[Prev] Page 1 / 2 / 3 / 4 [Next]
Formation history of the Japanese Islands
Formation of the islands
Early Miocene
The subsidence producing grabens since the end of the Paleogene
continued in the Miocene, making lakes, marshes, and rivers widely. The
formation of the Sea of Japan began as sea water entered into the
rifted depression zone about 20 million years ago. The Southwestern
Japan and Northeastern Japan Arcs were not translated from the
continental edge to the current positions. Paleomagnetic studies
revealed that southwestern Japan clockwise rotated about 45 degrees
from 16 million to 14 million years ago and northeastern Japan
counterclockwise rotated about 46 degrees from 17 million to 14 million
years ago. Paleomagnetic data show rotation angles but not horizontal
movement. Therefore, the past position of the Japanese Islands can be
inferred from geological data in addition to the paleomagnetic data.
The Sea of Japan expanded until 15 million years ago. The Sea of
Okhotsk was also formed in the Early Miocene.
The active volcanism in the Late Oligocene was more energetic in the
Miocene; the expansion of the Sea of Japan involved intensive volcanic
activity. The volcanism on the seafloor of the Sea of Japan was
basaltic. However, dacite and rhyolite volcanic products in addition to
basaltic lava extruded on the coast of the Japanese Islands, indicating
that the Japanese Islands were dominated by extension in the Early
Miocene (currently by compression). Therefore, normal faults caused
abrupt collapse to form half grabens on land. An enormous amount of
volcanic products were deposited in the grabens and altered into
greenish rock. This rock is called “green tuff”, found in the
continental side of the Japanese Islands, especially in northeastern
Japan.
(Izu-Bonin (Ogasawara) Arc and Shikoku Basin)
The paleo-Izu-Bonin arc was located off Kyushu 25 million years ago.
This ancient arc began to split into the Kyushu-Palau Ridge and the Izu-Bonin
Arc 19 million years ago. The Izu-Bonin Arc moved eastward with the
expansion of the Shikoku Basin, which reached the present position 15
million years ago.
Middle Miocene
Northeastern Japan was broadly covered with the sea between 17
million and 9 million years ago. A region from
Akita to Niigata (the
continental side of northeastern Honshu) and the
Ou Range were long and
narrow basins. The Ou Range was a most deep basin in the beginning of
the Middle Miocene. Submarine volcanic activity was active in the basin
of the Ou Range but not in the Akita-Niigata basin in which thick
deposits were formed. The Tertiary formations in this region produce a
little petroleum. The submergence of northeastern Japan is accounted
for which the expansion of the Sea of Japan stretched and thinned the
crust to reduce its buoyancy and east-west tension was on the island
arc for a long period.
In southwestern Japan, sea expanded into western Honshu (Chugoku
Region) to central Honshu during the formation of the Sea of Japan. The
inland became archipelagic because of the relief. Subsequently,
southwestern Japan tended to emerge on the whole through into the
Middle Miocene since this region was compressed by the Shikoku Basin
subducting under southwestern Japan resulted from the rotation during
the expansion of the Sea of Japan.
Volcanic activity occurred in Kyushu, Shikoku, and the south Kii
Peninsula through the southern part of the central Honshu (Kanto) in a
short period about 15 million years ago. The volcanism extended near a
trench, differing from common activity in the arc-trench system. The
reason seems to be because the newly created hot sea floor (Shikoku
Basin) heated up the island arc widely with the subduction.
In Hokkaido, the Kitami Mountains abruptly uplifted and a sea area with
a deep trough developed parallel to the west of the mountains in the
middle of the Middle Miocene. The Kuril Arc began to collide with the
Northeastern Japan Arc in the end of the Middle Miocene (10 million
years ago). This collision raised the
Hidaka Mountains, which were a
shallow-sea area, and the Yubari Mountains.
The Izu-Bonin Arc moved northwestward and collided with Honshu by the
northwestward movement of the Philippine Sea Plate 15 million years
ago. Blocks on the Izu-Bonin Arc accreted to the Northeastern Japan Arc
as they could not subside. The Misaka Mountains to the north of Mt.
Fuji were a block on this island arc. The collision and accretion of the
blocks on the island arc continued after that.
