Journal of Geography (Chigaku Zasshi)
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
Volume 117 , Issue 1
Showing 1-22 articles out of 22 articles from the selected issue
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Special Issue on “From the Crust to the Mantle: What is the Moho?”
I. Seismological Aspects of the Moho
Original Articles
  • Junzo KASAHARA, Sadao UNOU, Kayoko TSURUGA, Toshihiro IKE, Keita KODA
    2008 Volume 117 Issue 1 Pages 5-44
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     The thickness of the Moho transition zone (MTZ) at the boundary between the Earth's crust and the subjacent mantle has a significant effect on seismic responses from the Moho. We examined the seismic characteristics of Moho reflections (hereafter PmP) using Multi-Channel Seismic (MCS) records obtained from high-quality seismic experiments in the western Pacific by Japan Oil, Gas and Metals National Corporation (JOGMEC). The MCS records show clear reflections at ∼6-10km in depth from the ocean bottom in the north and south of Ogasawara Plateau; however, considering horizontal variations in PmP intensity, the nature of the MTZ varies by location. In seismic profile D00-D, across Ogasawara Plateau in the N-S direction, the PmP abruptly disappears far from the nearby seamount where the overlain sedimentary section shows less change. In another case, shown in D00-C located 130km west of D00-D, the PmP clearly shows a high-amplitude continuous reflection near the seamount's flank. Data acquisition is relatively constant for the Ogasawara MCS reflection lines; therefore, the difference in PmP intensity between D00-D and D00-C might relate to the nature of the Moho.
     We calculated synthetic seismograms to evaluate the effects of MTZ thickness on seismic reflection records. The results suggest that if the thickness of the Moho transition zone is less than 1km for the dominant frequency of 4Hz, then PmP can be observed with the current MCS survey equipment. If the dominant frequency of the MCS reflection survey is ∼15Hz, penetrating down to the Moho depth, then the thickness of the Moho required to identify the PmP should be less than a few hundred meters. Moreover, anisotropy assuming a strong olivine preferred orientation in peridotite might affect the change of PmP intensity. The MCS reflection records in the western Pacific and the western Philippine Sea Basin suggest that the thickness of MTZ varies from ∼100m to more than a few kilometers. This is consistent with petrological observations in Oman ophiolite, sections of oceanic crust, and possible mantle rock, showing that the thickness of the mafic crust to ultra-mafic mantle transition varies from an order of meters to a few kilometers.
     The next target of the IODP seems to be to obtain the mantle constituent materials below the Moho and to explore the nature of the Moho. Considering the large heterogeneity of MTZ even in the oceanic region, the IODP drilling site to drill to the Moho depth should be carefully selected based on an understanding of the geophysical background of the proposed sites.
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  • Katsuhiko SHIOMI, Makoto MATSUBARA, Kazushige OBARA
    2008 Volume 117 Issue 1 Pages 45-58
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     By the end of the last century, the rough configuration of the Moho discontinuity beneath the Japan Islands had been revealed based on explosion surveys and natural earthquake observations. Recently, however, some researchers have pointed out that local roughness of the Moho geometry or relative location between continental and oceanic Moho might provide important knowledge about the source regions of large earthquakes.
     Within the southern portion of the Kinki district, the Philippine Sea plate subducts beneath the continental plate at the Nankai Trough. We detect P-to-S converted wave energy from the Moho velocity discontinuity beneath the Kinki district with receiver function analysis, and compare the results of other recent investigations of the depth of Moho. Both oceanic and continental Moho discontinuities are detected in not only our receiver function analysis but also active-source seismic exploration survey and travel-time tomography analysis. The inferred depths of the subducting oceanic Moho beneath the Kii Peninsula, the southern Kinki district, and the continental Moho beneath the northern Kinki correspond well with each other. However, beneath the central Kinki district, no significant converted phases are observed corresponding to the Moho depth inferred from the travel-time analyses. We interpret that no sharp velocity discontinuity exists around the Moho in the central Kinki district.
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  • Akira HASEGAWA, Junichi NAKAJIMA, Saeko KITA, Yusuke TSUJI, Kyohei NII ...
