「九重-別府地溝」北西縁部の地質構造形成史  [in Japanese] Tectonic history of the north-western area of "Kuju-Beppu Graben"  [in Japanese]

中北部九州に発達する「九重-別府鮮新世沈降盆地」は, 大口ケ岳-犬ケ岳帯に北限を, 桑野-大行司断層と日田-白草線に西端を, 大分-熊本構造線に南限を画され, 約1000 m沈降している。これらの断層帯の活動は鮮新世前期に始まっている。この沈降盆地のうち久留米-日出線の南側は鮮新世に引き続いて更新世, 特に1 Ma以降最大の沈降をおこし, その沈降量は1400 mを越える。この地域が「九重-別府地溝」にあたる。大口ケ岳-犬ケ岳帯, 久留米-日出線および大分-熊本構造線は右横ずれ性断層である。北を久留米-日出線, 南を大分-熊本構造線で画される「九重-別府地溝」が, 膨大な量の火山岩類を噴出しながらきわめて短期間に形成されたのは, テクトニクス上, Pull-apart basin形成の場が, 火山構造性陥没地形成の場であったことによる。これらの現象は, フィリピン海プレートの沈み込みによって引き起こされたと考えられる。

In Landsat images of northern and central Kyushu, we can see large scale tectonic depressions named "Pliocene Kuju-Beppu Tectonic Basin" and "Kuju-Beppu Graben". In the beginning of the Pliocene, right-handed strike-slip faults striking E-W began to form. Then Dainichigadake-Inugadake Tectonic Zone was localized on the boundary between the northwest margin of the Pliocene Kuju-Beppu Tectonic Basin and the basement area. Dainichigadake-Inugadake Tectonic Zone of echelon faulting is due to a couple, the southern block moved west relative to the northern block.The total right-handed displacement of them is up to 1000 to 1400 meters. Lava piles and dikes of the Hikosan Volcanics and the Dainichigadake Volcanics arranged in this zone showing an ENE-WSW trend with an extension of an E-W trend respectively. Kuwano-Daigyouji Fault and Hita-Hakuso Fault striking NNW-SSE compose the west margin or the Pliocene Kuju-Beppu Tectonic Basin. They began to form as a gravity fault after the last early Pliocene, and cut the Dainichigadake-Inugadake Tectonic Zone. The eastern block of the Dainichigadake-Inugadake Tectonic Zone has sunk down more than 1000 meters relative to the western block. Kurume-Hiji Line striking E-W, the northern end and Oita-Kumamoto Tectonic Line, the southeast one of the Kuju-Beppu Graben, are essentially right-lateral strike-slip faults. The Kuju-Beppu Graben is bound on the west formed Oyama Fault which is a gravity fault. The horizontal compressive tectonic stress is due to the subduction of the Philippine Sea Plate under Southwest Japan along the Nankai Trough. Consequently, the area of the Kuju-Beppu Graben which is sandwiched between the Kurume-Hiji Line and the Oita-Kumamoto Tectonic Line, has been pulled apart north and south, and subsided over the last 1 Ma. Simultaneously, enormous volumes of lavas and pyroclastic rocks were ejected and formed volcano-tectonic depressions in this area.