Faculty Profiles - MICHIBAYASHI Katsuyoshi

B-type olivine fabrics developed in the fore-arc side of the mantle wedge along a subducting slab Reviewed

Miki Tasaka, Katsuyoshi Michibayashi, David Mainprice

EARTH AND PLANETARY SCIENCE LETTERS Vol. 272 ( 3-4 ) page： 747 - 757 2008.8

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

B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg+Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr+Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites,as their formation requires hot, hydrous, shallow mantle (> 1250 degrees C at < 30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880-1030 degrees C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions. (c) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2008.06.014

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

Isotropic quartz aggregate prepared using a sol-gel method Reviewed

Imoto Hisashi, Michibayashi Katsuyoshi, Oouchi Tomohiro, Nakamura Michihiko

Geoscience reports of Shizuoka University Vol. 35 ( 0 ) page： 45 - 54 2008.7

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

Synthetic quartz aggregates as a starting material for high-temperature / high-pressure deformation experiments were prepared using a high-temperature / high-pressure apparatus. Silica gel was prepared using a sol-gel method and subsequently dried and heated in air for three days at temperatures between 700 and 1000°C Various sets of the gels were then packed into Pt-Ni capsules with 0–2 wt% water and were sintered to form quartz aggregates at 800°C and 1 GPa for 5 hours. As a result, synthetic quartz aggregates with polygonal textures and very weak crystallographic preferred orientations were formed from a 1:4 mixture of two silica gels dried at 700°C and 1000°C, respectively.

DOI： 10.51053/shizuoka.35.0_45

CiNii Research

Structure Sensitivity and Elastic Anisotropy within Peridotites Reviewed

MICHIBAYASHI Katsuyoshi

Journal of Geography (Chigaku Zasshi) Vol. 117 ( 1 ) page： 93-109 - 109 2008.2

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

&emsp;Peridotites derived from the uppermost mantle consist dominantly of olivine and subsequently of pyroxene, spinel, garnet, and plagioclase. Crystal-plastic flow of mantle rocks results in various types of structure within peridotite being developed to varying degrees, depending upon the structure sensitivity of the different mineral phases. Plastic deformation leads to the simultaneous development of shape-preferred orientations and crystal-preferred orientations. A shape-preferred orientation is the expression of the average orientation of flattening (foliation) and elongation (lineation) directions, as defined by the orientations of individual grains. A crystal-preferred orientation (CPO) is the expression of crystallographic orientations of grains within the rock, as developed via dislocation creep and recrystallization. During intense homogeneous plastic deformation of a peridotite composed of minerals with a dominant slip system, the preferred orientation of the slip plane and slip direction tends to coincide with the plane of plastic flow and the flow direction, respectively. Recently, a new olivine CPO classification (A, B, C, D, and E types) has been proposed by Karato and co-workers to illustrate the roles of stress and water content as controlling factors of olivine slip systems. An additional CPO type (AG) has also been proposed in recognition of its common occurrence in nature. Given that olivine and the other constituent minerals in peridotites contain intrinsic elastic anisotropies, the development of CPO within peridotite during plastic deformation gives rise to seismic anisotropy in the upper mantle. Thus, the anisotropic properties of mantle rocks derived from the upper 100 km of the mantle, such as Ichinomegata peridotite xenoliths from the northeast Japan arc, have been calculated and applied with the aim of understanding the seismic anisotropy of the Earth's mantle.

DOI： 10.5026/jgeography.117.93

Katsuyoshi Michibayashi, Miki Tasaka, Yasuhiko Ohara, Teruaki Ishii, Atsushi Okamoto, Patricia Fryer

TECTONOPHYSICS Vol. 444 ( 1-4 ) page： 111 - 118 2007.11

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

We retrieved samples of peridotite from a dredge haul (K1192-1-D2) collected during Cruise KE92-1 undertaken by the research vessel (R/V) Hakuho in 1992 at the landward trench slope of the southern Mariana Trench (11 degrees 41.16'N, 143 degrees 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. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2007.08.010

Oceanic core complexes and crustal accretion at slow-speading ridges Reviewed International coauthorship

