ACTIVE FAULTS IN WESTERN AND SOUTHERN MARGINS OF THE KRFU BASIN, CENTRAL JAPAN

Abstract

The author investigated topography and geology in western and southern margins of the Kofu Basin, in order to consider a characteristic of active faults through late Quaternary in these areas, on the basis of deformed landforms. The Kofu Basin, about 20km accross, is one of the tectonic basins in Central Japan, and consists of a flight of dissected fans in the area less than 500 m high (Fig. 1). In the western (Ichinose Upland) and the southern (Bone Hill) margins of the basin, dissected fans are well developed and classified into several steps. As a time marker tephra for the classification of terraces, Pm-I, one of widely distributed pumice layer (ca. 80, 000 y. B. P., Machida and Suzuki, 1971) which was erupted from Ontake Volcano, and Nirasaki mud flow deposits (ca. 300, 000y. B. P.) which were erupted from Yatsugatake Volcano, were used (Table 1). In the Ichinose Upland, geomorphic surfaces are classified into five levels, Higher terraces, Ia, Ib, II and III surfaces in descending order (Fig. 3). In the Sone Hill, they are classified into six levels (Ia, Ib, II, IIIa, IIIb and IV surfaces in descending order) as shown in Fig. 5. Table 1 shows correlation of terraces in both areas. Many active faults which displace these geomorphic surfaces are observed in the Kofu Basin, especially in the both areas of the Ichinose Upland and Sone Hill (Figs. 1, 3 and 5). Active faults of the Kofu Basin are dip slip faults without strike slip component. Most of them are distributed in margins of the Upland and the Hill. Detailed features of deformed surfaces and terrace deposits are illustrated in the maps and cross sections (Figs. 6-13). All the active faults are expressed as fault scarps, scarplets and flexure scarps, and most of them are estimated to be reverse faults or thrust faults. Warped terrace surfaces and reverse tilting towards mountain area are the typical types of deformed landforms in these areas. Rate of faulting of active faults in these areas are listed in Table 2. Since older terraces are usually much more deformed than the younger ones, progressive faulting in the late Quaternary is indicated. Average rate of vertical displacement is calculated to be 0.3_??_1.4m/1, 000 yrs. for the Ichinose Upland and less than 0.6m/1, 000 yrs. for the Sone Hill. It is interesting to note that major active faults are located at the outer margins of terraces far away from the piedmont lines themselves, both in the Ichinose Upland and Sone Hill, although some fault scarplets can be observed along the piedmont in the Ichinose Upland. Due to such arrangement of the active faults, the oldest terraces with reverse tilting are located immediately adjacent to the modern alluvial fans, and their apex area are often covered with the younger terraces. Above-mentioned characteristics of fault topographies may be explained by “a bending model of fault plane” (Ikeda and Yonekura, 1979) (Fig. 14). In the Sone Hill, this model can be applied (Fig. 15-C). But, in the Ichinose Upland, another models are devised because active faults in piedmont area were not inactive (Figs. 15-A, B).