    2008 Volume 117 Issue 1 Pages 59-75
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     Transportation of H2O from the slab to the arc crust by way of the mantle wedge is discussed based on seismic observations in the northeastern Japan subduction zone. A belt of intraslab seismicity, perhaps caused by dehydration of eclogite-forming phase transformations, has been found in the Pacific slab crust at depths of 70-90 km parallel to iso-depth contours of the plate interface, showing the major locations of slab dehydration. H2O thus released from the slab may be hosted by serpentine and chlorite just above the slab and is dragged downward. DD seismic tomography detected this layer of serpentine and chlorite as a thin S-wave low-velocity layer. Serpentine and chlorite thus brought down to a depth of 150-200 km should decompose there. H2O released by this dehydration decomposition is then transported upward and encounters the upwelling flow directly above, which perhaps causes partial melting of materials within the upwelling flow. Seismic tomography studies have clearly imaged this upwelling flow as an inclined sheet-like seismic low-velocity zone at depths of 30-150 km in the mantle wedge subparallel to the subducted slab. This upwelling flow finally meets the Moho below the volcanic front, and melts thus transported perhaps stagnate directly below the Moho. Some of them further migrate into the crust, and are also imaged by seismic tomography as low velocity areas. Their upward migration and repeated discharge to the surface form the volcanic front. Seismic tomography study of the mantle wedge further revealed along-arc variations of the inclined low-velocity zone: very low velocity areas appear periodically every ∼80 km along the strike of the arc in the backarc region of northeastern Japan above which clustering of Quaternary volcanoes and topography highs are located, suggesting that melts could segregate from these very low velocity areas in the upwelling flow and rise vertically to form volcanoes at the surface in the backarc region.
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  • Junichi NAKAJIMA, Akira HASEGAWA
    2008 Volume 117 Issue 1 Pages 76-92
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     Recent studies on seismic velocity and anisotropy structures beneath the northeastern Japan arc and their quantitative analyses have deepened our understanding of fluid circulation and magmatism in the mantle wedge. A prominent inclined low-velocity zone is clearly imaged in the mantle wedge sub-parallel to the down-dip direction of the Pacific slab, which is considered to be an upwelling flow induced by subduction of the slab. The flow direction is inferred to be parallel to the maximum-dip direction of the slab from shear-wave splitting analyses. Quantitative analyses of the inclined low-velocity and high-attenuation zone in the mantle wedge reveal that temperatures in the upwelling flow are higher than the wet solidus of peridotite and melts with a volume fraction of 1-4% are generated in the flow. These observations suggest that the upwelling in the mantle wedge plays an important role in arc magmatism.
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Review Article
  • Katsuyoshi MICHIBAYASHI
    2008 Volume 117 Issue 1 Pages 93-109
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     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.
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II. Petrological Aspects of the Moho
Review Articles
  • Shoji ARAI, Natsue ABE
    2008 Volume 117 Issue 1 Pages 110-123
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     This article reviews interpretations of the geological and petrological nature of the Moho, which is defined as a discontinuity in terms of Vp, with a view to preparing for the Mohole on the ocean floor in IODP. We strongly propose discarding non-seismic terms for the Moho, such as “petrologic Moho”. The nature of the Moho has been controversial for a long time; an isochemical phase transition boundary between gabbro (crust) and eclogite (mantle) was favored for the Moho by some researchers, while a chemical boundary between mafic rocks (crust) and peridotite rocks (upper mantle) is now favored by a majority of researchers. Boundaries between completely or partially serpentinized peridotite and fresh peridotite may be applicable as the Moho at some parts of the ocean floors of a slow-spreading ridge origin. Antigorite serpentinite can be expected to be observed at the lowermost crust if the Moho is the serpentinization front at the stability limit of serpentine. The Moho beneath the Japan arcs can be estimated using mafic-ultramafic xenoliths in Cenozoic volcanics. Peridotitic rocks scarcely mix with feldspathic rocks, indicating that the Moho at that location is the boundary between feldspathic rocks (mostly mafic granulites; crust) and spinel pyroxenites (mantle). Possible fossil Mohos are observed in well-preserved ophiolites, such as the Oman ophiolite. Two types of Moho are distinct in the Oman ophiolite; gabbro-in-dunite Moho, where a gabbro band network in dunite changes upward to the layered gabbro within a few to several tens of meters, and dunite-in-gabbro Moho, where late-intrusive dunites intruded into gabbros. The former is of a primary origin at a fast-spreading ridge, and the latter is of a secondary origin at a subduction-zone setting in the obduction of the oceanic lithosphere as an ophiolite. The gabbro/peridotite (dunite) boundary as the primary Moho forms in embryo as a wall of melt conduit at fast-spreading ridges as well as at the segment center of slow-spreading ridges. The oceanic primary Moho is modified to various degrees by magmatism, metamorphism and tectonism in subsequent arc and continental environments. The gabbro-in-dunite Moho formation in the Oman ophiolite is an embryo of this modification.
