Geology of Japan

Japanese archipelago relief (including submerged parts)

The breakup of Rodinia about 750 million years ago formed the Panthalassa ocean, with rocks that eventually became Japan sitting on its eastern margin. In the Early Silurian (450 million years ago),[2] the subduction of the oceanic plates started, and this process continues to the present day, forming a roughly 400 km wide orogeny at the convergent boundary. Several (9 or 10) oceanic plates were completely subducted and their remains have formed paired metamorphic belts. The most recent complete subduction of a plate was that of the Izanagi Plate 95 million years ago. Currently the Philippine Sea Plate is subducting beneath the continental Amurian Plate and the Okinawa Plate to the south at a speed of 4 cm/year, forming the Nankai Trough and the Ryukyu Trench. The Pacific Plate is subducting under the Okhotsk Plate to the north at a speed of 10 cm/year. The early stages of subduction-accretion have recycled the continental crust margin several times, leaving the majority of the modern Japanese archipelago composed of rocks formed in the Permian period or later.

Around 23 million years ago, western Japan was a coastal region of the Eurasia continent. The subducting plates, being deeper than the Eurasian plate, pulled parts of Japan which become modern Chūgoku region and Kyushu eastward, opening the Sea of Japan (simultaneously with the Sea of Okhotsk) around 15-20 million years ago, with likely freshwater lake state before the sea has rushed in.[3] Around 16 million years ago, in the Miocene period, a peninsula attached to the eastern coast of the Eurasian continent was well formed. About 11 million years before present, the parts of Japan which become modern Tōhoku and Hokkaido were gradually uplifted from the seafloor, and terranes of Chūbu region were gradually accreted from the colliding island chains. The Strait of Tartary and the Korea Strait opened much later, about 2 million years ago. At the same time, a severe subduction of Fossa Magna graben have formed the Kantō Plain.[4]

Overall, the geological composition of Japan is poorly understood. The Japanese islands are formed of several geological units parallel to the subduction front. The parts of islands facing oceanic plates are typically younger and display a larger proportion of volcanic products, while the parts facing the Sea of Japan are mostly heavily faulted and folded sedimentary deposits. In north-west Japan, the thick quaternary deposits make determination of the geological history especially difficult.[5]

The Japanese islands are divided into three major geological domains:

• Northeastern Japan, north of Tanakura fault (which had high volcanic activity 14-17 million years before present[6])
  • Idosawa Fault
  • Senya Fault
  • Hidaka Mountains
  • Kitakami Mountains
  • Ōu Mountains
• Central Japan, between Tanakura fault and Itoigawa-Shizuoka Tectonic Line.
  • Fossa Magna graben
  • Tanna Fault
  • Bōsō Hill Range
• Southwestern Japan, south of Itoigawa-Shizuoka Tectonic Line. The Southwestern Japan is further subdivided into several metamorphic belts stretched along Japan Median Tectonic Line.[7] The parts of Japan north of Japan Median Tectonic Line ("Inner Zone") contains many granitoid fragments dating from Paleogene to Cretaceous period intruding the older material, while south of the line ("Outer Zone") is mostly accretionary complexes of Jurassic period or younger.
  • Urasoko fault
  • Fukozu Fault
  • Neodani Fault
  • Nojima Fault
  • Hida orogenic belt (Hida Mountains and Ryōhaku Mountains)
  • Sangun orogenic belt
  • Maizuru orogenic belt
  • Tanba-mino orogenic belt
  • Ryoke orogenic belt
  • Shimanto orogenic belt[8]
  • Sambagawa orogenic belt[9]
  • Chichibu orogenic belt[10]
  • Sambosan orogenic belt
  • Beppu–Shimabara graben