Constraints on the Time of the Evolution of the Japan Sea Floor Based on Radiometric Ages. Ichiro KANEOKA

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1 J. Geomag. Geoelectr., 38, , 1986 Constraints on the Time of the Evolution of the Japan Sea Floor Based on Radiometric Ages Ichiro KANEOKA Geophysical Institute, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo, Japan (Received January 14, 1986) Based on available radiometric ages on igneous rocks recovered from the Japan Sea area, the constraints on the time of the evolution of the Japan Sea floor are discussed. To evaluate the reliability of radiometric ages on submarine rocks, some general problems related to each dating method are also referred. As far as volcanic rocks recovered from seamounts in typical oceanic regions such as the Yamato Basin and the Japan Basin are concerned, they show relatively young K-Ar ages of less than 20Ma. However, rocks recovered from the Yamato Bank area, which is regarded to be a remnant land mass, suggest the occurrence of definitely older volcanic activity more than 20Ma ago. In the southwestern part of the Japan Sea, which has a continental crust, granitic rocks of more than 100Ma together with Precambrian gneisses were recovered. Thus radiometric age data support a relatively young formation of the typical ocean floor in the Japan Sea area. 1. Introduction The Japan Sea area is regarded to be a typical example of a backarc basin. To characterize the evolution of a backarc basin, it is quite significant to study the evolution mode of the Japan Sea. Thus, a number of studies have been made on the origin and evolution of the Japan Sea (e.g., HILDE and WAGEMAN, 1973; OKAMURA, 1927). Plate tectonic models suggest that the Japan Sea was formed by an opening at the continental margin. Then problems arise when and how the Japan Sea was formed. In this respect, it is very important to identify the formation age of the sea floor of the Japan Sea. To estimate the formation age of the Japan Sea, many kinds of approaches are possible. The most direct method is to date the basement rocks of the ocean floor. The fossils in the lowest sedimentary layer of the ocean floor would also give an estimate of the age when the place became an ocean floor. Along this line, sample recovery was tried by DSDP (KARIG et al., 1975), but drilling could not reach the bottom of the sedimentary layer in the Japan Sea. The lowest sediments recovered suggest a late ocean floor. Furthermore, the age of the oldest fossils in the sedimentary layer does not always cover the whole volcanic activity which related to form the ocean floor. Hence, ocean floor drilling into the basement rocks is most desirable. However, no typical ocean floor basalts have been recovered so far in the Japan Sea and we must wait until the next drilling by the ODP. 475

2 476 I. KANEOKA The volcanic rocks recovered from the seamounts bear also significant importance, since they can be recovered by dredging and their formation ages can also contribute serious constraints on the time of the formation of the ocean floor. The ages of seamounts should not be older than that of the ocean floor, as long as we admit the formation of the seamounts occurred on the ocean floor. The igneous rocks so far dredged from the Japan Sea include basic and acidic volcanic rocks together with granitic rocks. Some of them were dated by radiometric dating methods. The obtained ages scatter very widely, ranging from a few million years to more than 2000 million years for some gneisses recovered from the Korea Plateau. If we limit the case to volcanic rocks, the oldest age is obtained for a basalt from the Yamato Bank, indicating an age of 76Ma (VASILIEV, 1975). However, such an age does not seem to correspond to any volcanic activity related to the opening of the Japan Sea. Such points will be discussed later in more detail. Other methods to estimate the age of the opening of the Japan Sea are based on more indirect information. The magnetic lineation pattern of the ocean floor does not always show a clear pattern. In the case of the Japan Sea, the areas which show clear lineation patterns are rather limited (ISEZAKI and UYEDA, 1973) and they may not always retain the whole record of the history of the evolution of the Japan Sea floor. However, they would also give some limits on the age of the time of the opening of the Japan Sea. The relationships between the age of the ocean floor and the heat flow and/or the depth of the ocean floor are also often used to estimate the formation age of the ocean floor (e.g., KONG and YOSHII, 1975; PARKER and OLDENBURG, 1973). Although they can give rough estimates on the age of the ocean floor, they include some other factors which reflect regional heterogeneity. Furthermore, based on the sedimentation rate and the thickness of the sedimentary layers in the Japan Sea, we can also get an idea about the formation age of the ocean floor. However, in this case we must assume that the sedimentation rate was kept constant during the whole period, which is not always guaranteed. On the basis of one or of combined methods above mentioned, the estimated ages on the opening of the Japan Sea range from about 10Ma to 50Ma (e.g., HILDE and WAGEMAN, 1973; OKAMURA, 1927). In recent years, however, there have appeared other approaches. The paleomagnetic results on volcanic rocks and sedimentary rocks from the southwest Japan and the northeast Japan have revealed that the declination of the geomagnetic field largely changed around 15Ma ago, which are interpreted to correspond to the time of the opening of the Japan Sea (e.g., OTOFUJI et al., 1985). This age almost corresponds to the time of the Kuroko formation (KANEOKA, 1983). Thus the age data also implies a possibility of a close relationship between the opening of the Japan Sea and the Kuroko formation. On the other hand, Green Tuff activity in the Japanese Islands is estimated to have occurred at least since early Miocene and it may also be related to the formation of the Japan Sea. Hence it is significant to examine the available data on the radiometric ages of igneous rocks recovered from the Japan Sea area. This study is aimed at pursuing it from such a viewpoint. 2. Reliability of Radiometric Ages on Submarine Rocks If a volcanic rock of which the ocean floor is composed is recovered, the