     We expect in-situ sampling across the primary oceanic Moho formed at a fast-spreading ridge through the Mohole of IODP. Ultra-deep drilling at gabbro/peridotite complexes exposed on the ocean floor is indispensable for our understanding of the suboceanic upper mantle. Studies on appropriate ophiolites and deep-seated xenoliths from oceanic areas should complement the Mohole and other ultra-deep drillings to grasp the whole picture of the oceanic upper mantle.
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  • Hiroshi SATO, Hidenori KUMAGAI, Natsuki NEO, Kentaro NAKAMURA
    2008 Volume 117 Issue 1 Pages 124-145
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     Mid-ocean ridge basalt (hereafter, MORB) is a final product of melt generated from the partial melting of mantle peridotite, following reaction with mantle and/or lower crustral rocks, fractionation at a shallower crust and other processes en route to seafloor. Therefore, it is difficult to estimate melting processes at the upper mantle solely from any investigations of MORB. In contrast to the restricted occurrence of peridotite of mantle origin in particular tectonic settings (e.g., ophiolites, fracture zones, or oceanic core complexes), the ubiquitous presence of MORB provides us with a key to understanding global geochemical variations of the Earth's interior in relation to plate tectonics.
     In fact, MORB has been considered to show a homogeneous chemical composition. In terms of volcanic rocks from other tectonic settings (e.g., island arc, continental crust, ocean island), this simple concept seems to be true. However, recent investigations reveal that even MORB has significant chemical variations that seem to correspond to location (Pacific, Atlantic, and Indian Oceans). These observations suggest that the mantle beneath each ocean has a distinct chemical composition and an internally heterogeneous composition.
     In this paper, global geochemical variations of MORB in terms of major and trace element compositions and isotope ratios are examined using a recently compiled database. The compilation suggests that MORB has heterogeneous compositions, which seem to originate from a mixture of depleted mantle and some enriched materials. Coupled with trace element compositions and Pb-isotope ratios, there seems to be at least two geochemical and isotopic domain of the upper most mantle: equatorial Atlantic-Pacific Oceans and southern Atlantic-Indian Ocean. Material (melt and/or solid) derived from plume, subducted slab, subcontinental crust, or fluid added beneath an ancient subduction zone is a candidate to explain the enrichment end-member to produce heterogeneous MORB.
     Because MORB is heterogeneous, using a tectonic discrimination diagram that implicitly subsumes homogeneous MORB or its mantle sources should be reconsidered. Further investigations, particularly of off-axis MORB, are needed to understand the relationship between heterogeneous compositions of MORB and geophysical parameters (e.g., degree of melting, temperature, spreading rate, crustal thickness, etc). In addition, the role of the MOHO transitional zone should be investigated to interpret the chemical characteristics of MORB.
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  • Kazuhito OZAWA
    2008 Volume 117 Issue 1 Pages 146-167
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     The vicinity of the oceanic Mohorovicic discontinuity, transitional zone between the oceanic crust and mantle, is characterized by the common occurrence of dunite consisting mostly of olivine with small amounts of chromite. The most plausible formation mechanism of such dunite is believed to be an open-system reaction between pyroxene-bearing mantle peridotites, residues of partial melting, and basaltic silicate melts, partial melting products and the main ingredient of the oceanic crust. It is, therefore, important to specify the reaction stoichiometry and rates of influx and separation of basaltic melt involved in the reaction to better understand the formation mechanism of the transitional zone. Geological, petrological, and geochemical observations of ancient oceanic crust-mantle sections (ophiolites) and dredging and drilling of the current ocean floors provide key information for constraining the reaction processes. The status quo of studies on ophiolites and the ocean floor related to this subject is reviewed.
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Original Article
  • Sumio MIYASHITA, Yoshiko ADACHI, Susumu UMINO
    2008 Volume 117 Issue 1 Pages 168-189
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     Recent progress of studies on oceanic crust is reviewed focusing on the architecture and its formation process. Although crustal thickness does not vary with spreading rates with the exception of very-slow spread oceanic crust, architectures vary with spreading rates. The thickness of the upper oceanic crust (lava and sheeted dike complex) decreases with increasing spreading rates at intermediate to ultra-fast spread oceanic crusts. This implies that the gabbroic section thickens with spreading rates. The lava layer seems to become thicker, while sheeted dike complex becomes thinner with spreading rates. Thus, a systematic change of the crustal structures is expected with spreading rates for intermediate to ultra-fast spread oceanic crusts.
