The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 105, Issue 4
Displaying 1-7 of 7 articles from this issue
  • Hisatoshi Ito, Kazuhiro Tanaka
    1999 Volume 105 Issue 4 Pages 241-246
    Published: April 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    K-Ar and zircon fission-track dating was carried out on some granitic rocks in the Hida Range, central Japan. The samples analyzed were collected on both sides of one of the major faults in the Hida Range : the Kurobe-Takase fracture zone. Ages obtained west of the fault are ∼60 Ma, while those obtained to the east of the fault are less than ∼5 Ma.These results indicate a remarkable age difference across the fault. The Okukurobe granite, located west of the fault, cooled rapidly from ∼600°C to ∼240°C between 60-55 Ma, and the Kanazawa granodiorite, located east of the fault, cooled rapidly from ∼600°C to ∼240°C between 5-1 Ma. The Okukurobe granite has remained cooler than ∼240°C since ∼55 Ma. Thus, it was found that the granitic rocks across the fault have experienced a remarkable different cooling history.
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  • Wataru Hirose, Mitsuhiro Nakagawa
    1999 Volume 105 Issue 4 Pages 247-265
    Published: April 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Based on ; 348 K-Ar and 61 FT age data, volcanic stratigraphical and physical volcanological studies(volume, rate and mode of eruption, and edifice shape), and whole-rock geochemical data, we discuss temporal and spatial variation in volcanic activity after 19 Ma at central and eastern Hokkaido, southern end of the Kurile arc.The regional horizontal stress field in the area has changed from extension (19-14 Ma), extension∼neutral (14-1 Ma) to compression (1-0 Ma) and volcanic rocks with strong arc characteristics have been erupted since 14 Ma. Scale of volcanism of 19-14 Ma was much smaller in scale (several km3/My) than that of 14-0 Ma (several hundreds km3/My). The former produced bi-modal and non-arc type rocks with high TiO2 and Nb contents, suggesting that it was not related to subduction, but to plume induced by spreading of a back-arc basin (possibly the Japan Basin). Although volcanic rocks during 14-9 Ma show strongly arc characteristics, it is difficult to explain tectonic setting because of the scarcity of geological and geochemical data from the Kuril basin, which might influence volcanism on the stage. Clearly subduction-related volcanism occurred at least up to 9 Ma. However a small amount of high-TiO2 (high-Ti) and icelandite-like andesitedacite often occurred during 9-1 Ma, distinct from the youngest rocks (1-0 Ma), which are mostly low-TiO2 and calcalkaline types. High-Ti and icelandite-like rocks may represent differentiated arc type under extension∼neutral stress field (arc-extension type).Subduction-related volcanism of 9-1 Ma was developing under oblique subduction of the Pacific Plate. The back-arc (volcanic arc) sector could be under less compression thereby producing arc-extension type rocks, because that compressional stress from Pacific plate is largely converted into strike-slip motion of the fore-arc sliver. The obliquity of the subduction of the Pacific plate has temporally decreased and is very weak at present, suggesting that subduction-related volcanism changed by compression since less than 1 Ma and producing mainly calcalkaline rocks.
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  • Nobuaki Kuchitsu, Tetsuji Ozaki
    1999 Volume 105 Issue 4 Pages 266-272
    Published: April 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    At the Yoshimi Caves Historic Site, Saitama Prefecture, Japan, many kinds of evaporites are often observed on the walls. Observation of the distribution of the evaporites throughout the years and mineralogical analyses show that soluble salts such as alunogen are abundantly formed on the walls near the entrance in winter to early spring, whereas only gypsum, much less soluble salt, is formed only in very small amount at other parts or in the other seasons. Experiments suggest that the evaporites are formed due to water-rock interaction and subsequent evaporation, and the mineral composition and the amount of the evaporites are influenced by the environment. Therefore, establishment of tight doors for the entrances of the caves will contribute to reducing the evaporites and eventually to the conservation of the site. However further studies are needed before taking active measures.
