Introduction to the Landforms and Geology of Japan

GLGArcs

Outline of Landforms and Geology of Japan

[Hokkaido]

Geotectonic subdivisions in Hokkaido are distributed north-south, not parallel to the Kuril Trench. It is thought that the tectonic settings of areas in which the basement rocks of Hokkaido were formed differed from southwest and northeast Japan. Therefore, the characteristics of the geotectonic subdivisions are distinct from those in other regions.

Sorachi-Yezo Belt

The Sorachi-Yezo Belt on the east of the North Kitakami (Oshima) Belt is a Jurrasic-Cretaceous accretionary complex zone with Tertiary deposits. The ages of deposits range from 150 million to 80 million years old.

Kamuikotan Belt

The Kamuikotan Belt is a zone of high pressure type metamorphic rocks of which the protolith is a Jurassic-Cretaceous accretionary complex. In addition to crystalline schist, granite, ophiolite, and an abundance of serpentinite occur in this belt. The rocks were subjected to metamorphism caused by plate subduction twice, 130 million years ago and 60 million years ago.

Hidaka and Tokoro Belts

The Hidaka Belt and the Tokoro Belt are Cretaceous-Paleogene accretionary complex zones (140-50 million years ago and 150-50 million years ago, respectively). The Hidaka Belt (west side) borders on the Tokoro Belt (east side). However, rocks in the Hidaka Belt become stratigraphically younger eastward, while those in the Tokoro Belt become younger westward. Some rocks in the Tokoro Belt were metamorphosed under high pressure conditions.

Hidaka Metamorphic Belt

The Hidaka Metamorphic Belt is composed of low pressure type metamorphic rocks and plutonic rocks which are 50 million years old or younger. The Hidaka metamorphic rocks were formed by collision between the Eurasian Plate and the Okhotsk (North American) Plate.

Nemuro Belt

The Nemuro Belt located in easternmost Hokkaido is composed of volcanic rocks and tuff formed in the shallow sea near the land, turbidite and conglomerate, aged 70 to 50 million years.

Plutonic rocks

Granite is broadly distributed in the Japanese Islands, occupying 10% of the islands, while gabbro and diorite are in limited areas. Granite in Japan is classified by age: Paleozoic, Permian-Jurassic, Cretaceous-Paleogene, and Neogene granites. Cretaceous-Paleogene granite is most widespread, found from central Honshu (Chubu) to western Honshu (Chugoku and the Seto Inland Sea). Granite of this age metamorphosed Jurassic accretionary complexes in the Ryoke Belt and the Abukuma Belt by intrusion. The similar igneous activity is known in the Maritime Province of Siberia and the circum-Pacific area. The ages of Japanese granites become younger toward the Pacific side except Paleozoic granite, indicating that the continental crust developed toward the outside.

Paleogene rocks

Paleogene rocks are distributed in the southwestern Kii Peninsula, southern Shikoku, and southeastern Kyushu. These areas are the south part of the Shimanto Belt (accretionary complex). Paleogene rocks are also found in small areas of Hokkaido (central and eastern areas) and Kyushu (northern and western areas). Some of these rocks are characterized by coal seams. Large coal fields are in central Hokkaido and northern Kyushu.

Neogene and Quaternary rocks (after the islands separated from the continent)

Neogene deposits are broadly distributed in northeast Japan and the Fossa Magna (central Honshu), while, in southwest Japan, they are found only along the coast of the Sea of Japan and in some areas on the Pacific coast. This is because northeast Japan was under sea for a long period after the islands separated from the Asian continent.

green tuff area Fig. 16 Green tuff area [another window]

 

green tuff Photo 5: Green tuff [another window]

The arc-trench system has depositional areas including forearc and backarc basins. Neogene forearc deposits are found in the Pacific coast areas including the Boso Peninsula (Kanto), Shizuoka (central Honshu), Miyazaki (southeastern Kyushu), and the Nansei Islands. Sediments accumulated in the backarc basin during the formation of the Sea of Japan (Miocene) are distributed on the Sea of Japan side of the Japanese Islands and the Fossa Magna. Neogene deposits underneath coastal plains are extremely thick. In coastal plains of Akita and Niigata Prefectures, petroleum and natural gas are produced. The deposits have considerable volcanic rocks extruded during the Sea of Japan expanding (Early and Middle Miocene). These rocks are volcanic and volcaniclastic rocks of basalt, andesite and rhyolite. Most of them were altered into greenish rocks, known as “green tuff”. The green tuff area yields kuroko (black ore) including sphalerite, galena, barite, tetrahedrite, and pyrite. Kuroko deposits are hydrothermal deposits produced by reaction between seawater and magma attributed to submarine volcanic activity 17 to 14 million years ago.

Quaternary deposits are mainly distributed in plains and low hill areas. Volcanic products are usually observed only around volcanoes, but massive pyroclastic materials from large-scale eruptions of Quaternary volcanoes cover broad areas. For example, the Ito pyroclastic flow ejected from the Aira Caldera 25000 years ago expanded in southern Kyushu. The deposit of this pyroclastic flow is found to the 90 km north of the Caldera. In the Kanto Plain, the Kanto loam consisting of volcaniclastic material and its eolian secondary sediment is well-known, derived from volcanoes around the Kanto Plain such as the Fuji, the Hakone, the Asama, and the Akagi volcanoes.

References

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