Benoit Ildefonse, D. K. Blackman, B. E. John, Y. Ohara, D. J. Miller, C. J. MacLeod, N. Abe, M. Abratis, E. S. Andal, M. Andr?ani, S. Awaji, J. S. Beard, D. Brunelli, A. B. Charney, D. M. Christie, A. G. Delacour, H. Delius, M. Drouin, F. Einaudi, J. Escartin, B. R. Frost, P. B. Fryer, J. S. Gee, M. Godard, C. B. Grimes, A. Halfpenny, H. E. Hansen, A. C. Harris, N. W. Hayman, E. Hellebrand, T. Hirose, J. G. Hirth, S. Ishimaru, K. T.M. Johnson, G. D. Karner, M. Linek, J. Maeda, O. U. Mason, A. M. McCaig, K. Michibayashi, A. Morris, T. Nakagawa, T. Nozaka, M. Rosner, R. C. Searle, G. Suhr, A. Tamura, M. Tominaga, A. von, der Handt, T. Yamasaki, X. Zhao

Geology Vol. 35 ( 7 ) page： 623 - 626 2007.7

Language：English Publishing type：Research paper (scientific journal)

Oceanic core complexes expose gabbroic rocks on the seafloor via detachment faulting, often associated with serpentinized peridotite. The thickness of these serpentinite units is unknown. Assuming that the steep slopes that typically surround these core complexes provide a cross section through the structure, it has been inferred that serpentinites compose much of the section to depths of at least several hundred meters. However, deep drilling at oceanic core complexes has recovered gabbroic sequences with virtually no serpentinized peridotite. We propose a revised model for oceanic core complex development based on consideration of the rheological differences between gabbro and serpentinized peridobte: emplacement of a large intrusive gabbro body into a predominantly peridotite host is followed by localization of strain around the margins of the pluton, eventually resulting in an uplifted gabbroic core surrounded by deformed serpentinite. Oceanic core complexes may therefore reflect processes associated with relatively enhanced periods of mafic intrusion within overall magma-poor regions of slow- and ultra-dow-spreading ridges. ? 2007 The Geological Society of America.

DOI： 10.1130/G23531A.1

Development of a shear band cleavage as a result of strain partitioning Reviewed

Katsuyoshi Michibayashi, Masami Murakami

JOURNAL OF STRUCTURAL GEOLOGY Vol. 29 ( 6 ) page： 1070 - 1082 2007

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Microstructural analyses of shear band cleavages in a centimeter-scale shear zone within a metasomatic biotite band in the Teshima granite, Ryoke metamorphic belt, southwest Japan, show that strain partitioning Occurred between quartz and biotite-feldspar domains within the shear zone. Pre-tectonic hydrothermal alteration within the granite caused biotite replacement of both plagioclase and K-feldspar, resulting in the development of biotite-feldspar domains where K-feldspar mantles dominantly biotite-plagioclase aggregates. Subsequently, the altered granite was plastically deformed in simple shear, so that intra-layer shear band cleavages were passively developed within the biotite-feldspar domains, whereas intense dynamic recrystallization occurred in the quartz domains. The rotation and orientation of the intra-layer shear band cleavages can be explained by a finite strain ellipse model, which shows that strain in the biotite-feldspar domain requires only 10-20% of the bulk simple shear strain for the development of such cleavages, so that most of strain could be accommodated by deformation in the quartz domain. Consequently, the model suggests that the development of the shear zone resulted in strain partitioning between the quartz and the biotite-feldspar domains due to compositional variations via hydrothermal alteration within the granite. (c) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jsg.2007.02.003

Misorientations of garnet aggregate within a vein: an example from the Sanbagawa metamorphic belt, Japan Reviewed

