Earth Science (Chikyu Kagaku)
Online ISSN : 2189-7212
Print ISSN : 0366-6611
Volume 74 , Issue 1
Showing 1-2 articles out of 2 articles from the selected issue
Research Article
  • Toshima TENPAKU, Kanji SUGII, Satoshi NAKANO, Collaborative Research G ...
    Type: research-article
    2020 Volume 74 Issue 1 Pages 1-20
    Published: January 25, 2020
    Released: July 04, 2020
    JOURNALS FREE ACCESS

    Distributions, modes of occurrence and geochronological data are summarized for the LateCretaceous ring-forming felsic and/or pyroclastic dykes, and their host granites around Lake Biwa, southwest Japan. The granitic plutons and ring dykes can be regarded to be genetically related from their petrological and geochemical characteristics. They are produced from a great felsic magma chamber, which is the source of igneous activities related to the formation of the assumed Biwako Cauldron. Recent geochronological studies, however, require the large revision of this cauldron model concerning the timescale of the pyroclastic activity and supposed huge magma chamber as its source.

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  • Collaborative Research Group for Geology of Myogi
    Type: research-article
    2020 Volume 74 Issue 1 Pages 21-38
    Published: January 25, 2020
    Released: July 04, 2020
    JOURNALS FREE ACCESS

    The Myogi area in western Gunma Prefecture shows a jagged landform of thin ridges and deep gorges bounded by precipitous cliffs up to several hundred meters high. The rock formations of the area consist of Late Miocene subaerial volcanics (the Myogi Formation), which are surrounded with the Middle Miocene marine sediments.

    The Myogi Formation is composed of the Yotsuya, Ipponsugi, Nakanotake and Chosunokashira Members in ascending order. The latter two, called the upper Myogi Formation, consist mainly of alternation of ortho- and clino-pyroxene andesite lavas and andesitic volcaniclastics, over 1,800 m thick. Many synchronous andesitic to dacitic intrusions and volcano-plutonic complexes penetrate the volcanic formations. The voluminous Nakanotake M. is bounded in an extent of 7 km x 9 km, surrounded by mostly vertical faults. The member is segmented into many smaller blocks several hundred meters to one kilometer across. The blocks tilt at various angles up to 90° and are clino-unconformably overlain by the horizontal layers of the Chosunokashira M.

    It is thus inferred that, after the accumulation of the thick Nakanotake M., it was segmented and depressed over 1,000 m though the collapsing and was subsequently covered unconformably by the Chosunokashira M. This Late Miocene volcanic cauldron is newly named the Myogi Caldera. The deep structure of the caldera was subsequently excavated to form the present jagged landform of the area.

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