3 Constraints on the Time of the Evolution of the Japan Sea Floor 477 radiometric age would give the most direct information on the time of the ocean floor formation. In this case, the recovered samples should be guaranteed as in-situ rocks. Volcanic rocks from a seamount should also retain the criteria. Since submarine rocks are generally limited to volcanic rocks, the methods of acquiring the radiometric ages are rather limited. Among these the K-Ar and 40Ar-39Ar methods are most often used to date them (e.g., OZIMA et al., 1970). Glassy parts of pillow basalts were sometimes dated by the fission track method (e.g., FLEISCHER et al., 1968). However, it is argued that the track annealing at ambient sea-floor temperatures would cause the lowering of the obtained age and the reliable range for the determined age might be rather limited to a relatively young age of less than 1Ma (MACDOUGALL, 1976). Since the Rb-Sr method requires several phases in a rock to show different 87Sr/86Sr ratios resulted from variable Rb/Sr ratios with time, the application on submarine volcanic rocks are not easy. Except for special kinds of rocks such as a granitic rock as a remnant land mass recovered from the ocean area, the Rb-Sr method is not applied to submarine rocks in a general case. The K-Ar dating on submarine rocks also holds some serious problems. If a submarine rock is cooled rapidly in the seawater, the occurrence of excess 40Ar might be expected, resulting in an older age than the real age (e.g., DALRYMPLE and MOORS, 1968; NOBLE and NAUGHTON, 1968). A volcanic rock with a K content of Hence the glass content in a submarine rock gives us one of the criteria to judge whether the sample might be suitable for K-Ar dating or not. Although the 40Ar-39Ar method with stepwise heating can identify the occurrence of excess 40Ar in a sample from the age spectra, it can not recover a meaningful age from the samples. Hence the examination of the above mentioned point is important to evaluate an obtained K-Ar or 40Ar-39Ar age. If a sample is fresh and only susceptable to the occurrence of excess 40Ar, the obtained age indicates the older limit for its formation. Another serious problem is the alteration of a rock due to its reactions with the sea water. If a submarine rock reacts with a sea water, K-addition and/or radiogenic 40Ar loss often occurs (e.g., KANEOKA, 1971), In this case, the obtained K-Ar age becomes younger than the formation age. The alteration can be identified qualitatively by macroscopic and microscopic observations. To evaluate it in a more quantitative way, KANEOKA (1972) has tried to examine the relationship between the obtained K-Ar age and the H2O(+) content in basic volcanic rocks. An example is shown in Fig. 2, where K-Ar ages of volcanic rocks dredged from the Erimo were recovered at the time of the same dredging, we assume that they represent rocks of similar origin. The rocks with a H2O(+) content of less than 1% show K-Ar ages of about 80Ma, whereas those with larger H2O(+) content show definitely younger K-Ar ages. Paleontological evidences (TsUCHI and KAGAMI, 1967) and an isochron K-Ar age based on separated minerals (KANEOKA, 1971) suggest that the age of 80 Ma is a good estimate for the formation of these rocks. The 40Ar-39Ar method with stepwise heating would also overcome the problem

