Faculty Profiles - MICHIBAYASHI Katsuyoshi

Plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks Reviewed International coauthorship

Shaocheng Ji, Tongbin Shao, Matthew H. Salisbury, Shengsi Sun, Katsuyoshi Michibayashi, Weihua Zhao, Changxing Long, Fenghua Liang, Takako Satsukawa

JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH Vol. 119 ( 11 ) page： 8064 - 8088 2014.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust. Thus, the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust. To this end, we have measured P and S wave velocities and anisotropy in 17 plagioclase-rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650MPa. Anorthosites and gabbroic anorthosites containing >80vol% plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctive seismic anisotropy patterns: V-p(Z)/V-p(Y)1 and V-p(Z)/V-p(X)1 for anorthosites while 0.8<V-p(Z)/V-p(Y)1 and 0.8<V-p(Z)/V-p(X)1 for gabbros. Amphibolites lie in the same domain as gabbros, but show a significantly stronger tendency of V-p(X)>V-p(Y) than the gabbros. Laminated anorthosites with V-p(X)approximate to V-p(Y)<<V-p(Z) display a strong crystal preferred orientation (CPO) of plagioclase whose (010) planes and [100] and [001] directions parallel to the foliation. For the gabbros and amphibolites characterized by V-p(X)approximate to V-p(Y)>V-p(Z) and V-p(X)>V-p(Y)>V-p(Z), respectively, pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy. The Poisson's ratio calculated using the average P and S wave velocities from the three principal propagation-polarization directions (X, Y, and Z) of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent. The CPO-induced anisotropy enhances and decreases the foliation-normal incidence reflectivity at gabbro-peridotite and anorthosite-peridotite interfaces, respectively.

DOI： 10.1002/2014JB011352

A new method for calculating seismic velocities in rocks containing strongly dimensionally anisotropic mineral grains and its application to antigorite-bearing serpentinite mylonites Reviewed

The Yushugou granulite-peridotite terrane as a paleozoic continental crust-mantle transition zone exposed at the northern margin of the southern Tianshan (Xinjiang) Reviewed International coauthorship

Ji S.

Geotectonica et Metallogenia Vol. 38 ( 3 ) page： 473 - 494 2014.8

Tohru Watanabe, Yuhto Shirasugi, Katsuyoshi Michibayashi

EARTH AND PLANETARY SCIENCE LETTERS Vol. 391 page： 24 - 35 2014.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Seismic velocity is one of the most important sources of information about the Earth's interior. For its proper interpretation, we must have a thorough understanding of the dependence of seismic velocity on microstructural elements, including the modal composition, the crystal preferred orientation (CPO), the grain shape, the spatial distribution of mineral phases, etc. The conventional Voigt, Reuss and Hill averaging schemes take into account only the modal composition and the CPO. The information about the Earth's interior is thus poorly constrained. For a better interpretation, it is critical to have a calculation method which accounts for the grain shape and the spatial distribution of mineral phases, etc. We propose a calculation method which accounts for the grain shape of strongly dimensionally anisotropic minerals like micas and serpentines. Our method can be applied to a distributed geometrical orientation of mineral grains. Comparison was made between calculated and measured velocities in three antigorite-serpentinite mylonites. Judging from the root mean square relative error, our method provides velocities closer to measured values than the Voigt, Reuss and Hill averaging schemes. The input of the grain shape considerably improves the prediction of seismic properties. However, large discrepancies (>0.1 km/s) between measured and calculated velocities can be seen in some directions. The discrepancies might come from microstructural elements which were not considered in the calculation (layer structures and cracks). (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2014.01.025

Antigorite-induced seismic anisotropy and implications for deformation in subduction zones and the Tibetan Plateau Reviewed International coauthorship

Tongbin Shao, Shaocheng Ji, Yosuke Kondo, Katsuyoshi Michibayashi, Qian Wang, Zhiqin Xu, Shengsi Sun, Denis Marcotte, Matthew H. Salisbury

JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH Vol. 119 ( 3 ) page： 2068 - 2099 2014.3

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

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 V-p anisotropy decreases but shear wave splitting (Delta V-s) and V-p/V-s 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 degrees. 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 degrees C in the region of interest.