     However, such systematics are not followed at a slow to ultra-slow spread oceanic crust, where the sheeted dike complex might occur only locally. Therefore, the magmatic systems controlling the formation of the crustal architecture differ significantly between slow to ultra-slow and intermediate to ultra-fast spread oceanic crusts. The difference is also shown in lower oceanic crusts, i.e., small and isolated numerous gabbroic intrusions at slow to ultra-slow spread crusts, and thick successive gabbroic layer at intermediate to ultra-fast spread oceanic crusts.
     Recent dense sampling from extensive areas near the East Pacific Rise and drilling at Hole 1256D show that off-axis magmatism plays an important role to in making the lava layer thicker. It should be emphasized that the upper succession in the lava layer might correspond to magmatism from on-axis to off-axis toward the upside. Therefore, vertical variations in the upper portion of the lava layer show a lateral igneous variation from on-axis to off-axis magmatism.
     Another important result obtained at Hole 1256D is the appearance of granoblastic dikes that are recrystallized under high temperature conditions up to pyroxene hornfels facies. Petrographical and petrological observations suggest that dehydration partial melting occurred in the metamorphosed dikes due to invasion by gabbros.
     Gabbroic rocks recently obtained from Hole U1309D of the Atlantis massif near the Mid-Atlantic Ridge, give a new constraint for the formation process of slow spread oceanic crusts. Down hole variation at this site confirms that the lower crusts of slow spread crusts are composed of many gabbroic intrusions. However, it is noted that the most primitive gabbroic rocks such as troctolite occur in the upside of the drill hole as was the case for Hole 735B of the Atlantis bank. It is not understood yet what mechanism produces such primitive gabbros at the upside.
     The accretion fashion of the lower crust beneath intermediate to ultra-fast spreading ridges is still controversial with two end models proposed, gabbro glacier model and sheeted sill model. Hybrid models based on thermal analyses and petrological data have also been proposed. Segmentation structures might also affect also for the accretion style of the lower crust. Much deeper drillings and detailed studies of ophiolites are required to solve the accretion style of lower oceanic crusts beneath intermediate to ultra-fast spreading ridges.
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Review Article
  • Susumu UMINO, Nobuo GESHI, Hidenori KUMAGAI, Kiyoyuki KISIMOTO
    2008 Volume 117 Issue 1 Pages 190-219
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     A large number of intraplate volcanoes erupted two to several hundred kilometers off the fast-spreading East Pacific Rise (EPR). These volcanoes consist of large lava fields, monogenetic volcanoes, and linear chains of monogenetic volcanoes and volcanic ridges. Large lava fields of 7-26 km3 in volume are known at 8°N, 14°S, and 16°S within 2-19 km from the rise axis and from the top 75-100 m of ODP Site 1256 on the 15 Ma Cocos plate. Monogenetic volcanoes form within ∼20 km from the rise axis or on the basement < 200 kyr, and are evenly distributed over the rise axis. Linearly aligned volcanoes and volcanic ridges occur farther from the rise axis than large lava fields and monogenetic volcanoes, and run subparallel to the direction of the Pacific plate motion. The Sojourn Ridge, the largest volcanic ridge, extends up to 440 km in length and is several hundred cubic kilometers in volume. Eruptive ages along a volcanic ridge and a volcano chain contradict the hot-spot origin of these volcanic features. Negative free-air and residual mantle Bouguer anomalies correlate well with the linearly aligned volcanoes and volcanic ridges, suggesting excess magma supply beneath the volcanic edifices. Seismic experiments show volcanic ridges have no keel below the Moho, indicating compensation of surface loading by plate flexure and underplating.
     Whole rock compositions of off-ridge volcanoes have a much wider spectrum than the adjacent axial lavas, spanning from depleted NMORB through TMORB to isotopically fertile EMORB. Some off-ridge lavas could be produced by the fractional crystallization of the same parent magma as the adjacent axial lavas. However, most off-ridge lavas originate from different parent magmas than the neighboring axial lavas. Some TMORB magmas including the 14°S large lava field are the mixing product of the NMORB and EMORB magmas. Copious differentiated lavas of the large lava fields require a large magma chamber as a the site for crystallization differentiation and magma mixing. The lava geochemistry of off-ridge volcanoes strongly suggests the presence of a magma source that is independent of the axial magma plumbing system.
