The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 112, Issue 1
Displaying 1-11 of 11 articles from this issue
Articles
  • Hiroki Hayashi, Keiji Kasahara, Hisanori Kimura
    2006 Volume 112 Issue 1 Pages 2-13
    Published: 2006
    Released on J-STAGE: May 01, 2006
    JOURNAL FREE ACCESS
    Forty-nine deep borehole data and 31 seismic reflection profiles were used for understanding a distribution of pre-Neogene basement rocks beneath the Kanto Plain, central Japan. Pre-Neogene rocks drilled at each borehole are divided into seven geologic units, namely, Ashio, Tsukuba, Ryoke, Sanbagawa, Chichibu and Shimanto Complexes and Cretaceous sediments. P-wave velocities of pre-Neogene rocks underneath of the Kanto Plain suggest a relationship with each geologic unit. Boundary between pre-Neogene basements and covering sediments are well traced on some seismic reflection profiles, and a boundary between geologic units was suggested by geophysical difference in reflection patterns, such as shapes and intensities. Thus subsurface distribution of pre-Neogene geologic units beneath the Kanto Plain is synthesized based on both geological and geophysical data.
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  • Masaki Takahashi
    2006 Volume 112 Issue 1 Pages 14-32
    Published: 2006
    Released on J-STAGE: May 01, 2006
    JOURNAL FREE ACCESS
    Tectonic boundary between Northeast (NE) and Southwest (SW) Japan arcs during Japan Sea opening is discussed based mainly on geographical and geochronological distribution of Miocene sediments in and around the Kanto district, central Japan. Geological discontinuity, such as thick marine sediments vs. volcaniclastics, and E-W vs. N-S structural trend of island arc, is recognized between the Kanto Mountains and the Ashio Mountains. The abrupt change in geological characters indicates a tectonic boundary between Miocene fore-arc region of SW Japan arc and volcanic province of NE Japan arc. This tectonic boundary can be traced to the east, where a sharp, steep, and straight-lined submarine valley is recognized. Thus the previously presumed Tonegawa Tectonic Line is re-defined as a fault which was originated during Japan Sea opening in the Early Miocene, and divided the Japanese Islands into NE and SW Japan arcs in the Kanto district.
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  • −With special reference to the Yoshimi metamorphic rocks and western extension of the Tonegawa Tectonic Line−
    Masaki Takahashi, Hiroki Hayashi, Keiji Kasahara, Hisanori Kimura
    2006 Volume 112 Issue 1 Pages 33-52
    Published: 2006
    Released on J-STAGE: May 01, 2006
    JOURNAL FREE ACCESS
    Geologic structure was discussed on a seismic profile of Asaka-Konosu-Oura transect in the western part of the Kanto Plain, central Japan. A general model of stratigraphy and geologic structures, established based on surface geology in the Kanto District, was applied to subsurface geology. Both surface and subsurface geology can be divided into three stratigraphic units as "N.8 deposits (ca. 16.5-15.0 Ma)", "post N.8 deposits (ca. 15.0-3.0 Ma)" and the Kazusa-Shimosa Groups (<3 Ma). The "N.8 deposits" is composed of latest Early Miocene to earliest Middle Miocene submarine fan and overlying half-graben fill deposits. The "post N.8 deposits" is interpreted as Middle Miocene intra-arc basin deposits, which widely cover underlying strata unconformably (Niwaya Unconformity). The Kazusa-Shimosa Groups are latest Pliocene to Quaternary fore-arc basin deposits, which cover the "post N.8 deposits " horizontally. The base of this unit (Kurotaki Unconformity) is traced throughout transect. The basement structures underneath of the Kanto Plain are characterized by southerly-tilted basement blocks, which are cut by northward-dipping large normal faults. These structures were probably formed under extension stress field during Japan Sea opening (ca. 19-15 Ma). As metamorphic rocks in the Yoshimi Hills have been located at the crest of tilted-block since Miocene, such basement rocks are cropped out within the Quaternary sediments of the Kanto Plain.