4 478 I. KANEOKA Fig. 1. Excess 40Ar vs. glass content in submarine basalts. Different symbols indicate different localities. Data sources: DALRYMPLE and MOORE, 1968; DYMOND, 1970; FUNKHOUSER et al., 1968; NOBLE and NAUGHTON, of sample alteration in a favourable case, since the higher temperature fractions still retain the in-situ radiogenic 40Ar even if the lower temperature fractions lost some part of radiogenic 40Ar (e.g., OZIMA and SAITO, 1973). However this method requires

5 Constraints on the Time of the Evolution of the Japan Sea Floor 479 the subdivision of the analysed radiogenic 40Ar in each temperature fraction and the material should contain a larger amount of radiogenic 40Ar than that required for conventional K-Ar dating. Further the correction for neutron induced interference Ar isotopes causes some ambiguities. Thus the application of the 40Ar-39Ar method on relatively young rocks with poor K contents loses its merit due to the larger increase in the uncertainty of the obtained age compared with that determined by the conventional K-Ar method (DALRYMPLE and LANPHERE, 1971). Hence, the choice of dating method is also important to obtain reliable radiometric ages. In the case of dredged rocks from the Japan Sea area, most rocks have been dated by the K-Ar method so far. At least some of them have altered to some extent. To get more reliable data, 40Ar-39Ar dating is now undertaken by us for dredged rocks recovered from seamounts in the Yamato Basin and other areas. 3. Radiometric Ages of Igneous Rocks Recovered from the Japan Sea and Their Significance The number of radiometric ages so far reported for igneous rocks recovered from the Japan Sea floor does not exceed 50 in total (Table 1). All samples were recovered by dredging. Especially, the number of radiometric ages for volcanic rocks is rather limited, estimated to be less than 20. So far, all the samples have been dated by either Japanese or USSR investigators. However, most of the igneous rocks dated by the USSR investigators are concentrated to granitic rocks and gneisses from the central to the southwestern part of the Japan Sea where a remnant land mass is assumed. In order to estimate the formation age of the typical ocean floor in the Japan Sea, we need much more radiometric ages on volcanic rocks recovered from such regions as the Yamato Basin and the Japan Basin. For this purpose, we are now undertaking K-Ar and 40Ar- 39Ar dating on some volcanic rocks recovered from the seamounts in the Yamato Basin and preliminary results are included in the present discussion. In Fig. 3, radiometric age data are shown for samples which were dredged from the northern and central part of the Japan Sea. The data whose definite localities are not identified are excluded in Fig. 3. Thus, most data obtained by the USSR investigators are not included in the figure, because the USSR data indicate only the recovered areas. Hence, the results are discussed by separating them into five groups: a) Japan Basin, b) Yamato Basin, c) Yamato Bank, d) Southwestern part, e) other regions. 3.1 Japan Basin It has been established that the Japan Basin has typical oceanic crust (e.g., LUDWIG et al., 1975) and it covers the wide area of the northern part of the Japan Sea. Hence it is very significant to know the formation age of the rocks of which the ocean floor is composed. However, because of the lack of available samples for dating, radiometric age data are so scarce. A subalkaline, high-alumina basalt dredged from the Bogorov Seamount is dated to be about 18Ma by the K-Ar method (SAHNO and VASILIEV, 1974). If this sample represents the rocks which form the seamount and the obtained age corresponds to the time of the rock formation, this age has a significant meaning. Since the Bogorov Seamount is located in the central part of the Japan Basin, the age of the seamount gives the younger limit for the age of the ocean floor. If