A. Okamoto, K. Michibayashi

JOURNAL OF METAMORPHIC GEOLOGY Vol. 24 ( 5 ) page： 353 - 366 2006.6

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

In this study, the chemistry and microstructure of garnet aggregates within a metamorphic vein are investigated. Garnet-bearing veins in the Sanbagawa metamorphic belt, Japan, occur subparallel to the foliation of a host mafic schist, but some cut the foliation at low angle. Backscattered electron image and compositional mapping using EPMA and crystallographic orientation maps from electron-backscattered diffraction (EBSD) reveal that numerous small garnet (10-100 mu m diameter) coalesce to form large porphyroblasts within the vein. Individual small garnet commonly exhibits xenomorphic shape at garnet/garnet grain boundaries, whereas it is idiomorphic at garnet/quartz boundaries. EBSD microstructural analysis of the garnet porphyroblasts reveals that misorientation angles of neighbour-pair garnet grains within the vein have a random distribution. This contrasts with previous studies that found coalescence of garnet in mica schist leads to an increased frequency of low angle misorientation boundaries by misorientation-driven rotation. As garnet nucleated with random orientation, the difference in misorientation between the two studies is due to the difference in the extent of grain rotation. A simple kinetic model that assumes grain rotation of garnet is rate-limited by grain boundary diffusion creep of matrix quartz, shows that (i) the substantial rotation of a fine garnet grain could occur for the conditions of the Sanbagawa metamorphism, but (ii) the rotation rate drastically decreased as garnet grains formed large clusters during growth. Therefore, the random misorientation distribution of garnet porphyroblasts in the Sanbagawa vein is interpreted as follows: (i) garnet within the vein grew so fast that substantial grain rotation did not occur through porphyroblast formation, and thus (ii) random orientations at the nucleation stage were preserved. The extent of misorientation-driven rotation indicated by deviation from random orientation distribution may be useful to constrain the growth rate of constituent grains of porphyroblast that formed by multiple nucleation and coalescence.

DOI： 10.1111/j.1525-1314.2006.00642.x

Seismic anisotropy in the uppermost mantle, back-arc region of the northeast Japan arc: Petrophysical analyses of Ichinomegata peridotite xenoliths Reviewed

Katsuyoshi Michibayashi, Natsue Abe, Atsushi Okamoto, Takako Satsukawa, Kenta Michikura

GEOPHYSICAL RESEARCH LETTERS Vol. 33 ( 10 ) 2006.5

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

A dense network of seismic stations has been deployed across the northeast Japan arc to investigate mantle wedge structures. To attain independent petrophysical constraints, we determined the seismic properties of Ichinomegata mantle xenoliths from the back-arc region that were brought to the surface from the mantle lithosphere by volcanic eruptions. We calculated the seismic properties of the xenoliths from olivine and pyroxene crystal-preferred orientations and single crystal elastic constants. The small magnitude of measured S-wave splitting ( delay time of 0.22 s in the area where the xenoliths were entrained) can be explained by the average seismic properties of mantle xenoliths for an approximately 20-km thick horizontal anisotropic layer, indicating that the mantle lithosphere could be one of the dominant sources of seismic anisotropy; this layer is possibly related to deformation in the uppermost mantle lithosphere due to back-arc spreading along the northeast Japan arc.

DOI： 10.1029/2006GL025812

The effect of dynamic recrystallization on olivine fabric and seismic anisotropy: Insight from a ductile shear zone, Oman ophiolite Reviewed

The design and performance of the solid-medium deformation apparatus MK65S: evaluation of the internal friction Reviewed

清水以知子, 道林克禎, 渡辺悠太

Japanese journal of structural geology ( 49 ) page： 15 - 26 2006.5

K Michibayashi, T Ina, K Kanagawa

EARTH AND PLANETARY SCIENCE LETTERS Vol. 244 ( 3-4 ) page： 695 - 708 2006.4

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

Subhorizontal mantle structures subparallel to the Moho are rotated into NW-SE subvertical orientations across a shear zone in a sinistral sense of shear within the northern Fizh mantle section of the Oman ophiolite. Dynamic recrystallization resulted in grain size reduction of olivine and the development of porphyroclastic texture. Mean olivine grain size stabilized at similar to 0.7 mm within the shear zone center; this may reflect the steady-state grain size of dynamically recrystallized olivine, as determined by the deviatoric stress, which in this case was as low as 10 MPa. Crystal-preferred orientation (CPO) patterns of olivine are consistently [100]-fiber or partial fiber texture, indicating that olivine slip systems did not change during shearing. Dynamic recrystallization causes a weakening of olivine fabric intensity toward the shear zone center, but this weakening is counterbalanced by CPO strengthening due to dislocation glide. This process resulted in an abrupt decrease in seismic anisotropy at the center of the shear zone, in contrast to a gradual decrease in olivine fabric intensity and mean grain size.
The measured seismic anisotropy patterns did not change in ways that would be significantly measurable by seismological observations. Despite the development of the shear zone, dispersion of both P- and S-waves in the shear zone may be of little effect with respect to the overall seismic anisotropy. This is not only because the shear zone occurs substantially in a narrow region but also because the seismic anisotropy is weaker in the shear zone than the high-T-structure region. It suggests that a record of simple systematic seismic anisotropy observed in the upper mantle may indicate a simplified mantle flow structure, as localized structures may be obscured in the region of the observation. (c) 2006 Elsevier B.V. All fights reserved.

