Introduction to the Landforms and Geology of Japan

GLGArcs

Outline of Landforms and Geology of Japan

Division of island arcs

Major tectonic lines (fault zones)
Tectonic lines Fig. 1 Tectonic lines [another window]

 

Fault lineFig. 2 Fault line of MTL [another window]

Several major tectonic lines run in the Japanese Islands, including the Median Tectonic Line, the Itoigawa-Shizuoka Tectonic Line, and the Tanakura Tectonic Line. These are long fault zones and borders geotectonically dividing the island arcs. 

The Median Tectonic Line (MTL) is the longest tectonic line in Japan, about 1000 km long. The line of MTL clearly appears as linear relief on the land, running nearly east to west from eastern Kyushu to southwestern Honshu, and curve to the northeast in central Honshu. On the east of the Itoigawa-Shizuoka Tectonic Line, the MTL cannot be traced because of thick deposits in the Fossa Magna, but probably continues under the Kanto Plain. Present active faults in the MTL are mainly right-lateral strike-slip faults, but faulting before the Miocene was in a left-lateral sense. Various fault rocks, such as mylonite and cataclasite, are found along the MTL.

The Itoigawa-Shizuoka Tectonic Line (ISTL) runs north-south in central Honshu, which is the border line between the Southwest Japan Arc and the Northeast Japan Arc. Landforms and geological features in southwest Japan drastically change beyond the ISTL. The eastern area of ISTL is called the Fossa Magna; the ISTL is its western edge. The volcanic front that runs from the Tohoku region turns to the south in the Fossa Magna and continues to the Izu-Bonin Arc.

The Tanakura Tectonic Line (TTL) is a fracture zone formed by strike-slip faults in southern Tohoku. The east-trending distribution of the basement rocks changes into the north-trending distribution on the east of TTL. Therefore, the TTL is regarded as the border between southwest Japan and northeast Japan in terms of basement geology.

Dividing the island arcs

An island arc can be geomorphologically classified as follows. A volcanic front divides it into the outer arc (the ocean side) and inner arc (the continental side). The outer arc and the inner arc are also called forearc and backarc, respectively. The outer arc is a nonvolcanic, sluggish-uplift area, and the inner arc is a volcanic area with active crustal movement. A forearc basin, a sedimentary basin, is between the island arc and the trench, often bordered with an uplift zone called an outer ridge (Figure). A basin behind the arc (in the continental side) is known as a backarc basin. The Japanese Islands have collision zones where island arcs meet each other. In such areas, the landforms and geology are more complex than in other regions. The collision zone, therefore, can be regarded as an independent area. It is difficult that the Southwest Japan Arc is separated by a volcanic front because the indistinct volcanic front obliquely crosses over the Nankai Trough. Conventionally, southwest Japan is divided into the outer zone (the Pacific Ocean side) and the inner zone (the continental side) by the MTL because geological and geomorphological characteristics in the inner zone are markedly different from the outer zone. However, there is a controversy over the significance of the MTL in terms of the Japanese Islands formation. 

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