Central Shikoku Research Articles

Timing and displacement of holocene faulting on the median tectonic line in central shikoku, southwest japan

The Median Tectonic Line (MTL) extends about 900 km parallel to the general trend of the southwest Japan island arc. The central segment of the MTL has been active during the Quaternary. Although the average slip-rate along the MTL is up to several mm a -1, there is no historical record of destructive earthquakes along it. In order to collect data on the behavior of the MTL during late Holocene times, especially on the timing of seismic events and the amount of displacement during each event, trench studies were conducted on the active segment in central Shikoku in 1984 and 1988. The results are summarized as follows: (1) the latest two seismic events produced by slip on the MTL in central Shikoku occurred during the early 8th century A.D., and between 3250 and 2820 years B.P.; (2) the amount of displacement during the latest event is inferred to have been about 5.7 m; (3) the earthquake magnitude for the latest event is estimated at 7.9 (Richter scale), and a fault-segment trace about 70 km long ruptured during that event; (4) an average slip-rate along the active trace of the MTL in central Shikoku during the late Quaternary is estimated to be 5–8 mm a -1.

上八川―池川構造線に沿う珪長質火成岩類のＫ‐Ａｒ年代

K-Ar dating was carried out on the Yanagino dacite and Takaiwa rhyolite which form part of a roughly E-W chain of small felsic intrusions along the Kamiyakawa-Ikegawa tectonic line in central Shikoku. K-Ar whole rock ages of the Yanagino dacite range from 15.2 ± 0.4 Ma to 16.9 ± 0.4 Ma and its biotite yields a little younger age of 14.2 ± 0.4 Ma. A K-Ar whole rock age of 15.0 ± 0.5 Ma was determined for the Takaiwa rhyolite. Present study confirms the felsic intrusions are correlated in age with the Miocene Ishizuchi Group and related intrusions distributed to the north of the tectonic line.

四国中央部における三波川帯・秩父累帯間の地体構造境界

四国中央部における三波川帯・秩父累帯間の地体構造境界

中・西部日本の地殻応力

We compiled data on the direction of the crustal stress measured in-situ and estimated from shallow (depth≤35km) earthquake focal mechanisms in the central and western parts of Honshu. In the central part of Honshu, the stress data were compiled by Tsukahara and Ikeda in 1991. This report summarizes the stress state in a rather large area: the central and western parts of Honshu. The results are as follows. (1) The stress state estimated from the stress direction and the type of faulting in and around the Izu Peninsula are explained in terms of dynamic interaction between the Philippine Sea (PHS) plate and the Asian (ASA) plate. (2) In the central part of the Kanto plain and the Kanto mountains, the minimum horizontal compressive stress (SHmin) is equal to or smaller than the vertical stress (i. e., tensile stress), and the azimuthal direction is roughly east-west. This stress condition cannot be explained by the westward movement of the Pacific (PAC) plate or the northwestward movement of the PHS plate against the ASA plate. (3) In the areas of the northern part of Kanto, Shin'etsu, Gifu, Hokuriku, and Chugoku, the azimuthal direction of the maximum horizontal compressive stress (SHmax) is west-northwest=east-southeast. Although this direction is approximately the same as that of the movement of the PAC plate, the stress state in these areas is not attributed to the plate movement because (a) some provinces with quite different stress directions lie between the PAC plate and these areas, and (b) SHmin in these areas is tensile stress. It seems that the stress state of this area is dominated by a large scale crustal movement of the Japan Sea or Eastern Asia. (4) In the area of Lake Biwa to Osaka, the stress state is slightly different from that in the areas discussed in (3), i. e., (a) the azimuth of the SHmax direction is approximately east-west, and (b) the type of faulting is different. There are strike-slip and reverse faults as opposed to strike-slip faulting alone for the surrounding area. The reason why this area has its own unique characteristics is not yet understood. (5) In the central Shikoku region, the azimuth of the SHmax direction is approximately east-west and the stress state regime is of the strike-slip faulting type. The east to west SHmax azimuth agrees well with the maximum shortening direction obtained from geodetic surveys in the region during these 100 years, which is attributable to the large crustal deformation at the Nankai Earthquake (1946, M=8.1). In contrast, however, the maximum shortening direction after the earthquake has been north to south. Therefore, the east-west SHmaxazimuth will change to north-south in the future.