     Seismic tomography and seafloor compliance measurements beneath the northern EPR indicate that the presence of melt across the rise axis is restricted in a narrow zone ∼4 km in width through the crust, but has a 10-14 km wide distribution in the uppermost mantle. Broad distribution, volume, and geochemistry of off-ridge monogenetic volcanoes and large lava fields strongly suggest that the off-ridge volcanoes originated from the Moho transition zone (MTZ). The MTZ is formed by a reaction between the uprising magma and the host mantle peridotite, leaving replacive dunite that experienced variable depletion and enrichment processes. Passive asthenospheric upwelling beneath the fast-spreading ridges produces a broad partial melt zone, through which magma ascends and accumulates beneath the off-ridge lithosphere. More depleted off-ridge magmas than axial magmas differentiate and mix with residual magmas in the MTZ, and react with variably enriched, impregnated dunite, resulting in variety of off-ridge lava compositions.
     Small clusters of volcanoes and linear volcano chains are created by partial melting in asthenospheric return flows or local instability of the thermal boundary layer beneath the cooling lithosphere. Linear volcano chains will develop into long and robust volcanic ridges extending several hundred kilometers in length.
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Original Articles
  • Tomoaki MORISHITA, Kentaro NAKAMURA, Takashi SAWAGUCHI, Kaori HARA, Sh ...
    2008 Volume 117 Issue 1 Pages 220-252
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     The hydrothermal circulation of seawater in the oceanic lithosphere is an important factor controlling seawater chemistry, compositions of subducted materials returned to the mantle and microbial activity. We summarize the results of hydrothermally altered rocks taken directly from the ocean floor in terms of major and trace elements combined with petrographic descriptions. Hydrothermal circulation starts at the spreading axis where magmatic heat from a basaltic crustal formation is available (high temperature of > 350°C). Low-temperature alteration (< 150°C) may persist for > a million of years through the ridge flanks. Due to ridge flanks occupying large regions of the seafloor, changes in chemistry, mineralogy and physical properties of the oceanic lithosphere are accompanied by geochemical fluxes that may be even larger than those at the ridge axis. Two deep drill holes, DSDP/ODP 504B and 1256D, allow an examination of downhole variations of hydrothermal alteration in basaltic rocks, and dolerite in the extrusive and sheeted dike sequence. Recent direct sampling from the ocean floor reveals that gabbros and peridotites crop out in significant amounts on the ocean floor, particularly in the slow-spreading ridges. The chemical behavior of these originally deep-seated rocks during hydrothermal circulation thus has a large impact on global mass budgets for many elements.
     Previous studies on the ocean floor have been mainly conducted in the Atlantic Ocean and the Pacific Ocean. We present our results on hydrothermally altered basaltic rocks, gabbros and peridotites recovered from the Indian Ocean. Basaltic samples dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction are classified into three types—a fresh lavas, low-temperature altered rocks and high-temperature altered rocks. Petrological and geochemical features of these rocks are basically comparable to those of the basaltic rocks in DSDP/ODP Hole 504B, which suggests generalities in alteration processes and chemical exchange fluxes during hydrothermal activity across all world oceans. Gabbros and peridotites were sampled from an oceanic core complex, which was composed of tectonically exposed footwalls of detachment faults, from the Central Indian Ridge. Less deformed serpentinized and gabbros were recovered from the ridge-facing slope, whereas highly deformed schist-mylonites of a mixture of these rocks were recovered from the top of the surface (i.e., detachment fault). Efficient localization of strain was probably due to the formation of secondary minerals (e.g., talc, chlorite, serpentine) onto large, discrete shear zones where fluid was introduced locally. In-situ microanalysis of trace elements of the primary minerals and their secondary minerals revealed that selective elements, such as Rb, Sr, Ba, Pb and U, are enriched in the secondary minerals. Although oceanic core complexes are places that allow cross-sectional samplings of deep-seated rocks (i.e., gabbros and peridotites) in the oceanic lithosphere, we should keep in mind the implications of the results for the normal oceanic lithosphere. To understand the nature of the oceanic lithosphere, a close linkage between the ophiolite study and a number of deep holes in the oceanic lithosphere, including a deep hole through the crust-mantle boundary, is required.
