Geological Map Reading, I.

Abstract

A critical study of the geological map, 1 : 50, 000, Sheet “Yoshinoyama” by K. Hirayama and F. Kishimoto, published by the Geological Survey of Japan, 1957, led the writer to the following conclusion : I. The Median dislocation line of Southwest Japan, in the original sense of the writer, is divisible into three sections. The east section which extends from the south of the lake Biwa to the southern side of Mt. Kongô, lying north of Gozyô, Nara Prefecture, separates in its greater part the terrain of the Ryôke complex (Ryôke metamorphics, with grantitic rocks in an intimate association) in the north, from the terrains of the Nagatoro metamorphics and semi-to non-metamorphosed sedimentaries of the Paraeozoic Titibu system in the south. West of Wasika, Ogawa-mura, Nara Prefecture, the former terrain comes in direct contact with that of the Upper Cretaceous Izumi group on the south. This east section marks the front margin of a southerly, resp. southeasterly overthrust of the Ryôke complex.II. The middle section of the Median dislocation line, which divides the terrain of the Izumi group in the north from that of the Nagatoro metamorphics in the south, runs almost straight ENE-WSW, resp. E-W, from near Wasika successively through the western part of northern Kii Peninsula, off the south coast of Awazi-sima, and across the northern part of Sikoku to Saganoseki, Kyûshû, beyond the Kitan and Bungo Straits. The crustal disturbance along this section is dated post-Cretaceous or early Tertiary, since it displaced the Upper Cretaceous Izumi group on one hand, and preceded on the other the deposition of the Middle Eocene member with Eofabiania of the Kuma group of the Isizuti-yama, which rests almost horizontally on both the Izumi group and the Nagatoro metamorphics over the tectonic boundary.III. These terrains on either side of the middle section of the Median dislocation line are overlapped near Wasika and thence eastwards by the Ryôke complex displaced by the southerly, resp. southeasterly overthrust cited above. The behavior at the junction near Wasika of the two dislocation lines bordering the south and north sides of the Izumi terrain can be well understood in this way.IV. Close to the east section, the Ryôke complex is fringed by a tectogenic rock-complex, the Kasio gneiss, of various mylonites and porphyroids, etc. The boundary between the Kasio gneiss and the Ryoke complex is often quite meager, while that between the former and the Izumi group, Nagatoro metamorphics and semi-to non-metamorphosed Paraeozoic rocks are in general more distinct. The Kasio gneiss is not known along the middle section.V. The zone of mylonitization is a name applied by S. Tsuboi to the linear extention of the Kasio gneiss, and he believed its earlier genesis than the Median dislocation line in the original sense of the present writer ; T. Kobayashi also distinguished early Cretaceous Kasio phase from the post-Cretaceous Itinokawa phase of the tectonic disturbance of the Median dislocation line, based on Iwao's observation on the Izumi group lying over the Kasio gneiss at the southern side of Mt. Kongô, where later R. Sugiyama found their relation to be discordant, and emphasized the absence of the pebbles of the Kasio gneiss in the conglomerate of the Izumi group there exposed, while those of the Ryoke complex are common in it. In the present peper, it is maintained that the Kasio phase, if this term is acceptable, is rather later and never older than the post-Cretaceous movement, the Itinokawa phase of Kabayashi, of the middle section of the Median dislocation line.