Abstract
The Yuda Basin is a narrow lowland situated along the axial part of the Ou Backbone Range in northeast Japan. Stratigraphic study and fission-track datings reveal that the Oishi, Kotsunagizawa, Kurosawa and Hanayama Formations distributed in the Yuda Basin are almost continuous deposits ranging from Middle Miocene to Pliocene (16-3 Ma) with a significant age gap between 12 and 9 Ma.The Hanayama Formation is composed of high frequency sequences formed under shallow marine, deltaic and fluvial environments, suggesting that the basin had been connected with the Japan Sea until Early Pliocene.The subsidence curve of the Yuba Basin suggests that there were three phases of uplifting in the surrounding Ou Backbone Range after the rapid subsidence in early Middle Miocene. 1)Temporary and sudden uplift of the eastern sector of the Backbone Range at 12 Ma, followed by slow deposition within the basin between 12 and 9 Ma, 2) Differential uplifting of the Backbone Range between 6.5 and 3 Ma. 3) Uplifting of the whole Backbone Range by thrusting after 3 Ma, together with formation of angular unconformity within the basin. The eastern sector of the Backbone Range is, therefore, considered to have been uplifted earlier than the western sector. The resultant differential denudation histories of the two sectors may be attributed to difference in topography as well as in geologic structures between the two sectors.
