Introduction to the Landforms and Geology of Japan


Outline of Landforms and Geology of Japan

Hida Belt
The Hida Belt is the oldest zone mainly consisting of metamorphic rock (gneiss). Some of the rocks are of the Precambrian, which are considered fragments of South China (Yangtze) block. Rock aged two billion years was found in the belt. Rocks in the eastern Hida Belt were remetamorphosed at least three times and intruded by granite (220 Ma–180 Ma). The belt also includes Carboniferous-Permian sedimentary rocks. The part of Hida belt in the Chugoku region is often separated from the part in the Chubu region as the Oki Belt.

Hida Marginal, Renge, Joetsu, and Kurosegawa Belts

These belts consist of high pressure type metamorphic rock, ophiolite, terrigenous sedimentary rocks including tuff and limestone, and igneous rocks formed in the subduction zone. The ages of the rocks range from 500 to 300 million years old. The metamorphic rocks include metamorphic rock related to subducted mid-ocean ridge. The Hida Marginal Belt and the Renge Belt are adjacent to the Hida Belt but the Kurosegawa Belt is located in a Jurassic accretionary complex zone (Chichibu Belt), lying from the eastern Kii Peninsula to western Kyushu. The Kurosegawa Belt is considered a klippe without the root.

Akiyoshi, Maizuru, Sangun, and Ultra-Tamba Belts

These belts are zones of Permian to Triassic accretionary complexes aged 300 to 200 million years. The Akiyoshi Belt is dominated by limestone including fossils such as coral and fusulinid inhabiting in the tropics. Since it is underlain by oceanic basalt and includes no terrigenous deposit, the limestone derives from accreted seamounts formed in the tropical sea far away from the land. The Maizuru Belt is characterized by ophiolite. The Sangun Belt is a high pressure type metamorphic rock zone. Subducted accretionary deposits of the Akiyoshi Belt and the Maizuru Belt were metamorphosed under high pressure condition to be the Sangun metamorphic rocks. The Ultra-Tamba Belt consists of chert, sandstone, and mudstone.

Mino, Tamba, Chichibu, and Ashio Belts

These belts are zones of Jurassic accretionary complexes aged 200 million years or younger. Ocean-floor basalt, limestone, chert, mudstone, terrigenous sediment (sand and mud) constitute the belts. The Chichibu Belt situated to the south of the Mino Belt and the Tamba Belt is considered as a nappe.

Ryoke and Sambagawa Belts

These belts are metamorphic rock zones. The metamorphic rock type in the Ryoke Belt is the low-pressure and high-temperature type, formed by massively granitic intrusion about 100 million years ago. The metamorphic rock type in the Sambagawa Belt is the high pressure type. The parent rocks (protolith) of the Sambagawa metamorphic rock are accretionary complex formed 140 million years ago. The subducted accretionary complex was metamorphosed by about 110 million years ago, and uplifted 80 to 70 million years ago.

Shimanto Belt

This belt is a zone of accretionary complex formed 100 to 30 million years ago (Cretaceous to Paleogene), distributed in the outermost end (Pacific side) of southwest Japan from the Boso Peninsula in Kanto to the Nansei Islands (1800 km in length and 100 km in maximum width). Oceanic basalt, pelagic sediments (chert and limestone), and terrigenous sediments (sandstone and mudstone) constitute the belt. Trench-fill sediments of sand and mud principally occur as alternating beds of sand and mud (turbidite). Mélange zones containing blocks of oceanic crust (basalt and chert) are sandwiched in turbidite zones. (See "Tei mélange and Muroto")

[Northeast Japan (Tohoku and western Hokkaido)]

In this region, because thick Tertiary sediments cover basement rocks broadly, the detail basement structure is unknown in some areas. The boundary between southwest Japan and northeast Japan as basement geology is the Tanakura Tectonic Line, although the Itoigawa-Shizuoka Tectonic Line is the border between the Southwest Japan Arc and the Northeast Japan Arc. Geotectonic subdivisions obliquely across the island arc (not parallel to the trench), but these are considered to have formed in the margin of the Asian continent with accretionary complexes in southwest Japan.

Abukuma Belt

The Abukuma Belt is characterized by Gosaisho and Takanuki metamorphic rocks and Cretaceous granite. The Gosaisho metamorphic rock is low pressure type metamorphic rock, which is considered to be related to the Ryoke Belt. The Takanuki metamorphic rock, which is also low pressure type, was metamorphosed in the Cretaceous, the protolith of which was Jurassic to Cretaceous terrigenous sedimentary rocks containing clastic zircon 2 billion to 200 million years old.

South Kitakami Belt

The South Kitakami Belt is comprised of various type rocks including metamorphic rocks, igneous rocks, and terrigenous (shallow-sea) sedimentary rocks. The ages of rocks are from the Ordovician to the Early Cretaceous. The composition of Paleozoic rocks in the belt is similar to the Hida Marginal, Renge, Joetsu, and Kurosegawa Belts.

High pressure type metamorphic rocks (Matsugadaira-Motai metamorphic rocks) are distributed in the eastern margin of the Abukuma Belt and the north of South Kitakami Belt (southwestern Kitakami Mountains). The rocks probably constituted pre-Silurian basement.

North Kitakami Belt

The North Kitakami Belt is a Jurassic accretionary complex zone. This belt extends to southwestern Hokkaido (The part of this belt in Hokkaido is also called the Oshima Belt).

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