DOI： 10.1016/j.epsl.2006.02.019

Progressive shape evolution of a mineral inclusion under differential stress at high temperature: Example of garnet inclusions within a granulite-facies quartzite from the Lutzow-Holm Complex, East Antarctica Reviewed

かんらん岩の構造解析と地球内部のレオロジー Reviewed

道林克禎

日本レオロジー学会誌 34 page： 291 - 300 2006

固体圧変形試験機MK65Sの設計と性能 : 内部摩擦の評価 Reviewed

道林克禎

構造地質 49 page： 15 - 26 2006

A Okamoto, K Michibayashi

JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH Vol. 110 ( B11 ) page： 1-16 - 16 2005.11

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

Interfacial tension (gamma) and differential stress (sigma) affect the shape of a mineral grain included within a crystalline host. We present a simple model that predicts the progressive change in aspect ratio of an ellipsoidal inclusion. Three processes are considered in the model: dislocation creep, interface diffusion creep, and rounding by interface diffusion. The model reveals that (1) the inclusion aspect ratio (L) evolves toward a steady state value, (2) the time taken to achieve a steady state aspect ratio increases with increasing grain size (R), and (3) the dominant deformation mechanism varies from diffusion creep to dislocation creep with increasing grain size and/or differential stress. The L-R distribution pattern of garnets in a granulite-facies quartzite from the Lutzow-Holm Complex, East Antarctica is compared with the model result. The garnet aspect ratio systematically varies with respect to grain size, and the most elongate ones are of intermediate grain size (similar to 250 mm). A two-stage deformation with high and low differential stresses (stage A and B) best explains for the observed L-R pattern of garnets. The duration (t(A)) at stage A is determined only when the differential stress (sigma(A)) is assumed due to the size independency of dislocation creep. Assuming gamma(grtqtz) = 1.0 N m(-1), Dg(b),(A) = 1.0 x 10(-12) m(2) s(-1), D-gb,D-B = 1.0 x 10(-11) m(2) s(-1), and sigma(A) = 1.0 MPa, the garnet data are fitted to a theoretical curve under condition of sigma(B) = 1.4 x 10(-2) MPa, t(A) = 0.5 Myr, and t(B) = 14.4 Myr.

DOI： 10.1029/2004JB003526

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

IODP Expeditions 304 and 305: Oceanic core complex formation, Atlantis Massif Reviewed International coauthorship

Blackman D

Scientific Drilling Vol. 1 page： 28-31 - 31 2005.9

AM Lin, T Maruyama, S Aaron, K Michibayashi, A Camacho, KI Kano

TECTONOPHYSICS Vol. 402 ( 1-4 ) page： 21 - 35 2005.6

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

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, splierulitic 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), myloniterelated (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. (c) 2005 Elsevier B.V All rights reserved.

DOI： 10.1016/j.tecto.2004.10.016

Integrated Ocean Drilling Program Expedition 305 preliminary report oceanic core complex formation, Atlantis Massif oceanic core complex formation, Atlantis Massif, Mid-Atlantic Ridge: Drilling into the footwall and hanging wall of a tectonic exposure of deep, young oceanic lithosphere to study deformation, alteration, and melt generation Reviewed

Donna Blackman, Benoit Ildefonse, Barbara E. John, Yasuhiko Ohara, D. Jay Miller, Christopher J. MacLeod, Heike Delius, Natsue Abe, James S. Beard, Daniele Brunelli, Adélie G. Delacour, Javier Escartin, Patricia B. Fryer, Angela Halfpenny, Heidi Elisabeth Hansen, Amber C. Harris, Akihiro Tamura Hasebe, Eric Hellebrand, Satoko Ishimaru, Kevin T.M. Johnson, Garry D. Karner, Margaret Linek, Olivia U. Mason, Katsuyoshi Michibayashi, Toshio Nozaka, Martin Rosner, Guenter Suhr, Masako Tominaga, Toru Yamasaki, Xixi Zhao, Florence Einaudi, Michael W. Abratis, Eric S. Andal, Muriel Andreani, Shunsaku Awaji, Allison Charney, David Christie, Marion Drouin, Bryce R. Frost, Jeffrey S. Gee, Marguerite Godard, Craig B. Grimes, Nicholas W. Hayman, Takehiro Hirose, James Gregory Hirth, Jinichiro Maeda, Andrew M. McCaig, Antony Morris, Tatsunori Nakagawa, Roger C. Searle, Anette Von Der Handt, Alexander Simpson, Wayne Malone, Ronald M. Grout, Roy Davis, Lisa K. Crowder, Chieh Peng, Margo Cortes, Charlie Endris, Dennis Graham, Michael J. Hodge, Leah Shannon Housley, Eric Jackson, Jan Jurie Kotze, Lena Maeda, Erik Moortgat, Michael Murphy, Pieter Pretorius, Paula Weiss, Robert M. Wheatley, Yasmin Yabyabin, Javier Espinosa

