The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 108 , Issue 1
Showing 1-7 articles out of 7 articles from the selected issue
  • Takahiro Hosono, Kuniaki Makino
    2002 Volume 108 Issue 1 Pages 1-15
    Published: January 15, 2002
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    The Cretaceous granitic complex in the southern part of Lake Biwa occurs as a ring-shape complex. The complex is composed mainly of Hira, Hiei, Tanakami, Suzuka, and Yasu granitic plutons. The Yasu granitic pluton, which has stock shape and 10 km (N-S) to 8 km (E-W) extension, is situated at the most inner part of this complex.The rock facies of the Yasu granitic pluton varies significantly. The pluton consists mainly of four facies : Lower, Main, Upper, and Granite porphyry facies. The Lower, Main, and Upper facies are intruded by the Granite porphyry facies with remarkable porphyritic texture. Petrographic studies and the chemical compositions of whole-rock and plagioclase crystals indicate that the Lower, Main, and Upper facies are formed by fractional crystallization. A large quantity of pegmatite and aplite veins accompany the Lower and Main facies, while Upper and Granite porphyry facies are poor or absent. This evidence and analogy of volcanic rocks suggest that the porphyritic texture of the Yasu granitic pluton developed through degassing process in the granitic magma.
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  • Masakazu Niwa, Kazuhiro Tsukada, Satoru Kojima
    2002 Volume 108 Issue 1 Pages 16-23
    Published: January 15, 2002
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Melange in the Machikata area, Nyukawa Village, Gifu Prefecture, central Japan is composed of clasts and blocks of sandstone, siliceous mudstone, chert, and mafic volcanic rocks enclosed in a matrix of mudstone. Radiolarians from the matrix are of Early Jurassic age. The melange has generally been regarded as a part of the Middle Jurassic accretionary complex of the Mino Belt. However the new finding of Early Jurassic radiolarians from the melange matrix in the Machikata area suggests that this melange is older than the melange of the surrounding accretionary complex. In addition, there are also distinct differences in lithology between the melange in the Machikata area and that of the surrounding accretionary complex.
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  • Tetsuo Miwa, Hiroyuki Hoshi
    2002 Volume 108 Issue 1 Pages 28-36
    Published: January 15, 2002
    Released: April 11, 2008
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    Remanent magnetizations in andesite samples collected from 26 sites of the upper Cenozoic Enrei Formation in Nagano Prefecture, central Honshu, allow us to discuss the age of the formation and rotational motion in the study area. Stable magnetic components revealed by stepwise alternating-field and thermal demagnetization give a directional dataset that passes the reversal test. This reliable dataset delineates a sequence of reversed-normal-reversed magnetic polarity changes that is assigned to the younger part of the Matuyama Chron on the geomagnetic polarity time scale. The normal polarity is correlated either to the Jaramillo Subchronozone or to the Cobb Mountain Subchronozone. This magnetostratigraphic correlation suggests that most of the Enrei volcanic sequence belongs to the Lower Pleistocene. A paleomagnetic pole that is indistinguishable from the geographic north pole indicates no significant rotational motion and latitudinal movement in the study area.
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  • Gaku Kobayashi, Muneki Mitamura, Shusaku Yoshikawa, Dicky Muslim, Miek ...
