Southwestern Honshu and Shikoku
Southwestern Honshu (Kinki and Chugoku) and Shikoku are main part of
the Southwest Japan Arc, in which the long axes of major landforms and
zonal distributions of the basement rock units are parallel to the
Nankai Trough. The Median Tectonic Line (MTL), which is the Japanese
longest (over 1000 km long) fault zone consisting mainly right-lateral
strike-slip faults, 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 units), see also “Outline
of landforms and geology of Japan”.
Outer zone
Landforms
Fig.
Topographic map [
]
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 rock units and geological structure.
Geology
Fig.
Geological map []
The geology is characterized by eastward trending belts of accretionary
complexes formed from the Jurassic to Paleogene and metamorphic rocks.
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 found from the Boso
Peninsula in central Honshu to the Okinawa Islands.
Fig.
Cross section of southwest Japan []
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 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 archipelagic Seto Inland Sea is a depression zone between the
mountains of the outer zone and the inner zone. Areas congested with
islands alternate with no-island areas in the Seto Inland Sea. The
island areas are uplift areas and the no island areas are subsidence
areas.
The Chugoku Mountains and plateaus are situated on 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.
Fig.
Active faults
in central Honshu []
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 Honshu, 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
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 along 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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