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Central Honshu
Central Honshu (Kanto and Chubu) is the widest part of Honshu
Island, in which three island arcs, the Northeast Japan Arc, the
Southwest Japan Arc, and the Izu-Bonin Arc, 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 Southwest
Japan Arc and the Northeast 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 Southwest Japan Arc into the outer (Pacific side) and
inner (continent side) zones. The volcanic front running in the
direction of the extension of the Northeast 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 Northeast Japan inner
arc and the southern side is the region of the Northeast Japan outer
arc. However, features of the Northeast Japan Arc (outer arc) and of
Southwest Japan Arc (outer zone) are found together on the southern
side. The geomorphological structure in the outer zone of the Southwest
Japan Arc appears in northeast Japan beyond the ISTL.
For the basement rocks (geotectonic units), see also “Outline
of landforms and geology of Japan”.
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 the Fuji volcanoes. The deposited volcanic ash was weathered to
become clay known as the Kanto loam.
The Miura Peninsula and the 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 Mountains and the Mikuni Mountains, which are to the north of
the Kanto Plain, is in the southern end of the Northeast Japan inner
arc. These mountain ranges are composed of Paleozoic-Mesozoic
sedimentary rocks (accretionary complex) and felsic plutonic rocks.
Fig.
Basement geologic map [
]
The Kanto Mountains are located on the west of the Kanto Plain. Although
this mountain range is in the Northeast Japan Arc to the east of the
ISTL, their landforms and geology have the same features as the outer
zone of the Southwest Japan Arc. In the Kanto Mountains, high-pressure
type metamorphic rocks (Sambagawa Belt), a Jurassic accretionary complex
(Chichibu Belt), and a Cretaceous-Tertiary accretionary complex (Shimanto
Belt) are distributed from north to south. These belts extending from
southwest Japan are curved to be northward convex near the ISTL. The
Kanto Mountains are placed on 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. Therefore, the bending is thought of resulting from this
collision. Tertiary felsic plutonic rocks (granite) intruding the older
rocks are exposed in the southwestern Kanto Mountains.
The Tanzawa Mountains consisting of Neogene volcanic rocks spewed on a
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
eight 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 the
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.
