Formation history of the Japanese Islands
Pre-Neogene accretionary complexes and metamorphic rocks constitute
the Japanese island arcs as basement rocks, covered with Neogene and
Quaternary deposits and volcanic products. The Sea of Japan was created
by rifting from 20 million through 15 million years ago (Middle
Miocene). Before that, the Japanese island arcs were continental margin
arcs in the Asian Continent, like the present-day Andes. Therefore, the
basement rocks were produced in the edge of the continent, and then the
Japanese Islands were formed with the expansion of the Sea of Japan.
The formation history has not been established completely, especially
in pre-Tertiary eras. Different stories have been suggested besides the
description below.
Basement produced in a continental margin (Paleozoic-Paleogene)
Silurian-Carboniferous
Fig.
Devonian paleogeographic map
[
]
The Japanese Islands have geological zones yielding rocks and fossils
older than the Permian: the
Kurosegawa Belt (also called Kurosegawa
Tectonic Zone) and the Hida Marginal Belt in southwestern Japan, and the
Southern Kitakami Belt
in northeastern Japan. These zones, which are in
(or adjacent to) Jurassic accretionary complex zones, consist of unmetamorphosed sedimentary rock including clastic rock derived from the
continent and neritic limestone, volcanic rocks, metamorphic rocks, and
granites. The ages of the rocks range from over 400 million years to 200
million years. In the Kurosegawa Belt, sedimentary rock produced in
various environments and rocks metamorphosed in different conditions
occur chaotically as small brocks surrounded by serpentinite
(serpentinite mélange). Granite indicating 400 million years old is also
found in this zone. The other belts have similar kinds of rocks and
formations to the Kurosegawa zone.
Various fossils are yielded in these zones. Those are Silurian and
Devonian fossils of plants and animals that lived in the subtropics to
tropics. Some of the fossils are the same species found in Australia and
the Yangzi Platform. Middle Carboniferous marine animal fossils also
occur, which animals were living in the Tethys Sea. Some Permian plant
fossils are of the Cathaysia flora indicating the temperate zone.
In the Silurian and Devonian, continents gathered in the southern
hemisphere; Gondwana is well-known supercontinent. Although the location
where Japanese Paleozoic rocks were formed has not been identified,
there are some theories based on the fossils and the facies mentioned
above. A hypothesis explains that blocks which were to be the zones
including the Kurosegawa Belt were situated in the equator to middle
latitude of the southern hemisphere as part of Gondwana (south China?)
and then moved to the north. Various types of rocks contained in the
zones may have been taken in the blocks by faulting on the way. Another
story explains that the zones were located in the margin of the
Sino-Korean Platform to the north of Gondwana.
(The oldest rock)
The oldest rock in Japan is gneiss gravel in the Kamiasou conglomerate
(Gifu Prefecture in central Honshu), which is about two billion years old.
However, Triassic-Jurassic (240 million to 160 million years old)
formation encompasses this conglomerate bed. In addition, the paleocurrent preserved in layers over and under the bed indicates that
the conglomerate came from the north. Consequently, the oldest gravel
was produced from rock formed somewhere in the continent about two billion
years ago, and then transported from the north to be deposited along
with other sediment 200 million years ago.
The Hida Belt, to the north of the Kamiasou conglomerate, is the closest
belt to the continent, seeming to be the oldest basement rock because
basement rock belts are younger toward the Pacific side. This belt
consists of metamorphic rocks (mainly gneiss). Some parent rocks are of
the Pre-Cambrian but most of the metamorphic rocks gave an age of the
Jurassic. Granite in the belt was also intruded in the Early Mesozoic.
The geological structure suggests that the metamorphic rocks thrusted
over a Jurassic accretionary complex. It is unknown where the rocks of
the Hida Belt were produced although those were possibly part of the
continental crust. Moreover, the relation between the Hida Belt and
other belts remains unexplained. Therefore, it is doubtful whether the
Hida Belt can be regarded as a basement rock zone.
Permian-Triassic
Fig.
Late Permian
paleogeographic map []
The Yangzi Platform and Indochina Block that separated from Gondwana
moved northward in the Permian. The Yangzi Platform collided with the
Sino-Korean Platform to the north, and then these platforms combined
with other platforms and blocks including the Siberian Platform to form
the ancient eastern Asian continent. It is thought that the Permian
accretionary complex oldest in Japan were produced in the margin of
the Sino-Korean Platform.
A group of seamounts with reefs on top in which coral and fusulinid
lived was formed near the equator in the Panthalassic Ocean. Some of the
seamounts moved to the subduction zone in the margin of the Sino-Korean
Platform by plate movement and subsided underneath the platform. The
reef limestone on top was sliced off to accrete to the landward slope of
a trench while the seamounts were subducting, remaining in an accretionary
complex. The coral and fusulinid living on top of the seamounts are
also found in the Pacific side of the North American and South American
Continents. Therefore, the seamounts dispersed in various directions
from where they were.
