Japan Sea

In recent years, many hydrothermal solution venting and sulfide-sulfate precipitations have been discovered on the seafloor of back-arc basins and island arcs (e.g., Ishibashi and Urabe, 1995) (section 2.3). Therefore, it is widely accepted that the most Kuroko deposits have formed at back-arc basin, related to the rapid opening of the Japan Sea (Horikoshi, 1990). 

Geochemical features of sedimentary rocks formed in the Japan Sea as a proxy for hydrothermal activity... 

As noted already, intense submarine hydrothermal activity took place in the Japan Sea in 15-12 Ma, associated with Kuroko mineralization. However, it is uncertain that submarine hydrothermal activities associated with the Kuroko mineralization took place in the other periods from middle Miocene to present in the Japan Sea. Therefore, the geochemical features of sedimentary rocks which formed from the Japan Sea at these ages have been studied by the author because they are better indicator of age of hydrothermal activities than those of hydrothermally altered igneous rocks because the samples of continuous age of sedimentation are able to be collected and the ages are precisely determined based on microfossil data (foraminiferal, radioralian and diatom assemblages). 

Thick sedimentary pile from middle Miocene to late Pliocene is exposed in the Oga Peninsula, northern Honshu, Japan (Fig. 1.153). Age of the sedimentary rocks has been determined by microfossil data. Thus, the sedimentary rocks in the Oga Peninsula where type localities of Miocene sedimentary rocks in northern Japan are well exposed have been studied to elucidate the paleoenvironmental change of the Japan Sea (Watanabe et al., 1994a,b). Kimura (1998) obtained geochemical features of these rocks (isotopic and chemical compositions) and found that regional tectonics (uplift of Himalayan and Tibetan region) affect paleo-oceanic environment (oxidation-reduction condition, biogenic productivity). However, in their studies, no detailed discussions on the causes for the intensity and periodicity of hydrothermal activity, and temporal relationship between hydrothermal activity, volcanism and tectonics in the Japan Sea area were discussed. They considered only the time range from ca. 14 Ma to ca. 5 Ma. 

The geochemical features of the sedimentary rocks in the Oga Peninsula and the hydrothermal activity in Japan Sea deduced from these features are described below. 

Figure 1.159. Eu/Eu values of (A) modern sediment, hydrothermal solution and seawater and (B) mid-Miocene to early Pliocene Japan Sea see in text). Modern data are from the Pacific ocean,...

In contrast to Southwest Japan, opening of northeastern part of the Japan Sea is unclear, compared with the southwestern part of Japan. Tosha and Hamano (1988) made a paleomagnetic study of Tertiary rocks of Oga Peninsula (northern Honshu) and considered that a counterclockwise rotation of Northeast Japan with respect to eastern Asia took place between about 22 Ma and 15 Ma and the before the rotation. Northeast Japan was situated along the east coast of the Asian continent. 

Lallemand and Jolivet (1986) have interpreted the opening of the Japan Sea as a pull-apart basin between two right-lateral strike-slip fault zones the Yagsan-Tsushima fault to the west and the Tartary-Hidaka shear zone to the east. 

Although many discussions on the origin of the Japan Sea have been carried out since the pioneer works by Tokuda (1927), and Terada (1934) who considered that the Japan Sea formed by southward migration from the Asian continent, the problem of the mechanism of the opening of the Japan Sea remains unsolved as mentioned above. 

Kuroko formation occurred at middle Miocene (15-16 Ma). But the opening of Southwest and Northeast Japan Sea occurred probably from the age earlier than this age. Therefore, it is thought that the beginning of the opening of the Japan Sea did not directly relate to the Kuroko formation. 

As mentioned above, formation of back-arc basins and marginal seas may be important for the formation of Kuroko and vein-type deposits, although genetic relationship between Kuroko formation and opening of the Japan Sea is not clear. For example, Horikoshi (1977) insists that vein-type deposits in Northeast Hokkaido did not form without the opening of Ohotsuku back-arc basin. 

Injection of the asthenosphere into the mantle wedge and the dam-up effect of the subducted slab explains the rifting process in the Japan Sea (Fig. 1.165). 

Otofuji (1996) proposed a double door opening mode with a fast spreading rate of 21 cm/year for the evolution of the Japan Sea, caused by the injection of asthenosphere with a low viscosity beneath the Japan Sea area. 

