Northeastern Honshu
This
region of Honshu (Tohoku) is in the Northeastern Japan Arc. This island arc has typical features of the arc-trench system, such as clear separation of
the outer arc and inner arc, volcanoes distributed parallel to the
trench, a distinct 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 three lines of mountain ranges and
depression zones between the lines: the Kitakami and Abukuma Mountains
as the first line, the Ou Mountain Range as the second line, and Dewa
Mountains as the third line. The alternation of mountain ranges
and depression zones 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.
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 Abukuma Mountains have gentle slopes resulted from
dissection of long duration, because neither the mountain ranges have
been under the sea, excepting the margin of the mountains, nor 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
remarkably associated with shallow-marine fossils in the Yangtze Block
in China. Therefore, the southern Kitakami Mountains were probably
exotic blocks from the ancient continent.
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 in scale than 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 of 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 (called “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 Mountain 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 Mountain 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 and
Asahi Mountains) comprises pre-Tertiary formations and Cretaceous
granite intruding the formations. The Iide and Asahi Mountains are
higher and steeper than the main part.
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