The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 115, Issue 1
Displaying 1-5 of 5 articles from this issue
Articles
  • Tomohiro Tsuji, Masayuki Sakakibara
    2008 Volume 115 Issue 1 Pages 1-16
    Published: January 15, 2009
    Released on J-STAGE: June 05, 2009
    JOURNAL FREE ACCESS
    A Jurassic accretionary complex within the Northern Chichibu Belt, Yanadani area, western Shikoku, is divided into three distinct geologic units (units I, II, and III) based on the constituent rocks and geologic structure. Unit I is characterized by pelitic mélange containing blocks of greenstone, chert, limestone, and terrigenous clastic rocks. Unit II consists mainly of psammitic and pelitic phyllites, and Unit III consists of large blocks of greenstone-limestone complex with alternating beds of sandstone, mudstone, and pelitic mélange. The contacts between the units are northward-dipping faults. Here, we propose a method of analyzing grading structure, and there by younging direction, which involves grain-size measurements under the microscope. The younging directions of coherent sequences and pillow lava were determined at 63 localities. These data reveal consistent younging directions within large-scale blocks, demonstrating that the strata of Units I and II, and the terrigenous clastic rocks of Unit III are overturned, younging to the south. In contrast, the greenstone-limestone complex within Unit III is not overturned, and youngs to the north. This large-scale overturned structure within the Northern Chichibu Belt is interpreted to represent a large-scale overturned fold.
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  • Shuji Mizoguchi, Kazuo Kiminami, Teruyoshi Imaoka, Atsushi Kamei
    2008 Volume 115 Issue 1 Pages 17-30
    Published: January 15, 2009
    Released on J-STAGE: June 05, 2009
    JOURNAL FREE ACCESS
    The Early Miocene Shijujiyama Formation in the Cape Muroto area is presumed to have accumulated in a trench-slope basin formed on the Hioki accretionary complex. The lower part of the formation is dominated by mudstone intercalated with pillow lava, volcanic breccia, and volcanic sandstone. A dolerite (Shijujiyama dolerite), inferred to be a dyke, occurs in the mudstone, and a dyke (Maruyama dolerite) intrudes the Hioki melange along the Maruyama coast. The pillow basalt, volcanic breccia (basaltic andesite and dolerite clasts), and dolerite dykes are geochemically sub-alkaline basalt/andesite. The basaltic andesite breccia and pillow basalt (together referred to as the Shiina volcanic rocks), and Shijujiyama dolerite have geochemical affinities with island arc basalt, whereas the Maruyama dolerite and dolerite breccia have affinities with MORB. The MORB magmas originated from the subducting, shallowly buried, active spreading center of the Shikoku Basin. The genesis of the Shiina volcanic rocks, which involved being extruded upon the floor of a trench-slope basin, can be attributed to the assimilation of accreted mudstone by MORB magma and fractional crystallization. In conclusion, the Maruyama and Shijujiyama dolerites and the Shiina volcanic rocks of the Cape Muroto area could be explained as products resulting from near-trench magmatism related to ridge subduction at about 15 Ma.
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  • Yudai Sato, Kazuhiko Kano, Kenshiro Ogasawara, Takeshi Ohguchi, Norihi ...
    2008 Volume 115 Issue 1 Pages 31-46
    Published: January 15, 2009
    Released on J-STAGE: June 05, 2009
    JOURNAL FREE ACCESS
    This paper details the stratigraphic framework of the Miocene Daijima Formation, southwest Oga Peninsula, Akita Prefecture, NE Japan, and proposes a new stratigraphic division of the Daijima Formation into five members: the Sugoroku-sawa Conglomerate, Sugoroku-sawa Dacite, Sugoroku Basalt, Tateyamazaki Dacite, and Tsubaki Sandstone and Conglomerate. The Tateyamazaki Dacite has previously been regarded as the representative rock of the Daijima Formation, but is now interpreted as caldera infill distributed between Cape Tateyamazaki and Cape Shiosenomisaki. Volcanic rocks of the Daijima Formation are similar in lithology, stratigraphy, and isotopic age to those of the Nomuragawa Formation that occur immediately north of the Daijima Formation. The Daijima and Nomuragawa Formations are stratigraphic equivalents being underlain by the Late Eocene to Early Oligocene Monzen Formation and overlain by the Middle Miocene Nishikurosawa Formation. Isotopic age data indicate that volcanic rocks of the Daijima and Nomuragawa formations erupted at about 21 Ma. These rocks are voluminous and bimodal in composition, and have a northwesterly paleomagnetic declination, in contrast with the present-day northerly declination. The volcanism represented by these formations is likely to have marked the beginning of the rapid opening of the Japan Sea.
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