Kimura, S., (1998) Paleoceanographic Environment of Japan Sea Deduced from Chemical and Isotopic Features of Miocene-Pliocene Sedimentary Rocks. Doctoral Thesis, Keio University, 224 pp. 

Lallemand, S. and Jolivet, L. (1986) Japan sea a pull apart basin. Earth Planet. Sci. Lett., 76, 375-389. 

Nakajima, M. and Nakagawa, T. (1994) Twice rotation of southwest Japan and opening of Japan sea. Monthly Earth, 16, 143-146 (in Japanese). 

Otofuji, Y, Hashida, A. and Torii, M. (1987) When was the Japan Sea opened. Paleomagnetic evidence from southwest Japan. In Nasu, N. et al. (eds.). Formation of Active Ocean Margins. D. Reidel Publ., pp. 551-566. 

Otsuki, K. and Ehiro, M. (1978) Major strike slip faults and spreading of the Japan sea (Uyeda, S., Morphy, R.W. and Kobayashi, K., eds.). J. Phys, Earth Suppl. Issue, 26, 537-555. 

Shuto, K. (1989) Tertiary volcanism of the Northeast Japan arc in view of hypothesis of the spreading of the Japan sea. Earth Science (Japan), 43, 28-42 (in Japanese). 

Tamaki, K. (1995) Opening tectonics of the Japan Sea. In Taylor, B. (ed.), Backarc Basins Tectonics and Magmatism, Plenum Publ., pp. 407-420. 

Tatsumi, Y., Maruyama, S. and Nohda, S. (1990) Mechanism of back arc opening in the Japan sea role of asthenospheric injection. Tectonophysics, 181, 299-306. 

Taylor, H.P. Jr. (1997) Oxygen and hydrogen isotope relationship in hydrothermal mineral deposits. In Barnes, H.L. (ed.). Geochemistry of Hydrothermal Ore Deposits, 3rd ed. Wiley and Sons, pp. 229-302. Terada, T. (1934) On bathymetrical features of the Japan Sea. Bull Earths. Re.s. Inst., 12, 650-656. 

Intense submarine and subaerial volcanic activities during the Tertiary at Green tuff regions took place not only at the Japan Sea but also at marginal basins in the circum-Pacific Region. 

According to the summary of the development of back-arc basins in the Cenozoic age by Tamaki and Honza (1991) (Figs. 3.3 and Fig. 3.4) and Kaiho and Saito (1994) (Fig. 3.5), many back-arc basins (Japan Sea, Kuri, Shikoku, Parece Vela, South China, Sulu, Makassar, Central Scotia, Cayman) widely and rapidly developed during 30-15 Ma. 

The zoogeographic history of the Japan Sea proposed by Chinzei (1991) (Fig. 4.5 and Fig. 4.6) indicated that (1) from 17 to 15 Ma, before the spreading of the Japan Sea, tropical and subtropical molluscs invaded the area (2) the benthic and planktonic faunas changed sharply into low temperature faunas at about 15 Ma (3) from about 10 to 5 Ma, sedimentary deposits formed that were barren of benthic molluscs as a result of the stagnation of the Japan Sea basin (4) cold-water molluscan fauna reappeared in the coastal areas at about 5 Ma and (5) this has been followed by a period of marked cyclicity of cold and warm water faunas since about 1.3 Ma. 

Fig. 4.6. Distribution of benthic molluscs associations during the middle Miocene and presumed paleogeogra-phy of the Japan Sea (Chinzei, 1991).

Several ions (e.g., manganese, iron (II), iron (III), cobalt, nickel, copper, zinc, cadmium, lead, and uranyl) react with pyrocatechol violet, and to some extent are extracted together with aluminium. The interferences from these ions and other metal ions generally present in seawater could be eliminated by extraction with diethyldithiocarbamate as masking agent. With this agent most of the metal ions except aluminium were extracted into chloroform, and other metal ions did not react in the amounts commonly found in seawater. Levels of aluminium between 6 and 6.3 pg/1 were found in Pacific Ocean and Japan Sea samples by this method. 

Macrolides boromvcin-like Mar. aplasmomycin from Streptomycesffiseus, Actinom., Bact. frmn Japan sea mud Land boromycin from African soil Streptomyces antibioticus, Actinom., Ba<4. Stout 1991 antimvcin-like Mar. and Land Streptomyces spp. Imamura 1993. 

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