The older Mesozoic Akiyoshi folded mountains

Abstract

The Akiyoshi folded mountains on the continental side of the Japanese islands are composed of geosynclinal sediments, their metamorphosed and granitized facies and the metaorogenic basin deposits. The first of these consists chiefly of the Middle Palaeozoic rocks of the intrageosynclinal volcanism in the lower part, in the middle of the Upper Palaeozoic of the limestone-rich Akiyoshi facies and the chert-rich Yamaguchi facies, the two facies indicating the differentiation of the geosyncline by the embryonic folding and in the upper part of the Permo-Triassic formations in the Maizuru zone.The embryonic Akiyoshi geanticline was bisected in the Permian by the Maizuru syncline into the Sanin anticline in the west and the Hokuroku anticline in the east which were aligned en echelon. The Akiyoshi mountains which reveal the best display in the inner side of West Japan consist of the axial metamorphosed zone and the Yamaguchi folded zone with the Nagato tectonic line at their boundary. The Sangun metamorphic complex in the former was produced from the Pre-Permian rocks by regional metamorphism. Subsequent to the metamorphism the axial zone was granitized, causing the dislocation of the median line in front of the granitized regime. In the Sanin anticline the zone was divided into the Hida gneiss zone on the rear side and the Sangun schist zone on the fore side. In the Hokuroku anticline or Hida plateau region the axial zone was largely granitized and dislocated along the median line in front, leaving its northeastern part where the Sangun complex exists. Because the Moscovian of Itoshiro which is located far inside of the median line is a roof pendant on the Hida gneiss, the Hida plutonism cannot be older than Carboniferous. It can be distinguished into three stages, i.e. the Amo injection, the Shimonomoto subconcordant intrusion and the discordant intrusion of the Funatsu granite, the last of which took place along the median line, intruding the Permian. Therefore its age must be either late Permian or Triassic.In the early and middle Triassic period sea flooded into the palaeo-Maizuru bay from the Pacific side, as far as Ussuri. The Ladinic stage is absent in the Maizuru zone and the base of the Carnic Nabae series marked by a strong discordance, but the zonal structure of Maizuru was not completed by the Carnic because the Nabae is folded in within the zone. The Shidaka and Nariwa series, Noric or late Carnic in age, were deposited in basins in the zone and behind it. The mountain-building was completed earlier on the west side of the zone where the Ladino-Carnic Atsu and later Triassic Mine series are all metaorogenic sediments. On the east side of the zone the Noric of Myogadani, Mt. Ibuki is folded together with the Paraeozoic and the Liassic Kuruma series deposited in a basin along the Nagato line is tremendously thick.It is a remarkable fact that the Maizuru zone as well as the folded arcs on its two sides are distinctly truncated by the Ryoke gneiss zone on the rear side of the axis of the Sakawa folded mountains.No evidence showing either Pre-Cambrian age of the Hida gneiss or its being an ancient massif has as yet been discovered.