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  • ニチ ズメ ケセンヌマ ダンソウ トクニ ソノ セイカク ト コウゾウ ハッタ
  • The Hizume-Kesennuma Fault : With Special Reference to its Character and Significance on the Geologic Development

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The present study is undertaken to elucidate the movement of the Hizume-Kesennuma fault extending from Hizume on the north to Kesennuma on the south which is one of the major faults traversing the Southern Kitakami Massif in a NNW-SSE trend (Fig. 1). Along the southern half of the Hizume-Kesennuma fault, highly deformed sedimentary rocks ranging their age from Carboniferous to Jurassic are distributed (Fig. 2). The stratigraphic succession of these formations is shown in Fig. 3 and the geologic structure in Fig. 11. The characteristics of the geologic structures in this area are as follows : On the western side of the Hizume-Kesennuma fault the Permian is folded with NNE-SSW trending axis which plunge southward. The folds are associated with longitudinal faults and traversed by left lateral strike-slip faults of a NNW-SSE trend. On the eastern side a thick sequence of the Carboniferous to Jurassic rocks forms a N-S trending large synclinorium with an axis plunging southward. The folds which constitute the synclinorium are associated with longitudinal faults and traversed by left lateral strike-slip faults of a NW-SE to NNW-SSE trend like on the western side. Furthermore, the folds are traversed by ENE-WSW trending faults in the southern area. Most of the folds in the studied area were formed during the Oshima orogeny (in Early Cretaceous) and the faults associated with these folds are formed during or shortly after the folding. The Hizume-Kesennuma fault, originated in Early Cretaceous Period, had been developed during the Oshima orogeny as well. The Hizume-Kesennuma fault is characterized by a steeply dipping fault plane, straight trace, many smaller associated faults trending in NW-SE to NNW-SSE direction in echelon arrangement and other parallel faults. En echelon faults have the left lateral strike-slip component like the faults of a NW-SE to NNW-SSE trend which developed on the both sides of the Hizume-Kesennuma fault. The latter are thought to be the second order faults associated with the main fault. The strain picture of the Hizume-Kesennuma faults is quite similar to those reported on a few precedents of strike-slip faults and model experiments. Considering this similarity, as well as the nature of second order faults associated with the Hizume-Kesennuma fault and the distribution pattern of the formations along the fault, it may be concluded that the Hizume-Kesennuma fault is a left lateral strike-slip fault. The strike-slip displacement of the fault is estimated at about 30 km from the following evidences : (1) strike separation indicated by the synclinal structure composed of Jurassic and Lower Cretaceous formations (Karakuwa belt) and (2) strike separation of the Permian Ubaishi Formation and its equivalent mainly consisting of tuff (Fig. 15). The dip-slip displacement is very little as compared with the strikeslip one and is estimated to be less than 1 km.

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