南西諸島周辺海域の堆積盆地の分布と性格

抄録

The Nansei-Shoto (Ryukyu Islands) is one of the island arcs along the West Pacific continental margins and has typical topographic features as an island arc, where marginal seas, volcanic fronts, island arcs and trenches are regularly and zonally arranged toward the Pacific Ocean.
Geology and structures of the Ryukyu Islands are partly discortant with the zonal arrangements, and the northern and southern parts of the Ryukyu Islands were under different tectonic conditions before the latest Neogene time. But finally both of them were combined into a present status by the Quarternary
tectonism.
Around the Ryukyu island arc, there are several sedimentary basins mainly consisting of the Neogene sediments.
The Shimajiri basin, a fore arc basin along the outer side of the island arc, is about 1100km long and contains up to 6000m of sediments deposited during the late Miocene to Pleistocene time, and is forming abyssal plain between the island arc and trench.
The Yaeyama basin is also situated in the outer side of the southern Ryukyu arc, but a trend of this fore arc basin is different from that of the himajiri basin. Plio-Pleistocene deposits are unconformably resting on possible early to middle Miocene layers and a total thickness of these sediments attain to 4000 m. Small basins within the older uplift zone such as the Amami trough are collapse basins formed by normal faults and tiltings due to pull-apart. They are filled mostly with Quarternary deposits.
The Okinawa trough is a typical back arc basin, which is formed by extensional tectonic forces and supplied with deltaic or submarine fan deposits from the continent to the west. Sediments in this trough will exceed 5000m at maximum. Many step faults and roll-over anticlines caused by them are commonly
observed on seismic profiles. Characteristically high heat flows are reported in the trough.
The Tokai basin is an older back arc basin compared with the Okinawa trough, and formed between Fukien-Ryeongnam and Goto-Senkaku belts. Presumably, this basin contains early to midde Miocene sand-shale sequences deposited in lagoonal shallow water environments. The presence of large anticlinal structures with gentle dips suggests significant hydrocarbon accumulations in the basin.