Omine and Odai Cauldrons -Arcuate and semicircular faults, dike swarms and collapse structure in the central area of the Kii Mountains, Southwest Japan-

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  • 大峯・大台コールドロン - 紀伊山地中央部にみられる弧状および半円形の断層・岩脈群と陥没構造 -
  • オオミネ オオダイ コールドロン キイサンチ チュウオウブ ニ ミラレル コジョウ オヨビ ハンエンケイ ノ ダンソウ ガンミャクグン ト カンボツ コウゾウ

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Description

The Chichibu Terrane overlies tectonostratigraphically on the Shimanto Terrane by the Omine-Odai thrust, in the central area of the Kii Mountains. The Omine-Odai thrust is shifted by arcuate fault and semicircular fault. The Chichibu Terrane is underlain inside the arcuate fault, which is over 30km east and west in width and semicircular fault, which is 15km in diameter. Both faults show the concentric circles-shaped. The arcuate dike swarm composed of andesite and andesite-quartz porphyry complex dike intrudes inside area of the arcuate fault. The tuffite dike swarm intrudes side by side with outside area of the semicircular fault. The Mesozoic system (The Chichibu and Shimanto Terrane) collapsed several hundred meters inside both faults and tuffite dike swarm. They are named the Omine Cauldron and the Odai Cauldron. The former cauldron is surrounded by the outside arcuate fault. The semicircular fault and the tuffite dike swarm surround the latter. The feature described above suggests that these cauldrons continuously formed duplex cauldron. Isotopic ages of dike swarm with the cauldron indicate them to be formed the middle Miocene in age. The mechanism of the formation of the Omine and Odai Cauldrons are considered piston cylinder type subsidence caused by large volume eruption of pyroclastic rocks and especially the Odai Cauldron is inferred to be trap-door type subsidence. The Chichibu Terrane in the central area of the Kii Mountains is the relic of the Mesozoic system inside of these cauldrons. Because of the uplift and erosion of the Kii Mountains, the caldera-fill pyroclastic sediments derived from these cauldrons are not found surrounding the cauldrons and the Mesozoic systems composed calderas floor were exposed.

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