Earth Science (Chikyu Kagaku)

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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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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