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Japan in a subduction zone
Seismicity
Although shallow-focus earthquakes occur at many places on the Earth, earthquakes whose focuses are at more than 70 km deep only occur in subduction zones. Focuses are distributed with increasing depth with a constant angle from a trench to an island arc as shown the figure. This focus zone is called Wadati-Benioff zone, which corresponds with the leading edge of a descending plate. The depth contours of earthquakes in the figure below are parallel to trenches. Focuses in and around Japan are distributed in two zones parallel to the two series of trenches.
Volcanism
Volcanism resulting from plate subduction plays an important role in
forming island arcs. In a subduction zone, the interaction between the
subducted part of a plate (called a slab) and mantle over the slab
(called a mantle wedge) causes generation of magma in a mantle wedge. It
is thought that yielding magma is involved with water derived from
sediment on a subducted plate and from a plate which was altered by
seawater under seafloor. Magma begins to be generated by partial melting
of the mantle at a certain depth and rises by buoyancy. Magma, then,
spews out on the ground to form volcanoes, or stops rising under the
ground resulting in plutonic rocks. In the eastern Japan arc system, the
start of magma production is at about 110 km in depth. Thus, volcanoes
on an island arc are distributed in a zone parallel to the trench with
the distinct border of the ocean side. This border is called a
volcanic front.
Volcanoes on island arcs often erupt explosively because of andesitic
magma and its high viscosity. (See [here]
for the characteristics of
Japanese volcanoes)
Accretionary prisms
In a subduction zone, accretionary prisms are formed on the landward
side of some trenches, which consist of basalt produced at mid-ocean
ridges, oceanic sediment, and sediment derived from landward and
deposited in the trench.
The figure shows the forming process of accretionary prisms. With an
oceanic plate moving from a mid-ocean ridge to a trench, oceanic plate
stratigraphy develops orderly as follows: from the bottom, basalt
composing the oceanic crust, pelagic sediment (chert), hemipelagic
sediment (siliceous mudstone), and terrigenous sediment including
mudstone and sandstone that deposited in the trench. As the oceanic
plate descends, the oceanic sediments and basalt are scraped off the
plate, and then are fractured and mixed to become mélange comprised of
rock fragments of all sizes in a sheared muddy matrix. These slices add
to the landward wall of the trench (under older accretionary prisms)
with the terrigenous sediment (turbidite) by underthrusting. The
accretionary prisms are intensively folded and faulted by tectonic
force. Thus, they are characterized by extremely deformed beds
containing mixed rocks derived from the land and ocean, and the age of
which the upper accretionary prism is older then the lower prism
(however, the upper layer is younger than the lower layer within
each accretionary prism bordered with faults). Accretionary prisms are
well developed in the Nankai Trough, but little in the Kuril, Japan, Izu-Bonin
Trenches.
An accretionary complex is defined as accretionary prisms
formed along a past trench. The basement in the Japanese Islands
consists mainly of accretionary complexes formed from the Permian to
Tertiary. These complexes are zonally distributed, especially in
southwestern Japan, the ages of which become older toward the continent
(Fig.). These features indicate that the Japanese island arcs have
been formed in subduction zones (see Formation
history of the Japanese Islands).
Metamorphic rock
Metamorphic rocks are commonly found in island arcs and some of them
are distributed in zones (regional metamorphic rock). Rocks dragged into
the depths by plate subducting along the trench are metamorphosed under
high pressure to become high-pressure and low-temperature type
metamorphic rocks. Under island arcs, low-pressure and high-temperature
type metamorphic rocks are formed in high temperature areas heated up by
rising magma. Both the types are zonally yielded along the trench. Rocks
around magma at relatively shallow depths of under the ground are also
transformed into contact metamorphic rocks (Fig.).
In Japan, the two different metamorphic zones appear in pairs in some
areas: for instance, the Ryoke Belt (low-pressure and high-temperature
type) and the Sambagawa Belt (high-pressure and low temperature type)
(#15 and 16 in Fig.). This is one of the characteristics in the subduction zone. Sierra Nevada Belt (low-pressure and high temperature
type) and Franciscan Belt (high-pressure and low temperature type) in
western North America are another example outside Japan. Paired zones
consisting of the different metamorphic types are thought to reflect the
subduction of the oceanic plate at the time of the metamorphism.
Geological constitution of island arcs
Geology of regions that were in a subduction zone generally comprises: from the trench side, accretionary complexes or metamorphic rocks, forearc sediments, igneous rocks, and backarc sediments. Formations and rocks in Japan have the characteristics of the subduction zone.
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