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  • Kei Odawara, Yasufumi Iryu
    1999 Volume 105 Issue 4 Pages 273-288
    Published: April 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Ryukyu Group, composed of Pleistocene reef complex deposits laterally passing into terrigenous sediments, extensively crops out on Yoron-jima, Ryukyu Islands, southwestern Japan. Our investigation leads to a major revision of the previous stratigraphic scheme. This paper aims to provide a formal stratigraphic description of the Group on Yoron-jima. The Pleistocene sequences comprise three groups of strata. The lowest Ugachi and Mugiya Formations consist of partly-karstified coral limestone ; their surface exposures are confined in the type localities. The stratigraphic relationship between them remains unknown. The Yoronjima Formation which unconformably overlies the Ugachi and Mugiya Formations reaches 55 m in thickness and is exposed extensively, covering most of the island. It is divisible into three units, each consisting of proximal coral limestone and distal rhodolith, Cycloclypeus-Operculina, and detrital limestone. The Chichizaki and Tomori Formations unconformably resting on the Yoronjima Formation are composed mainly of thin (<10 m) coral limestone. They crop out on the southern and eastern peripheries of the island at elevations less than ca. 20 m. The relation between them is uncertain. On stratigraphic position and known age-diagnostic nannofossils, the Yoronjima Formation may be correlative to the main body of the Ryukyu Group on Okierabu-jima (lower and upper units) and Toku-no-shima (units 1, 2 and 3), ranging in age from 390 to 890 ka, while the ages of the other formations are left open.
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  • Akihiro Kano, Terumi Kambayashi, Hidenori Fujii, Jun Matsuoka, Koji Sa ...
    1999 Volume 105 Issue 4 Pages 289-304
    Published: April 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Water chemistry and hydrology of tufa-bearing stream of Shirokawa Town, Ehime Prefecture were studied. The streamwater is originated from a limestone underground water system and contains a large amount of calcium carbonate (2.1∼3.1 millimol/l in alkalinity). The Pco2 of the water quickly decreases from a high value (reaches 3, 000 μ atm in summer) and saturation index for calcite (SIC) increases to values exceeding 0.5 after issuing from the spring. Throughout the year, CaCO3 was deposited along the stream, as indicated by the general decreasing trends of alkalinity and Ca-contents to the downstream direction. According to the results of calculation, daily deposition of calcite was largest in late July, whereas the water in winter was inefficient in depositing tufas. In this tufa-bearing stream, 5.8 tons of calcite were deposited, and 72.6 tons of calcium carbonate flowed out in a year.Water chemistry and hydrology seasonally changed under the controls of weather conditions and underground processes. Water temperature and flow in the stream varied generally following the changes in temperature and rainfall at the nearest weather observation point. Subsurface processes were discussed based on spring water data. Alkalinity of the spring water was high in summer and autumn that is probably associated with evaporation of water in underground and active dissolution of limestone in summer due to high Pco2 in soil air. The amount of flow is negatively correlated to SIC of the spring water. Probably, CO2-degassing in underground is an important control to the SIC. During the periods of larger water flow, the high underground water level may have restricted gas exchange across the issuing point and the underground CO2-degassing.
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  • Koichiro Narita, Atsushi Yamaji, Takahiro Tagami, Hiroshi Kurita, Akik ...
    1999 Volume 105 Issue 4 Pages 305-308
    Published: April 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Cenozoic clastic sediments in the Kuma area, Shikoku, southwest Japan, previously designated as the Kuma Group, are here redefined as the Hiwada-toge Formation and the overlying Kuma Group sensu stricto, considering a significant time-gap between them. The Hiwada-toge Formation is Early Eocene in age on the basis of dinoflagellate cysts, while the lower part of the Kuma Group s.s. (Sagayama Formation) was dated as late Early Miocene by fission-track dating. Organic microfossil assemblages show that the Hiwada-toge Formation contains marine strata and the lower part of the Kuma Group s.s. (Sagayama Formation) is of non-marine origin.These results provide two geochronological constraints critical to the regional tectonic history.The age of the Hiwada-toge Formation indicates that the Sanbagawa metamorphic rocks came under subaerial erosion by Early Eocene in its provenance area. The relationship between the Median Tectonic Line (M.T.L.) and the northern extension of the Miocene Kuma Group s.s. and its contiguous strata shows that a compressional activity of the M.T.L. occurred during a relatively short period in late Early to Middle Miocene.
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  • Yoshihiro Sawada, Akane Toume, Sinji Kobayashi, Hiroaki Komuro, Hiroya ...
    1999 Volume 105 Issue 4 Pages VII-VIII
    Published: 1999
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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