領家南縁剪断帯の研究

The Ryoke metamorphic terrain is placed in the southern front of acid volcano-plutonism during the Late Mesozoic age in Japan. The Median Tectonic Line (MTL) is the southern boundary of the present Ryoke terrain (=the boundary between the Ryoke and the Sambagawa metamorphic terrains), but, judging from pattern of zonal arrangement of metamorphic facies, it was he initially developed within the southern marginal part of the initial Ryoke terrain. Namely, the part of the Ryoke terrain developed initially in the southern side of MTL was lost through Late Mesozoicrecent tectonic movements along MTL. This paper deals with the initial form and initial movement picture of MTL. The Ryoke metamorphic terrain shows a syntaxis like arrangement on both sides of Fossa Magna. The structural trends in the Ryoke terrain, except for those around Fossa Magna, which are shown by gneissosity and linear structures of granitic rocks and metamorphics, are at low to moderate angles to the general trend of the Ryoke terrain and MTL, forming en echelon structures. The main structure around Fossa Magna shows horizontal axial trend subparallel to them, but it is superposed by new mineral lineation of dip-direction. These structural trends continue into a narrow shear belt (=southern marginal shear belt) developed along MTL. Around Fossa Magna the structure of later phase continues into the shear belt. The granitic rocks and metamorphics in the shear belt are generally mylonitic in texture. However, their constituent minerals such as brown biotite, hornblende, K-feldspar and plagioclase are stably recrystallized, though considerably decreased in size. This shows that the shear belt was formed under the high-temperature condition. On the basis of observed c-axis fabrics of quartz (pseudo-two-girdle type), the gneissosity (mylonite band) in the shear belt is regarded as corresponding to the plane including the longest and the intermediate principal axes of strain, and the lineation on it to the longest principal axis of strain. The longest and the shortest principal axes of strain (X and Z) in the shear belt are generally approximately horizontal, and the angle between X and dexceases MTL gradually towards MTL, except for in the shear belt around Fossa Magna, where X is oriented in dipdirection. The orientation patterns of the principal axes of strain in the shear belt are comparable with those in the half sides of the shear belts described by RAMSAY et al. (1970) and HARA et al. (1973). MTL is placed along the part of the highest shear strain, and it shows that the southern half side of the initial southern marginal shear belt was lost by tectonic movements of later phase along MTL. The sense of shear strain in the shear belt is left-lateral 011 western side of Fossa Magna, and its magnitude appears to increase with increase of distance from Fossa Magna, showing lateral displacement of ca. 30km near Deai (Central part of Kinki Province). In the shear belt around Fossa Magna, lateral displacement does not occur but dip-direction displacement. On eastern sidd of Fossa Magna the Ryoke granitic and metamorphic rocks are so sporadically developed that it is difficult to examine their structures. It has been concluded in this paper that the geological structures (en echelon structures, southern marginal shear belt etc.) of the Ryoke metamorphic terrain was produced through the process of formation of syntaxis-like arrangement of the terrain under non-uniform compression from the Pacific Ocean side, and that the stress concectration along the southern margin of the Ryoke terrain (=the formation of southern marginal shear belt) occurred when the syntaxis-like arrangment of the terrain was produced to some extent and the shear movement of dip-direction began around Fossa Magna.