The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 122, Issue 9
Displaying 1-5 of 5 articles from this issue
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Articles
  • Metamorphism and mylonitization in sinistral shear zones found in the Poroshiri ophiolite
    Akiyuki Kato, Kyuichi Kanagawa, Yoshikuni Hiroi
    2016 Volume 122 Issue 9 Pages 461-475
    Published: September 15, 2016
    Released on J-STAGE: December 15, 2016
    DOIhttps://doi.org/10.5575/geosoc.2016.0017
    JOURNAL FREE ACCESS
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    We found meter-wide, NNW-trending sinistral shear zones in the Poroshiri ophiolite which has been dextrally sheared during thrusting with the Hidaka metamorphic rocks. Metagabbro in the Poroshiri ophiolite was transformed into porphyroclastic amphibolite and dextrally sheared during the amphibolitization, the latter of which in turn was mylonitized in the sinistral shear zones through dynamic recrystallization of plagioclase and metamorphic reactions to produce fine-grained hornblende and plagioclase. Foliation deflection, σ- and d-type mantled plagioclase porphyroclasts, shear bands, and shape-preferred orientations of hornblende porphyroclasts as well as of dynamically recrystallized plagioclase grains oblique to mylonitic foliation all indicate a sinistral sense of shear. Hornblende-plagioclase geothermometry suggests the metamorphic temperatures of porphyroclastic amphibolite and amphibolite mylonite to be 708±7℃ and 678±6℃, respectively. Thus the mylonitization occurred during the retrograde metamorphism. In contrast to the preceding dextral shearing with a thrust component, the sinistral shearing has a normal slip component, which possibly occurred by local backward movement during the retrograde metamorphism.

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  • Stratigraphy and conodont and ammonoid ages of Upper Triassic Nakijin Formation in Hedomisaki area, Okinawa-jima, Japan
    Daisuke Yamashita, Chika Yasuda, Takeshi Ishibashi, Rossana Martini, T ...
    2016 Volume 122 Issue 9 Pages 477-493
    Published: September 15, 2016
    Released on J-STAGE: December 15, 2016
    DOIhttps://doi.org/10.5575/geosoc.2016.0018
    JOURNAL FREE ACCESS
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    We present a lithostratigraphic, biostratigraphic (conodont and ammonoid), and petrographic study of Upper Triassic bedded limestones (Nakijin Formation) from Hedomisaki, northern Okinawa Island, Japan. The Hedomisaki limestones comprise two formations: Upper Triassic bedded limestone of the Nakijin Formation and massive limestone of the Hedomisaki Formation of unknown age. The Nakijin Formation is divided into three members: (1) lower = dark gray micritic limestone (ca. 110 m thick); (2) middle = clastic limestone (ca. 180 m thick); and (3) upper = clastic limestone with slump beds (ca. 110 m thick). Biostratigraphic datums based on conodont and ammonoid fossils show that the Nakijin Formation was deposited in the early Carnian through the middle Norian. Microfacies in the Nakijin Formation are classified into four major facies: (1) radiolarian limestone-mudstone; (2) thin-shelled bivalve mudstone-wackestone; (3) litho-bioclastic packstone; and (4) peloidal-bioclastic packstone. Facies 1 and 2 are dominated by planktonic biota, indicating a deep-water pelagic environment. The clastic limestones (3, 4) in the middle to upper member of the Nakijin Formation were likely deposited in a shallow-water environment on and around an oceanic seamount in an open ocean setting.

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  • U-Pb age of the tuff bed in the Urahoro Group, eastern Hokkaido, northern Japan
    Takahiro Katagiri, Hajime Naruse, Takafumi Hirata, Kentaro Hattori
    2016 Volume 122 Issue 9 Pages 495-503
    Published: September 15, 2016
    Released on J-STAGE: December 15, 2016
    DOIhttps://doi.org/10.5575/geosoc.2016.0021
    JOURNAL FREE ACCESS
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    The Paleogene Urahoro Group crops out in the Kushiro area of eastern Hokkaido, Japan. The depositional history of the Urahoro Group probably has significant implications for interpreting Paleogene arc-arc collisional processes that are considered to have occurred on Hokkaido Island. However, the depositional age of the Urahoro Group remains uncertain. Here, we present the U-Pb zircon depositional age of an acidic tuff bed of the Tenneru Formation in the middle of the Urahoro Group in the coastal Kushiro area. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we obtained a weighted mean age of zircon grains in the tuff bed of 39.06 ± 0.23 Ma (late Eocene). The tuff bed is stratigraphically higher than the horizon where provenance of gravels in the Urahoro Group have changed. This indicates that the tectonic event that caused uplift of the sedimentary provenance area of eastern Hokkaido occurred in the late Eocene.

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