DOI： 10.1002/2013JB010661

Flow in the uppermost mantle during back-arc spreading revealed by Ichinomegata peridotite xenoliths, NE Japan Reviewed

Seismic velocities, anisotropy and elastic properties of Xiuyan jade and its geological implications Reviewed International coauthorship

Wang Q

Geotectonica et Metallogenia Vol. 38 ( 1 ) page： 12-26 2014.2

Takako Satsukawa, Katsuyoshi Michibayashi

LITHOS Vol. 189 page： 89 - 104 2014.2

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Spinel peridotite xenoliths from the Ichinomegata Volcano (NE Japan) have distinct foliations defined by compositional layering between olivine-rich and pyroxene-rich layers as well as lineations defined by elongated spine! grains. Crystallographic preferred orientations (CPOs) of olivine are consistent with slip on (010)[100] and {0kl} [100]. The angles between the foliation and the olivine slip planes decrease with increasing values of the J-index (i.e. CPO strength). Such composite planar relationships within the peridotite xenoliths could result from shearing in the uppermost mantle, so that shear strains can be estimated by the angles between the foliation and the olivine slip plane in terms of simple shear strain (0.31-4.26). From these observations, we argue that a suite of the peridotite xenoliths recorded a rare snapshot of uppermost-mantle flow related to back-arc spreading during the opening of the Japan Sea. The peridotite xenoliths with higher J-indices (i.e. higher shear strain) tend to have slightly lower minimum temperatures, possibly defining a vertical strain gradient in the uppermost mantle section at the time of the volcano's eruption. The CPO data have been used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and are compared to field geophysical data from the literature. Our results are consistent with a roughly EW-oriented fastest P-wave propagation direction in the uppermost mantle beneath the northeast part of the Japan arc. Average samples are calculated based on three different structural reference frames; horizontal plane parallel to 1) foliation, 2) the plane containing the maximum concentration of olivine [100], and 3) P-wave maximum direction. S-wave anisotropy deduced from CPOs requires a reasonable thickness of the anisotropic layer (24.1-26.6 km), and the structural reference frame does not have significant effect on the estimation of thickness. Consequently, Ichinomegata peridotites record a long and complicated tectonic history; they preserve deformation 'frozen in' during back-arc spreading and this deformation also has an effect on present-day mantle flow. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.

DOI： 10.1016/j.lithos.2013.10.035

Dehydration breakdown of antigorite and the formation of B-type olivine CPO Reviewed

Takayoshi Nagaya, Simon R. Wallis, Hiroaki Kobayashi, Katsuyoshi Michibayashi, Tomoyuki Mizukami, Yusuke Seto, Akira Miyake, Megumi Matsumoto

EARTH AND PLANETARY SCIENCE LETTERS Vol. 387 page： 67 - 76 2014.2

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). The lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form as a result of solid-state deformation. Instead, the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable. The Happo Ol CPO has a strong a-axis concentration perpendicular to the lineation and within the foliation commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the mantle wedge of these regions. Experimental work has shown that B-type CPO can form by dislocation creep under hydrous conditions at relatively high stresses. There are, however, several discrepancies between the characteristics of natural and laboratory samples with B-type Ol CPO. (I) The formation conditions (stress and temperature) of some natural examples with B-type CPO fall outside those predicted by experiments. (2) In deformation experiments, slip in the crystallographic c-axis direction is important but has not been observed in natural examples of B-type CPO. (3) Experimental work suggests the presence of H2O and either high shear stress or relatively low temperatures are essential for the formation of B-type CPO. These conditions are most likely to be achieved close to subduction boundaries, but these regions are also associated with serpentinization, which prevents strong olivine CPO patterns from forming. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of foliated serpentinite. These results resolve the discrepancies between experimental and natural examples of B-type CPO and show the need to rethink the formation process of olivine CPO in convergent margins. Topotactic growth of olivine after antigorite can account for the inferred distribution of B-type Ol CPO in the mantle wedge more successfully than dislocation creep. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2013.11.025

Influence of mineral fraction on the rheological properties of forsterite plus enstatite during grain size sensitive creep: 3. Application of grain growth and flow laws on peridotite ultramylonite Reviewed

