The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 124, Issue 10
Displaying 1-7 of 7 articles from this issue
SPECIAL ISSUE Greentuff Renaissance
Preface
Review
  • Kazuhiko Kano
    2018 Volume 124 Issue 10 Pages 781-803
    Published: October 15, 2018
    Released on J-STAGE: January 24, 2019
    JOURNAL FREE ACCESS

    Green Tuff is an informal term that has been widely applied to the volcanic successions distributed mainly along the Japan Sea coast of Honshu, Japan. As Green Tuff underlies the middle Miocene marine sediments and sparsely contain Miocene-type fossil floras, these successions have been generally considered to be Early to Middle Miocene in age. However, recent stratigraphic revisions, based on the facies analysis and more reliable radiometric dating of selected volcano-sedimentary successions, suggest that the Green Tuff successions span from Late Eocene to Middle Miocene in age and can be divided into six time-stratigraphic units: the Upper Eocene to Lower Oligocene (44 to 28 Ma); the Upper Oligocene (28 to 23 Ma); the lower Lower Miocene (23 to 20 Ma); the middle Lower Miocene (20 to 18 Ma); the upper-Lower Miocene (18 to 15.3 Ma); and the lower Middle Miocene (15.3 to 12.3 Ma). These geological units likely reflect the evolution of a continental arc to the present-day island arc through the following stages: 1) onset of continental arc rifting and volcanism accompanied by a local marine transgression; 2) thermal crustal doming accompanied by local rifting and volcanism; 3) onset of spreading in the central doming area, with the propagation of uplifting, rifting, and bimodal volcanism to the surrounding region by successive upwelling of hot asthenosphere, followed by a local marine transgression onto the back-arc region; 4) acceleration of spreading, rifting, and the inflow of a warm ocean current into the back-arc region; 5) rapid rifting along the eastern margin of the Japan Sea by further invasion of hot asthenosphere to the trench side; and 6) onset of collision against, and subduction beneath SW Japan, by the Philippine Sea Plate, alongside migration of the volcanic front to the Pacific coast, and onset of crustal shortening. In this scenario, the last event appears to represent the beginning of the present-day island arc system of Japan.

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  • Hiroyuki Hoshi
    2018 Volume 124 Issue 10 Pages 805-817
    Published: October 15, 2018
    Released on J-STAGE: January 24, 2019
    JOURNAL FREE ACCESS

    The Kanto Syntaxis in central Japan is a cuspate, convex-northward deformation structure of zonal geologic belts and belt-bounding faults, and its formation can likely be attributed to the Neogene collision of the Honshu and Izu-Ogasawara (Bonin)-Mariana (IBM) arcs. However, it remains unclear as to how and when the deformation occurred. To determine the age of onset of syntaxis formation, this review summarizes recent geological and paleomagnetic studies, published during the past 25 years, in and around the collision area. The climax of tectonic rotation of both sortheast and southwest Japan during the back-arc opening of the Japan Sea has previously been estimated at ~15 Ma, with this age potentially representing the onset of syntaxis formation. However, sedimentary records in the earliest accreted volcanic edifice indicate that arc-arc collision began at 17 Ma or shortly later. This edifice, named the Kushigatayama (or Koma) block, originally formed on the IBM arc and is now present in the Izu collision zone. Paleomagnetic analyses, which included an orocline test that utilized early Miocene data from both limbs of the Kanto Syntaxis, suggest that it began to grow between 17 and 15 Ma. This is consistent with the age estimate of the onset of arc-arc collision inferred from the geology of the collision zone. Thus, it is concluded that the arc-arc collision, which began between 17 and 15 Ma, caused the coeval onset of the syntaxis formation. Hypotheses relating to the development of the syntaxis are introduced, and these need to be tested in future studies.

