even though the strain increases within more strongly mylonitized rocks. Some variation of mean grain sizes may result from secondary effects due to hydrothermal activity associated with the Median Tectonic Line. Despite the development of a steady-state mean grain size, individual grains were not stable, as revealed by log-normal grain-size distributions. Consequently, dynamic recrystallization and grain growth were competing processes that achieved a balance and hence resulted in the development of the steady-state grain size. The actual grain size that resulted probably depended on both the water content and strain rate as well as the flow stress. Approaching the centre of the shear zone, mylonitic fabrics (e.g., S-C fabrics, ‘mica-fish’) were established well before a steady-state grain size was reached. A steady-state grain shape, shape orientation and crystallographic fabrics may not be developed until even more mylonitization had occurred. Variable grain sizes result from heterogeneous deformation as a consequence of deformation partitioning even under a stable stress prior to the development of a steady-state grain size. Consequently, it should be determined whether a steady-state grain size has been achieved within a shear zone, before the grain size is used as a paleopiezometer.

Antigorite‐induced seismic anisotropy and implications for deformation in subduction zones and the Tibetan Plateau

The present study, which is a follow-up of the Journal of Geophysical Research paper by Ji et al. (2013a), provides a new calibration for both seismic and fabric properties of antigorite serpentinites. Comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic-preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high T serpentinite, which is essentially controlled by the antigorite c axis fabric, is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock. Extrapolation of the experimental data with both pressure and temperature suggests that Vp anisotropy decreases but shear wave splitting (ΔVs) and Vp/Vs increase with increasing pressure in either cold or hot subduction zones. For a cold, steeply subducting slab, antigorite is most likely deformed by nearly coaxial flattening or trench-parallel movements, forming trench-parallel seismic anisotropy. For a hot, shallowly subducting slab, however, antigorite is most likely deformed by simple shear or transpression. Trench-normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30°. The geophysical characteristics of the Tibetan Plateau such as strong heterogeneity in Vp, Vs and attenuation, shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks, hydrated zones of subducting lithospheric mantle, and the crust-mantle boundary if the temperature is below 700°C in the region of interest.

Shearing during progressive retrogression in granitoids: Abrupt grain size reduction of quartz at the plastic-brittle transition for feldspar

The Kashio shear zone, Chubu district, Japan, affects the southeast margins of the Ryoke granitic plutons as well as the Ryoke metasedimentary rocks, and is truncated by the Median Tectonic Line near the shear zone centre. Coarse quartz grains within the granitic rocks have been dynamically recrystallized to quartz aggregates towards the pluton margins. The grain size data show an abrupt reduction from various sizes (59–148 μm) to a steady-state size around 35 μm at approximately 400 m from the boundary of the granitic plutons without any tectonic boundary such as a fault being present. This abrupt grain size reduction accompanies brittle deformation in feldspar that overprints earlier plastic deformation and develops strong mylonitic fabrics. In contrast, feldspar away from the pluton margins contains only weak microstructural evidence for plastic deformation and still locally preserves magmatic polycrystal structures amongst the inhomogeneously recrystallized quartz grains. These features, combined with an isochronological study, suggest that the Kashio shear zone began at high temperature (≥450°C) as a ductile event that affected relatively wide regions in the Ryoke granitic plutons before rapid cooling by ca 70 Ma. At this latter stage, strong mylonitization at lower temperatures (≥300°C) in a semiductile regime occurred in a narrow zone towards the shear zone centre. Here, recrystallized quartz aggregates reached a steady-state grain size by ca 60 Ma. That is, cooling within the granitic plutons resulted in strain localization, with the abrupt grain size reduction of quartz aggregates correlating with the plastic-brittle transition of feldspar.

Subduction initiation and ophiolite crust: new insights from IODP drilling

ABSTRACT International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.