Earth Science (Chikyu Kagaku)
Online ISSN : 2189-7212
Print ISSN : 0366-6611
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Volume 71, Issue 2
Displaying 1-5 of 5 articles from this issue
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Research Article
  • Basement of Chuseki-so and sedimentary surface of the MIS 5e deposit in the Ishikari Plain, Hokkaido, Japan
    Tsumoru SAGAYAMA, Yukio IZIMA, Yoshiki FUJIWARA, Satoshi OKAMURA, Goro ...
    2017 Volume 71 Issue 2 Pages 43-61
    Published: April 25, 2017
    Released on J-STAGE: January 14, 2018
    DOIhttps://doi.org/10.15080/agcjchikyukagaku.71.2_43
    JOURNAL OPEN ACCESS
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    In order to clarify stratigraphy and geologic age of the late Pleistocene to Holocene sediments, analyses of volcanic ash, diatom and pollen were carried out on five drilling cores, from the Teine (SB-1), the Moere-numa (MB-4, MB-5 and MB-11) and the Tarukawa (ISK-1) areas in the Ishikari Lowland. The Toya volcanic ash, fallen in ca. 113 ka, is intercalated in SB-1, MB-4, MB-5 and ISK-1. The Shikotsu Pumice Flow deposits, erupted in ca. 41 ka, are found in the MB-4, MB-5 and MB-11. Our data indicate basement of the latest Pleistocene to Holocene sediments, so-called Chuseki-so, and geologic horizon of the Toya volcanic ash. The sedimentary surface, formed by the Momijidai Formation in ca 120 ka, widely extends under the Ishikari Lowland. The Toya volcanic ash covers the surface of the formation. Latter, sediments from MIS 5d epoch to the last glacial stage piled up on the paleo-Ishikari Lowland. The latest Pleistocene to Holocene sediments deposited on the paleo-lowland after the Last Glacial Maximum, ca. 20 - 30 ka, and formed the Ishikari Lowland.
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Research Article
  • Neogene volcanism associated with back-arc basin tectonics at the northern Fossa Magna, NE Japan
    Satoshi OKAMURA, Mitsuru INABA, Yoshiko ADACHI, Ryuichi SHINJO
    2017 Volume 71 Issue 2 Pages 65-73
    Published: April 25, 2017
    Released on J-STAGE: January 14, 2018
    DOIhttps://doi.org/10.15080/agcjchikyukagaku.71.2_65
    JOURNAL OPEN ACCESS
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    Isotopic and trace element data imply a temporal change in magma sources and thermal conditions beneath the northern Fossa Magna from the Oligocene to the Pliocene. Less radiogenic 176Hf/177Hf and Nd/144Nd, and high Zr/Hf characterize the Early Miocene volcanism in the northern Fossa Magna region. Based on out isotope proxies, we propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling demonstrates that slab-derived melt mixed with mantle wedge produces the observed isotopic and trace elemental characteristics. The Early Miocene Kiriake Basalt has similar geochemical composition to the basalts of the Japan Sea basement, suggesting the melting of asthenospheric mantle beneath the extensional rift system within the northern Fossa Magna region during the opening of the Japan Sea. In the Middle Miocene, the injection of hot and depleted asthenospheric material replaced the mantle beneath the northern Fossa Magna region. The mantle wedge was gradually cooled during the Middle Miocene to the Pliocene with back-arc opening ending in the Late Miocene. Slab surface temperatures were still high enough for sediments to melt but not too high (<~780 °C) to lose zircon as a residual phase.
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  • Geochemical diversity of Neogene plutonic rocks, North Fossa Magna, Japan
    Yoshinobu KAWANO
    2017 Volume 71 Issue 2 Pages 75-86
    Published: April 25, 2017
    Released on J-STAGE: January 14, 2018
    DOIhttps://doi.org/10.15080/agcjchikyukagaku.71.2_75
    JOURNAL OPEN ACCESS
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    Neogene plutonic rocks disperse in three areas, Makihata-Tanigawa (MT), Suzaka-Ueda (SU) and Utsukushigahara-Kirigamine (UK) areas in North Fossa Magna, central Japan. The rocks consist of tonalite and granodiorite with granite and become young to MT via SU from UK areas. The plutonic rocks in UK area show higher Sr, Nb, ΣLREE/ΣHREE and LaN/YbN than those of MT area and the rocks of SU have both characteristics. It is indicated that partial melting degree for UK area was smaller than that for MT area. Initial Sr isotopic ratios of the rocks display positive correlation with Tin contents, suggesting that source magma affected by crustal materials. In εSrI –εNdI diagram, the plutonic rocks from UK and SU areas plot between fields of the Izu arc volcanic rocks and gabbroic xenoliths, whereas those of MT area indicate the same trend of Neogene volcanic rocks in northeast Japan arc, which are affected by continental crust or originated from lower crustal materials. It is considered that the plutonic rocks of UK and SU areas were derived from source magma for the Izu arc volcanic rocks affected by gabbro or gabbroic magma, and those of MT area were formed by the source magma reacted with continental or lower crustal materials.
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