中央アルプスとその西域の地質 : その3:領家帯の地質構造

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Ryoke metamorphics of which typical representatives are banded gneiss and schistose hornfels, grade into non-metamorphosed Paleozoic strata, passing through biotite slate. The transition is so gradual that the boundary line between the metamorphic and non-metamorphic zones can not be drawn untill close microscopic observations are made on samples from lots of localities. This line is revealed to cross with the trend of the Paleozoic formations, and this phenomenon is especially clear in the northern part of the area under question. Moreover, the zonal arrangement of metamorphic zoning crosses with the trend of the Paleozoics. Due to the structural as well as lithological differences, Ryoke zone could be divided into the " central " and " outer. " zones. The former zone corresponds to the distribution area of Ryoke granites, and the metamorphics here are mainly of high-grade (schistose hornfels and banded gneiss). The latter zone is the territory of lowgrade metamorphics (biotite slate), and herein no Ryoke granites exist. The northeastern area is occupied exclusively by the outer " zone. On the contrary, in the soutwestern area is widely developed the "central" zone and the "outer" zone is very narrow and, in places, lacking at all. This peculiar pattern of geographical distribution of both zones is chiefly due to northeasterly-pitching of the axis of Ryoke zone. Concerning this disposition, however, such effects as the strike faults cause the highergrade metamorphics to contact directly with the non-metamorphics without intervening the lowgrade rocks should not be negligible. The geologic structures of Paleozoic deposits in the non-metamorphic and low-grade metamorphic ("outer") zones are characterized by folding with high-angle inclination, and the stratigraphic formations are arranged zonally with the general trend of NNE-SSW. Structurally, both zones are continuous and form one single unit. The most conspicuous of the fold structures are the anticlinorium in the transitional area and the synclinorium within the interior part of the metamorphic zone. These folds are distinctly observable throughout the northeastern area, namely, the main field of "outer" zone. But they disappear in the southwestern area where "central" zone is widely developed. In the "central" zone, granites are predominant, while metamorphics are merely subordinate. Therefore, the structure of this zone is nothing but a structure of granite body or a structure formed at the time of granite emplacement. Most of the granites bear schistosity, and there exists the intimate relation among the schistosity of granite, the schistosity of xenolithic metamorphics included within, and the external form of xenoliths, all being harmonic and parallel with each other. The measurements of them reveal the "dome and basin" structure. Granites bodies without schistosity have outlines harmonic with the structures above-mentioned and have a tendency to occupy the center of each dome or basin. Therefore, the structural pattern of the "central" zone is concluded to be in a marked contrast with that of "outer" zone. Both zones are separated by a shear zone characterized by the intrusion of the protoclastic Otagiri granite (Otagiri-Kisokoma Line). But some characteristic Paleozoic formations distributed continuously in the "outer" zone (northeastern area) are tracable across the Line in the northern part of the "central" zone. The fact that the boundary line between the metamorphic and non-metamorphic zones and, also, boundaries between subzones within metamorphic terrain run obliquely with fold axis, as well as the fact that the Ryoke zone has the characteristics of metamorphism under static condition with comparatively low pressure, suggest that the age of the metamorphism is not penecontemporaneous with but posterior to the main phase of folding of the Paleozoic formations. At the time of metamorphism, pre-existing structures of these formations were hardly disturbed rior modified not only in the "outer" zone but also in the "central" zone. Before the metamorphisim took place, nearly all the folds and a few of the faults had been already formed. After zonal arrangement of the Ryoke zone had been accomplished, the Ryoke granites were intruded, independently of the metamorphic zonal arrangement, in the "central" zone, where the pre-exting rocks had been already changed into plastic condition in consequence of the highly advanced metamorphism. The two events, however, were very successive. Thus, the emplacement of Ryoke granites gave rise to the "dome and basin" structure of the "central" zone. The age of the main folding phase of the Paleozoic geosynclinal deposits is early Mesozoic. Concerning the age of Ryoke metamorphism, exact dating by field evidence has been impossible, but, according to the dating by the K-A method, the age of some Ryoke granites is reported as middle Cretaceous. Therefore, it is probable that the Ryoke zone was formed at Middle Cretaceous time. It is very striking that there exists a very long interval between the main folding of the Paleozoic strata and forming of the Ryoke zone. Consequently, the Ryoke metamorphism and plutonism are concluded to have been closely related to an intense geanticlinal upheaval. In fact, the protoclastic nature of Otagiri granite proves the existence of a faulting movement or a differential movement within the Ryoke zone contemporaneous with the plutonism, and this diastrophism, we suppose, may be a reflection of pitching of the axis of the Ryoke zone at the time. From the standpoint of the wider view, the parallelism of the metamorphic trend of Ryoke zone to the direction of Median Tectonic Line suggest the intimate genetical relation between the two. Since the age of the plutonism, the upheaval continued, and the constituents of Ryoke zone were exposed on land surface as early as at late Cretaceous time. In this age, faulting was very active, and the pre-existing structures were slightly modified. Subsequently the granites of latest Cretaceous age was chiefly intruded along the pre-existing faults in the area of transtion from the metamorphic to the non-metamorphic zone as well as in the area of non-metamorphics nearby. In consequence, the former transional relation, whether an imperceptibly gradual change or a stepwise change due to parallel faults, was masked especially in the southwestern area, by this plutonism. Upheaval of the blocks went on, accompanying faulting, in Tertiary and Quaternary. In this way, the Ryoke zone has come to show an altitude as high as 3,000m. in the present Japanese Central Alps.
地学団体研究会の論文
1961-11-25

中央アルプスとその西域の地質 : その3:領家帯の地質構造

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