西南日本弧前縁の圧縮テクトニクスと中期中新世カルデラ火山(<特集>紀伊半島における中新世火成作用とテクトニクス)  [in Japanese] Middle Miocene ash-flow calderas at the compressive margin of southwest Japan arc: Review and synthesis(<Special Issue>Miocene igneous activity and tectonics in the Kii Peninsula)  [in Japanese]

平均噴出率・楕円率・ルーフアスペクト比を用いた検討から,圧縮テクトニクスと西南日本の中期中新世カルデラ火山の関係について,以下のような考察が得られた.カルデラ火山とテクトニクスの関係では,マグマの噴出しやすさが重要である.陥没構造の違いは,陥没の仕事量の相違である.西南日本のカルデラ火山群は,圧縮と引張の中間的性質を示す極めて大きな平均噴出率である.楕円率・規模・ルーフアスペクト比とトレンチ距離の関係では,トレンチ側で楕円率×活動規模が大きく,ピストン型カルデラが発生しやすいことがわかった.古応力方位はNNE-SSW〜NE-SWの最大圧縮応力方位(σ_<Hmax>)と推定され,この時代の中央構造線(MTL)の左横ずれ変位と一致する.さらに,火山フロントの後退条件を検討した結果,マグマ供給率の低下とテクトニック応力の増加によって起こると考えられる.

We reviewed and synthesized middle Miocene caldera volcanoes at the frontal margin of southwest Japan in order to reveal possible relationship between the caldera formation and the compressive tectonics. Cumulative frequency diagram for fault-slip can be used for determining the deformation style of caldera floor. The deformation style is defined as the ratio of caldera collapse energy to the total eruptive energy. The changing of style is to modify the ratio. Mechanical collapse energy for the piston model is rather reduced than the piecemeal deformation or the funnel explosion. This character is suitable to the larger caldera formation. Long-term eruption rate of the Miocene calderas indicates larger rate for the compressive stress regime. The ellipticity, eruptive volume and roof aspect ratio of calderas in relation to distance from subduction trench was used to reveal an effect of strong compression. Both ellipticity and eruptive volume increase toward the trench area, and the upper limit of roof aspect ratio decreases toward there. These suggest that near the trench region, only the piston collapse caldera is capable to erupt against the strong compression. Orientation of dyke swarm, caldera shape, and magma chamber were used to infer the orientation of paleostress at the time of caldera formation. The optimal paleostress orientation of horizontal compressive stress (σ_<Hmax>) lies on NNE-SSW. The sinistral (left-wrenching) fault-slip of Median Tectonic Line (MTL) had occurred in the same period and is concordant to the NNE-SSW orientation of σ_<Hmax>. The clockwise rotation of Japan-Sea Opening, however, occurred in the same period and it might have modified the original orientation into the NNE-SSW. In the southwest Japan, the migration of volcanic front took place backward after middle Miocene period. We concluded that the backward migration could be induced by the decrease of magma supply rate and increase of compressive stress at frontal margin after middle Miocene.