Introduction to the Landforms and Geology of Japan

GLGArcs

Outline of Landforms and Geology of Japan

Geology

Rocks in the Japanese Islands are roughly classified into pre-Neogene rocks, plutonic rocks including granite, and Neogene and Quaternary rocks. Pre-Neogene rocks and plutonic rocks constitute the Japanese Islands as basement rocks. These rocks are well exposed in southwest Japan but are broadly covered with Neogene rocks in northeast Japan and Hokkaido. The Itoigawa-Shizuoka Tectonic Line clearly distinguishes between pre-Neogene rock and Neogene rock distributions. Strata subjected to intense force have complex structures such as folds and faults. Horizontal strata are only seen in younger deposits in Japan.

Basement -- accretionary complexes and metamorphic rocks

Basement of the Japanese island arcs mainly comprises accretionary complexes produced from the Cambrian to the Paleogene and regional metamorphic rocks.

Accretionary complexes generally include basalt, pelagic chert consisting mainly of radiolarian shells, siliceous mudstone, and thick sandstone and mudstone. Chert of various ages and basalt are contained in muddy matrices as fragments of all sizes (mélange) (see “Accretionary prism”). In addition, accretionary complexes may have blocks of basalt and reef limestone derived from seamounts, and ophiolite. The seamounts were basaltic volcanoes with reef limestone on top, formed on the seafloor far from a subduction zone and moved to a trench. Ophiolite is regarded as fragments of an oceanic plate and the upper mantle, consisting of ultramafic rocks (serpentinite), mafic rocks (gabbro, dolerite, and basalt), and pelagic sediments such as chert.

Metamorphic rocks regionally distributed are metamorphosed accretionary deposits, mainly crystalline schist characterized by the parallel arrangement of the constituent minerals (schistosity). Gneiss also occurs in metamorphic rock zones. Metamorphic rock in island arcs is briefly described in the section, “Metamorphic rock”.

The basement rocks are divided into some units based on their formation ages and geological characteristics. These subdivisions are zonally distributed in order of which their ages generally become younger oceanward, indicating that the Japan Islands have been developed oceanward in the subduction zone. On a basement geologic map, Jurassic accretionary complexes most widely occupy the Japanese Islands. However, an amount of pre-Cretaceous rocks is much less than Cretaceous and Cenozoic rocks (Figure 14). Most of the zones are bordered by thrust faults; an older zone overlies the adjacent younger zone. (See also “Japan in a subduction zone” and “Formation history of the Japanese Islands”).

The characteristics of basement subdivisions are different in southwest Japan, northeast Japan, and Hokkaido.

[Southwest Japan (from Kanto to Kyushu)]

In southwest Japan, the geologic structure formed in the subduction zone are well preserved. Basement rock zones are distributed clearly parallel to the Nankai Trough in order of age. However, the Chichibu Belt, a Jurassic accretionary complex zone is situated on the Pacific side of the Median Tectonic Line (MTL), to the south of the Mino Belt and the Tamba Belt which are also the Jurassic accretionary complex zones. The Chichibu Belt is considered a nappe which is allochthon that migrated from the birthplace by thrusting. See also “200 to 140 million years ago (Jurassic)" (Formation history of the Japanese Islands).

Figure 14 shows a cross section of southwest Japan. This cross section was made on the basis of seismic profiling data as well as geological surveys of exposures. The profile is markedly different between the inner and the outer zones bordered by the MTL which cuts the crust northward at an angle of 40 degrees. The inner zone (the Sea of Japan side) is characterized by almost horizontal structure. The uppermost layer is a nappe group of acrretionary complexes such as the Mino, the Tamba, and the Maizuru Belts. Granitic batholiths underlie the uppermost layer and granite intrudes into the layer, but the seismic profiling data suggests geologic body of which the components are unknown under the nappe group layer in the Chugoku region. The lower crust is underneath the granitic batholiths (Ito et al., 2010).

In the outer zone, the upper crust is occupied with the Shimanto accretionary complex gently dipping northward and the lower crust is extremely thin. The Shimanto and younger accretionary complexes intrudes into the inner zone. The Sambagawa metamorphic rocks are distributed along the MTL, traced to a depth of about 20 kilometers.

The characteristics of major basement rock zones are briefly described as follows.

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