1989 Volume 43 Issue 6 Pages 366-391
The author investigates the crustal movement and the formative process of the Muikamachi Basin through an analysis of the land-form in the Uonuma district, central Japan. The results are summarized as follows: 1) The detailed examination of summit level in this district and land-forms of the Sumon and liji volcanos, shows that the basement mountain of pre-Middle Miocene on the east of the Muikamachi Basin was devided into a number of blocks 20 to 30 kilometers long and uplifted with independent behavior, mostly tilting, for each blocks, resulted in the various elevations of block. 2) The comparative study among summit level and remnants of low-relief erosional surfaces in this district, indicates that a low-relief erosional furface named the Uonuma Surface was formed as the primary peneplain all over the surveyed area during the early Middle Pleistocene, and that the Uonuma Surface has been differenciated into the western Uonuma Hills and the eastern Muikamachi Basin due to fault displacement of the Shibata-Koide Tectonic Line since the late Middle Pleistocene. 3) The terraces in the Tokamachi Basin on the west of the westward-tilting Uonuma Hills are classified into four groups, that is, the higher terraces (Takaba, Taniage and Maibara I furfaces), the middle terraces (Maibara II and Unoki surfaces), the lower terraces (Honokizaka, Kaisaka and Shomen surfaces) and the Holocene terraces (Owarino I-II surfaces). The examination of extents scale and longituninal profiles of the terraces and the analysis on provenances of the terrace gravels and paleocurrents, show that the terrace surfaces from the Takaba to the Maibara II were formed as extensive fans of river that flowed into the Tokamachi Basin from the basement mountain on the east of the Muikamachi Basin. It means that the Muikamachi Basin did not yet appear in that time. 4) The terraces distributed in the Muikamachi and the Horinouchi Basin are classified into ten steps in order from higher to lower and correlated respectively to the terrace groups, in the Tokamachi Basin. The examination of longitudinal profile of the terraces in the Horinouchi Basin, indicates that the rivers in the Muikamachi Basin were flowing into the Horinouchi Basin beyond the divide of the Uonuma Hills during forming period of the Maibara II surface in the Tokamachi Basin, and then the rivers connecting these two basins disappeared through displacement of the Shibata-Koide Tectonic Line as at present. 5) Longitudinal profiles of the terrace surfaces and colluvial slopes in the Tokamachi, Muikamachi and Horinouchi Basin, lineaments and subsurface geology around the Muikamachi are investigated synthetically. The investigation shows that the mean vertical slip rate of the Sibata-Koide Tectonic Line, which is composed of some faults arranged in echelon of right hand, became a muximam when the middle terraces were formed and the Muikamachi Basin was remarkably deepened. Activity of the faults are estimated to be of the A rank. 6) The formative process of the Muikamachi Basin is divided into the four stages, that is, peneplanation of the Uonuma Surface (early Middle Pleistocene), germinating of the basin (late Middle Pleistocene), deepening of the basin (early to late Late Pleistocene) and filling of the basin with fan sediments (latest Late Pleistocene to Holocene).
