Northeastern Honshu
This region of Honshu (Tohoku) is in the Northeast Japan Arc. This
island arc has typical features of the arc-trench system, such as
distinction between an outer arc and inner arc, volcanoes distributed
parallel to the trench with a clear volcanic front, and geological
structure parallel to the long axis of the island arc. Strong
earthquakes frequently occur owing to active crustal movement in the
region.
The landforms are characterized by mountain ranges running north-south
and depression zones between these mountains: the Kitakami Mountains
and the Abukuma Mountains on the Pacific side, the Ou Mountain Range as a
central backbone, and the Dewa Mountains on the Sea of Japan side. The
repetition of a mountain range with a depression zone continues to the
eastern margin of the Sea of Japan.
The
geology consists mainly of: in the outer arc, Paleozoic-Mesozoic
sedimentary rocks and accretionary complexes, and Cretaceous felsic
plutonic rocks; in the inner arc, volcanic rocks and sedimentary rocks
yielded in the Late Oligocene or later and volcanic products spewed
from Quaternary volcanoes. For the basement rocks (geotectonic units),
see also “Outline of landforms and geology
of Japan”.
The distribution of volcanoes makes double lines: the first volcanic
line on the Ou Mountain Range and the second line on the Dewa Mountains. The volcanism of the first line is more active than that of the
second.
Outer arc
The outer arc is a non-volcanic region in which sluggish upwarping
and downwarping with long wavelength are dominant. The Kitakami
Mountains and the Abukuma Mountains have gentle slopes resulted from
dissection of long duration, because the mountain ranges have never
been under the sea, excepting the margin of the mountains, and have not
been subjected to intensive crustal movement during the Cenozoic.
The Kitakami Mountains are geologically divided into the northern and
southern parts. The northern Kitakami Mountains consist mainly of a
Jurassic accretionary complex intruded by Early Cretaceous felsic
plutonic rocks. The southern Kitakami Mountains comprise shallow-marine
sediments accumulated during the Silurian to the beginning of the
Cretaceous and felsic plutonic rocks intruded into the sediments in the
Early Cretaceous. These sedimentary rocks and felsic plutonic rocks are
also found in the eastern margin of the Abukuma Mountains. High
pressure type metamorphic rocks older than 300-400 million years ago
are distributed in the southern Kitakami Mountains. The shallow-marine
sediments found in the southern Kitakami Mountains, which were
deposited in the margin of the continent in the Paleozoic, differ from
other Mesozoic and Paleozoic formations containing pelagic sediments
and turbidite (trench-fill sediment). In addition, Carboniferous and
Permian fossils yielded in the southern part are associated with
shallow-marine fossils in the South China (Yangtze) block.
In the Abukuma Mountains, felsic plutonic rocks are widely distributed
and low-medium pressure type metamorphic rocks and a Jurassic-Early
Cretaceous accretionary complex are found in the southern part.
Inner arc
In the inner arc region, short-wavelength folding and faulting are
dominant, forming mountains and basins smaller than those in the outer
arc region. The reason is thought that this region being compressed
east-west is composed of formations (mainly of the Neogene) younger
than those of the outer arc region and the high ground temperature
because rising magma facilitates deformation of the strata.
Neogene sediments widely distributed in this region were deposited in
the backarc basin by subsiding accompanied with the expansion of the
Sea of Japan in the Miocene. Altered volcanic rocks (known as “green
tuff”) that were produced in the Late Oligocene-Early Miocene are also
broadly found. These andesitic-rhyolitic volcanic rocks with basalt
spewed while the Japanese Islands separated from the continent.
The Ou Range about 500 km long runs north-south in the almost center of
northeastern Honshu, which is the longest range in Japan. The altitudes
are higher in volcanic areas; the highest peak is 2038 m of Iwaki
Volcano and 1440 m in the non-volcanic area. The range is characterized
by the single ridge with many Quaternary volcanoes and the landforms
consistent with the geological structure. High elevation areas in the
range correspond with anticlines (the axis of uplifting). The Ou Range
began to uplift in the Late Miocene and the uplift rate increased in
the Quaternary.
The Dewa Mountains is young fold mountains, which started to upheave in
the Late Pliocene. The Dewa Mountains is separated into several
mountain ranges by antecedent valleys. The main part of the range
consists of Neogene rocks, but the southern part (Iide Mountains and
Asahi Mountains) comprises pre-Tertiary formations and Cretaceous
granite intruding the formations. The Iide Mountains and the Asahi
Mountains are higher and steeper than the main part.
References
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