40Ar/ 39Ar mineral age constraints for the tectonothermal evolution of the Sambagawa metamorphic belt, central Shikoku, Japan: a Cretaceous accretionary prism

The Sambagawa metamorphic belt exposed in central Shikoku, Japan, is tectonostratigraphically represented by the Oboke and structurally overlying Besshi nappe complexes. Oboke units consist of metamorphic rocks largely derived from clastic protoliths (sandstone and conglomerate) whereas Besshi units are dominated by rocks with oceanic protolith affinities (shale, chert, and greenstone). Whole-rock schist samples from low-grade Oboke units record intermediate temperature. 40Ar 39Ar plateau ages of 70–77 Ma, which are interpreted to closely date attainment of maximum metamorphic conditions. Whole-rock schist samples from Besshi units (chlorite zone) record plateau ages of 85–94 Ma suggesting an earlier metamorphic climax within this structural level. Amphibole within the albite-biotite zone in Besshi records 36Ar 40Ar vs. 39Ar 40Ar plateau isotope correlation ages of 84–87 Ma, which are interpreted as dating the post-metamorphic cooling through temperatures required for intracrystalline retention of argon. A slightly older isotope correlation age (94 Ma) is recorded by amphibole from the higher grade oligioclase-biotite zone. Contrasts in amphibole cooling ages are interpreted as reflecting thrust-related metamorphic inversion and resultant earlier cooling of higher grade structural units. Muscovite from all structural levels of the Besshi nappe complex records 40Ar 39Ar plateau ages of 76–89 Ma. These are interpreted to date post-metamorphic cooling through appropriate closure temperatures following structural assembly of the Besshi nappe complex. Protoliths of the Besshi units originated at approx. 130 Ma and attained maximum metamorphic conditions during entrainment in an accretionary complex at approx. 90–100 Ma. Higher grade sequences developed at deeper levels along the hanging wall of the accretionary complex. During subsequent structural uplift to shallower crustal levels the Besshi nappe complex was tectonically emplaced over Oboke units.

Prograde and retrograde metamorphism of hematitebearing basic schists in the Sanbagawa belt in central Shikoku

The mineral assemblages of hematite‐bearing basic schists in intermediate high‐pressure metamorphism are temperature dependent. For assemblages with excess hematite, albite, muscovite and quartz, the paragenetic relations can be dealt with in terms of a four‐component system, without omitting or grouping major components.In the Sanbagawa belt in central Shikoku, the dominant amphibole in the hematite‐bearing basic schists changes from winchite, via crossite and barroisite to hornblende. The stability of amphibole is described chemographically within a pseudoternary system with another excess phase, epidote. Many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low‐T/P amphibole around the prograde amphibole. The examination of amphibole zoning makes it possible to draw a retrograde P‐T trajectory which passes on the lower pressure side of the prograde one.

Clasts of high‐grade Sanbagawa schist in Middle Eocene conglomerates from the Kuma Group, central Shikoku, south‐west Japan

Middle Eocene conglomerates which overlie the Sanbagawa metamorphic rocks contain clasts of metamorphic rock with isotope ages of 120‐85 Ma, which fall within the age range reported from the Sanbagawa metamorphic rocks. They were derived from the chlorite to oligoclase zones of the Sanbagawa metamorphic belt. Clasts of garnet amphibolite and oligoclase‐biotite schist show a mineral assemblage similar to the highest grade Sanbagawa schists. However, the metamorphic temperatures estimated by various mineralogical thermometers show that some of the clasts were formed at higher temperatures than the in situ Sanbagawa metamorphic rocks. Such higher grade rocks were at the surface by the Middle Eocene and for the most part they have been eroded away. Cretaceous and post‐Cretaceous sediments overlie, or are in fault contact with, the Sanbagawa metamorphic rocks which suggests that rocks in the belt were uplifted and eroded from the latest Cretaceous to Middle Eocene time after strike‐slip movement along the Median Tectonic Line. Since the Middle Eocene, the belt has experienced relatively slow uplift which was locally around 2 km in central Shikoku.

