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Central Honshu
Central Honshu (Kanto and Chubu) is the widest part of Honshu Island, in which three island arcs, the Northeastern Japan, Southwestern Japan, and Izu-Bonin Arcs, meet one another and intensive crustal movement makes the geomorphological and geological structures extremely complicated. Central Honshu is divided by the Itoigawa-Shizuoka Tectonic Line (ISTL), the Median Tectonic Line (MTL), and the volcanic front. The ISTL is a fault zone running north-south in the central part of Honshu and is the boundary between the Southwestern Japan Arc and Northeastern Japan Arc. The MTL, the length of which is over 1000 km, is a fault zone consisting mainly of right-lateral strike-slip faults. It is clearly traced from Kyushu to the ISTL. The MTL separates the Southwestern Japan Arc into the outer (Pacific side) and inner (continent side) zones. The volcanic front running in the direction of the extension of the Northwestern Japan Arc turns toward the Izu-Bonin Arc in front of the ISTL. In Central Honshu, the northern side of the volcanic front is the region of the Northeastern Japan inner arc, but the southern side is a region where features of the Northeastern Japan Arc (outer arc) and of Southwestern Japan Arc (outer zone) are found together. For example, the geomorphological structure in the outer zone of the Southwestern Japan Arc appears beyond the ISTL.
Eastern side of ISTL
The Kanto Plain spreads to the south of the Abukuma Mountain, which
is the largest plain in Japan. The Kanto Plain is in a large subsidence
area depressed more than 1000 m during the Quaternary. The plain is
covered with thick Quaternary sediments including a large amount of
volcanic ash derived from volcanoes to the west of the plain such as the
Hakone and Fuji volcanoes. The deposited volcanic ash was weathered to
become clay called the Kanto loam.
The Miura Peninsula and Boso Peninsula are situated in the southern
Kanto Plain. Hills consisting of Tertiary rocks are distributed in the
Miura Peninsula and the southern part of the Boso Peninsula. These hills
are regarded as an outer ridge (uplift zone on the boundary between a
forearc basin and a trench) formed by the Philippine Sea Plate
descending along the Sagami Trough. The Kanto Plain to the north of the
hills, therefore, can be considered as a forearc basin.
The Echigo and Mikuni Mountains, which are to the north of the Kanto
Plain, is in the southern end of the Northeastern Japan inner arc. These
mountain ranges are composed of Paleozoic-Mesozoic sedimentary rocks (accretionary
complex) and felsic plutonic rocks.
Fig.
Basement geology map [
]
The Kanto Mountains are located on the west of the Kanto Plain. Although
this mountain range is to the east of the ISTL is in the Northeastern Japan
Arc, their landforms and geology have the same features as the outer
zone of the Southwestern Japan Arc. In the Kanto Mountains,
high-pressure type metamorphic rocks (Sambagawa Belt), a
Paleozoic-Mesozoic accretionary complex (Chichibu Belt), and a Mesozoic-Paleogene
accretionary complex (Shimanto Belt) are distributed from north to
south. These belts extending from southwestern Japan are curved to be
northward convex near the ISTL. The Kanto Mountains are placed in the
east side of the curved belts. The top of the convex is located in the
area where the Izu-Bonin Arc collides with Honshu. The bending is
thought of resulting from this collision. Tertiary felsic plutonic rocks
intruding the older rocks are exposed in the southwestern Kanto
Mountains.
The Tanzawa Mountains consisting of Neogene volcanic rocks spewed on the
seafloor and quartz diorites intruding the rocks are situated on the
south of the Kanto Mountains. The Tanzawa Mountains, which were part of
the Izu-Bonin Arc moving northward, collided and accreted to Honshu six
million years ago, and then was uplifted by continuing collision of the
Izu-Bonin Arc.
The Izu Peninsula also collided with Honshu two to one million years ago,
which was located far off the south of the Japanese Islands as Izu block
on the Izu-Bonin Arc. The basement of the peninsula comprises lava and
volcaniclastic rocks (Middle Miocene-Pliocene) including shallow-marine
sediments. Quaternary volcanoes are formed on the basement.
