別府-島原地溝の発想とその後の発展および課題  [in Japanese] Conception of the Beppu-Shimabara graben, its development and problems  [in Japanese]

別府湾から島原半島にかけて, 中部九州を横断する別府-島原地溝の存在は, 重力異常, 先中新統の分布, 活断層, 現世の地殻変形, 基盤岩深度などから推定される。造構史として, 先行隆起と撓曲→深部断裂→陥没→火山活動(ときに火山活動→陥没)の大きく3回の繰り返しが考えられる。これらの時代は中新世中後期, 鮮新世〜更新世中期前半, 更新世中期後半〜現世である。別府-島原地溝は中央構造線(とくに大分-熊本線)とも密接に関係し, その東方延長は松山-西条凹地から吉野川地溝, 南西〜南南西延長は天草灘地溝から沖縄トラフへと追跡され, 非火山性地溝と火山性地溝の組み合わせとも考えられる。別府-島原地溝の実像を解明し, その形成過程や形成要因の真実を探求するためには, 地質学, 地球化学, 測地学, 地球物理学, 火山学, 地熱学, その他の分野を総合した真の意味の総合研究が必要である。

Based on the Miocene and post-Miocene volcanic geology, distribution of the pre-Miocene system, depth of the basement rocks, Bouguer anomalies, active faults and recent crustal deformation in central Kyusyu, there is a conjecture that a graben structure exists with a general trend of ENE-WSW from Beppu Bay to the Shimabara Peninsula in the post-Miocene age. This depression structure is named the Beppu-Shimabara graben formed the author (Matsumoto, 1979), The scale of this graben is about 150 kilometers in length and 20 to 30 kilometers inwidth. The total head of this depression is 1,500 to 2,500 meters or more. The Beppu-Shimabara graben is divided into three parts by the discontinuous areas, as the Kujyu-Beppu, Aso-Kujyu, and Shimabara-Kumamoto grabens. Moreover, it is considered that this depression structure changes its direction southwestwards to a NNE-SSW trend, continuing to the Amakusanada graben. The history of geologic development of the Beppu-Shimabara graben can be summarized in the following three stages: First stage (Green tuff disturbance stage):- Middle Miocene to late Miocene. Elevation and warping of ground → deep fissuring (deep fracturing) → first depression of the Beppu Shimabara graben → Setouchi volcanic activities and green tuff activities with vigorous volcanism deposition (repetition in some area). Second stage (early Island arc disturbance stage):- Early Pliocene to early Pleistocene and middle Pleistocene. Elevation and warping of ground → deep fissuring (deep fracturing) → second depression of the Beppu-Shimabara graben and small graben and caldera → Hohi and Bungo volcanism → deposition (repetition in some area). Third stage (late Island arc disturbance stage):- Middle Pleistocene to Holocene. Elevation and warping of ground → deep fissuring (deep fracturing) → San-in and Ryukyu volcanic activities → local depression (Aso caldera, Kujyu caldera?, Unzen graben and Chijiwa caldera) → San-in and Ryukyu volcanic activities (Yufu and Tsurumi volcanoes, Kujyu volcano, Unzen volcano, and Aso central coneetc.). Consequently, each stage is recognized as the elevation and warping of ground formed magmatic activities, deep fissuring and fracturing, depression, volcanism and deposition of volcanic materials. The geologic structure in the volcanic field of central Kyusyu has been formed by repetition of the above geologic phenomena. Recently, a great deal has been reported about opinions and suggestions for the Beppu-Shimabara graben. According to Kimura (1982, 1983), Tada (1984, 1985) and others, the Beppu-Shimabara graben is a rift of the crust and north-eastern terminal part of the Okinawa Trough which is the activeback arc basin of the Ryukyu arc and is spreding in the N-S direction, and is formed by subduction of the Philippine Sea plate. There are other papers as follows: the eastern part of the Beppu-Shimabara graben is a volcano-tectonic depression from the Pliocene to Quaternary (Kamata, 1985a, 1958b), a pull-apart basin (Kido, 1991), and a doubled cauldron (Kubota, 1991). However, the many problems of the graben or depression structure in central Kyusyu have not been fundamentally settled even today. In order to make clear the above problems, further synthetic investigation of geosciences is necessary.