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Formation history of the Japanese Islands
Before 500 million years ago
Continents repeatedly aggregated and dispersed through the earth’s history. The Japanese Islands are situated in the margin of the Eurasian continent formed by the combination of smaller continents (blocks and platforms) including the Lhasa, the Indochina, the India, the Siberia, the North China (Sino-Korean), and the South China (Yangzi) blocks.
Fig. 3
Rodinia [
]
Rodinia is a supercontinent existing from 1300 to 700 million years ago. Blocks constituting the present Eurasian continent were pieces of Rodinia. The North China block and the South China block formed the present China region, but these blocks were away from each other on Rodinia. It is thought that the South China block was situated near the center of Rodinia and the formation of proto-Japan started on the margin of the block. About 700 million years ago, the Rodinia supercontinent began to break. The rising of the Pacific superplume split the central part of Rodinia into the South China block and the North American block. A rift zone was formed in this region because of stretching the crust. The oldest rocks of Japan are fragments of rocks in the edge of the continental crust in the rift zone. This region further expanded to form the oceanic crust by ejected basaltic magma, resulting in the formation of the paleo-Pacific ocean. The margin of the ocean was a passive margin where the plate was not subducted like the present Atlantic Ocean. The sea-floor spreading lasted until about 500 million years ago.
Fig. 4
Gondwanaland []
After Rodinia was broken up, these pieces reaggregated to form the
Gondwana supercontinent 600 million years ago. Gondwana began to split
up 500 million years ago. This split caused another sea-floor spreading
and the beginning of the subduction of the paleo-Pacific sea floor.
Therefore, the tectonic settings around proto-Japan converted from the
passive margin to an active margin (convergent plate boundary) at this
time point.
Ophiolites giving ages of 580 to 520 million years are found at Nomo
Point in Nagasaki Prefecture (Kyushu), Oeyama in Kyoto Prefecture
(western Honshu), and Mount Hayachine in Iwate Prefecture (northeastern
Honshu). These ophiolites are considered fragments of the oceanic crust
of the paleo-Pacific Ocean. An intra-oceanic arc was probably present in
the Oeyama zone. Moreover, the Hitachi metamorphic rocks yielded in
northern Ibaraki Prefecture include rocks aged 500 million years. Their
protolith of the metamorphic rocks were sedimentary rocks derived from
continental shelf sediments and igneous rocks produced by plate
subduction. Therefore, the Hitachi metamorphic rocks were formed in a
subduction zone in the continental margin.
500 to 300 million years ago (Cambrian - Carboniferous)
Fig. 5
Early
Ordovician paleogeographic map []
When the Gondwana supercontinent began to split up around 500 million years ago, the South China block including the proto-Japan was situated between the equator and the middle-latitude of the southern hemisphere. The South China block accompanied by the subduction zone drifted in the paleo-Pacific Ocean. Rocks found in the Hida Marginal Belt, the Renge Belt, the Joetsu Belt, the Kurosegawa Belt, and the South Kitakami Belt were formed the margin of this small continent, including non-metamorphosed sedimentary rocks (clastic rocks derived from the continent and neritic limestone), volcanic rocks, and granites. Various fossils also occur in these belts. 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 South China block. Middle Carboniferous marine animal fossils also occur, which animals were living in the Tethys Sea. Some Permian plant fossils belong to the Cathaysia flora indicating the temperate zone.
The Kurosegawa Belt is distributed on the Pacific side of southwest Japan, and the South Kitakami Belt also on the Pacific side of northeastern Honshu (Tohoku) (Figure 1). The Kurosegawa Belt and the South Kitakami Belt have been regarded as strange geotectonic units because these zones have various fossils and non-metamorphosed rocks as mentioned above and are encompassed younger accretionary complexes. There is an argument about the origin of the belts. In the above explanation, rocks in the Kurosegawa Belt and the South Kitakami Belt were formed in the continental margin. Accretionary complexes subsequently developed oceanward. If this development process was preserved as it was, the Kurosegawa Belt and the South Kitakami Belt would be found on the continental side of younger accretionary complexes. The reason of their distributions is because these are klippes that moved onto the younger accretionary complexes by thrust faulting (Isozaki et al., 2010). Rocks with the same characteristics as in the Kurosegawa Belt are distributed on the continental side, supporting the reason. However, there is a different hypothesis that rocks in the Kurosegawa Belt and the South Kitakami Belt were produced in an area far from the proto-Japan and transported by plate movement to accrete to the proto-Japan.
[Oldest rock]
The oldest rock in Japan is gneiss gravel about two billion years old in
the Kamiasou conglomerate (Gifu Prefecture, central Honshu). However,
Triassic-Jurassic (240 to 160 million years old) formation encompasses
this conglomerate bed. The paleocurrents preserved in layers over and
under the bed indicate 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 oldest sedimentary rock is found in the Hitoegane formation in the
Hida Marginal Belt. Conodont fossils giving an age of 460 to 440 million
years occur in this formation.
[Vanished granitic batholiths]
Zircon is abundantly contained in granite in continents and island arcs.
Zircon is also found as clastic particles in sedimentary rocks such as
sandstone. Middle-Paleozoic to Triassic sandstone in Japan is rich in
igneous zircon grains aged 520 to 400 million years, suggesting the
presence of granitic rocks of the Early Paleozoic era. Nevertheless,
granites contemporary with the zircon grains are rare in Japan; those
are found as small blocks only in southwest Japan including the
Kurosegawa Belt.
The zircon grains are thought to be derived from granitic batholiths
formed by igneous activity caused by subduction in a volcanic arc at
that time. The volcanic arc may have been an island arc because granites
520 to 400 million years old have not been found in eastern Asia,
especially in southern China. It is also known that granitic batholiths
aged 190 to 160 million years and 110 to 90 million years were produced
in a continental margin arc.
The Early Paleozoic granitic batholiths are considered to have
disappeared in the Triassic period. The vanishing process is proposed as
follows. Exposed granites were rapidly eroded to be clastic materials
that were carried to the sea and deposited in a trench. These sediments
were incorporated into accretionary prisms being formed at that time
and/or were brought into the deep part of the crust with a subducting
plate.
