信濃川左岸，時水背斜東翼の露頭に現れたタイプを異にする活断層群とその解釈

Abstract

The area at the middle to lower reaches of the Shinano River is a well-known major Neogene thrust and fold belt in Japan. Deformed fluvial terraces, such as anticlinal ridges, synclinal valleys, and fault scarps along the Shinano River, provide a good record of recent tectonic activity in this belt. A large exposure (ca. 150 m long, and up to 10 m deep) was excavated by construction work on the eastern limb of the Tokimizu anticline, giving us an opportunity to observe various types of fault geometry. Four faults—F1, F2, F3, and F4—cut terrace deposits of ca. 130-150 ka (Koshijippara terrace) and underlying early Pleistocene Uonuma Formation. The westernmost fault, F1 is represented as a remarkable flexure dipping westward, suggesting the presence of a low angle thrust underneath. We found a very low angle fault dipping eastward from an additional 2 m deep excavation. The vertical slip at F1, judged from the height difference with the top of the gravel bed (Bed V), is 12 m. In contrast, faults F2 and F3 to the east of F1 follow the bedding plane of the steeply dipping Uonuma Formation, and are high angle reverse faults with the upthrown side to the east. The vertical slip is 3-4 m for F2 and 7.5 m for F3. Profiling across these faults shows that F1 is clearly expressed as a deformed terrace, but the topographical expression of F2 and F3 is not necessarily obvious. Similar faults to F2 are recognized in the study area from observations of the other three large exposures. We classify the faults in the study area into three types: Type 1 is a blind fault assumed at the base of the eastern limb of the Tokimizu anticline. This fault might be the most important contributor to the formation of the major tectonic relief in the study area, although we have no data to prove the nature of the fault plane itself from this study. F1 fault, demonstrated by Type 2, was found for the first time in this study, and is a low angle reverse fault truncating the structure of the Uonuma Formation with a vertical slip rate of 0.1 m/ka. The Type 3 fault is represented by F2, F3, and F4, and these are interpreted to be flexural slip faults along the bedding plane of the Uonuma Formation. Repeated faulting is confirmed from the progressive deformation of different beds not only for the F1 fault (Type 2) but also for the fold-related secondary faults, F2 and F3. No faulting has occurred since ca. 7,500 years BP, however.