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  • Hideo Takagi, Hiroyoshi Suzuki, Masaki Takahashi, Takuji Hamamoto, Hir ...
    2006 Volume 112 Issue 1 Pages 53-64
    Published: 2006
    Released on J-STAGE: May 01, 2006
    JOURNAL FREE ACCESS
    The eastern extension of geotectonic belts and of the Median Tectonic Line (MTL) in Southwest Japan becomes ambiguous in the Kanto Plain region because of the thick cover of Neogene-Quaternary sediments. Deep well core samples provide significant clues to clarifying the location of the MTL. More than 3500 m samples of the Iwatsuki borehole drilled in 1971 by the National Research Institute for Earth Science and Disaster Prevention were re-examined petrologically and geochronologically. The deepest part of the core (Layer H: 3505.0-3510.5 m) is mainly composed of garnet-hornblende tonalitic mylonite and mylonitized epidote amphibolite. EPMA analytical data for garnet, hornblende and plagioclase all suggest that the rocks are not derived from a high P/T Sambagawa metamorphic belt as previously suggested, but from a higher T plutono-metamorphic complex. K-Ar ages of hornblende from two samples are 77.4 and 83.1 Ma, and a Rb-Sr mineral age gives 69.8 Ma. Consequently, correlation of the deepest core samples with the Paleo-Ryoke (Abukuma) Terrane (existing as nappes with the 105-112 Ma high T Yorii plutono-metamorphic complex on the Sambagawa rocks) is neglected, and the rocks are instead correlated with the Ryoke belt. The second deepest part of the core (Layer G: 2943-3327 m) is composed of tonalitic quartz porphyry. It contains biotite phenocrysts and their K-Ar age is 17.7 Ma. This age suggests that the quartz porphyry is difficult to correlate with the felsic intrusions of Setouchi volcanic rocks along the Median Tectonic Line in SW Japan. The degree of mylonitization in Layer H and of cataclasis in both Layer G and H suggests that the MTL extends within 500 m to the south of the core site, and a possible superficial extension of the estimated MTL is slightly oblique to the NW-SE trending Ayasegawa-Fukaya Fault, major active fault in the Kanto Plain. Further thought should be given to clarify the significance of the relationship between the MTL and the Ayasegawa-Fukaya Fault in the Kanto Plain.
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  • Hideo Takagi, Masaki Takahashi
    2006 Volume 112 Issue 1 Pages 65-71
    Published: 2006
    Released on J-STAGE: May 01, 2006
    JOURNAL FREE ACCESS
    Deep well core samples provide significant clues to clarifying the location of the Median Tectoic Line (MTL) in the Kanto Plain which is covered by thick Cenozoic sediments. Pre-Miocene basement cores from the Matsubushi SK-1 drilled in 1957 by Japan Petroleum Exploration Co., Ltd. were re-examined petrologically and geochronologically. Eight core samples collected from 1,922 m to 1,948 m are mainly composed of weakly mylonitized hornblende-biotite tonalite and fine-grained mafic rock. A K-Ar age of hornblende from weakly mylonitized tonalite is about 67 Ma. The core samples are correlative geochronologically with the plutonic rocks of the Ryoke belt. The mineral composition and microstructures of core samples are quite similar to those of the Hiji gneissose tonalite and associated basic rocks distributed along the MTL in the Chubu Region and Hiki Hills in the Kanto Mountains. Degree of mylonitization estimated by grain size of recrystallized quartz suggests that the MTL extends within 500-1,500 m to the south of the borehole site, and a possible superficial extension of the estimated MTL position is also about 1-3 km south if the MTL has a dip of moderate angle.
