Kyushu
Fig.
Topographic map [
]
The Southwest Japan Arc meets the Ryukyu Arc in Kyushu. The
Southwest Japan Arc parallel to the Nankai Trough turns in the
direction parallel to the Ryukyu Trench. Features of the Southwest
Japan Arc and the Ryukyu Arc are found together in Central Kyushu.
Volcanism in Kyushu is extremely active, forming large volcanoes.
Kyushu can be separated into three parts: northern Kyushu, central
Kyushu, and southern Kyushu. In central Kyushu, a subsidence area
called the Beppu-Shimabara Graben lies and the Usuki-Yatsushiro
Tectonic Line regarded as the extended MTL crosses Kyushu is on the
south of the Beppu-Shimabara Graben. Therefore, each region is defined
as follows: in and around the Beppu-Shimabara Graben as central Kyushu,
on the south of the Usuki-Yatsushiro Tectonic Line as southern Kyushu,
and the other as northern Kyushu.
For the basement rocks (geotectonic units), see also “Outline
of landforms and geology of Japan”.
Northern Kyushu
Fig.
Geological map []
This region is in the Southwest Japan Arc (inner arc).
Mesozoic-Paleozoic accretionary complexes, high-pressure type
metamorphic rocks, and Cretaceous felsic plutonic rocks intruding those
rocks constitute mountain ranges, which are also distributed in the
western part of Honshu. Main faults trend north or north-northwest,
dividing and transferring the zonal basement rock distribution trending
east in the Southwest Japan Arc. The fault valleys run north-south in
the mountain ranges (Tsukushi Mountain Ranges) in the eastern part of
northern Kyushu, differing from east-striking valleys of the Chugoku
Mountains.
Paleogene-Neogene sediments and volcanic rocks that filled forearc
basins or backarc basins are found in northern Kyushu. The sedimentary
rocks are characterized by interbedded coal.
In the lowlands, poorly developed alluvial lowlands and Pleistocene
terraces as well as few faults bordering mountains suggest that crustal
movement in the Quaternary is dormant in northern Kyushu.
In the area on the west of the line joining Karatsu Bay and Ariake Bay,
the landforms mainly comprise low mountains/hills about 500 m high and
small islands. Volcanism intermittently yielded basaltic lava in places
in the Late Miocene or later and formed some plateaus and hills in this
area. This volcanism also occurred in the Goto Islands in the Late
Quaternary. The characteristic of alkali basaltic lava and monogenetic
volcanoes, which is rare in Japan, differs from other volcanoes along
the volcanic front. Volcanoes in this area and in northeastern Asia
such as Cheju Island in South Korea have many common features. The
basement rocks in the westernmost area consist of high-pressure type
metamorphic rocks including crystalline schist and metagabbro. The
distribution and structure trending north of these rocks are
dissimilar from those of the Southwest Japan Arc.
Central Kyushu
Fig.
Aso caldera []
Central Kyushu is characterized by large volcanoes in the
Beppu-Shimabara Graben and extensive volcanic products. The
Beppu-Shimabara Graben has subsided along normal faults trending east
and has expanded north-south since the Late Miocene or Pliocene.
Although this subsidence area regarded as the extended Okinawa Trough
leads to the Seto Inland Sea, it is not a sea area because extensive
volcanic products filled the graben. Active large volcanoes, the
Yufudake, the Kujyu, the Aso, and the Unzen volcanoes are arranged in this order
from east to west in the graben. Pyroclastic flow deposits also covered
the graben widely.
Plateaus, hills, and mountains formed by volcanism during the Pliocene
to Middle Pleistocene are located on the north of the active volcanoes.
The Chikuhi Mountains consisting of pre-Miocene rocks are on the west of
the older volcanoes. The rocks are high-pressure type metamorphic rocks
and felsic plutonic rocks.
The Tsukushi Plain and Ariake Bay, which are the largest lowland in
Kyushu, are a basin subsided between the Beppu-Shimabara Graben that
sheared by right-lateral movement and the stable northwestern region of
Kyushu.
Southern Kyushu
The Kyushu Mountains are situated on the south of the
Usuki-Yatsushiro Tectonic Line. The mountains with steep slopes are
non-volcanic and comprise the same rock formations as the Shikoku
Mountains and the Kii Mountains which are mountain ranges in the outer
zone of the Southwest Japan Arc.
Low-pressure type metamorphic rocks and granite (Ryoke Belt) are
distributed along the north of the Usuki-Yatsushiro Tectonic Line.
Cretaceous sedimentary rocks (Izumi Group) are also found on the north
of the tectonic line. A Jurassic accretionary complex (Chichibu Belt)
lies on the south of the tectonic line, and a Cretaceous-Paleogene
accretionary complex (Shimanto Belt) is next to the Chichibu Belt.
Fig.
Kagoshima Graben []
The region on the south of the Kyushu Mountains has the character of the Ryukyu Arc. The distribution trend of the Chichibu Belt and the Shimanto Belt turns from the northeast to the north (parallel to the Ryukyu Arc). Mountain ranges on the south of the Miyazaki Plain are not in the outer zone of the Southwest Japan Arc but in the Ryukyu outer arc, although they comprise the Shimanto Supergroup and felsic plutonic rocks which are the same as the Kyushu Mountains. In this region, Quaternary volcanoes including active large volcanoes appear again in parallel to the Ryukyu Arc. Large calderas are distributed north-south in the Kagoshima Graben in the southern part of southern Kyushu. Enormous eruptions with pyroclastic flow during the Pleistocene formed these calderas, and pyroclastic flow deposits broadly covered lowlands around Kagoshima Bay to form plateaus.
References
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