Integrated Ocean Drilling Program ( 305 ) page： 1 - 78 2005

Language：English Publishing type：Research paper (scientific journal)

Integrated Ocean Drilling Program Expedition 305, a joint science program with Expedition 304, was designed to investigate the processes that control formation of oceanic core complexes, as well as the exposure of ultramafic rocks in very young oceanic lithosphere. Prior studies indicated that two main drill sites on Atlantis Massif, on the western rift flank of the Mid-Atlantic Ridge (MAR) at 30°N, could provide key constraints on the structure of the detachment fault zone, rock types exposed at shallow structural levels in the footwall, and their alteration history, as well as that of the volcanic succession in the hanging wall. Expedition 305 deepened Hole U1309D in the footwall of Atlantis Massif to 1415.5 meters below seafloor, with high recovery (average = 74.8%) of dominantly gabbroic rocks. Hole U1309D was logged twice, providing the opportunity for unprecedented core-logging integration for a deep borehole in the oceanic lithosphere. The recovered rocks range from dunitic troctolite, troctolite, (olivine) gabbro, and gabbronorite to evolved oxide gabbro that locally contains abundant zircon and apatite, and diabase. The texture of the dunitic troctolite suggests a cumulate origin. The gabbroic suite from Hole U1309D is among the most primitive recovered from the MAR, with Mg# ranging from 67 to 87. Although alteration mineral assemblages record cooling of gabbroic rocks from magmatic conditions to zeolite facies, a low-temperature phase that reflects alteration at temperatures <500°C is most significant. The overall trends in alteration and the changes in secondary mineralogy downhole suggest that there may be two separate secondary processes that have affected the footwall in the vicinity of Hole U1309D. In the upper ∼840 m, seawater-rock interactions may pervade the gabbroic sequence. Below that depth, the nature of and the fluctuations in degree and style of metamorphism are related to fluids of a different composition percolating along fault/ductile deformation zones. Hence, the core records an extensive history of gabbroic rock-fluid interaction, possibly including magmatic fluids. One of the prominent features of the rocks from Hole U1309D is the lack of extensive amphibolite facies alteration and deformation. This contrasts strongly with the gabbroic suite recovered from Ocean Drilling Program Hole 735B, at the Southwest Indian Ridge. The rocks recovered in Hole U1309D show very little deformation, and any deformation related to a major detachment fault system must have occurred at low temperature and must be strongly localized in the very upper part of the hole. This, together with very minor deformation in the amphibolite facies, is not consistent with the classical "core complex" interpretation of the corrugated, domal massifs on the seafloor resulting from surface exposure of a detachment fault that roots deeply at the base of the lithosphere. In addition, shipboard paleomagnetic measurements indicate there has been no significant net tectonic rotation (5°) of the footwall. This seems to preclude a rolling hinge model for the uplift of the core of Atlantis Massif along a single concave, normal fault. The ∼ 1.4 km sequence of dominantly gabbroic rocks is inconsistent with the initial prediction that the footwall was composed of an uplifted mantle section where serpentinization was responsible for lower densities/seismic velocities in the upper few hundred meters. A more complex model than that put forward before Expeditions 304 and 305 will be required. The fact that we did not reach fresh mantle peridotite, together with the known exposures of serpentinized mantle along the southern ridge of the massif, supports models of complicated lateral heterogeneity in slow-spreading oceanic crust. We have, however, placed a constraint on the magnitude of this heterogeneity - gabbro bodies in this setting can exceed 1.5 km in thickness.