Introduction to the Landforms and Geology of Japan

GLGArcs

Central Honshu

Fossa Magna

The Fossa Magna is a tectonic zone that was depression, crossing the central Honshu (figure below). The western boundary of the Fossa Magna is the ISTL. However, the ISTL is not one long fault forming the edge of the depression zone but a fault zone including different type faults. The eastern boundary is undefined because no clear tectonic lines indicating the border have been found. The Fossa Magna can be divided into the north Fossa Magna and the south Fossa Magna by geological characteristics. The north Fossa Magna consists of thick submarine-volcanic products and marine clastic rocks, which deposited while the Japanese Islands moved from the margin of the continent to the current position. Crustal movement uplifted this region accompanied by intensely folding after the Islands settled. The south Fossa Magna consists of accreted rocks (mainly volcanic rocks) of the Izu-Bonin Arc and clastic rocks derived from the land (Honshu Island). The Neogene sediments in the Fossa Magna are more than 5000 m thick. The volcanic zone turns toward the south in the Fossa Magana, continuing to the Izu-Bonin Arc.

Fossa Magna

Fig. Fossa Magna region (hatched lines)
Non-dashed line area, North Fossa Magna; dashed line area, South Fossa Magna; yellow, Neogene rocks; green and purple, basement rocks; light blue, Quaternary sediments; red, Quaternary volcanic rocks (see "Geologic map of Japan" for the detailed legend)

Western side of ISTL

Three mountain ranges with peaks more than 3000 m high are placed along the ISTL: the Hida, the Kiso, and the Akaishi Ranges in the order from the north. These mountain ranges with northward or northeastward trending main ridges 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.

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.

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 (protolith) 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, the Hida Highland, and the Ryohaku Mountains. Late Cretaceous-Paleogene volcanic rocks (Nohi Rhyolite) covered rocks of the Mino 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 protolith 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 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 sedimentary rocks such as those of 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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