Geological, geochemical research of bottom samples, from continental shelf surveys, H. D. Japan (Part 1) .
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Alternative Title一一PRELIMINARYSTUDY FOR OCEAN FLOOR ON THE JAPANESE CONTINENTAL SHELVES
AbstractS/S Takuyo (2600t) which belonging to the Hydrographic Department of Japan, is cruising several times in the southern ocean area off Mainland Honshu, Japan since October 1983 for the purpose of collecting precise data on the continental shelves. One of this survey object is to collect scientific and technological data in order to determine outer limit of Continental Margin beyond 200 nautical miles from Japanese base line. So the cruise have been carried out as geological and geophysical survey. In addition, bottom samples are collected in order to study for the petrological and geochemical characters of ocean floor. A hundred and nineteen dredge hauls or coring attempts were made. Almost sampling were circular chain bag dredge with a small pipe dredge. A few coring install with heat flow meter were also attempted. They had been obtained core samples and determined heat flow value. Samples are collected from the East China Sea including Okinawa Trough, to Philippine Basin, Shikoku Basin, Izu-Ogasawara Ridge and to the Northwest Pacific Ocean. Obtained samples from the ocean floor in the East China Sea, Philippine Sea and the Northwest Pacific Ocean were examined by petrological microscopic observation and chemical analyzed results. Main 13 rock forming elements and main 9 metal elements components in manganese crust were obtained. In addition,40K-40Ar radio isotope dating have been attempted to selected samples. These result reveals tectonical history and geological environment of the ocean floor in southward ocean off Central Japan as a continental margin. Ocean floor is essentially classified three tectonic zone which are as follows. 1 . Spreading axis and Back arc basin as a divergent plate boundary, e. g. Okinawa Trough, Kinan Seamount Chain, Ogasawara Trough 2 . Volcanic arc as a mid plate edifices e. g. Ryukyu arc, Izu-Ogasawara arc 3. Convergent plate margin, Plate subduction zone e.g. Nansei-shoto(Ryukyu) trench, Izu-Ogasawara trench. Accordingly we classified investigated area into four subdivisions because of their geological character in addition to geomorphology. From the view point of petorology, volcanic rock character recognized genrally three rock type series, so called, tholeiite rock series, calk-alkali rock series, and alkali olivine rock series which are controlled by each essential magma series. Collected samples from investigated area, mainly andesite and basalt, which is mainly classified as calk-alkali rock series and tholeiite rock series. Therefore sample represent regional area from petrological character above mentioned. Obtained dating data from selected fresh rocks are concordance with previous results. Reliable dating data are as follows, North Okinawa trough is 4.39 Ma, Middle part of the Trough axis shows 10.30 Ma, Oki Daito-Ridge is 65.60 Ma, Kinan kaizanretu(Komahasi No.3 S.m.) is 7.92 Ma. Sitito Ridge(Ansei S. m.) is 6.33 Ma, Ogasawara Ridge is 39. 7～ 40.4 Ma, Hahajima S.m. is 34.14～ 52.05 Ma, Ogasawara Plateau (Hanzawa S.m.) is 47.3～48.5 Ma. These results means not so different from previous data.
JournalReport of Hydrographic and Oceanographic Researches