北部フォッサマグナにおける褶曲形成と火山活動  [in Japanese] Tectonic relation between folding and volcanic activity in the North Fossa Magna  [in Japanese]

From recent geological study of the Higashiyama hill and subsurface data by deep drilling, volcanic successions and tectonic situations of volcanic rocks of the middle Miocene toearly Pleistocene around the Nagaoka City are sumarized. Calc-alkali dacite and andesites of the latter half of the middle Miocene to late Miocene occured in the Nigoro and Minami-Nagaoka anticlines but these are absent in the anticlines of the Chuo and Nishiyama oil fields. Calc-alkali andesites of the late Miocene to Pliocene occured along the Shibata-Koide tectonic line. On the other hand, tholeitic basalts-andesites and calc-alkali andesites of the Pliocene in the western part of the Niigata Prefecture and the northern part of the Nagano Prefecture occured in synclinal parts and contain many gabbroic xenolithes. From studies on the development of the Niigata sedimentary basin, it is concluded that folding started in the late Miocene and continued from the late Pliocene to the early Pleistocene. The late Miocene volcanic rocks are assumed to be related to fractureing in embryo of folding which might be brought about magma generation. The Pliocene volcanic rocks are assumed to be not related to folding but to be related to more regional fracturing which formed tectonic lines or sedimentary basins which also brought about magma generation.