1990 年 28 巻 5 号 p. 369-387
The purpose of this paper is to investigate the amount and mode of crustal movement in the Hidaka mountains, Hokkaido Island. It is known that longitudinal proflies of almost all rivers have changed in accordance with periodic changes in climate and sea level during the Quaternary. In the upper reaches, an erosional river bed formed in the Last Interglacial Age correspond to the bottom of the Last Glacial fill terrace deposit, and its longitudinal profile is often parallel to that of the present one. Therefore the author hypothesized that homologic river profiles were formed in two periods with similar climatic conditions and sea level heights, corresponding to the Last Interglacial Age and the Post Glacial Age. Based on this hypothesis, the difference in height between the Last Interglacial buried valley bottom and the present river bed is explained by the amount of uplift during this period.
Fluvial and marine terraces are well developed, and their surfaces are covered with a number of marker tephra layers around the Hidaka mountains (Table 2). Figure 9 shows the difference between the height of the Last Interglacial buried valley bottom and that of the present river bed in the study field. The difference in height between the Last Glacial terrace surface and the Penultimate Glacial terrace surface (Fig. 10) is also useful for cross-checking of the data on height differences between interglacial profiles.
The characteristics of crustal movement in the study field are summarized as follows. Almost all of the region on the east side of the Umaoi hills has a tendency to uplift. The uplift rate of the mountainous and hilly region on the west side of the Hidaka mountain range is estimated to have been 0.2-0.4mm/yr., and the rate increases toward the western Hidaka coast. This tendency is also supported by evidence that the shoreline heights of the Last Interglacial marine terrace along the Hidaka coast decrease toward the Erimo cape, where the present coast traverses the axis of the Hidaka mountain range. The east side of the Hidaka mountain range and the Tokachi plain have slowly upheaved at the rate of 0.1mm/yr. Short wave deformation of fluvial terrace surfaces accompanied by rapid uplift (more than 0.7mm/yr.) is expected around the Yubari mountains.