TECTONIC MORPHOGENESIS IN THE NOSAKA MOUNTAINS, NORTH OF LAKE BIWA, CENTRAL JAPAN

Abstract

The Nosaka Mountains are situated on the western side of the Tsuruga Bay-Ise Bay Tectonic Line which is one of the main tectonic lines in Southwest Japan, and occupy the northeasternmost part of the Setouchi tectonic depression. In this mountain area, tectonic landforms of various kinds are remarkably developed. This fact has been recognized by many investigators, but the process of tectonic morphogenesis in this area has not inten-sively been analysed since the Yamazaki & Tada's study (1927). The author investigated the tectonic m_orphogenesis in this mountain area, chiefly based on interpretations of summit level maps and aerial photographs, together with field surveys. The followings are the major results obtained ; (1) On summits of the Nosaka Mountains, erosion surfaces with low relief are recognized. After these erosion surfaces with low relief had been formed, the Nosaka Mountains were undulatingly upwarped trending WNW-ESE, and then disintegrated into many small mountain blocks by faulting trending N-S, NE-SW and NW-SE. However, the WNW-ESE alignment of the undulating tectonic relief, has not been dislocated horizontally and is clearly recognized even after the disintegration. This fact suggests that these faults are dip-slip faults. (2) Many active faults are ascertained by photo-interpretations and field surveys in the Nosaka Mountains. It is relatively easy to find active faults in this area because each of them is characterized by a remarkable photogeologic linca.ment and tectonic landforms such as offset stream, shutter ridge, fault trench, scarplet, wind gap and so on. These active faults are trending NE-SW and NW-SE, and most of them are strike-slip faults, among which the NE-trending ones are dextral and the NW-trending ones sinistra.l. (3) Except several parts, these active faults are inconsistent in distribution from the above-mentioned faults by which the Nosaka Mountains were disintegrated into many blocks. And, as already mentioned, most of the former are strike-slip faults, while the latter are dip-slip faults. Besides, one of the active faults is confirmed by field survey to have displaced one of the latter. From these facts, it can be concluded that these active faults have been active since the Nosaka Mountains were disintegrated into many blocks.