Introduction to the Landforms and Geology of Japan

GLGArcs

Outline of Landforms and Geology of Japan

Strike-slip faults concentrate in central and southwestern Honshu (Chubu and Kinki) to the west of the Itoigawa-Shizuoka Tectonic Line. Heights of fault scarps formed by these faults are generally lower than those of reverse and normal fault scarps because of the small vertical offsets. Strike-slip fault blocks are distributed only in western Chubu and northern Kinki, such as the Hida Highland and the Tamba Highland. Northeastern trending and northwestern trending active faults run reticulately in these areas. Mountains in the Hida Highland show clearly accordance of summit levels. Although mountains have been well dissected to make many valleys, the topography looks like plateau when taking bird’s-eye view. The landform of the Hida Highland is considerably different from that of the adjacent Hida Range with sharp ridges (knife-edges).

Many mountain ranges began to rise in the Pliocene, and the uplift rates increased in the Quaternary. The present mean uplift rates of the mountains are estimated <1 to 5 mm per year. The highest peak of nonvolcanic mountains in Japan is 3192 m high in the Akaishi Range (the highest point in Japan is the top of Fuji-san (volcano), 3776 m high).

[Morphology]

Japanese mountains are much smaller compared to the Himalayas and the Andes; heights of highest mountain ranges in Japan slightly exceed 3000 meters, which are a third height of the Himalayas and a half height of the Andes. Some small mountains may be regarded as hills. Small geological structure units are responsible for the mountain sizes. The mountain morphology is characterized by dense valleys (busy alternation of valley and ridge), convex or straight slopes, erosional low-relief surfaces on summits, and few knife-edges and independent peaks (horns). Pluvial climate and high uplift rates in the Quaternary are responsible for active erosion making deep and dense valleys. In mountains with high relief, deep V-shaped valleys are formed. Photo 1 shows the profile of common mountains in Japan. The tops of mountains are roundish and slope lines are convex.

Erosional low-relief surfaces of Japanese mountains are principally remnants of uplifted peneplain formed during the mild crustal movement period in the Late Tertiary. Some of them are upland denudation surfaces or piedmont benchlands. The low-relief surfaces are broadly distributed in mountain ranges in central Hokkaido, the outer arc side of Tohoku, the inner zone of southwest Japan, especially in the Kitakami Mountains, the Abukuma Mountains, and the Chugoku Mountains, but those are limited in the inner arc of Tohoku and in the outer zone of southwest Japan. In addition to the presence of erosional low-relief surfaces, early dissection stage due to recent uplift of mountains and poor experiences of glacial erosion resulted in that mountains have few independent peaks (horns). Therefore, most mountain ranges have flat skylines (accordance of summit levels).

Although Japanese mountains possess the characteristics mentioned above, there are regional variations reflecting local crustal movement and climates.

The Sea of Japan side from central Honshu to Hokkaido is a heavy snow fall region in which maximum snow depths are over one meter (two to three meters in places). The quantities of snow affect erosion in addition to running water agent. For example, straight or concave slopes are found rather than convex slopes in the Echigo Range situated in the region. Concave mountainsides are found in mountains dominated by glacial erosion, such as the Himalayas and the Alps.

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