Fossa Magna

The Fossa Magna is a major geological zone in central Japan, where a broad sedimentary basin separates northeastern and southwestern Japan.

Itoigawa–Shizuoka Tectonic Line: A major tectonic boundary running through central Japan from the Sea of Japan to Suruga Bay. It marks the western boundary of the Fossa Magna and separates distinct geological regions.
Sedimentary basin: A region of long-term subsidence where sediments accumulate and become preserved. The Fossa Magna is characterized by a broad basin filled with thick sedimentary deposits.
Naumann’s map of Japan: A nineteenth-century geological interpretation of Japan by German geologist Heinrich Edmund Naumann. Naumann introduced the Fossa Magna concept through his interpretation of Japan’s geological structure.
Matsumoto Basin: An intermontane basin in Nagano Prefecture, central Japan, containing the city of Matsumoto. It lies within the Fossa Magna’s central region and illustrates its basin landscape.
Median Tectonic Line: A major fault zone extending across southwestern Japan, separating the Inner and Outer Zones. Its extension toward central Japan helps define the tectonic setting around the Fossa Magna.
Forearc basin: A sedimentary basin between a volcanic arc and its offshore subduction trench. Forearc settings provide a useful comparison for interpreting sediment accumulation in Japan’s tectonic zone.
Heinrich Edmund Naumann: A German geologist who conducted foundational geological surveys in Japan during the Meiji period. His fieldwork and writing established the name and early interpretation of the Fossa Magna.
Nagano Prefecture: A mountainous prefecture in central Honshu, Japan, surrounded by several major mountain ranges. Much of the Fossa Magna’s inland terrain and basin deposits occur in Nagano.
Japan Trench: A deep ocean trench east of Japan where the Pacific Plate descends beneath the island arc. Subduction east of central Japan contributes to the deformation that shaped the Fossa Magna.
Accretionary prism: A wedge of scraped-off sediment and crust assembled at a subduction zone. Accreted material around Japan provides geological context for the basin’s surrounding structures.

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Japanese AlpsRelated: Its tectonic setting helps explain the complex geology surrounding the Japanese Alps.