GEOMORPHIC DEVELOPMENT OF THE HIDA-MINO HIGHLAND, CENTRAL JAPAN

Abstract

To the south of the Hida Mountains, so-called Northern Alps of Japan, there extends the Hida-Mino Highland, where the Kiso and Hida Rivers drain, The highland is bordered by the Kiso Mountains, so-called Central Alps, at its eastern margin and abruptly descends from the Mikawa Highland in its southern margin. The Hida-Mino Highland is subdivided from northeast to southwest into the Adera, Futatsumori-san and Kengyo-san blocks by the Adera and Ako fault scarps (Fig. 1). Geomorphic surfaces were distinguished in three levels in each block. In the Kengyo-san Block these surfaces are 700-900, 350-700 and 250-500 meters high, and in the Futatsumori-san Block 800-1, 000, 600± and 300-400 meters high, while in the Adera Block Block the higher ones are 1, 400-1, 700 meters high and the middle and lower ones could not be clearly classified, ranging in the height between 900-1, 200 meters (Fig. 3). The higher surfaces are represented by flat summits of the mountains in every block, and the middle ones are gently declining foot-slopes of these mountains and the lower ones depositional surfaces, composed of the Toki Gravel Beds. These geomorphic surfaces of three levels are respectively correlated among three blocks, judging from sand and gravel beds that have taken part into their formation (Fig. 5 and 7). According to the results of the correlation of the geomorphic surfaces and further analyses of geomorphic and geologic evidences, the author concluded on the geomorphic history of the Hida-Mino Highland as follows. After the formation of the geomorphic surfaces of the higher and middle levels, the Adera Block bordered by the Adera fault scarp was uplifted, tilting northeast, and the Hida-Mino Highland was differentiated into two blocks. In consequence of dissection of the Adera fault scarp and its back slope, the Toki Gravel Beds were deposited in depressions located downstream of the Adera Block. The southern part of the Hida-Mind Highland was further differentiated into two blocks by the Ako fault. While the northeastern block, that is, the Futatsumori-san Block, was warped down southwards, forming the Boa Basin bordered by the Byobu-san Fault Scarp along its southern margins the southwestern block, namely Kengyo-san Block, was tilted to southwest after the deposition of the lower horizons of the Toki Gravel Beds, and was uplifted about 100 meters higher than the Ena Basin. The Kiso River consequently has cut an antecedent valley into the block. The Hida-Mino Highland gently descends to the Nobi Plain at its western margin. There are several terraces and erosion surfaces along the margin of the highland. The geomorphic relation of these surfaces to ones in the Hida-Mino Highland was discussed in this paper, too.