Hydration of basic granulite to garnet-epidote amphibolite in the Sanbagawa metamorphic belt, central Shikoku, Japan

Granulite in the Iratsu metagabbro of central Shikoku, with color-banding defined by the presence and absence of plagioclase, is surrounded by a narrow epidote amphibolite, which itself is encased in garnet-epidote amphibolite. The epidote amphibolite formed metastably under low P H 2O conditions based on thermodynamic considerations. The granulite-epidote amphibolite boundary is formed by the hydration reaction of plagioclase to zoisite, kyanite and quartz, and that of clinopyroxene and orthopyroxene to amphibole. If it is assumed that the temperature range of the Sanbagawa epidote amphibolite facies is 600–650°C, we obtain a lithostatic pressure of 12–15 kbar and a H 2O pressure of 1–3 kbar for the hydration conditions. The boundary between epidote amphibolite and garnet-epidote amphibolite is defined by the formation of garnet, which is more stable than the epidote amphibolite assemblages. Pargasitic amphibole occurs at the garnet rim. Up to 0.32 wt.% Cl is present in the amphibole from both of the hydrated facies, but is < 0.03 wt.% Cl in the granulite amphiboles. During hydration, there was an overall increase in bulk-rock Na 2O and Cl contents, and localized migration of Mg and Fe across the layering of the hydrated derivatives. It is concluded that hydration did not take place in the presence of an excess fluid phase, based on the difference of the Cl contents of amphibole between observed and expected values from a qualitative infiltration metasomatic model and also on the estimated low fluid pressure. The formation of a sharp boundary between the granulite and epidote amphibolite suggests intracrystalline diffusion.

Nappe structure of the Sambagawa belt

The thermal structure of the Sambagawa belt, which is reflected by minerals produced during the highest temperature of metamorphism, generally is characterized by the occurrence of the highest grade schists in the middle of the structural pile. The origin of this structure has been analysed in the upper horizon of the Sambagawa schist sequence in Central Shikoku as an example for the structure of the whole belt. The upper horizon of the Sambagawa schist sequence in Central Shikoku consists of three nappes, the Saruta nappe II, the Saruta nappe I and the Fuyunose nappe in descending order of structural level. The Saruta nappe II shows a downward increase of metamorphic grade from the garnet zone, through the albite‐biotite zone, to the oligoclase‐biotite zone. The Saruta nappe I consists mainly of rocks in the albite‐biotite zone and partly in the oligoclase‐biotite zone, but the direction of the increase of metamorphic grade is not clear. The Fuyunose nappe shows an upward increase in metamorphic grade which changes from the glaucophane zone to the barroisite zone. It is concluded that the Saruta nappe II and Saruta nappe I were overturned and then mechanically coupled with the Fuyunose nappe. The Sambagawa metamorphic field, which is of the highest temperature phase of metamorphism, appears to have had an inverted thermal gradient and a thermal structure comparable with that expected in the deeper parts of a subduction complex.

The higher grade metamorphic zonation of the Sambagawa metamorphic belt in central Shikoku, Japan

The higher grade metamorphic zonation of the Sambagawa (= Sanbagawa) belt is established for the first time for the whole area of central Shikoku. As discontinuous reactions to define the isograd are absent, the metamorphic grade is primarily determined by the Mg‐Fe partitioning between garnet and chlorite along representative traverses. However, for regional mapping, mineralogical features of the pelitic schists, such as using mineral assemblages of more than divariant equilibrium, the modal garnet to chlorite ratio, and the optical properties of chlorite, are employed as auxiliary criteria.The presence of the highest grade mineral zone in the middle of the structural level is confirmed, but its spatial distribution is far more complex than hitherto accepted. Thermal axes are now confirmed at three different structural levels. A model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress. Thermal readjustment brought a continuous metamorphic temperature gradient across and within the thrust sheets. Tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re‐equilibrated. Local anomalies of metamorphic grade, represented by mixing of schists of different metamorphic grade, exist, but they are due to a later stage event.