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Articles
  • Jun Hosoi, Takeshi Nakajima, Tohru Danhara, Hideki Iwano, Takafumi Hir ...
    2018 Volume 124 Issue 10 Pages 819-835
    Published: October 15, 2018
    Released on J-STAGE: January 24, 2019
    JOURNAL FREE ACCESS
    Supplementary material

    We carried out fission-track (FT) and U-Pb double dating on zircon grains from six Green Tuff samples collected from Nishiwaga Town, Iwate Prefecture (northeast Japan). Based on these data, our revised stratigraphy is as follows: the Oarasawa Formation (the early Miocene ca. 20-18 Ma), which unconformably overlies the Paleozoic and Mesozoic basement; the main part of the Oishi Formation (16-15.4 Ma), which unconformably overlies the Oarasawa Formation; the Kawashiri Tuff Member (15.4-13.6 Ma) of the upper part of the Oishi Formation; and the Kotsunagizawa Formation (13.6-12 Ma), which conformably overlies the Oishi Formation. These results, which are consistent with the recently revised stratigraphy for the type section of the Green Tuff on Oga Peninsula, indicate that a tectonic change which is coupled with rapid tectonic subsidence started at ca. 16 Ma, accompanied by active felsic volcanism and counter-clockwise rotation of the region at ca. about 15.4-13.6 Ma, and then ended at ca. 12 Ma in the study area.

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Regular Section
Articles
  • Masataka Yamada, Takayuki Kawai, Fumikatsu Nishizawa, Takehiko Suzuki
    2018 Volume 124 Issue 10 Pages 837-855
    Published: October 15, 2018
    Released on J-STAGE: January 24, 2019
    JOURNAL FREE ACCESS
    Supplementary material

    This study describes the revised stratigraphy and correlations of the middle Pleistocene tephra units around northern Tochigi and southern Fukushima prefectures. We have identified three pyroclastic successions consistent with the volcanic deposits So-OT, So-TN, and So-KT (from youngest to oldest). So-OT consists of a fall pumice deposit, a surge deposit, and three pyroclastic flow units. We observed the fall deposits of So-OT at six localities, with the northernmost occurrence in southern Fukushima city. So-TN is divided into four pyroclastic flow units on the basis of observed differences in the refractive index values of the orthopyroxene and the glasses. Importantly, the refractive index values of the So-TN orthopyroxene are higher than those of the So-OT and So-KT tephra units. From the distribution of pyroclastic flow deposits, and trends in grain size and thickness of the fall deposits, the So-OT tephra is confidently attributed to the Shiobara caldera. The So-TN and So-KT tephra units become thicker and coarser toward the Shiobara caldera; furthermore, similarities in the glasses to those of So-OT indicate a shared volcanic provenance. The stratigraphy of these pyroclastic flow deposits and other important tephra units is reconstructed as follows (from youngest to oldest): the Tobiyama tephra (Az-MiP), So-OT, So-TN, A4Pm, So-KT, and A1Pm or A2Pm. The So-TN and the stratigraphically younger So-OT tephra units are constrained to between the dated A4Pm (337 ka) and Az-MiP (300 ka). The stratigraphically older So-KT tephra is constrained to below the A4Pm (337 ka) and above the A1Pm or A2Pm (337-410 ka).

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Report
  • Keisuke Eshima, Masaaki Owada
    2018 Volume 124 Issue 10 Pages 857-862
    Published: October 15, 2018
    Released on J-STAGE: January 24, 2019
    JOURNAL FREE ACCESS

    A diorite stock and mafic-felsic dikes, which intrude Cretaceous sedimentary rocks of the Wakino sub-group, are exposed on the western side of Mt. Shaku-dake. In terms of modal mineralogy and chemical composition, the diorite stock is classified as a two-pyroxene diorite and contains euhedral clinopyroxene and orthopyroxene. The dikes are dolerite, porphyritic fine-grained tonalite, fine-grained clinopyroxene granodiorite, and fine-grained biotite granite. Whole-rock chemical analyses reveal that some of the diorites and dolerites possess high Mg-numbers (100×MgO/(MgO+FeO*) in moles) of >64 and that these rocks are geochemically similar to Sanukitic high-Mg andesites (HMA).

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