BULLETIN OF THE GEOLOGICAL SURVEY OF JAPAN
Online ISSN : 2186-490X
Print ISSN : 1346-4272
ISSN-L : 1346-4272
Volume 71 , Issue 2
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  • Ayumu MIYAKAWA, Kazunari NAWA, Yusuke YAMAYA, Toshiki OHTAKI, Mituhiko ...
    2020 Volume 71 Issue 2 Pages 63-76
    Published: April 27, 2020
    Released: May 07, 2020
    JOURNALS FREE ACCESS

    To reveal the Bouguer gravity anomaly which reflects underground geological structures, we conducted absolute and relative gravity measurements in the Nagura River Basin, western Ishigakijima Island, Japan. The gravity acceleration at the gravimeter pier of the VERA-Ishigaki station measured with an absolute gravimeter was 979002.518 mGal. We further obtained gravity values using a relative gravimeter at 62 stations on Ishigakijima Island. Subsequently, we constructed gravity anomaly maps of the Nagura River Basin by compiling gravity data obtained in this study and an existing database. The Bouguer gravity anomaly decreased from Nagura Bay to the foot of Mt. Omoto. The decrease of the Bouguer gravity anomaly is considered to be due to the lower density of the Oligocene felsic plutonic rocks, which constitute Mt. Omoto, than the surrounding Jurassic accretionary complex. Furthermore, a negative gravity anomaly zone was observed from the foot of Mt. Omoto. This reflects the local basement shape from Nagura Bay to Mt. Omoto.

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  • Takayuki UCHINO, Toru YAMASAKI
    Type: Report
    2020 Volume 71 Issue 2 Pages 77-83
    Published: April 27, 2020
    Released: May 07, 2020
    JOURNALS FREE ACCESS

    Purple to pale-blue amphibole regarded as alkali amphibole ubiquitously appears in the Mikabu Greenstones of the Sambagawa Belt in the Shima Peninsula, Mie Prefecture. The amphibole was proved to be magnesio-riebeckite of Na-amphibole and ferri-winchite of Na-Ca-amphibole based on chemical compositions measured using the EPMA system. Judging from the mineral assemblage of these alkali amphiboles and the other metamorphic minerals such as chlorite, epidote, albite, actinolite, hematite, etc., the dolerite probably underwent the metamorphism of the transitional zone facies between the blueschist facies and pumpellyite–actinolite facies or greenschist facies.

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