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Central Honshu
Fossa Magna
Fig.
Fossa
Magna area [
]
A tectonic depression zone called the Fossa Magna lies on the east side of the ISTL. The western boundary of the Fossa Magna is the ISTL. Although the eastern boundary is undefined, it seems to be the Kashiwazaki-Chiba Tectonic Line and the Shibata-Koide Tectonic Line shown in the Figure. The Fossa Magana had subsided while the Japanese Islands moved from the margin of the continent to the current position, and then uplifted. The depression is extremely deep; the Neogene sediments accumulated in the zone is more than 5000 m thick and over 10000 m thick in some places in the southern part of the zone. The large uplift rate yielded many folds and faults in these formations. The Neogene formations in the region on the west of the ISTL has no this feature. The volcanic rocks produced by intensive volcanism during the expansion of the Sea of Japan were also deposited in the Fossa Magna. The volcanic zone turns toward the south in the Fossa Magana, continuing to the Izu-Bonin Arc.
Western side of ISTL
Three mountain ranges with peaks more than 3000 m high are placed from north to south along the ISTL: the Hida, the Kiso, and the Akaishi Ranges in the order from the north. These northward or northeastward trending ranges were formed by the collisions of three arcs, which have significantly uplifted in the Quaternary. The amount of the uplift is 1000 to 1500 m. The Hida and the Kiso Ranges belong to the inner zone of the Southwest Japan Arc and the Akaishi Range to the outer zone. The Hida Range has volcanoes.
Fig.
Active faults
in central Honshu []
The Hida Highland and the Mikawa Highland located on the west of the
mountain ranges have ridges with constant height, so that they look like
a plateau when viewed from a distance. The highlands have deep dissected
valleys and considerably many faults. The Ryohaku Mountains with
volcanoes are situated to the west of the Hida Highland, and the Nobi
Plain is placed to the west of the Mikawa Highland.
In the region on the west of the ISTL, the basement rock units are
zonally distributed and become younger toward the Pacific Ocean. These zones
trend nearly east in Western Honshu, but they curve toward the north in
Central Honshu.
Fig.
Basement geologic map []
The oldest geological belt (Hida Belt), consisting of metamorphic rocks
and Early Mesozoic (220 to 180 million years ago) felsic plutonic rocks,
is placed in the northern part of the Hida Highland and the Ryohaku
Mountains. The metamorphic rocks are gneiss, the parent rocks of which
are pre-Cambrian rocks (one to two billion years ago). Paleozoic
high-pressure type metamorphic rocks and serpentinite are distributed
with Ordovician-Permian sediments in a narrow zone on the south and east
of the Hida Belt (Hida Marginal Belt).
A Jurassic accretionary complex (Mino-Tamba Belt) is widely exposed in
the southern part of the Hida Mountains, the Hida Highland, and the Ryohaku Mountains. Late Cretaceous-Paleogene volcanic rocks (Nohi
Rhyolite) covered rocks of the Mino-Tamba Belt and the Hida Belt.
Low-pressure type metamorphic rocks and granite intruded in the Late
Cretaceous (Ryoke Belt) are distributed in the Kiso Range and the Mikawa
Highland. The age of the parent rocks is estimated almost the same age
as the Mino-Tamba Belt.
High-pressure type metamorphic rocks (Sambagawa Belt) are found in the
outer zone along the MTL, facing the low-pressure type metamorphic rocks
of the Ryoke Belt. The metamorphic rocks of the Sambagawa Belt are
derived from part of Mesozoic accretionary complex brought into a deep
part of the crust. Non- or weakly-metamorphosed rocks
(mudstone/sandstone) (Chichibu Belt) are distributed in a narrow zone on
the Pacific side of the Sambagawa Belt. A Cretaceous-Paleogene
accretionary complex (Shimanto Belt) constitutes the main part of the
Akaishi Range in the outer zone.
Cretaceous lacustrine or shallow-marine deposits are found in the Hida
Range (northern part), the Hida Highland, and the Ryouhaku Mountains.
These formations such as the Tedori Group include fossils of dinosaurs.
Neogene rocks are distributed on the Sea of Japan and in hills in the
eastern and southern side of the Nobi Plain. Altered Neogene volcanic
rocks (known as "green tuff" on the Sea of Japan are broadly found in
northeast Japan.
References
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