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Formation history of the Japanese Islands
300 to 200 million years ago (Carboniferous - Triassic)
Small continents including the South China, the North China, and the Tarim blocks in the southern hemisphere moved to the northern hemisphere and gathered to an area from the Carboniferous to the Triassic. The Siberia block and the Kazakhstan block also came to the area. The descent of huge cold plume in the mantle under the area is thought to be responsible for the gathering. In this period, Laurasia in the northern hemisphere and Gondwana in the southern hemisphere combined together to form the Pangaea supercontinent.
Fig. 6
Middle
Permian paleogeographic map [
]
The Siberia block, the North China block, and the South China block,
which formed the Asian continent, were approaching to the location of
the present Asia about 280 million years ago. There were the Paleo-Asian
ocean (Mongolian seaway) between North China and Siberia and the Paleo-Tethys
Sea between North China and South China.
Rocks in the Maizuru Belt, the Ultra-Tamba Belt, the Akiyoshi Belt, and
the Sangun
Belt were produced between 300 and 200 million years ago. The Maizuru, the Ultra-Tamba, and the Akiyoshi Belts are accretionary
complex zones and the Sangun Belt is a high P/T type metamorphic rock
zone (see here for high P/T type metamorphic rock). The accretionary
complexes were formed at a trench to the southeast of the South China
block by the subduction of the Farallon plate. Part of the accretionary
complexes were subducted and metamorphosed to be the Sangun metamorphic
rocks. The Akiyoshi Belt is rich in limestone with fossils including
corals and fusulina inhabiting tropic coral reefs. The limestone
overlies oceanic basalt and has no terrigenous sediments, suggesting
that the limestone was produced on top of seamounts formed in a tropic
(equatorial) area distant from land. The seamounts traveled to the
trench in the margin of the South China block on the moving Farallon
plate and subducted with the plate. Their limestone was scraped off and
accreted to the landward plate.
The South China block collided with the North China block 250 million
years ago. Ultra high-pressure and middle-pressure metamorphic zones
were formed at the border of the blocks. Rocks in the Hida Belt and the
Hitachi-Takanuki Belt were subjected to this metamorphism.
200 to 140 million years ago (Jurassic)
Fig. 7
Early Jurassic paleogeographic map []
The Izanagi plate was subducting in the eastern margin of the paleo-Asia
continent, forming accretionary complexes. Jurassic accretionary
complexes are most widely distributed in Japan (the Mino-Tamba,
the Chichibu, the Ashio, and the North Kitakami-Oshima Belts). These were
added to the ocean side of Permian to Triassic accretionary complexes.
The Chichibu Belt is situated to the south (Pacific Ocean side) of the
Mino-Tamba Belt across the Median Tectonic Line (MTL) in southwest
Japan. Because accretionary complexes developed toward the ocean, there
is controversy over the cause of the arrangement of these Jurassic
accretionary complexes. The MTL is the longest fault zone mainly with
right-lateral strike-slip faults in Japan, about 1000 km long. The
faulting before the Miocene was in a left-lateral sense. Therefore, the
Chichibu Belt (the accretionary complex in the outer zone) came from
the south by the strike-slip faulting. However, there is another
explanation that the Chichibu Belt is a part of Jurassic accretionary
complex situated on the continental side, that is, a nappe moved to the
current location by thrust faulting apart from the accretionary
complex. Moreover, a combined hypothesis with the strike-slip fault and
the nappe theories has been proposed. Although the argument is still
unsettled, the nappe theory is likely conclusion based on surveys
including seismic profiling of the crust (Isozaki
et al.,
2010).
140 to 60 million years ago (Cretaceous)
Fig. 8
Middle
Cretaceous paleogeographic map []
Part of accretionary complexes produced 140 million years ago were
subducted and metamorphosed under a high pressure condition until 110
million years ago. This metamorphism produced metamorphic rocks of the
Sambagawa Belt. The Izanagi plate, the Kula plate, and the Pacific plate
were subducting along trenches in the eastern margin of Asian continent
about 90 million years ago. Mid-ocean ridges dividing these plates were
also subducting. The cause of the Sambagawa metamorphic rock rising to
the surface has not been elucidated, but some hypotheses have been
proposed. One of them is that the subduction of the mid-ocean ridges (80
to 70 million years ago) was related to the regional metamorphic rock
rising (Isozaki, 2000).
Moreover, the subduction of the mid-ocean ridge between the Kula plate
and the Pacific plate was responsible for generating a massive volume of
granitic magma. Rocks surrounding the magma were thermally metamorphosed
to be low P/T type metamorphic rocks (Ryoke Belt). The activity of
granitic magma continued until 60 million years ago and formed granite
in the Chugoku region and the Nohi rhyolite in the Chubu region. The
formation of Shimanto accretionary complexes relating to the subduction
of the Pacific plate began 100 million years ago and lasted for about 70
million years.
The Hidaka Belts and the Tokoro Belt (Cretaceous-Tertiary accretionary
complexes) of Hokkaido were formed. Rocks in the Hidaka Belt in central
Hokkaido become stratigraphically younger eastward. Thick forearc sediments
accumulated on the west of the Hidaka Belt (Yezo Supergroup). The Yezo
Supergroup is well-known for an abundance of fossil ammonites and
inoceramus. On the other hand, rocks in the Tokoro Belt in eastern
Hokkaido become stratigraphically younger westward. Therefore, it is thought
that the accretionary complex of the Tokoro Belt was formed in a
subduction zone different from that of the Hidaka Belt. The Kamuikotan
Belt on the west of the Hidaka Belt is a metamorphic rock zone. The
protolith was rocks subducted from a trench and was metamorphosed twice,
130 million years ago (related to the Izanagi plate subduction) and 60
million years ago (related to the Kula plate subduction).