    2002 Volume 108 Issue 1 Pages 37-47
    Published: January 15, 2002
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    The Quaternary stratigraphy of subsurface geology of the Osaka Plain has been mainly built up with deep drilling cores and detailed lithostratigraphy, tephrostratigraphy, and biostratigraphy has become clear. On the other hand, seismic reflection surveys in the Osaka Plain have been carried out along 13 lines. But, acoustic features and similarity of reflectors of each seismic profiles were not synthetically discussed until now. In this paper, seismic stratigraphic classification of seismic profiles in the Osaka Plain is carried out from detailed description of reflectors, and the relations between this seismic stratigraphic division and lithostratigraphic division are examined. As a result, it has become clear that there are similarities of acoustic facies in the different seismic profiles in the Osaka Plain. The seismic zones of Quaternary sediments in the Osaka Plain are divided into two zones, seismic zone N and seismic zone P. In seismic zone N shows very poor to non-continue and non-clear reflectors, which can be correlated with the Miyakojima Formation. In seismic zone P shows good to poor continue and clear reflectors, which can be correlated with the Tanaka Formation. On the basis of continuity of reflectors, variety of reflector interval, and strength of reflection, the seismic zone P is divided into three sub seismic zones as seismic zone Pa, seismic zone Pb, and seismic zone Pc which can be correlated with the horizon between Ma -1 to Ma 2, Ma 2 to Ma 7, and above Ma 7, respectively. It is considered that these seismic stratigraphic divisions will expect to become important basic data in the Osaka Plain.
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  • Yasuhiro Fujisawa, Mitsuru Okuno, Toshio Nakamura, Tetsuo Kobayashi
    2002 Volume 108 Issue 1 Pages 48-58
    Published: January 15, 2002
    Released: April 11, 2008
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    Tsurumi volcano located in northeastern Kyushu, Japan is the stratovolcano which consists mainly of lava domes and lava flows. Several tephra layers from the volcano are also distributed on its flank. In this paper, the eruptive activities of Tsurumi volcano during the past 30, 000 years are examined by tephrochronology and radiocarbon dating. Furthermore, the eruptions during historical time are considered. During 29∼7.3 cal ka BP, the lava flow eruptions occurred repeatedly at Tsurumi volcano. In the 10.5 cal ka BP eruptions, block-and-ash-flows were generated by dome collapse. Subsequently, small-scale eruptions without lava flowage occurred in 1.8 cal ka BP, 1.2 cal ka BP and 1.0 cal ka BP. In historical time, eruptive phenomena were recorded in AD 711 and 867. Judging from geological data and interpretation of ancient documents, we estimate that these events correspond to 1.2 cal ka BP (probably 711 AD) and 1.0 cal ka BP (probably 867 AD) phreatic eruptions at Garan-dake, respectively.
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  • Yuya Kobayashi, Tsutomu Otsuka
    2002 Volume 108 Issue 1 Pages 59-73
    Published: January 15, 2002
    Released: April 11, 2008
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    The Samondake Unit of the Mino terrane in central Japan is characterized by a large quantity of sandstone. The structure, age and tectonics of this unit have been poorly known because of its monotonous lithofacies, remarkable deformation and scarce evidence of fossils.The Samondake Unit in the Neo-Izumi area in the northern part of the Mino terrane is divided into two subunits-the Kuzawadani Subunit and the Kochidani Subunit on the basis of lithology and structure. The Kuzawadani Subunit exposed in northeastern part of the study area has been thrust over the Kochidani Subunit in the southwestern part. The Kuzawadani Subunit is mainly composed of sandstone and mudstone without chert. This subunit is gently folded and not strongly deformed.The Kochidani Subunit is composed of strongly-sheared strata and mudstone-dominant melanges including chert blocks.The deformation of rocks and the low-grade metamorphism based on illite crystallinity of the Samondake Unit are investigated in the Neo-Izumi area. The Kubler index of illite crystallinity ranges from 0.33Δ°2θ to 0.64Δ°2θ. Both illite crystallinity and deformation degree of the beds increase toward the southeastern area where the structurally lower part of the unit is exposed. This suggests that the thermal structure of the Samondake Unit is not a result of igneous intrusion but of the burial metamorphism in the subduction process. The structurally lower part showing high illite crystallinity was uplifted as a result of accretion of the seamount which made up the Funafuseyama Unit now located to the south of the Samondake Unit.
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  • Nobuhiro Kotake, Masakazu Nara
    2002 Volume 108 Issue 1 Pages I-II
    Published: 2002
    Released: November 26, 2010
    JOURNALS FREE ACCESS
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