6 480 I. KANEOKA Table 1. Summary of radiometric ages on igneous rocks dredged from the Japan Sea, N. B. 1)Age data are tabulated as reported values except for those with asterisks which are recalculated by using the recommended values for decay constants (STEIGER and JAPER, 1977). 2)(1) SAHNO and VASILIEv (1974), (2) LELIKOV and BERSENEV (1975), (3) UENO et al. (1974), (4) Kaneoka, Takigami et al. (unpublished), (5) GNIBIDENKO (1979), (6) VASILIEv (1975), (7) SHIMAZU (1968), (8) Tamaki (personal communication), (9) Geol. Surv. Japan (unpublished), (10) YUASA et al. (1978). we take the apparent value, the Japan Basin already existed at least 18Ma ago. Furthermore, if the rock is not glassy and not completely fresh, there remains a possibility that the real formation age for this rock may be still older to some extent. Unfortunately, no detailed description on the condition of the sample is known and the data are so limited. Hence, it is premature to conclude from this sample alone. One granitic rock from the Gabass Seamount, which is located to the west of the Japan Basin, is reported to show a K-Ar age of about 110Ma (LELIKOv and BERSENEV, 1975). We do not know whether the sample represents that of the Seamount as a remnant land mass or it was transported there from somewhere else. In either case, this age would have no direct relation with that of the formation of the ocean floor of the Japan Basin.

7 Constraints on the Time of the Evolution of the Japan Sea Floor 481 Fig. 3. Radiometric ages (in Ma) on igneous rocks dredged from the Japan Sea. Onlythose samples are plotted whose dredging localities are well identified. A numerical figure with a dagger indicates a Rb-Sr age. The other ages are obtained by the K-Ar method. Those ages in parentheses are preliminary ones. All ages in this figure are recalculated by using the recommended values for decay constants (STEIGER and JAGER, 1977). Closed circle indicates a dated volcanic rock. Closed square indicates a dated granitic rock. Open circle indicates a volcanic rock to be dated. Data sources: Geol. Surv. Japan, unpublished; Kaneoka, Takigami et al., unpublished; SAHNO and VASILIEV, 1974; SHIMAZU, 1968; UENO et al., Yamato Basin This region is also regarded to have oceanic crust (e.g., LUDWIG et al., 1975) and a series of seamounts exist in a chain from the southwest to the north in the midst of the Basin. It is quite interesting to know the formation ages of these seamounts.

8 482 I. KANEOKA observation indicates that their glass contents are less than 30%. Accordingly, as described in a previous section, the possibility for the occurrence of excess 40Ar is less likely. Although they are relatively fresh among the samples dated at that time, they still show some signs of alteration to some degree. There remains a possibility that the formation age of the seamounts are older than these ages to some extent. It is worth mentioning that the K-Ar age for the Meiyo Daini Seamount is close to that of the time for the opening of the Japan Sea as suggested on the basis of paleomagnetic studies (e.g., OTOFUJI et al., 1985). Since the Matsu Seamount seems to be located at a different unit from the chain of the seamounts to which the Meiyo Daini Seamount belongs, these seamounts may represent different phases of volcanic activity in the Yamato Basin. As mentioned previously, a series of seamounts are located in a chain in the midst of the Yamato Basin. In order to reveal the evolution of the Yamato Basin, it is very significant to characterize the properties of rocks which form these seamounts, including their ages. The origin of these seamounts would probably reflect some phases of the evolution of the Yamato Basin. To clarify these points, we are investigating the volcanic rocks dredged from these seamounts during the cruise of the Hakuho-maru of the Ocean Research Institute of the University of Tokyo (KH84-3). The investigation includes petrographical studies, geochemical analyses of the major and trace elements including isotope studies and their radiometric age studies by the K-Ar and 40Ar-39Ar methods. Preliminary results of the K-Ar dating are also shown in Fig. 3. The dated rocks show K-Ar ages ranging from about 6Ma to 13Ma. Judging from their macro and microscopic observations together with their H2O(+) contents which exceed mostly 1%, we cannot neglect a possibility that radiogenic 40Ar loss might have occurred to some extent in these rocks. They are mostly andesites and some basalts are also dredged. If we take the apparently oldest K-Ar ages for these these seamounts might have been formed during a series of volcanic activities in this region. Such volcanic activities might represent the latest stage for the formation of the Yamato Basin. As long as these data are concerned, they are not incompatible with the suggestion that the opening of the Japan Sea might have occurred around 15 Ma ago. Since we are now examining their ages by the 4OAr-39Ar method, we will discuss the situation in more detail when we get the results. 3.3 Yamato Bank On the basis of the seismological work to study the structure of the Japan Sea floor (e.g., LUDWIG et al., 1975) together with some other evidence, the Yamato Bank region is regarded to be a remnant land mass when the Japan Sea was formed. In effect, many granitic rocks were dredged from this region. In this paper, the Yamato Bank includes the Yamato Bank itself in a narrow sense, the Kita-Yamato Bank and the Takuyo Bank. Although only two examples are shown for granitic rocks in Fig. 3,