Miki Tasaka, Takehiko Hiraga, Katsuyoshi Michibayashi

JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH Vol. 119 ( 2 ) page： 840 - 857 2014.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Microstructures of a layered peridotite ultramylonite from the Oman ophiolite are compared with that of experimentally deformed samples. Average grain sizes and grain size ratios of olivine and pyroxene from each layer are compared with respect to the fraction of pyroxene (f(px)) in the layer. Grain size of the pyroxene is almost constant among different f(px) layers, whereas olivine grain size decreases significantly with increasing f(px), both of which were characteristic features found in forsterite+enstatite aggregates after grain growth experiments (Tasaka and Hiraga, ). Furthermore, the Zener relationship (log d(ol)/d(px) versus log f(px)) found in the ultramylonite is remarkably comparable to that observed in our experiments. These observations indicate effective pinning of olivine grain growth due to the presence of pyroxene grains during the deformation of the rocks. Olivine grains in layers with f(px)0.03 do not exhibit lattice-preferred orientation (LPO), whereas the grains in layers with f(px)<0.03 exhibit LPO, indicating that deformation proceeded via diffusion- and dislocation-accommodated creep in the former and the latter layers, respectively. We simulated the evolution of grain size and viscosity in the shear zone based on our grain growth and flow laws obtained for diffusion creep of forsterite+enstatite (Tasaka and Hiraga, ; Tasaka et al., ) and successfully reproduced the observed grain sizes in the ultramylonite. We therefore conclude that the relative values of the kinetic parameters, some of which are functions of the f(px), are applicable to nature.

DOI： 10.1002/2013JB010619

A new calibration of seismic velocities, anisotropy, fabrics, and elastic moduli of amphibole-rich rocks Reviewed

A database of plagioclase crystal preferred orientations (CPO) and microstructures - implications for CPO origin, strength, symmetry and seismic anisotropy in gabbroic rocks (vol 4, pg 511, 2013) Reviewed International coauthorship

T. Satsukawa, B. Ildefonse, D. Mainprice, L. F. G. Morales, K. Michibayashi, F. Barou

SOLID EARTH Vol. 5 ( 1 ) page： 509 - 509 2014

Shaocheng Ji, Tongbin Shao, Katsuyoshi Michibayashi, Changxing Long, Qian Wang, Yosuke Kondo, Weihua Zhao, Hongcai Wang, Matthew H. Salisbury

JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH Vol. 118 ( 9 ) page： 4699 - 4728 2013.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

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 (V-p and V-s) and anisotropy of 17 amphibole-rich rock samples containing 34-80vol% amphibole at hydrostatic pressures (P) up to 650MPa. Combined petrophysical and geochemical analyses provide a new calibration for mean density, average major element contents, mean V-p-P and V-s-P coefficients, intrinsic V-p and V-s anisotropy, Poisson's ratios, the logarithmic ratio R-s/p, and elastic moduli of amphibole-rich rocks. The V-p values decrease with increasing SiO2 and Na2O+K2O contents but increase with increasing MgO and CaO contents. The maximum (0.38-0.40km/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 V-p(X)/V-p(Y)-V-p(Y)/V-p(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.

DOI： 10.1002/jgrb.50352

Deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust Reviewed

Daeyeong Kim, Ikuo Katayama, Katsuyoshi Michibayashi, Tatsuki Tsujimori

PHYSICS OF THE EARTH AND PLANETARY INTERIORS Vol. 222 page： 8 - 21 2013.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust, and here we report on our systematic fabric analyses of glaucophane, lawsonite, and epidote in naturally deformed blueschists from the Diablo Range and Franciscan Complex in California, and the Hida Mountains in Japan. Glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations (CPOs) characterized by a (1 00)[001] pattern. These characteristics, together with a bimodal distribution of grain sizes from some samples, possibly indicate the occurrence of dynamic recrystallization for glaucophane. Although lawsonite and epidote display high aspect ratios and a strong CPO of (001)[010], the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism. The P-wave (AV(P)) and S-wave (AV(S)) seismic anisotropies of glaucophane (AV(P) = 20.4%, AV(S) = 11.5%) and epidote (AV(P) = 9.0%, AV(S) = 8.0%) are typical of the crust; consequently, the fastest propagation of P-waves is parallel to the [001] maxima, and the polarization of S-waves parallel to the foliation can form a trench-parallel seismic anisotropy owing to the slowest V-S polarization being normal to the subducting slab. The seismic anisotropy of lawsonite (AV(P) = 9.6%, AV(S) = 19.9%) is characterized by the fast propagation of P-waves subnormal to the lawsonite [001] maxima and polarization of S-waves perpendicular to the foliation and lineation, which can generate a trench-normal anisotropy. The AV(S) of lawsonite blueschist (5.6-9.2%) is weak compared with that of epidote blueschist (8.4-11.1%). Calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench-parallel and trench-normal seismic anisotropy beneath NE Japan, but not to that beneath the Ryukyu arc. Our results demonstrate, therefore, that lawsonite has a strong influence on seismic velocities in the oceanic crust, and that lawsonite might be the cause of complex anisotropic patterns in subduction zones. (c) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.pepi.2013.06.011

The earliest mantle fabrics formed during subduction zone infancy Reviewed International coauthorship