四国中央部および紀伊半島西部黒瀬川地帯北縁の弱変成岩類のＫ‐Ａｒ年代 西南日本における黒瀬川地帯の広がりについて

四国中央部および紀伊半島西部黒瀬川地帯北縁の弱変成岩類のＫ‐Ａｒ年代 西南日本における黒瀬川地帯の広がりについて

四国中央部三波川変成帯の変成分帯

四国中央部三波川変成帯の変成分帯

Discovery of late Triassic radiolarians from the Ino Formation, central Shikoku

Discovery of late Triassic radiolarians from the Ino Formation, central Shikoku

Muscovite K-Ar ages of the Sanbagawa schists, Japan and argon depletion during cooling and deformation

Seventy muscovites from schists in the Sanbagawa terrain in central Shikoku were dated by the K-Ar method. The muscovite ages are consistently older with increasing metamorphic grade. Within the same zone the ages are significantly younger in schists which have been more severely deformed. These K-Ar age variations could be due to systematic argon depletion during deformation i.e., to the dynamic recrystallization of muscovites during ductile deformation that formed a large-scale recumbent fold during the uplift and cooling. Argon loss was greater in schists that were more extensively deformed and in the lower grade zone that experienced a longer period of low-temperature deformation than the higher grade zone. The relationships between age and grain size in a pelitic schist suggest that coarse-grained muscovites lost more argon than the finegrained ones. There was no significant resetting of ages in the vicinity of major strike-slip faults, such as the Median Tecotonic Line or near thrust faults. The combination of geochronological and geological data constrains the cooling rate of the Sanbagawa schists to 9–12° C/Ma in the oligoclase-biotite zone in central Shikoku, Japan.

四国中央部別子地域の三波川変成岩中に産するドロマイト

Dolomite was found in Sanbagawa pelitic and basic schists from the Bessi and Asemigawa-Sarutagawa areas in central Shikoku. The dolomite occurs in two modes: (1) as independent and coarse crystals of 0.2-1.0mm in size; and (2) as rounded and small (0.01-0.1mm in size) crystals dispersed in large calcite crystals. The independent crystals of dolomite are always replaced by secondary calcite and included in garnet porphyroblasts, and hence they are regarded as a product of the Sanbagawa prograde metamorphism. The small dolomite crystals of the second type are probably an exsolution product formed by re-equilibration of Mg-rich primary calcite during retrograde metamorphism. The coexisting independent crystals of dolomite and calcite show equilibrium temperatures of about 540-570°C and 580-620°C in the albite-biotite and oligoclase-biotite zones, respectively. Dolomite+quartz assemblage occurs mostly in the oligoclase-biotite zone of the highest grade, and was not observed in the chlorite and garnet zones of lower grade. On the other hand, tremolite+diopside+calcite+quartz and talc+ calcite+quartz assemblages occur in crystalline limestones and basic schists in the garnet and albite-biotite zones. These facts suggest that (1) Xco2 of metamorphic fluid was limited to low value in the low grade zone of the Sanbagawa terrain, and (2) Xco2, value of the metamorphic fluid in the oligoclase-biotite zone tends to be higher than that of the lower grade zones.

K-Ar ages of carbonate- and mantle nodule-bearing lamprophyre dikes from Shingu, central Shikoku, Southwest Japan.

K-Ar age determinations were carried out on two lamprophyre dikes intruded into the Sambagawa metamorphic rocks at Shingu in the central part of Shikoku Island, Southwest Japan. These dikes are very distinctive among Neogene igneous rocks in Shikoku because of their abundant mantle xenoliths and carbonate minerals in the fresh groundmass, and of their unique chemical compositions. In order to eliminate the effect of extraneous argon from xenoliths and carbonates, removal of large crystals and acid leaching experiments were done for three grain size fractions. Apparent ages ranging from 18.6±0.7Ma to 21.6 ±2.0Ma were obtained for both rocks. However, positive correlation between apparent ages and atm. 40Ar% suggests that samples contain initial argon which is not atmospheric. The well defined isochron age of 17.7±0.5Ma is obtained from all samples with the initial 40Ar/36Ar ratio of 310±5 in the 40Ar/36Ar-40K/36Ar diagram. As the initial 40Ar/36Ar ratio is higher than that of atmosphere, apparent ages do not show any geologic meaning, and the isochron age of 17.7±0.5Ma gives the intrusive age of dikes. The Shingu lamprophyres are much older than other Miocene igneous rocks in Shikoku ranging in age from 14 to 12Ma, and both magmatisms could be genetically independent. As the Shikoku Basin kept opening in a northeast-southwest direction at 18Ma, it could not subduct significantly beneath Southwest Japan from the south when the Shingu lamprophyres intruded into the Sambagawa metamorphic belt. Petrogenesis of the Shingu magma could be related to the rifting of the back arc basin, the Japan Sea.

