金剛断層系の平均変位速度分布と奈良盆地南西縁地域における地形発達

Abstract

The Nara basin, located in the central part of the Kinki triangle in central Japan, mainly consists of Plio-Pleistocene sediments (Osaka group), Pleistocene terrace deposits, and Holocene alluvial deposits.The upper most part of the Osaka group extending in this area consist of fluvial deposits, and is correlated with the Ma3, which is includes the Azuki tephra.The terrace surfaces have developed along the eastern foot of the Kongo-katsuragi Range, in the southwestern part of the basin. Based on the distribution of surface heights and stratigraphies, these terraces are classified into the H, Ml, M2, M3, Li, and L2 surfaces in descending order.The Kongo fault system striking in the north-south direction extends along the eastern foot of the Kongo-katsuragi Range. This fault system consists of the Yamaguchi, Kongo, Nakato, and Yamada faults. These faults have displaced fluvial terrace surfaces, and the slip rates of the vertical component are about 0.03-0.3 m/ky.The Median Tectonic Line striking in the west-east direction and extending southward of the Nara basin, separates the Izumi Range and the Kinokawa lowland. The Shobudani fault, one of the fault systems of the Median Tectonic Line, had a tendency of reverse faulting activity in the former half of the Quaternary. On the contrary, right lateral faulting activity of the Gojodani fault started from the middle Pleistocene, and this tectonic movement has continued to the present along the Median Tectonic Line.Based on the stratigraphy of the Osaka group and fault activity around the study area, the fact that the Nara basin was divided from the Kinokawa lowland by reverse faulting of the Shoubudani fault at least the early Pleistocene, was, clarified. This faulting also formed the, present drainage pattern and the watershed located in the southwestern part of the Nara basin.Fault activity of the Kongo fault system started after the middle Pleistocene. The central part of the fault system shows the highest slip rate. On the contrary, the slip rate at the tips of fault system show a small value compared with the slip rate in the central part of the fault system. Both the height distribution of the terraces and that of the Kongo-katsuragi range, extending to the southwestern part of the Nara basin, have same trend in comparison with the distribution of slip rate along the Kongo fault system.In conclusion, tectonic movement with faulting of the Kongo fault system has affected the formation of landforms in the southwestern margin of the Nara basin.