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Review Article
  • Toshio NOZAKA
    2008 Volume 117 Issue 1 Pages 253-267
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     Gabbroic rocks recovered from deep holes in the oceanic crust significantly vary in the abundance and assemblage of alteration minerals, showing a close association with the original lithology and distribution of dikes and veins. The mineralogical variation is considered to reflect the durability of primary minerals, accessibility and composition of alteration fluids, and alteration temperature. Textural relationships of alteration minerals suggest a common cooling history of oceanic gabbros from granulite or pyroxene hornfels facies to zeolite facies conditions. It is considered that regardless of spreading rate, the static formation of upper greenschist- to lower amphibolite-facies minerals is the dominant alteration process at the lower crust near oceanic ridges, whereas subgreenschist-facies alteration represents the exhumation histories of gabbroic masses from depth. High-temperature plastic shear zones with almost anhydrous recrystallization of primary minerals develop locally at slow-spreading ridges, and possibly provide pathways for later hydrothermal fluids.
     In contrast to the gabbroic rocks, oceanic peridotites have a monotonous mineralogy formed during low-temperature serpentinization processes, making it difficult for us to depict their cooling histories or in-situ alteration processes at the upper mantle.
     The hypothesis that oceanic Moho represents a serpentinization front in peridotites is suitable for the uniformity of crustal thickness inferred from seismological observations, but lacks a rationale for supplying a constant amount of water to the upper mantle or for the cessation of serpentinization at a constant degree. Alternatively, preferential alteration of pyroxene at relatively high-temperature conditions might form the oceanic crust of uniform thickness.
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III. Explanation Notes
  • Yukio ISOZAKI
    2008 Volume 117 Issue 1 Pages 268-269
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     Andrija Mohorovičić (1857-1936) is a world-famous Croatian geophysicist and the discoverer of the Earth's crust/mantle boundary known as the Moho-discontinuity. The historical seismometer used by A. Mohorovičić to detect the Moho in 1909 is still maintained in working conditions, and is displayed in the Department of Geophysics of the University of Zagreb, Croatia, together with memorabilia from his office.
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Notes
Report of the Spring Meeting, 2007
  • Hikaru IWAMORI
    2008 Volume 117 Issue 1 Pages 292-298
    Published: February 25, 2008
    Released: February 10, 2010
    JOURNALS FREE ACCESS
     The origin of the Sanbagawa-Ryoke metamorphic rocks and associated granitic rocks is discussed with respect to thermal structure and water cycling in subduction zones. First, the relationship between thermal structure and water cycling is summarized based on previous studies of numerical modeling for three different settings along the Japan arcs. This shows that water cycling becomes shallower in a warmer environment, which has been demonstrated or supported by studies on seismic structures and distribution of volcanoes. In a very hot environment associated with a young (<10 Ma) plate, including ridge subduction, arc magmatism is shut down to switch to granitic magmatism and regional metamorphism in the forearc region, as both heat and water are supplied to the region. Numerical modeling studies show that a paired metamorphism can occur associated with ridge subduction in a single forearc domain within a relatively short period (a few tens of million of years), which explains the spatial association of Sanbagawa (high-P type) and Ryoke (high-T type) metamorphic rocks associated with granitic batholiths, their similar metamorphic ages, and a common mode of syn-metamorphic deformation (prolate strain). Therefore, in terms of thermal structure and water circulation, arc magmatism and regional paired metamorphism associated with granitic magmatism are regarded as representing different stages of a series of thermal and fluid processes in subduction zones. At any stage of these processes, even after major dehydration of the subducting slab and the overlying mantle wedge, nominally anhydrous minerals, such as olivine, pyroxene, and garnet, can carry a significant amount of water (up to several 1000 ppm) to the deep mantle. Recent geochemical and statistical studies have revealed that such signatures of deeply subducted fluid and associated elements are indeed identified in oceanic basalts as exits of global material circulation. Based on an understanding of the thermal structure and water circulation in subduction zones, it is argued that the fluid processes and slab configuration (e.g., cold hard slab vs. hot soft slab) exercise great control on large-scale dynamics in subduction zones, such as formation of backarc basins (e.g., Japan Sea), and wet region representing extensive magmatism with abundant volatile components over eastern Asia. Regional metamorphism, arc magmatism, large-scale dynamics in Eastern Asia, and global material circulation are potentially all connected with each other, irrespective of qualification or disqualification of the model or working hypothesis discussed in this paper.
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