FAULT TOPOGRAPHY AND QUATERNARY FAULTING ALONG THE MIDDLE AND EASTERN PARTS OF THE ARIMA-TAKATSUKI TECTONIC LINE, KINKI DISTRICT, CENTRAL JAPAN

Abstract

The Arima-Takatsuki Tectonic Line (ATL), located on the northwestern rim of the Kinki Triangle, is an active fault zone developed along the geologic boundary separating the Tanba Zone of Paleozoic rocks to the north and the Ryoke Zone of granite to the south. Along the middle and eastern parts of this tectonic line, limiting the northern border of the Osaka Plain, displacements of terrain features, mainly terrace surfaces (dissected fans), due to fault activities can be clearly observed. The writer has investigated these displaced topographies and attempted to elucidate the nature of fault activities of the middle and eastern parts of the ATL. The findings can be summarized as follows. 1) Faults in this area, trending either E-W or ENE-WSW, can be classified into three types, those having: 1) displaced terrace surfaces vertically, and valleys cutting these surfaces dextrally, such as Kiyoshikojin and Satsukioka Faults; 2) formed shallow depression zones such as Hanayashiki, Koyaike and Ibaraki Depressions in the E-W direction on the terrace surfaces; and 3) cut through and displaced valleys in the mountains on the north, such as Ishizumitaki, Satsukioka, Minoo and Nyoidani Faults. 2) Rates of displacements have been computed by estimating ages of the terrace sur-faces and valleys dissecting these surfaces; for type 1), 0.5-1.5m/103 years dextrally and 0.06-0.2m/103 years vertically, and for type 2), a maximum of 0.8m/103 years in vertical direction. 3) The active fault system of the Rokko Mountains is continuous to that of the study area. Active faults of both regions are distributed in the shape of E-W extending wedge, with the tip pointing to the eastern end of the study area. These fault systems have been active in the second half of the Quaternary under the stress field of E-W compression. Accordingly, NE-SW trending faults have greater amounts of vertical displacements than ENE-WSW trending faults, with both having dextral movements. 4) The Median Tectonic Line (MTL), bordering the base of the Kinki Triangle, is also an E-W trending geologic boundary. Along active faults associated with the MTL, the rates of displacements both in vertical and dextral directions are twice as great as those found for the active faults of the ATL. Distribution of type 3) faults of the ATL is similar to that of the MTL in the sense that both are located on the northern side of geologic boundaries.