Introduction to the Landforms and Geology of Japan

GLGArcs

Formation History of the Japanese Islands

See Note on Page 1 for paleogeographical maps

200 to 140 million years ago (Jurassic)

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, the 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 Belt and the 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)

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 at 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-pressure and high-temperature 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 Belt 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).

60 to 23 million years ago (Paleogene)

Paleogene deposits, especially marine sediments, are distributed in limited areas in the Japanese Islands, indicating that most of the region to be the Japanese Islands was land during this period. The central Hokkaido area that was a sea in the Cretaceous emerged with 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. The Yubari area was a famous for large coal mines (almost abandoned at present). Paleogene coal fields are also found in Tohoku (northern Honshu) and Kyushu. A rift zone was formed with lakes and deltas in the edge of the continent from the Oligocene to the beginning of the Miocene (after 25 million years ago). Coal was produced in sediments of the lakes and the deltas.

The volcanic activity dormant 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.

23 to 16 million years ago (Early Miocene)

The subsidence responsible for the rift zone 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 Southwest Japan Arc and the Northeast Japan Arc separated and shifted from the continental edge to the current positions with rotation. Paleomagnetic studies revealed that southwest Japan clockwise rotated about 45 degrees from 16 to 14 million years ago and northeast Japan counterclockwise rotated about 46 degrees from 17 to 14 million years ago (however, different angles and times of the rotations have been also proposed). 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 sea floor 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 tensional stress in the Early Miocene (currently by compressive stress). 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 on the continental side of the Japanese Islands, especially in northeast 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.

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