西南日本の第四紀地殻変動から導かれた地殻の流動

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In this paper, mechanical behaviors of the rock during the long period of million years are considered on the scale of the earth's crust of Southwest Japan. As the result of neotectonical studies by Huzita (1969), the structural elements of the Quaternary crustal movements named "Rokko Movements" in Southwest Japan are shown schematically in Fig. 3. The anticlinal uplifts and synclinal depressions due to foundation folding (Makiyama, 1956) have developed associated with thrust faults, which run parallel to the folding axes, and are perpendicular to the direction of the Japan Island Arc. Another fault system is represent-ed by conjugate sets of strike-slip faults of NW-SE and NE-SW trends, of which the former is left-lateral and the latter right-lateral. Such fold and fault systems suggest that Southwest Japan has been under the horizontal compressive stress state of E-W trend. On the other hand, the recent studies of Hagiwara (1967) on the Bouguer anomalies in Japan give us valuable informations on the bottom surface of the crust, which may have been deformed isostatically corresponding to the foundation folding of the crustal surface in the Inner Zone of Southwest Japan. On the basis of the geological data in Rokko and Osaka areas, let us assume 1) that "Rokko Movements" began one million years ago and the foundation folding has developed at uniform rate as the first approximation, and 2) that the maximun amount of depressions and uplifts formed during the period of one million years reaches 600 m in each. Provided that the thickness of the crust is 40 km, the second assumption gives rise to vertical expansions of the crust reaching 7% on the average, from which horizontal contraction e_n of the crust is derived to be 10%. From the first assumption, we get the strain rate e^^・_n=3×10<-15>/sec. Kumagai and Ito^^^ have been continuing the experiments extending over 10 years on the flow of granite under small stresses, and they have reached the conclusion that granite undergoes viscous flow or plastic flow with a very small yield stress. In order to explain the above mentioned crustal contraction e_n=10% based on this conclusion, it seems to be proper to consider that the crust makes viscous flow or plastic flow with very small yield stress. Assuming the viscous flow of the crust, we get the simple relation p_n=3ηe^^・_n, where p_n is compressive stress horizontally acting on the crust and is its viscosity. Using this relation, we can get the viscosity of the crust without consideration of the motion of the mantle. The compressive stress p_n may be estimated by using the data of stress releases calculated by the analyses of earthquake faults and those of absolute rock-stress measurements performed in mines. Thus we have been enabled to get the viscosity of the crust to be in the order of 10<22> poises.
社団法人日本材料学会の論文
1971-02-15

西南日本の第四紀地殻変動から導かれた地殻の流動

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