Sulfur and oxygen isotope ratios of anhydrite from the Iratsu peridotite-hornblendite-epidote amphibolite complex, central Shikoku, Japan.

Sulfur and oxygen isotope ratios were determined for eight anhydrite samples (seven from silicate rocks and one from a marble) from the Iratsu peridotite-hornblendite-epidote amphibolite complex, central Shikoku, Japan. The δ34S values of anhydrite are in the range from +4.7 to +11.0‰. The δ18O values of anhydrite from the silicate rocks are in the range from +8.6 to +10.2‰ and that from the marble is +12.4‰. The fairly uniform δ18O values may indicate that these anhydrites were re-equilibrated with respect to oxygen isotopes with host rocks during the Sambagawa metamorphism. The rather variable, high δ34S values of anhydrite suggest that it is not cognate with the original rocks from which the host rocks were derived. The obtained δ34S values do not provide a straightforward solution concerning the source of sulfur for these anhydrites.

Ardennite in a quartz schist from the Asemi-gawa area in the Sanbagawa metamorphic terrain, central Shikoku, Japan.

Ardennite is found to occur in a piemontite-bearing quartz schist from the Asemigawa area in the Sanbagawa metamorphic terrain, central Shikoku. The ardennite-bearing quartz schist contains quartz, piemontite and garnet as major constituents with subordinate amounts of talc, chlorite, albite, calcite, hematite, braunite, rutile and apatite, together with ardennite. Ardennite is pale golden yellow and occurs as subhedral and prismatic crystals up to 1 mm in length. It is optically positive with 2VZ=57±3°. It is orthorhombic and unit cell parameters are a=5.828(2) A, b=18.540(8) A, c=8.692(3) A and V=939.1(6) A3. As2O5 and V2O5 contents of ardennite reach 10.8 and 1.3 wt.% in maximum, respectively. The empirical formula of the ardennite is (Ca0.53Mn3.67)(Mg1.02Cu0.03Fe0.203+Al4.47Ti0.01)(As0.80V0.09) Si5.25O22 (OH)6. It is slightly poor in (As + V) and rich in Si compared with the ideal formula of ardennite. Piemontite contains SrO of 0.7wt.% in maximum.

Pressure-temperature trajectory of the Sanbagawa metamorphism deduced from garnet zoning

Most garnets in a small volume of the pelitic Sanbagawa schists nucleated simultaneously, grew, and then ceased with their growth, maintaining near-equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism. Chemical trends of zoned garnets systematically shift from the pyrope-poor to pyrope-rich portions in a ternary MnFeMg system in order of ascending metamorphic grade. A ternary (MnFeMg) model system suggests that the higher-grade garnets started to grow at lower pressures and higher temperatures than the lower-grade ones during prograde metamorphism. It follows that high-grade metamorphism took place at lower pressures in the garnet to oligoclase-biotite zones of the Sanbagawa metamorphic terrain in central Shikoku.

Growth history of garnet in pelitic schists of the Sanbagawa metamorphic terrain in central Shikoku

During the prograde metamorphism of the Sanbagawa metamorphic terrain in central Shikoku, garnet has grown in the pelitic schists whilst maintaining surface equilibrium with coexisting minerals such as chlorite and epidote, etc. In the quartz-albite layer, it has grown euhedrally. In the micaceous layer, its crystal morphology varies according to the different minerals in contact and their different orientations. Garnet enclosed in or sandwiched between micas is always ellipsoidal, extended parallel to the basal planes of the micas. However, when the edge of a garnet has intruded at an acute angle into the micas, euhedral garnet has grown. It is concluded that the growth of garnet at the interface with micas ceased when the {110} surface of garnet came into contact with the basal plane of the micas in the tectonic c-axis direction. Other models, such as chloritization-resorption, pressure-solution, and mechanical-erosion or grinding, do not adequately explain the origin of the flattened garnet. During retrograde metamorphism, chloritization and resorption of garnets took place in the matrix, resulting in the rough and irregular surface of garnet. Garnets included in albite porphyroblasts have been largely unaffected by the retrograde metamorphism.