Yumiko Harigane, Katsuyoshi Michibayashi, Tomoaki Morishita, Kenichiro Tani, Henry J. B. Dick, Osamu Ishizuka

EARTH AND PLANETARY SCIENCE LETTERS Vol. 377 page： 106 - 113 2013.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Harzburgites obtained from the oldest crust-mantle section in the Philippine Sea plate (similar to 52 Ma) along the landward slope of the southern lzu-Ogasawara Trench, preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The harzburgites have relatively fresh primary minerals despite of their heavy serpentinizations, and show inequigranular interlobate textures, and crystal preferred orientation patterns in olivine (001)1100] and Opx (100)1001]. The harzburgites have the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. We propose that the (001)1100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.eps1.2013.06.031

Olivine fabric evolution in a hydrated ductile shear zone at the Moho Transition Zone, Oman Ophiolite Reviewed

Katsuyoshi Michibayashi, Tatsuya Oohara

Earth and Planetary Science Letters Vol. 377-378 page： 299 - 310 2013.9

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal)

The Fizh massif, Oman Ophiolite, contains a ductile shear zone at the Moho Transition Zone. The dunites in the shear zone are classified based on microstructures into coarse granular texture, medium-grained texture, mylonite, and ultramylonites toward a gabbro contact. The average grain size of olivine decreases toward the shear zone, which contains a zone of high strain (~15 m wide). The proportion of hydrous minerals (amphibole and chlorite) in the shear zone show an increase toward the gabbro contact, suggesting that water infiltrated the shear zone from the gabbro contact. Equilibrium temperatures indicate a higher deformation temperature (~900°C) outside of the high strain zone compared with inside this zone (~750°C). Under these geochemical and temperature conditions, the temporal evolution of olivine crystal-preferred orientations (CPO) indicates the following continuous deformation scenario. First, deformation by dislocation creep under higher temperatures resulted in slip by D-type {0. kl}[100] and then weak E-type (001)[100] slip. Next, deformation by dislocation creep under lower temperatures and higher stress conditions produced a C-type (100)[001] CPO. Finally, superplastic deformation by grain boundary sliding resulted in a random CPO. © 2013 Elsevier B.V.

DOI： 10.1016/j.epsl.2013.07.009

Whole-rock chemical composition analysis of the mafic rocks in a ductile shear zone developed in the Moho Transition Zone, the Oman ophiolite Reviewed

Furuhata Keisuke, Michibayashi Katsuyoshi, Yamashita Hiroyuki

Geoscience reports of Shizuoka University Vol. 40 ( 0 ) page： 13 - 19 2013.7

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Institute of Geosciences, Faculty of Science, Shizuoka University

The whole-rock chemistries of the mafic rocks in the Moho Transition Zone, Fizh Massif, the Oman ophiolite have been analyzed by X-ray fluorescence spectrometry. The mafic rocks were classified into three texture types based on their microstructures: weakly deformed gabbro, moderately deformed gabbro and ultramylonites. There is no systematic variation of the whole-rock chemistries among the three texture types. The ultramylonites show various chemistries, although they consist mostly of very fine-grained matrix with minor relatively coarser porphyroclasts. It may suggest the effect of fluid-rock interaction during ductile shearing in the Moho Transition Zone or variation of the mafic rock chemical compositions.

DOI： 10.51053/shizuoka.40.0_13

CiNii Research

Progressive deformation partitioning and recrystallization of olivine in the lithospheric mantle Reviewed

Katsuyoshi Michibayashi, Makoto Suzuki, Naoaki Komori

Tectonophysics Vol. 587 page： 79 - 88 2013.3

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal)

Intense serpentinization within the Machinoyama ultramafic body, eastern-most part of the Yakuno ophiolite in the Paleozoic Maizuru belt, SW Japan, resulted in the development of block-in-matrix structure in the brittle regime, possibly associated with exhumation along a fault. Various microstructures are heterogeneously distributed throughout peridotite blocks in a serpentinite matrix. The microstructures are classified into four domains according to olivine grain size: coarse (1.0-1.5. mm), medium (0.5-1.0. mm), small (0.2-0.5. mm), and fine-grained (0.01. mm). Even in a single peridotite block, the medium-, small- and fine-grained domains occur at various scales. In particular, the fine-grained domain occurs in thin zones that cut across the other domains. These observations suggest that deformation in the peridotites before the serpentinization was partitioned into anastomosing zones during progressive inhomogeneous non-coaxial shear, resulting in the simultaneous development of a range of microstructures from the coarse-grained domain to the small-grained domain. Olivine crystal-preferred orientations tend to be weaker with decreasing olivine grain size, indicating a change in deformation mechanism from dislocation creep to grain-size-sensitive creep. Moreover, the fine-grained domains contain much more second phase than the small-grained domain even in the same sample, suggesting the role of the second phases on strain localization during dynamic recrystallization of olivine. © 2012 Elsevier B.V.

