Introduction to the Landforms and Geology of Japan

GLGArcs

 

Nansei Islands

The Nansei Islands about 1200 km long are an island chain of the Ryukyu Arc lying from Kyushu to Taiwan (some islands are outside the Ryukyu Arc). The islands are divided into three groups at the Tokara Strait and the Kerama Gap. The region in and around the islands can be also geomorphologically classified into the Ryukyu Trench, slope of the outer arc, uplift area of the outer arc, uplift area of the inner arc, Okinawa Trough, and continental shelf of the East China Sea. The Nansei Islands are characterized by coral reefs and terraces of emergent coral reefs.

In the outer arc, large islands including Tanegashima, Yakushima, Amamioshima, Okinawa Island, and Ishigaki Island are distributed from the south of Kyusyu. Generally, islands in the northern part of the Nansei Island have higher elevations and steeper slopes than those in the southern part; for example, Yakushima near Kyushu has the 1935 m peak higher than any mountains of Kyushu.

In the inner arc, volcanic islands are distributed from the Kikai caldera located off the south of Kyushu to Iwo-Torishima situated to the west of between Amamioshima and Okinawa Island. Although no volcanic island appears to the south of Iwo-Torishima, knolls and seamounts are found from the Iwo-Torishima through the eastern or central part of the Okinawa Trough. Volcanoes in the inner arc become smaller toward the south. The width of the volcanic arc is about 70 km from the south of Kyushu to the west of Okinawa Island and becomes markedly narrow toward the south.

The Okinawa Trough lies along the Ryukyu Arc from the west of Kyushu to the northeast of Taiwan. The trough sharply subsided in the Pliocene and is still subsiding.

In the Nansei Islands, accretionary complexes and metamorphic rocks are zonally distributed as the basement in the same manner of the Southwest Japan Arc, and sediments accumulated on the basement in the forearc in the Eocene or later. Accretionary complexes are correlated with the basement rock subdivisions of the Southwest Japan Arc as follows; the Permian accretionary complex is correlated with the Akiyoshi Belt, the Jurassic-Early Cretaceous accretionary complex with the Chichibu Belt, and the Cretaceous-Tertiary accretionary complex with the Shimanto Belt (for the basement rocks [geotectonic subdivisions], see also “Outline of landforms and geology of Japan”). The northern and central areas of the Nansei Islands comprise Late Paleozoic-Mesozoic olistostrome, the Late Cretaceous-Paleogene accretionary complex (Shimanto Supergroup), and sediments of the continental slope or island arc slope. The southern area consists of Late Paleozoic-Mesozoic metamorphic rocks, Eocene volcanic rocks, and Miocene shallow-marine or bay sediments. Lava and pyroclastic rocks produced in the Middle Miocene or later are zonally distributed from the Tokara Islands to Kume Island (to the west of Okinawa Island). Felsic plutonic rocks yielded in the Eocene, Oligocene, or Middle Miocene are broadly scattered.

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