過去200万年間における宮崎平野の地層形成と陸化プロセス

―海面変化とテクトニクスに関連して―

抄録

 The Miyazaki Plain is situated at the western end of the forearc of the Southwest Japan Arc, and is one of the standard areas of the Quaternary system in Japan. Many tephra layers were supplied from volcanoes in the Ryukyu and Southwest Japan arcs during Quaternary. The authors establish the stratigraphy and chronology of sediments during middle Pleistocene to Holocene in the Miyazaki Plain using tephrochronological methods, and discuss landform development and tectonics.
 The sediments are subdivided into Kariya formation, Nojiri formation (the Highest terraces), the Higher terrace deposits, the Middle terraces deposits, the Lower terraces deposits, and the Holocene formation (alluvium plain). Kariya formation of 1 Ma-500 ka, is fluvial sediments which are partially accompanied by marine sediments. Nojiri formation is 500-300 ka old and fluvial and marine sediments form the highest terraces. Shiinoki member in Nojiri formation has a transgressive sediment and is correlated with MIS 9. The higher terraces are composed of Kukino fluvial terrace of 330-240 ka and Chausubaru fluvial terrace of 240 ka. The middle terraces consist of the four fluvial and marine terraces: Sanzaibaru terrace of 140-120 ka, Baba terrace of 110 ka, Nyutabaru terrace of 110-90 ka, Karasebaru terrace of 90 ka, and Saitobaru terrace of 90-80 ka. Sanzaibaru terrace with a thick transgressive sediment and well-developed marine terraces correlates with MIS 6-5e. Nyutabaru terrace is mainly fluvial accompanied partially by marine terraces, and emerged in MIS 5c. Baba and Saitobaru terraces are fluvial only. Saitobaru terrace with a gravel bed over 10-m thick indirectly corresponded to the transgression in MIS 5a. All of the lower terraces of 70-10 ka are fluvial, and are subdivided into ten terraces: Shimizu, Okatomi, Toyobaru 1, Toyobaru 2, Ikazuno, Oyodo, Fukadoshi 1, Fukadoshi 2, Mikazukibaru 1, and Mikazukibaru 2. Their steep longitudinal profiles indicate that sea-level when the terraces formed was lower than the recent one. This lower sea-level stage chronologically corresponds to MIS 4-2. Fukadoshi 1,2 terraces, the profile of which is the steepest of all the terraces, were formed in MIS 2, the last glacial maximum. Shimotajima marine terraces and recent alluvium correspond to Holocene high sea level and transgression in MIS 1.
 The tectonics of the Miyazaki Plain during the past 2 Ma are estimated from landforms and geological structures. During 2-1 Ma, the forearc basin beneath the sea was uplifted slowly and the Miyazaki Plain emerged. Then the left-lateral movement of Wanitsuka Mts block bordering the plain on the north formed the two pull-apart basins of the Kariya and Nojiri formations at the southern part of the plain during 1-0.3 Ma. The left-lateral movement was caused by the opening of the Okinawa Trough in the back arc of Ryukyu Arc. At 0.3 Ma, the Miyazaki plain suddenly began uplifting rapidly due to E-W trend compression. During 0.8-0.6 Ma, the movement direction of the Philippine Sea Plate changed from NW to WNW, and then the plate subducted obliquely along the Nankai trough. Consequently, the Southwest Japan forearc including the Miyazaki Plain decoupled from the inner arc, moving westward, and finally caused the E-W trend compression and uplifting around the Miyazaki Plain. The uplift rate of the southern part of the plain accelerated from 0.1 m/ka to 1 m/ka during the last 0.3 Ma at the southern part of the plain. In detail, the tectonical mode is domical uplifting the center, which is several kilometers off Miyazaki City in the Pacific Ocean. The domical uplifting and the increasing uplift rate relate to the isostasy of the subducting Kyushu-Palau Ridge on the Philippine Sea plate under the Miyazaki Plain or a rising serpentinite diapir in the crust of the Eurasia plate.