Introduction to the Landforms and Geology of Japan

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Northeastern Honshu

Northeastern Honshu (the Tohoku region) is in the Northeast Japan Arc. This island arc has typical features of the island arc-trench system, such as distinction between an outer arc and inner arc, volcanoes distributed parallel to the trench with a clear volcanic front, and geological structure parallel to the long axis of the island arc. Strong earthquakes frequently occur owing to active crustal movement in the region.

The landforms are characterized by mountain ranges running north-south and depression zones between these mountains: the Kitakami Mountains and the Abukuma Mountains on the Pacific side, the Ou Mountain Range as a central backbone, and the Dewa Mountains on the Sea of Japan side.

The major geological setting is as follows: in the outer arc, Paleozoic-Mesozoic sedimentary rocks and accretionary complexes, and Cretaceous felsic plutonic rocks; in the inner arc, volcanic rocks and sedimentary rocks yielded in the Late Oligocene or later and volcanic products spewed from Quaternary volcanoes. For the basement rocks (geotectonic subdivisions), see also “Outline of landforms and geology of Japan”.

The distribution of volcanoes makes double lines: the first volcanic line on the Ou Mountain Range and the second line on the Dewa Mountains. The volcanism of the first line is more active than that of the second.

Topographic map Geological map
Fig. Topographic map [Another window] Fig. Geologic map [Another window]

 

Outer arc

The outer arc is a non-volcanic region in which sluggish upwarping and downwarping with long wavelength are dominant. The Kitakami Mountains and the Abukuma Mountains have gentle slopes resulted from dissection of long duration, because the mountain ranges have never been under the sea, excepting the margin of the mountains, and have not been subjected to intensive crustal movement during the Cenozoic.

The Kitakami Mountains are geologically divided into the northern and southern parts. The northern Kitakami Mountains consist mainly of a Jurassic accretionary complex intruded by Early Cretaceous felsic plutonic rocks. The southern Kitakami Mountains comprise shallow-marine sediments accumulated during the Silurian to the beginning of the Cretaceous and felsic plutonic rocks that intruded into the sediments in the Early Cretaceous. These sedimentary rocks and felsic plutonic rocks are also found in the eastern margin of the Abukuma Mountains. High pressure type metamorphic rocks older than 300-400 million years ago are distributed in the southern Kitakami Mountains. The shallow-marine sediments found in the southern Kitakami Mountains, which were deposited in the margin of the continent in the Paleozoic, differ from other Mesozoic and Paleozoic formations containing pelagic sediments and turbidite (trench-fill sediment). In addition, Carboniferous and Permian fossils yielded in the southern part are associated with shallow-marine fossils in the South China (Yangtze) block.

In the Abukuma Mountains, felsic plutonic rocks are widely distributed and low-medium pressure type metamorphic rocks and a Jurassic-Early Cretaceous accretionary complex are found in the southern part.

Inner arc

In the inner arc region, short-wavelength folding and faulting are dominant, forming mountains and basins smaller than those in the outer arc region. The reason is thought that this region being compressed east-west is composed of sedimentary rocks (mainly of the Neogene) younger than those of the outer arc region and the high ground temperature attributed to rising magma facilitates deformation of the rocks.

Neogene sediments widely distributed in this region were deposited in the backarc basin by subsiding accompanied with the expansion of the Sea of Japan in the Miocene. Altered volcanic rocks (known as “green tuff”) that were produced in the Late Oligocene-Early Miocene are also broadly found. These andesitic-rhyolitic volcanic rocks with basalt spewed while the Japanese Islands separated from the continent.

The Ou Range about 500 km long runs north-south in the almost center of northeastern Honshu, which is the longest range in Japan. The altitudes of volcanic areas are higher than those of non-volcanic areas; the highest peak is 2038 m of Iwaki Volcano and 1440 m in the non-volcanic areas. The range is characterized by the single ridge with many Quaternary volcanoes and the landforms consistent with the geological structure. High elevation areas in the range correspond with anticlines (the axis of uplifting). The Ou Range began to uplift in the Late Miocene and the uplift rate increased in the Quaternary.

The Dewa Mountains is young fold mountains, which started to upheave in the Late Pliocene. The Dewa Mountains is separated into several mountain ranges by antecedent valleys. The main part of the range consists of Neogene rocks, but the southern part (Iide Mountains and Asahi Mountains) comprises pre-Tertiary rocks and Cretaceous granite intruding the pre-Tertiary rocks. The Iide Mountains and the Asahi Mountains are higher and steeper than the main part.

References

 

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