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"Geology" from Page 5
Outline of landforms and geology of Japan
Division of island arcs
Major tectonic lines (fault zones)
Fig.
Tectonic lines
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Fig.
Fault line of MTL
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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 Southwestern Japan Arc and
the Northeastern Japan Arc, and also between the Eurasian Plate and the
North American Plate. Landforms and geological features dramatically
change at 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
southwestern Japan and northeastern Japan in terms of basement geology.
Dividing the island arcs
Fig.
Division of the island arcs []
An island arc can be classified based on geological and geomorphological features as follows. A volcanic front divides it into the outer arc (the ocean side) and inner arc (the continental side). The outer arc and 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 (Fig.). A basin behind the arc (in the continental side) is named a backarc basin. The Japanese Islands have collision zones where island arcs meet each other. In such areas, the characteristics of arc mix with those of another arc, and the landforms and geology are more complex than other regions. This area, therefore, can be separated as an independent area. Although the island arcs in Japan can be divided in this way, the Southwestern Japan Arc is separated into the outer zone (the Pacific Ocean side) and inner zone (the continental side) by the Median Tectonic Line, because the indistinct volcanic front obliquely crosses over the Nankai Trough and geological and geomorphological characteristics in the inner zone are different from the outer zone.
