討論の要約と論評

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To consider the structural unit of the North-eastern part of the Honshu island only from the viewpoint of present day state is not at all correct. The structural unit itself is a product of long geohistory, and has been changing greatly through ages. (II) According to my view, Volcano stratigraphy of the fossiliferous Palaeozoic formations developed in the Kitakami and the Abukuma mountains seems to have been greatly advanced but can not be viewed as the same level of study as done in the southwestern part of Japan. It is therefore too early or almost impossible to bring forth any conclusion such as that submarine volcanic activities might be probably different between the Northeast and southwestern part of Japan, either in stratigraphic sequence or in lithology. (III) The Permian Usuginu type of conglomerates include various boulders and pebbles: granite, basic plutonics, dyke rocks, altered pyroclastics and various sedimentary rocks of Devonian to Mississippian age, etc. They may represent deposits of the Molass phase of the Abean Orogeny (≒Variscan) in Late Palaeozoic time of Japan. (IV) The Tsubonosawa gneiss is clearly observed in the field to be intruded by the Higami granite, presumably of the Lower Carboniferous of the Latest Devonian by K-Ar dated age. The entire Higami granite itself is intensely mylonitized. (V) The presence of sandstone layer with kyanite and staurolite crystal grains in the Lower Devonian formation developed around the Hitachi coppermine, located in the southern Abukuma mountains is worth mentioning (J. Watanabe, 1971). (IV) and (V) suggest that Precambrian rocks may be exposed both in the Kitakami and Abukuma mountains. The Tsubonosawa gneiss and the Nishidohira gneiss may be the examples. Actually, staurolite and kyanite bearing schists are found in the Nishidohira gneiss typically developed in the west of the Hitachi mine. The structural trend of this gneiss and that of the Palaeozoic in that district is remarkably different. (VI) Sedimentary basins seem to have shifted east-wards through Mesozoic time, at least so far as their maximum thickness of deposits of those basins are concerned. Such tendency is also noticeable in the Palaeozoic basins in Northeastern Honshu. (VII) To state the relationship between the shallow subsurface structure and deeper crust of the upper mantle is not wise, without detailed informations on the structure and the lithology of both units: shallow and deep structural units. (VIII) Alteration of the Neogene deposits developed along the Japan sea coast may have occurred at least twice: the earlier phase was more extensive, and the later phase was local. New informations were given by various speakers at this symposium. These new informations seem, according to my view, to be well incorporated with the known facts to build a more reasonable framework of the structure of the Northeastern part of Japan, than before. On behalf of the organizers of this symposium, I would like to express my sincere thanks to all the speakers, and to all the members of our society who kindly participated in this meeting and brought the symposium to great success.
日本地質学会の論文
1974-07-31

討論の要約と論評

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