関東平野の基盤構造

Abstract

The basement rocks distributed in Central Honshu are divided from the Sea of Japan to the Pacific side into the Hida, Hida Marginal-Joetsu, Mino-Ashio, RyOke, Sambagawa, Chichibu, and Shimanto Belts. The Median Tectonic Line forms the boundary between the Ryoke and the Sambagawa Belts. The Hida and the Mino-Ashio Belts are characterized by some Helium-rich natural gases. Some geologists has postulated the existence of the Kashiwazaki-Choshi Tectonic Line trending NW-SE from Kashiwazaki in the Sea of Japan side to the southern part of the Kashima-nada in the Pacific side. The line is confirmed on the subsurface geology known from many deep wells and some geophysical data. Although there are three tectonic belts, that is the Sambagawa, Chichibu, and Shimanto Belts on the southern side of the Kashiwazaki-Choshi Tectonic Line, the Mino-Ashio and Ryoke Belts are recognized on the northern side of the line.At present Northeast Japan belongs to the north American plate. The front of the plate was situated at the Itoigawa-Shizuoka Tectonic Line by the middle Miocene age. Afterward the front has migrated from the line to the Kashiwazaki-Chashi Tectonic Line, on which the nest of earthquakes on the southwestern part of Ibaraki Prefecture rests.