Introduction to the Landforms and Geology of Japan

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Southwestern Honshu and Shikoku

Southwestern Honshu (the Kinki and Chugoku regions) and Shikoku are main part of the Southwest Japan Arc, in which the long axes of major landforms and zonal distributions of the basement rocks including accretionary complexes are parallel to the Nankai Trough. The Median Tectonic Line (MTL) running east-west in Shikoku and the Kii Peninsula is the Japanese longest (over 1000 km long) fault zone consisting mainly right-lateral strike-slip faults. The MTL can be readily traced as lineaments on the ground. The southwest Japan is conventionally divided into the outer zone (Pacific side) and the inner zone (continental side) by the MTL.

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

Outer zone

Landforms

Mountain ranges with steep slopes occupy in the outer zone: the Kii Mountains and the Shikoku Mountains. These ranges are upwarped mountains with domed morphology, the highest peaks of which are about 2000 m high. Antecedent rivers have dissected the mountains. The Shikoku Mountains have two uplift areas in the eastern and the western parts. Ridges and valleys extend east-west, reflecting the distribution of basement rocks and geological structure.

Geology

The geology is characterized by eastward trending belts of accretionary complexes formed from the Jurassic to Paleogene and metamorphic rocks. High pressure type (high P/T type) metamorphic rocks are distributed along the MTL (Sambagawa Belt). The metamorphic rocks are crystalline schist, the parent rocks of which are Carboniferous-Jurassic mudstone, sandstone, and volcanic rocks. A Late Jurassic accretionary complex (Chichibu Belt) lies on the south of the Sambagawa Belt. Sedimentary rocks in this belt were weakly metamorphosed. The Kurosegawa Belt consisting of various Paleozoic rocks is distributed in the Chichibu Belt as a narrow zone. A Cretaceous-Paleogene accretionary complex (Shimanto Belt) is situated on the south of the Chichibu Belt. The Shimanto Belt is a very long zone from the Boso Peninsula in central Honshu to the Okinawa Islands.

The Shimanto Belt is regarded as a typical accretionary complex. The belt consists of turbidite (alternating beds of sand and mud) and mélange including basaltic pillow lava, limestone, chert, and red shale. The formations of sedimentary rocks strongly folded and faulted dip northward as a whole, although they become younger southward (see "Accretionary prism" and "Tei melange and Muroto").

Inner zone

Landforms

The inner zone can be geomorphologically divided into three parts: the Seto Inland Sea, the region including the Chugoku Mountains, and the Kinki Triangle.

The Seto Inland Sea is a depression zone between the mountains in the outer zone and those in the inner zone. Archipelagic areas and no-island areas are alternately arranged in the Seto Inland Sea. The archipelagic areas are uplift areas and the no-island areas are subsidence areas.

The Chugoku Mountains and plateaus are situated to the north of the Seto Inland Sea. The Chugoku Mountains with peaks more than 1000 m is surrounded by plateaus including the Tamba Highland, the Kibi Highland, and the Iwami Highland. The heights of tops of these plateaus are 400-600 m. Erosional low-relief surfaces regarded as an uplifted peneplain are remarkably developed in the mountains and plateaus. The mountains in the inner zone have gentle slopes in contrast to those in the outer zone.

The Kinki Triangle is located to the east of the Chugoku Mountains. In this region, many faults, which are reverse faults and strike-slip faults, separated landforms into small units and complicated the arrangement of them. Mountain ranges (uplift areas) are bordered with mainly northward trending faults, differing from other mountain ranges lying east-west in the inner zone. Those ranges comprise small blocks 20 to 50 km long, 5 to 15 km wide, and 800 to 1200 m high, with erosional low-relief surfaces. The mountains in the eastern area of the region are higher than in the western area. Basins (subsidence areas) are placed between the mountain ranges.

Geology

In the inner zone, high-pressure type metamorphic rocks (Sangun Belt) are distributed in the northern part of Chugoku, the parent rocks are mud stone, chert, and volcanic rocks (Late Paleozoic accretionary complex), metamorphosed during the Late Carboniferous to Jurassic depending on places. A Jurassic accretionary complex is situated on the south of the Sangun Belt. Volcanic rocks (rhyolite) and felsic plutonic rocks (granite) were produced in the Cretaceous to Neogene. They broadly covered or intruded the metamorphic rock zone and the accretionary complex. Therefore, the zonal distribution of the basement rocks is unclear.

Late Cretaceous sedimentary rocks (Izumi Group) are distributed on the north of the MTL. The formations consist of marine sediments that filled a basin formed by left-lateral movement of the MTL. Neogene formations are found on the Sea of Japan.

Tectonics

Forearc basins
Fig. Forearc basins (blue circles)

Forearc basins are divided by heights.

In southwestern Honshu and Shikoku, the subsidence zone (Seto Inland Sea) trending east is between the uplift zones (Shikoku Mountains and Chugoku Mountains). In addition to this east-west structure, the alternating structure of uplift areas and subsidence areas with the axes trending north or northeast is found in the region. This alternation, for example, appears in the distribution of peak heights of the mountain ranges and the uneven spread of islands in the Seto Inland Sea as mentioned above. Forearc basins off southwest Japan also exhibit this structure. The features of the region, such as the alternating structures and landforms of the Kinki Triangle, are attributed to the movement of the Philippine Sea Plate and the Pacific Plate.

The outer zone and the inner zone on the west of the Kinki Triangle are affected by oblique compression resulted from the Philippine Sea Plate slantwise descending. The movement of the Philippine Sea Plate drags the blocks between the MTL and the Nankai Trough and leads to the right-lateral movement in the MTL. The alternating structure of uplift areas and subsidence areas with the axes trending north or northeast is also affected by the oblique subduction and the right-lateral movement.

The region in and around the Kinki Triangle is compressed east-west by the subduction of the Pacific Plate at the Japan Trench and the collision of the Izu-Bonin Arc with Central Honshu. This crustal movement formed the landforms in this region.

References

 

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