Nansei Islands
Fig.
Map of 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 Kerama Gap. This region is 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 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 immediately decreases to 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.
Fig.
Geological
map of Okinawa []
In the Nansei Islands, accretionary complexes and metamorphic rocks are zonally distributed as the basement in the same manner of the Southwestern Japan Arc, and sediments accumulated in the forearc in the Eocene or later covering the basement. Accretionary complexes are correlated with the geologic belts of the Southwestern Japan Arc as follows: the Permian accretionary complex with the Akiyoshi Belt, the Jurassic-Early Cretaceous accretionary complex with the Chichibu Belt, and the Cretaceous-Tertiary with the Shimanto Belt. The northern and central regions of the Nansei Islands comprise Late Paleozoic-Mesozoic olistostrome, the Late Cretaceous-Paleogene accretionary complex (Shimanto Supergroup), and sediments of the continent slope or island arc slope. The southern region 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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