Page2/2 [Previous page]
Central Honshu
Fossa Magna
Fig.
Fossa
Magna area [
]
A tectonic depression zone called the Fossa Magna lies in 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: from north, the Hida, Kiso, and Akaishi Ranges. These north-northeast-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 Kiso Ranges belong to the inner zone of the Southwestern 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 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 rocks 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 geology map []
The oldest geological belt (Hida Belt), consisting of low-pressure type
metamorphic rocks and Early Mesozoic felsic plutonic rocks, is placed in
the northern part of the Hida Highland and 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 Belt) is widely exposed in the
southern part of the Hida Mountains, Hida Highland and Ryohaku
Mountains. Late Cretaceous-Paleogene volcanic rocks (Nohi Rhyolite)
covered the Mino and Hida Belts.
Low-pressure type metamorphic rocks and granite intruded in the Late
Cretaceous (Ryoke Belt) are distributed in the Kiso Range and Mikawa
Highland. The age of the parent rocks is estimated almost the same age
as the Mino 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 Mesozoic accretionary prisms brought into the depths 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), Hida Highland, and 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. Neogene volcanic rocks on
the Sea of Japan are green tuff broadly found in northeastern Japan.
Page2/2 [Previous page] [PageTop]
