The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 120, Issue 8
Displaying 1-4 of 4 articles from this issue
Articles
  • Kazuki Sako, Hiroyuki Hoshi
    2014 Volume 120 Issue 8 Pages 255-271
    Published: August 15, 2014
    Released on J-STAGE: November 27, 2014
    JOURNAL FREE ACCESS
    We report on a new paleomagnetic direction from early Miocene (18-17 Ma) sedimentary rocks of the Tomikusa Group in southern Nagano Prefecture. Rock samples collected from 24 sites were demagnetized stepwise by means of alternating-field and thermal methods. Site-mean directions of normal, reverse, and intermediate polarities were determined for 23 sites. Rock magnetic experiments suggest that the remanent magnetization is carried by magnetite, as well as by iron sulfide (pyrrhotite or greigite) or maghemite. The directional data pass a reversals test and a baked contact test, indicating they are reliable and can therefore be used to investigate magnetic polarity stratigraphy and tectonic rotation. Magnetostratigraphic correlation shows that the Tomikusa Group is younger than the base of Chron C5Er (18.7 Ma) and older than the top of Chron C5Dn (17.2 Ma). The north-seeking overall mean direction is indistinguishable from the geocentric axial dipole field direction, as well as from the direction calculated from the paleomagnetic pole for the North China Block in the Asian continent. The paleomagnetic orocline test was applied to a data set consisting of the Tomikusa and other early Miocene directions reported from near the curved Median Tectonic Line (MTL) in central Honshu. The result demonstrates that the surface trace of the MTL was straight in the late early Miocene (ca. 17 Ma).
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  • Jinichiro Maeda, Tetsuya Ueno, Kohei Yamashita, Takehiro MATSUDA, Sato ...
    2014 Volume 120 Issue 8 Pages 273-280
    Published: August 15, 2014
    Released on J-STAGE: November 27, 2014
    JOURNAL FREE ACCESS
    We present K-Ar ages of biotite (20.2±0.5 Ma, 19.0±0.4 Ma, 17.8±0.4 Ma) from garnet-cordierite-bearing biotite hornfels adjacent to the Tadoshunai, Sakkuru Higashi, and Sakkuru Nishi masses of the Okushibetsu plutonic complex, Hidaka magmatic belt, central Hokkaido. The Sakkuru Higashi and Sakkuru Nishi masses are composed of troctolite, olivine gabbro, and diorite, and the Tadoshunai mass of quartz diorite. It is likely that some or all of the masses were the heat source for the thermal metamorphism, thereby indicating that mafic and/or intermediate magmatism in the northern half of the Hidaka magmatic belt occurred during the early Miocene.
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