Japan in a subduction zone
Metamorphic rocks
Metamorphic rocks, as well as igneous rocks, are major components of the continental crust. They are rocks transformed under different conditions including temperature and pressure from the original conditions of the rock formation (metamorphism). Metamorphic 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 P/T type and low P/T type (P/T refers to the ratio of pressure to temperature). High P/T 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 P/T metamorphic rocks. Low P/T metamorphic rocks are formed under high temperature and relatively low pressure conditions by contacting magma.
Photo 4
Crystalline schist (Nagatoro)
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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 arc-trench systems, regional metamorphic rocks are common, zones of which are classified into a high P/T metamorphic rock zone and a low P/T metamorphic rock zone. In Japan, the Sambagawa Belt is well-known as the high P/T metamorphic rock zone, and the Ryoke Belt as low P/T metamorphic rock zone. These zones are parallel to the Nankai Trough.
Why do high P/T 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 P/T 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 P/T 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 prisms. 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 prisms. 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 P/T
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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