Southwestern Honshu and Shikoku
Southwestern Honshu (Kinki and Chugoku) and Shikoku are main part of the Southwestern
Japan Arc, in which the long axes of major landforms and zonal
distributions of the basement rocks 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 MTL divides the Southwestern Japan Arc into the
outer (Pacific Ocean side) and inner (continent side) zones.
Outer zone
Landforms
Fig.
Topographic map [
]
Mountain ranges with steep slopes occupy in the outer zone: the Kii Mountains and 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 western parts. Ridges and valleys strike east-west, reflecting the basement distribution and geological structure.
Geology
The geology is characterized by east-trending belts of accretionary complexes formed from the Jurassic to Paleogene and
metamorphic rocks. High-pressure, low-temperature 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. 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.
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 prisms).
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 those of 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, Kibi, and Iwami Highlands.
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 or strike-slip faults, separated landforms into small units and complicated the arrangement of them. Mountain ranges (uplift areas) are bordered with mainly north-trending faults, differing from other mountain ranges lying east-west in the inner zone. Those ranges comprise small blocks 20-50 km long, 5-15 km wide, and 800-1200 m high, with erosional low-relief surfaces. The mountains in the eastern side of the region are higher than in the western side. 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 (the Seto
Inland Sea) trending east is between the uplift zones (the 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 southwestern 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.
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