Introduction to the Landforms and Geology of Japan


Japan in a Subduction Zone

Igneous activity

Igneous activity resulting from plate subduction plays an important role in forming island arcs. Magma produced under island arcs form felsic plutonic rocks (granite), some of which erupts on the surface to make volcanoes. Large scale formation of granites develops the crust of island arc. See the chapter “Continental crust and development of island arcs” and the section "Volcanoes" for the details of Japanese volcanoes.

Accretionary prism

An accretionary prism (also called accretionary wedge) is a sedimentary body formed by which ocean floor sediments and trench-fill sediments provided from the land accrete to the landward slope of trench by off-scraping accretion and underplating accretion. Oceanic plate subduction is responsible for such accretion. Slices (thrust sheets) pile next to one another with reverse faults (thrust faults). They are characterized by inward younging in each sheet and overall outward younging.

Oceanic plates are produced by basaltic magma upwelling at mid-ocean ridges and move away from the ridges toward trenches. Magma ejected onto a seafloor forms pillow lavas. In temperate and tropical regions, shells of calcareous nannoplanktons deposit on the seafloor shallower than carbonate compensation depth (CCD) to form limestone. As the plate moves away from the ridge, the seafloor becomes denser and deeper because of cooling. When the depth of seafloor exceeds the CCD, calcareous shells no longer deposit because they are dissolved, and only siliceous shells of radiolarian settle on the seafloor. Chert, therefore, is formed on limestone beds (pelagic sediments). As the plate comes close to the land, mud and tuff from the land deposit on the chert beds to be siliceous shale (hemipelagic sediments). In trenches, clastics including sand and mud flow down to the bottom of trench on the landward slope as turbidity current and deposit (trench-fill sediments). This sedimentation makes alternating beds of sand and mud. Accordingly, oceanic plate stratigraphy, consisting of basalt (pillow lavas), limestone, chert (pelagic sediments), siliceous shale (hemipelagic sediments), and alternating beds of sand and mud (trench-fill sediments) in order from the bottom, is formed on the plate travelling from the mid-ocean ridge to the trench.

Chert Turbidite
Photo 1  Bedded chert [Another window Photo 2  Turbidite [Another window]
Fig. 8 Accretion [Another window]


MelangePhoto 3 Melange []

Rocks and sediments composing the oceanic plate stratigraphy are deformed by plate movement and split into an accreted part and subducted part (Figure 8). The border between the parts is called decollement (horizontal slip plane). The sediments of accreted part are scraped off and accreted to the front of accretionary prism; this process is called “off-scraping accretion”. Thus, the accretionary prism develops outward and the trench migrates seaward. Under the accretionary prism, the decollement steps down and subducted sediments accrete to the bottom of the prism with forming duplex structure. In addition, part of the oceanic crust subducted underneath the prism is scraped off and added to the prism. This accretion is defined as “underplating”. Moreover, thrust faults (out of sequence thrusts) cut a part of the accretionary prism and bank up sliced parts to thicken the prism. Accreted materials including basalt, limestone, chert, siliceous shale, and turbidite are sheared and mixed to be fragments of all sizes in this process. These fragments are found in muddy matrix as mélange. Since sliced sediments are added under the pre-existing thrust sheets by plate subduction, the lower sheet is younger than the upper one.

Accretionary prisms are not always formed in trenches. About 40% of trenches all over the world possess developed accretionary prisms. Tectonic erosion is in active rather than accretion in other trenches. The formation of accretionary prism requires large volume of sediments in a trench. Therefore, the onset of accretion depends on the convergence rate of plate and the supply rate of sediments because sediments are taken to under the landward plate if the plate subduction rate is too fast. The conditions for which accretion preferentially occurs are the thickness of trench-fill sediments of >1 kilometer and/or convergence rate of <7.6 centimeters per year (Clift and Vannucchi, 2004). In Japan, accretionary prisms are well developed only in the Nankai Trough, and little in the Kuril, the Japan, the Ryukyu, and the Izu-Bonin Trenches.

Most of Japanese basement is composed of accretionary complexes and metamorphosed accretionary complexes. An accretionary complex is defined as a former accretionary prism, characterized by a mix of ocean-floor basalt, pelagic and hemipelagic sediments and terrigenous sediments (turbidite) with complex structures such as mélange and duplex. See “Tei mélange and Muroto” for examples of accretionary complex exposures.

Metamorphic rocks

Metamorphic rocks, as well as igneous rocks, are major components of the continental crust. Metamorphic rocks are produced by transformation under different conditions including temperature and pressure from the original conditions of the rock formation (metamorphism). These rocks are broadly exposed in shields which have been very stable regions of the continents during the past 600 million years, some of which are extremely old, over three billion years old. Metamorphic rocks are also found in orogenic belts. Two types of metamorphic rocks are commonly present in subduction zones: the high pressure type and low pressure type. High pressure type metamorphic rocks are formed under high pressure and relatively low temperature conditions. In subduction zones, rocks taken into a deep part of the crust by plate subduction are transformed into high pressure type metamorphic rocks. Low pressure type metamorphic rocks are formed under high temperature and relatively low pressure conditions by contacting magma.

Metamorphic rocks that result from regional metamorphism occurring over a large area are known as regional metamorphic rock; crystalline schists are typical. Contact metamorphic rock such as hornfels is locally formed around intrusive rocks. In island arc-trench systems, regional metamorphic rocks are common, zones of which are classified into a high pressure type metamorphic rock zone and a low pressure type metamorphic rock zone. In Japan, the Sambagawa Belt is well-known as the high pressure type metamorphic rock zone, and the Ryoke Belt as low pressure type metamorphic rock zone. These zones are parallel to the Nankai Trough.

Why do high pressure type metamorphic rocks formed in a deep part of the crust come up toward the surface? Subduction of mid-ocean ridges is proposed as one of the reason. Surveys of accretionary complexes reveal the ages of oceanic plate stratigraphy. The survey results in Japan show that mid-ocean ridges were subducted several times and the times of the subduction are consistent with those of which high pressure type metamorphic rocks rose. However, the detail mechanism of the metamorphic rock rising is unknown. Moreover, the subduction of mid-ocean ridges provides large amount of heat to the crust, resulting in the generation of granitic magma and the formation of low pressure type metamorphic rocks. (See also “Formation history of the Japanese Islands [p.3]".)

Continental crust and development of island arcs

Development of island arcs is closely related to the growth of continental crust. Accretionary prisms develop toward the ocean as mentioned in the chapter of accretionary prism. Accretionary complexes in Japan become younger toward the Pacific Ocean, indicating that the basement of the Japanese Islands has been increased by accretion on the margin of the continent. However, only the growth of accretionary prisms is not enough to explain the increment of the crust in the Japanese Islands. Accretionary prisms consist of terrigenous deposits and materials scraped away from an oceanic plate but the terrigenous deposits extremely dominate the prisms. The terrigenous sediments were produced by erosion of the land and/or pre-existing prism. Therefore, the continental crust does not increase essentially with the accreted sediments because the sediments are recycled materials. Accreted rocks and sediments derived from the oceanic plate contribute to the growth of the continental crust but the amount of them is a little. The addition of granite (felsic plutonic rock) largely develops the continental crust. Thus, substances provided from the mantle thicken the continental crust by igneous activity in island arcs.

Igneous activity, the formation of accretionary prisms, and high pressure type metamorphic rock rising develop island arcs, while tectonic erosion reduces them. In Japan, recent zircon chronology suggests that granitic batholiths disappeared in the past (see “Formation history of the Japanese Islands [p.2]”). It is, therefore, thought that island arcs do not grow toward the ocean constantly.


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