Introduction to the Landforms and Geology of Japan

GLGArcs

 

Kyushu

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.

A subsidence area called the Beppu-Shimabara Graben lies in central Kyushu. The graben is distinctive in geology and geomorphology. Therefore, Kyushu can be separated into three parts as follows; northern Kyushu is to the north of the graben, central Kyushu is in and around the graben, and southern Kyushu is to the south of the graben bordered by the Usuki-Yatsushiro Tectonic Line regarded as the extended MTL crossing Kyushu.

Topographic map Geologic map
Fig. Topographic map [Another window] Fig. Geologic map [Another window]

For the basement rocks (geotectonic subdivisions), see also “Outline of landforms and geology of Japan”.

Northern Kyushu

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. Volcanic activities intermittently yielded basaltic lava in places in the Late Miocene or later and formed some plateaus and hills in this area. The volcanic activities 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. The features of volcanoes in this area are similar to those in northeastern Asia such as Cheju Island in South Korea. 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

Aso caldera
Fig. Aso caldera [Another window]

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 the graben is regarded as part of a long subsidence zone from the Okinawa Trough 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 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 have the same rock types 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) is on the south of the tectonic line and is bordered on the south by a Cretaceous-Paleogene accretionary complex (Shimanto Belt).

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.

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