On the Yamashiro-Yamato Trough

Abstract

The Kyoto basin, the Yamato basin and the Ikoma hilly land may be included in a trough running north to south, which is called as the Yamashiro-Yamato Trough after the provincial names of this district. The trough is defined by the Ikomagawa fault on the west, and the Kasuga-Obaku on the east, but northward by the Paleozoics on the north of the Kyoto basin, while southward by the Ryoke metamorphic rocks developed along the Median line. The Ikoma hilly land is a depressed block, cut off from the Ikoma metamorphosed granite running north to south, by the Ikomagawa fault, and partly covered by the Osaka series. It may be considered to have been sliced by the north south faults into many blocks which are dropped stepwise toward the central line from the two sides. The Kyoto basin is the terminal of the Yamashiro-Yamato Trough which is delimited by the Nishiyama fault on the west and the Obaku on the east. The Hanaore fault runs northeast-ward and passes the western foot of Higashiyama. It is evident that more than half of southern depression of the Kyoto basin has occurred between the Nishiyama fault and the Higashiyama, but the remaining has occurred between the branching faults of the former and the Hanaore. The Yamashina basin may be a depression that occurred between the Obaku fault and the branch-ing fault of Higashiyama, running north to south through the eastern foot of Higashiyama. The Ikoma and the Katsuragi are the horst ranges lying between Osaka Bay and the Yamashiro-Yamato Trough. The former presents steep cliffs to Osaka Bay, whereas the latter offers similar cliffs toward the Yamato Plain, Steep cliffts may be an indication of depression of great magnitude in Osaka Bay as well as in the Plain. The Yamashiro Yamato Trough was produced by the pulling force of the Fossa Magna Disturbance during the Plio-Pleistocene times. Though the making of the Trough is on a small scale it may be applied to such a large Trough as Osaka Bay, Biwa Lake and Ise Bay which were built at the same time and of the same origin. The Fossa Magna Disturbance was brought about by the enormous force of the Pacific applied to the Fossa Magna in the Plio-Pleistocene times.