DOI： 10.1016/j.tecto.2012.07.008

Rheological contrast between glaucophane and lawsonite in naturally deformed blueschist from Diablo Range, California Reviewed

Preface to "Deformation, porphyroblasts and mountain building: A special issue in honour of the career contributions of TH Bell" Reviewed International coauthorship

Domingo G. A. M. Aerden, Scott E. Johnson, Katsuyoshi Michibayashi

TECTONOPHYSICS Vol. 587 page： 1 - 3 2013.3

Daeyeong Kim, Ikuo Katayama, Katsuyoshi Michibayashi, Tatsuki Tsujimori

ISLAND ARC Vol. 22 ( 1 ) page： 63 - 73 2013.3

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-BLACKWELL

Deformation microstructures for a lawsonite blueschist from the New Idria serpentinite body, Diablo Range, are investigated to clarify rheological behaviors of glaucophane and lawsonite, which are main mineral assemblages of subducting oceanic crust at relatively cold geotherm. Developments of crystal-preferred orientations (CPOs) with small grain size, irregular grain boundary and high aspect ratio of glaucophane indicate deformation mechanism as recovery and dynamic recrystallization possibly accommodated by dislocation creep, while lawsonite deforms by rigid body rotation based on euhedral grains with angular or straight grain boundaries. Higher aspect ratios, lower angle to foliation, and stronger CPOs of both minerals in the glaucophane-rich layer rather than those in the lawsonite-rich layer suggest the strain localization into the glaucophane-rich layer. Additionally fabric strength (the degree of crystal alignment) and seismic anisotropy are higher in the glaucophane-rich layer than that of the lawsonite-rich layer, which is consistent with the microstructural analyses. All our results imply, therefore, the dominant role of glaucophane rather than lawsonite for rheological behavior and seismic anisotropy of blueschist.

DOI： 10.1111/iar.12003

T. Satsukawa, B. Ildefonse, D. Mainprice, L. F. G. Morales, K. Michibayashi, F. Barou

SOLID EARTH Vol. 4 ( 2 ) page： 511 - 542 2013

Language：English Publishing type：Research paper (scientific journal) Publisher：COPERNICUS GESELLSCHAFT MBH

This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO characteristics as a function of the deformation regime (magmatic or crystal-plastic) are outlined and discussed. The studied samples are dominantly from slow-and fast-spread present-day ocean crust, as well as from the Oman ophiolite. Plagioclase is the dominant mineral phase in the studied samples. Plagioclase CPOs are grouped into three main categories: Axial-B, a strong point alignment of (010) with a girdle distribution of [100]; Axial-A, a strong point maximum concentration of [100] with parallel girdle distributions of (010) and (001); and P-type, point maxima of [100], (010), and (001). A majority of CPO patterns are Axial-B and P-type, in samples showing either magmatic or crystal-plastic deformation textures. Axial-A CPOs are less common; they represent 21% of the samples deformed by crystal-plastic flow. Although fabric strength (ODF J index) does not show any consistent variation as a function of the CPO patterns, there is a significant difference in the relationship between the ODF and pole figures J indices; the magmatic type microstructures have high (010) pole figures J indices, which increase linearly with ODF J index, whereas the high [100] pole figures J indices of plastically deformed samples vary in a more scattered manner with ODF J index. The multistage nature of plastic deformation superposed on a magmatic structure compared with magmatic flow, and the large number of possible slip-systems in plagioclase probably account for these differences. Calculated seismic properties (P wave and S wave velocities and anisotropies) of plagioclase aggregates show that anisotropy (up to 12% for P wave and 14% for S wave) tends to increase as a function of ODF J index. In comparison with the olivine 1998 CPO database, the magnitude of P wave anisotropy for a given J index is much less than olivine, whereas it is similar for S wave anisotropy. Despite a large variation of fabric patterns and geodynamic setting, seismic properties of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a small difference in the aggregate elastic symmetry, with magmatic microstructures having higher orthorhombic and hexagonal components, whereas plastic deformation microstructures have a slightly higher monoclinic component, possibly correlated with predominant monoclinic simple shear flow in plastically deformed samples. Overall, plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, although our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry (triclinic) structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs.

DOI： 10.5194/se-4-511-2013