琉球弧と沖縄トラフの発達史 とくに沖縄トラフの形成年代について

Abstract

The Ryukyu Arc-Okinawa Trough system is one of several arc-backarc systems alongthe margin of the western Pacific and east Asia. Recent results of seismic refractionand reflection studies have revealed that the origin of the trough is the continental rifting, and the trough is in a state of the active rifting stage (e. g. Hirata et al., 1991; Furukawa et al., 1991). Basement rocks and sediments to appraise for the age of the trough, however, have notbeen sampled by deep sea drilling. Then the formation age of the trough was mainlyexamined by the geological studies of the Ryukyu Arc and Okinawa Trough areas andpaleomagnetic stud -ies of the Ryukyu Arc.Geological sequence of the Ryukyu Arc is generally divided into two units, thepre-Miocene basement complex and post-middle Miocene sediments (e. g. Kizaki, 1986). Thenorth and central Ryukyu Arcs represent the geological continuation of the Outer Beltof the south west Japan Arc, composed of the Mesozoic to Eocene sedimentary sequences, whereas thesouth Ryukyu Arc is characterized by high-pressure metamorphic rocks and the Eocenevolcanics and limestone. The geological and structural contrasts between the north-central andsouth Ryukyu Arcs are conspicuous before the late Miocene. These observations suggest that theformation of the Okinawa Trough have been originated since the Miocene, and the Ryukyu Archave been established since then.Paleomagnetic studies for the the Eocene to Pliocene volcanics and sedimentscollected from the Ryukyu Arc and northeast Taiwan suggest that the south Ryukyu Arc rotatedclockwise 19° with respect to the central Ryukyu Arc and Taiwan between 10 Ma and 4 Ma.Paleomagnetic studies attribute this rotation to the opening of the southern part of theOkinawa Trough during 10 and 4 Ma (Miki et al., 1990; Miki, 1991).The acoustic sequence in the trough can be divided into three units. And the eachunit are correlated to the Pliocene to early Pleistocene, middle to late Pleistocene, and late Pleistocene to Recent sediments in descending stratigraphic order. Widespreaddeformation by normal faulting is especially developed in the lowest unit which displays horst andgraben structures. The deformation would have originated at about the Plio-Pleistoceneboundary and formed the prototype of the trough (Furukawa et al., 1991). Synthesizes of the above studies have resulted in interpretativedevelopment of the trough as follows; a major phase of formation of the trough occurred during 10 and4 Ma, and reactivation of the formation of the trough occurred after 2 Ma. Recent studies fortherifting of several continental margins, however, suggest that the intra-arc rifts evolved 10times as first as major intra-continental rifts, and the life time of the rapid rifting is onlyabout 3 to 5 Myr (Yamaji and Takeshita, 1989). The middle Pleistocene reefal limestone dredged fromsome knolls in the Okinawa Trough indicate that the subsidence of the trough was about500-1, 000m after the middle Pleistocene. As a preliminary conclusion, therefore, the age ofthe trough could be estimated to be roughly 2 Ma. The present age estimation is againstrecently proposed clockwise rotation of the south Ryukyu Arc due to the opening of thetrough from 10 Ma to 4 Ma which was reported by the paleomagnetic study.