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Japan in a subduction zone
Convergent boundary
The Japanese Islands are situated in the northwestern margin of the
Pacific Ocean surrounded by plate boundaries. Most of these boundaries are of the convergent type,
where two plates collide. At a convergent boundary, an oceanic plate heavier than the
other plate subducts underneath the lighter plate. Thus, such area is called a subduction zone. However,
when both of the plates approaching each other are continental, neither descend because of their great
thickness and low and equal densities. Convergent boundaries are characterized by crustal movement making
geomorphological and geological structures complex, such as deformation, volcanism, earthquakes, mountain
building, and metamorphism.
The active crustal movement zone surrounding the Pacific Ocean is called the circum-Pacific orogenic belt,
and also known as Ring of Fire. The Japanese Islands in a subduction zone are emerged parts of volcanic
island arcs, extending for about 3000 km.
Arc-trench system
Fig.
Arc-trench system [
]
In a subduction zone, a subducting plate forms a trench and uplift area parallel to the trench.
Such uplift area is called a volcanic island arc (hereafter referred to as an island arc) or a
continental margin arc because it is a curved chain of islands/volcanoes. A continental margin arc
is formed along an oceanic-continental plate boundary without seas between the arc and
the continent.
The arrangement of major landforms, distribution of volcanoes, and geological belts (terrane) in Japan
is parallel to trenches off the Japanese Islands, indicating that
island arcs are closely related with trenches. Therefore, island arcs
and trenches are regarded as the arc-trench system. It is mainly
characterized by the following: (1) major landform arrangement,
ocean–trench–arc–marginal sea (backarc basin)–continent, (2) seismicity
occurring at depths of ≥70 km, (3) volcanism, and (4) accretionary
prisms and metamorphic rocks produced by plate subducting.
In and around Japan
Fig.
Map in
and around Japan []
A region in and around Japan consists of four major islands (Hokkaido, Honshu, Shikoku, and Kyushu), the Kuril Islands extending to the Hokkaido, the Izu-Bonin (Ogasawara) Islands chaining southward from Central Honshu, and the Nansei Islands situated to the southwestern of Kyushu. In the Pacific Ocean side, trenches run parallel to these islands: from north, the Kuril Trench, Japan Trench, Izu-Bonin Trench, Nankai Trough, and Ryukyu Trench. Short troughs, the Suruga and Sagami Troughs, are situated on the west and east sides of the Izu Peninsula, respectively. The Suruga Trough connects with the Nankai Trough. Trenches and troughs are long, narrow submarine depressions; a trench is 6000 m deep or more and a trough is shallower than 6000 m.
Fig. Plates in Japan
The Japanese Islands are in marginal areas of the Pacific, Philippine
Sea, North American, and Eurasian Plates. Oceanic plates, the Pacific
and Philippine Sea Plates, are subducted along trenches/troughs. The
Pacific Plate is subducted underneath the North American Plate along
the Kuril and Japan Trenches and underneath the Philippine Sea Plate
along the Izu-Bonin Trench. The Philippine Sea Plate descends beneath
the Eurasian Plate along the Suruga-Nankai Troughs and Ryukyu Trench.
The Sea of Okhotsk, the Sea of Japan, and the East China Sea are
located between the islands and the Asian Continent as marginal seas.
Marginal seas are expanded on basins in the back arc side (back arc
basin), generally situated between island arcs and the continent. The
Philippine Sea is surrounded by the Philippine Islands, Taiwan, Nansei
Islands, southwestern Japan, Izu-Bonin Islands, and Mariana Islands.
This sea consists of the Shikoku Basin and Philippine Basin,
geomorphologically divided from the northwestern Pacific Basin.
Although the Philippine Sea corresponds to the ocean side (forearc
side) of the Southwestern Japan and Ryukyu Arcs, it is also a marginal
sea (backarc side) of the Izu-Bonin and Mariana Arcs.
Island arcs
Five island arcs are located in and around Japan: from north, the
Kuril, Northeastern Japan, Izu-Bonin, Southwestern Japan, and Ryukyu
Arcs (Fig.). The Kuril Arc collides with
the Northeastern Japan Arc in Central
Hokkaido. The Northeastern Japan Arc meets the Southwestern Japan Arc in
Central Honshu. The Izu-Bonin Arc collides with those two arcs in
Central Honshu. The Southwestern Japan Arc meets the Ryukyu Arc in
Central Kyushu. In these areas called the arc-arc collision zone, the
directions of the island chains change and geomorphological and
geological structures are complicated.
Each arc is accompanied with a trench in parallel: Kuril Arc–Kuril
Trench, Northeastern Japan Arc–Japan Trench, Izu-Bonin Arc–Izu-Bonin
Trench, Southwestern Japan Arc–Nankai Trough, and Ryukyu Arc–Ryukyu
Trench. These trenches are divided into two series according to
continuity. The first series is the line of the Kuril, Japan, and Izu-Bonin
Trenches, and the second is the line of the Nankai Trough and Ryukyu
Trench. The arc-trench system in Japan, therefore, is classified into
two systems: the eastern Japan arc system (Kuril, Northeastern Japan,
and Izu-Bonin Arcs) and the western Japan arc system (Southwestern
Japan and Ryukyu Arcs). Tectonism and volcanism in the eastern Japan
arc system and in the western Japan arc system are mainly regulated by
the Pacific Plate movement and the Philippine Sea Plate movement,
respectively.
