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Formation history of the Japanese Islands
Jurassic
Jurassic accretionary complex belts including
the Mino-Tamba, Abukuma, and Norhtern Kitakami Belts occupy the Japanese Islands most
widely, which were formed on the outside (Pacific side) of the Permian-Triassic complex. Exotic blocks including Paleozoic sediments (the Hida Marginal Belt, the Southern Kitakami Belt, and the Kurosegawa
Belt), collided with the Jurassic accretionary prisms. The
proto-Asian continent was formed by the collision of the southern Asian
continent and the Siberian continent.
Cretaceous
Fig.
Cretaceous paleogeographic map [
]
Strike-slip movement began in the eastern margin of Asia in the
Early Cretaceous. The oblique subduction of the Izanagi Plate is
thought to cause this movement.
The Median Tectonic Line (MTL) running in southwestern Japan is a major
fault zone 1000 km long. Accretionary complex belts in southwestern Japan
are younger toward the Pacific side but the Jurassic complex belts are
distributed on both sides of the MTL (Fig.). The faults of the MTL slip
right-laterally at present. However, before the Miocene, the paleo-MTL
was a considerably larger-scale fault system with a left-lateral sense
and a much lower dip angle than the present faults. Accordingly, the
formation process of the double Jurassic accretionary complex belts is
thought as follows. The accretionary prisms were originally formed
in a single zone in the eastern margin of the paleo-Asian continent.
The left-lateral activity of the fault system moved a part of the
complex northward and placed it on the Pacific side of the northern
complex to be double belts (the formation of
the inner and outer
zones). However, there is a different opinion; a portion of the
Jurassic complex in the inner zone was moved to the outer zone by
thrusting rather than the strike-slip movement.
In the Late Cretaceous, accretionary prisms started to be formed (Shimanto
Belt). The Jurassic accretionary complex markedly uplifted because
the formed accretionary prisms were pushed into underneath the
complex. The Sambagawa Belt also rose, which consists of rock derived
from Jurassic accretionary prisms that metamorphosed under a high
pressure condition in the depths of the lithosphere.
Volcanism in the Cretaceous was active in the continental margin
volcanic arc. Early Cretaceous sedimentary rock in Japan is nonmarine
and interbeds volcanic rocks. Many plutonic rocks are younger than the
volcanic rocks. They are distributed to the Pacific side of the
volcanic rock area, suggesting that the volcanic area expanded toward
the Pacific side in the Early Cretaceous. In the Late Cretaceous, the
volcanic front backed toward the continental side, but active volcanism
caused granite intrusion in the inner zone. The volcanic front moved
farther continentalward and the volcanic activity declined in the end
of the Cretaceous.
Fig.
Hokkaido in the Cretaceous []
(Hokkaido in the Cretaceous)
The tectonic setting in Hokkaido was different from that in
southwestern and northwestern Japan in the Cretaceous.
The Oshima, Hidaka, and Tokoro Belts are accretionary complex zones.
The Oshima Belt (Jurassic accretionary complex) is an extended part of
the Northern Kitakami Belt in northern Honshu. The Hidaka Belt in
central Hokkaido contains a Cretaceous-Tertiary accretionary complex
which is 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, the Tokoro Belt in eastern Hokkaido also
comprises a Cretaceous-Tertiary accretionary complex but it is stratigraphically younger westward. Consequently, it is thought that
two trenches were in the location where the Hidaka and Tokoro Belts
were formed: one with westward plate subduction and another with
eastward plate subduction. The paleo-Pacific Plate (Kula Plate) was
subducted along with the two trenches (Figure). The Eurasian Plate was
on the west of this plate and the North American Plate on the east.
Volcanic rocks yielded by volcanism related to the westward subduction
are found in the eastern edge of the Oshima and Northern Kitakami
Belts, and the other volcanic rocks related to the eastward subduction
are found in the Nemuro Belt. The Euraian Plate and the North American
Plate approached each other and collided in the Middle Miocene. The two
subduction zones disappeared because of this collision.
Paleogene
Fig.
Oligocene paleogeographic map []
Paleogene deposits, especially marine sediment,
are distributed in limited areas on the Japanese Islands, indicating that most of the region
that became the Japanese Islands was land. In central Hokkaido, the sea
area in the Cretaceous uplifted to be land by the North American Plate
approaching from the east. Sand and mud including coal deposited in
this uplifted area. Large coal fields were produced in the west of the
Yubari Mountains, in which major coal mines developed before. Paleogene
coal fields are also found in Tohoku (northern Honshu) and Kyushu.
These coal fields were yielded where lakes and deltas appeared because grabens were formed in the edge of the continent from the Oligocene to
the beginning of the Miocene (after 25 million years ago).
The dormant volcanic activity between the Late Eocene and the Early Oligocene
became active in the Late Oligocene again. The volcanic area expanded
to the Pacific side beyond the present-day volcanic front.