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  • Ken-ichiro Hisada, Kanako Ota, Koichi Okuzawa, Atsushi Tanase
    2006 Volume 112 Issue 1 Pages 72-83
    Published: 2006
    Released on J-STAGE: May 01, 2006
    JOURNAL FREE ACCESS
    The Edosaki observation well was located at Edosaki Town, Ibaraki Prefecture, and reached at the depth of 1234.0 m. The basement rocks of the northeastern part of the Kanto Plain have been known as the Ryoke belt on the northern side of the Median Tectonic Line. The stratigraphy at this well is the Shimosa Group, Kazusa Group and the pre-Neogene clastics in descending order. The cores were recovered from three intervals; 843.40~845.15 m (pebbly mudstone), 1037.00~1039.70 m (fine-grained sandstone), and 1205.30~1207.70 m (black mudstone) . Among them, the sandstones from the middle interval were examined by modal composition and chemistry of detrital garnet. The sandstones were also collected from several localities in order to compare with the pre-Neogene, that is, sandstones from the Choshi, Nakaminato Groups, Sanchu Cretaceous, and Yorii and Atokura Formations. Our study proves that the sandstone of the Edosaki core is mainly classified into feldspathic wacke in QFR diagram and Dissected to Transitional Arc in QmFLt diagram, and the sandstone corresponds probably to the Atokura Formation. The detrital garnets seem to be less different from those of the Atokura Formation. The Atokura Formation lies on the Sambagawa metamorphic rocks as a Klippe in the Kanto Mountains. Therefore the Edosaki observation well is situated not above the Ryoke belt but above the Sambagawa belt.
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  • Hisao Ando
    2006 Volume 112 Issue 1 Pages 84-97
    Published: 2006
    Released on J-STAGE: May 01, 2006
    JOURNAL FREE ACCESS
    The Lower Cretaceous Choshi Group, Upper Cretaceous Nakaminato Group and Paleogene Oarai Formation show isolated distribution in the eastern end of the Kanto Plain along the Pacific coast, central Japan. The current status of their geological studies and significance are reviewed, because they are important for reconstructing the tectonic relation between Northeast Japan and Southwest Japan. On the basis of age, stratigraphy and depositional facies, there are no correlative strata with the above in the Cretaceous-Paleocene Yezo forearc basin sediments of Northeast Japan. Despite the difference in facies, the Barremian to Upper Aptian Choshi Group seems to be an eastern extension correlative with the Sanchu Group of the Chichibu Belt in the northern Kanto Mountains, Southwest Japan. The Upper Campanian to Lower Maastrichtian Nakaminato Group has some characteristics common to the Izumi Group along the southern part of the Inner Zone of Southwest Japan. It forms a fault-bounded basement-uplifting block associated with the Oarai Formation and Miocene sediments within the southern extension of the Tanakura Tectonic Zone. The Oarai Formation is thought to be Paleogene in age based on isotopic age of gravel, plant fossils and geologic setting. It may be correlative with the Yorii and Kanohara Formations in the northern Kanto Mountains, judging from lithofacies and tectonic setting.
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  • Eri Tobe, Hideo Takagi, Masaki Takahashi
    2006 Volume 112 Issue 1 Pages 98-103
    Published: 2006
    Released on J-STAGE: May 01, 2006
    JOURNAL FREE ACCESS
    Choshi Peninsula, is one of the very limited localities in the Kanto Plain where pre-Cretaceous strata are exposed and thus is a significant location for considering the geological connection of SW Japan and NE Japan. Pre-Cretaceous Atagoyama Group is composed of clastic sediments, chert and limestone, and considered to be correlative with Jurassic accretionary terrane which is widely distributed in SW Japan. We described and dated granitic clasts from the conglomerate in the Atagoyama Group, and discussed their significance. The granitic clasts are medium-grained hornblende-biotite granodiorite to tonalite. They contain primary magnetite grains, which cause high magnetic susceptibility showing magnetite-series characteristics. The K-Ar ages of two clasts are 255 Ma (biotite) and 270 Ma (hornblende). Such Permian granitic clasts can be correlative with the Kinshozan quartzdiorite constituting Paleo-Ryoke terrane forming a nappe structure on the Sambagawa metamorphic belt and with the granitic clasts in Permian Usuginu-type conglomerate in the South Kitakami and Kurosegawa terranes.
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