Geological and geophysical survey in the Okinawa Trough and adjoining seas of Nansei Syoto.
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AbstractThe hydrographic Department, Maritime Safety Agency initiated a series of systematic depth sounding, seismic reflection profiling, magnetic and gravity measurements in the seas around Nansei Syoto (Ryukyu Islands) in 1913. In 1983 the department placed a newly built survey vessel “Takuyo"（ 2,600 tons) in commission. Takuyo is installed a narrow multi-beam echo sounder (Sea Beam), a 3.5 KHz subbottom profiler and a deep sea camera etc. to cover the Okinawa Trough, Nansei Syoto Trench and the other adjoining seas of Japan. Dense survey tracks were laid out to cover the special areas of interest in the middle Okinawa Trough. The survey results were compiled in the detailed bathymetric, magnetic anomaly and free-air gravity anomaly maps. Seismic reflection profiles show characteristic small fault groups in the Okinawa Trough which indicate block faulting. Well developed grabens composed of active normal faults exist in the axis of middle and southwest Okinawa Trough. Knolls of igneous rock were identified in the grabens. In the middle Okinawa Trough. Knolls in a graben form ENE-WSW trending small ridges. The linear patterns of magnetic anomalies in the graben are attributable to the normal magnetization of the small ridges. The magnetic anomaly map reveals two magnetic anomaly belts fringing the both side of the Okinawa Trough. One of the magnetic anomaly belt extends between Goto Retto and Senkaku Syoto along the southeast margin of the shallow shelf of East China Sea, which indicate igneous basement high damming sediments from the continent. The other magnetic anomaly belt extends between Tokara Gunto and northern vicinity of Yaeyama Retto. The latter is considered to be a magnetic expression of the Ryukyu volcanic belt including the recent volcanic front of the Ryukyu Arc. Magnetic anomalies of the knolls in the graben are adjacent to the magnetic anomalies of the Ryukyu volcanic belt. Dredging attempts in the Okinawa Trough recovered rhyolite, dacite and high alumina basalt. Chemical examination and dating by K-Ar method of these rock samples confirmed Quaternary arc type volcanic activity in the trough. Limestone was sampled at a site 1,050 m deep on the upper part of Miyako Seamount in the southwestern part of the Okinawa Trough. Forarninifera of the limestone suggest shallow coastal ambient of Late Pliocene (N.21) to Middle Pleistocene (N.22) time at the dredge site. This fact indicates an about few hundred meters subsidence of the seamount. These results summarize the recent tectonic activity of the Okinawa Trough as subsidence of the basement accompanied by arc-type volcanism, faulting and sedimentation, bearing some resemblance to a back arc depression.
JournalReport of Hydrographic and Oceanographic Researches