Large Scale Slope Failure during Caldera Collapse : Structural Development of the Late Miocene Takagawa Caldera, Northeast Japan

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  • カルデラ陥没に伴う大規模斜面崩壊 : 会津若松市南方,後期中新世高川カルデラの内部構造
  • カルデラ カンボツ ニ トモナウ ダイキボ シャメン ホウカイ アイズワカマツ

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The Late Miocene Takagawa Formation, filling an elliptical large caldera measuring 10 by 15 km, located in the Aizu district, is exposed at an advantageous level where both remnants of a caldera wall and upper parts of accumulations during caldera collapse are preserved. The caldera-forming stage deposits of this formation consist of welded dacite ash-flow tuff and intercalated debris avalanche deposits comprising fragments of basement rocks which are exposed on the caldera wall. The post-caldera stage deposits, which rests on the caldera-forming stage ones without significant erosion, consist of caldera lake sediments (Unit 1), unwelded dacite ash-flow tuff (Unit 2), andesite lava flow and resedimented volcaniclastic rocks (Unit 3); the volcanism of this stage is characterized by discontinuous ring of intrusive bodies indicating a rim of subsidence. These structural features of the Takagawa caldera are common for large caldera complexes. The debris avalanche deposits in the caldera-forming stage consists of two structurally different facies : a block facies and a matrix facies. The block facies is made up of monolithic breccia in which polyhedral clasts loosely fit together as in a three-dimensional jigsaw puzzle, and preserves original stratigraphic structures of ement rocks after the pulverization. The matrix facies is an ill-sorted mixture of various basement rocks, and embeds the block facies. These sedimentary structures suggest that the rock debris initially descended as rockslides from the caldera wall and moved without major turbulence and dispersion of grains. Furthermore, a consideration of the distribution of various block facies indicates that the caldera wall was about 1000 m high. This debris avalanche deposits interbed with welded ash-flow tuff ; it is interpretable as resulting from large scale slope failure from the wall of subsidence during the caldera-forming eruption. Therefore, the topographic caldera wall is the scarp of slope failure ; its rim is significantly larger in diameter than the rim of subsidence indicated by the alignment of eruption centers in the post-caldera stage.

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