火山構造性陥没地としての豊肥火山地域とその形成テクトニクス : 西南日本弧・琉球弧会合部におけるフィリピン海プレートの斜め沈み込み開始が引き起こした3現象  [in Japanese] The Hohi volcanic zone as a volcano-tectonic depression and its formation tectonics : Three tectonic events caused by subduction of the Philippine Sea plate under the junction of the Southwest Japan Arc and the Ryukyu Arc  [in Japanese]

西南日本弧と琉球弧の会合部にあたる九州では, フィリピン海プレートが, 約6 Maに北西方向への沈み込みを開始し, 以下の地質構造形成の原動力となった。(1) 約5 Maから, 豊肥火山地域の火山活動が急激に活発になり, 大規模なグラーベンを形成した。(2) 中央構造線の九州における西方延長と考えられる大分-熊本構造線が, 遅くとも約5 Maに, 右横ずれ断層運動を開始した。(3) 約6 Ma以降に, 南九州地塊が, 反時計廻りに約30゜回転した。これらの現象の同時代性は, 日本海の拡大(15 Ma頃)以後の6-5 Maに, 九州で重要なテクトニクスの変化があったことを示唆する。また, フィリピン海プレートの, 島弧会合部における沈み込み様式の違い(西南日本弧に対する右斜め沈み込みと, 琉球弧北部に対する垂直沈み込み)が, 上記の3現象を生じさせた原因と考えられる。

The northwestward subduction of the Philippine Sea (PHS) plate at the junction of the Southwest Japan Arc and the Ryukyu Arc since about 6 Ma induced three major geologic events in Kyushu: (1) Dextral fault has been active since no later than 5 Ma along the Oita-Kumamoto Tectonic Line, awestern extension of the Median Tectonic Line. (2) A large graben (70 km×40 km) has been formed in the Hohi volcanic zone, a volcano-tectonic depression, since about 5 Ma, accompanied by eruptions of >5,000 km^3 magmatic material in an extensional stress field. (3) South Kyushu has experienced an about 300 counter-clockwise rotation since about 6 Ma. The initiation of these events (6-5 Ma) is synchronous with that of the subduction of the PHS plate underneath the two arcs, suggesting that Kyushu experienced a significant tectonic change at 6-5 Ma. Due to the difference in orientation of these arcs, the PHS plate has subducted oblique to the Southwest Japan Arc and that normal to the Ryukyu Arc. The oblique subduction possibly made a strong coupling between the fore-arc sliver and the subducting slab and hence caused dextral faults and associated graben. By contrast, the subduction under the Ryukyu Arc initiated back-arc spreading at the Okinawa Trough and hence crustal extension in the initial stage of rifting caused a counter-clockwise rotation for South Kyushu and a clockwise rotation for the south Ryukyu Arc.