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Outline of landforms and geology of Japan
The oldest belt is the Hida Belt mainly consisting of metamorphic
rock (gneiss) of which some parent rocks are of the Pre-Cambrian.
However, the rocks in this belt are thought to have been
remetamorphosed in the Early Mesozoic. The belt also includes
Carboniferous-Permian sedimentary rocks. The Hida Belt is doubtful
whether it can be regarded as a basement rock zone because the relation
between the Hida Belt and other belts remains unexplained and it is
also unknown where it was produced.
The following belts are arranged toward the Pacific Ocean: the Hida
Marginal Belt (mentioned later), Permian-Triassic accretionary complex
belts (Sangun Belt, Maizuru Belt, and Yamaguchi (Akiyoshi) Belt),
Jurassic accretionary complex belts (Mino-Tamba Belt and Ashio Belt), a
low-pressure type metamorphosed Jurassic accretionary complex belt (Ryoke
Belt), high-pressure type metamorphosed Jurassic accretionary complex
belts (Sambagawa Belt, Nagasaki Belt), a Jurassic accretionary complex
belt (Chichibu Belt), and a Cretaceous-Tertiary accretionary complex
belt (Shimanto Belt). Features of each belt are as follows:
The Sangun Belt consists of high-pressure type metamorphic rock
although it is classified into the Permian-Triassic accretionary
complex in the figure. The parent rock is Permian accretionary prisms.
This metamorphic rock zone adjoins the low-pressure type metamorphic
rock of the Hida Belt. The Yamaguchi Belt is well-known for large
limestone blocks and the Maizuru Belt for the largest ophiolite zone (Yakuno
ophiolite). The Ryoke Belt was formed by broadly contact metamorphism
with extensive granite intruding. The Sambagawa Belt is the longest
metamorphic rock zone (high-pressure type), over 1000 km long from
Kanto to Kyushu. This belt is adjacent to the Ryoke Belt of
low-pressure type metamorphic rock bordered with the Median Tectonic
Line (MTL). The Shimanto Belt is typical of the accretionary prism
structure, distributed from the Boso Peninsula in Kanto to Okinawa
Island (1800 km long). The age difference between turbidite and chert
of the Shimanto Belt is shorter than those of the Chichibu and Mino-Tamba
Belts, 60 million years for the Shimanto Belt and 150 million years for
the Chichibu and Mino-Tamba Belts. This is because the Shimanto
accretionary prisms were formed by ocean floor subducting that started
shortly after the ocean floor was produced.
[Northeastern Japan (Tohoku and western Hokkaido)]
The boundary between southwestern Japan and northeastern Japan as basement geology is the Tanakura Tectonic Line, although the Itoigawa-Shizuoka Tectonic Line is the border between the Southwest Japan Arc and Northeast Japan Arc. The basement geology of northeastern Japan is unknown in detail because Tertiary and Quaternary deposits broadly cover basement rocks. Jurassic accretionary complex zones including the Abukuma, Northern Kitakami, and Oshima Belts occupy most of the region, obliquely across the island arc (not parallel to the trench). The Abukuma Belt is characterized by low-pressure type metamorphic rock and granite, which may be related to the Ryoke Belt. The characteristics and age of the accretionary complex of Northern Kitakami Belt are similar to those of the Mino-Tamba and Chichibu Belts. The Oshima Belt is regarded as the extension of the Northern Kitakami Belt.
[Hokkaido]
Basement geological belts are distributed north-south, not parallel to the Kuril Trench. The Sorachi-Yezo Belt on the east of the Oshima Belt has thick Cretaceous-Tertiary deposits with Jurassic pelagic sediment in lower formations (Sorachi Group). The Kamuikotan Belt is a high-pressure type metamorphic rock zone, the parent rock of which is probably Jurassic-Cretaceous accretionary prisms. The Hidaka and Tokoro Belts are Cretaceous-Tertiary accretionary complex zones. The Hidaka Metamorphic Belt consists of low-pressure type metamorphic rocks. The Nemuro Belt includes Cretaceous-Tertiary sedimentary rocks. The Hidaka Belt is stratigraphically younger eastward, while the Tokoro Belt is younger westward. Therefore, it is thought that two trenches existed in central Hokkaido in the Cretaceous: one with westward plate subduction and another with eastward plate subduction (see “Formation history of the Japanese Islands”).