9 Constraints on the Time of the Evolution of the Japan Sea Floor 483 the number of the dated granitic rocks from this region exceeds 10 samples. All these rocks show K-Ar ages of more than 100Ma up to about 300Ma (LELIKOV and BERSENEV, 1975; UEN0 et al., 1974; VASILIEV, 1975). Hence, it is almost certain that the rocks represent some parts of the continental crust which are relatively old. On the other hand, some volcanic rocks were dredged, whose K-Ar ages range from about 20Ma to 76Ma (GNIBIDENKO, 1979; UENO et al., 1974; VASILIEV, 1975). They include both basalt and andesite. It seems that the volcanic activity which produced these rocks might have already occurred before the opening of the Japan Sea. Hence, the volcanic activity observed in this region might have different characteristics from those of the seamounts in the Japan Basin and the Yamato Basin. This point should be clarified further in a future study. 3.4 Southwestern part Seismologically, this region is regarded to have a continental crust structure (e.g., LUDWIG et al., 1975). All dated rocks dredged from this region include gneisses from the Korea Plateau and granitoids from the Ullung Plateau (LELIKOV and BERSENEV, 1975). Some granites were dredged from the slope off Primore. K-Ar ages of gneisses dredged from the Korea Plateau show typical Precambrian ages ranging from about 2000Ma to about 2700Ma. Such old ages are observed in the northern part of the Korea Peninsula. Although K-Ar ages of granites recovered from the slope off Primore range from 60Ma to 90Ma, two granites dredged from the Ullung Plateau show K-Ar ages of about 100Ma (LELIKOV and BERSENEV, 1975). All these rocks suggest that they formed some portions of the old remnant land mass and the situation is quite compatible with the inference of the structure of the ocean floor in this region. 3.5 Other regions One granitic rock dredged from the Kita Oki Bank shows a K-Ar age of about 142Ma (Geol. Surv. Japan, unpublished), which suggests that the Kita Oki Bank would be a remnant land mass (Fig. 3). indicates a K-Ar age of about 7.9Ma. However, this sample is altered to some extent and the obtained age should represent the younger limit for the formation age of the rock. As shown in Fig. 3, the dredged place seems to be located at the base of the Japanese Islands. Hence, this sample might represent a volcanic activity which related to that in the Japanese Islands. Furthermore, from the western cliff of the Oshima Plateau, which is located to the west of Hokkaido, some welded tuffs were dredged during the cruise of Hakuhomaru (KH84-3) (Fig. 3). As preliminary results, they show K-Ar and 40Ar-39Ar ages related to form the basement of the Japanese Islands and would not have direct relationship with the formation of the ocean floor. One welded tuff recovered from the Musashi Bank was dated to be about 78Ma by the K-Ar method (YUAsA et al., 1978). This rock has also similar meaning as those recovered from the western cliff of the Oshima Plateau. Hence, some rocks which were dredged from the fringing area of the Japanese Islands seem to be related to the volcanic activities of the Japanese Islands, but might not be related to the formation of the ocean floor of the Japan Sea.

10 484 I. KANEOKA 4. Summary As long as radiometric ages on volcanic rocks dredged from the oceanic region of the Japan Sea are concerned, the apparent K-Ar ages show relatively young ages of less than 20 Ma. Especially those from the Yamato Basin show the ages ranging from about 4Ma to 14Ma. A series of seamounts in a chain in the midst of the Yamato represent the latest volcanic activity which might form the ocean floor of the Yamato Basin, the apparent K-Ar ages are not incompatible with the inference that the opening of the Japan Sea might have occurred around 15Ma ago (OTOFUJI et al., 1985). If the K-Ar age (18Ma) of the rock of the Bogorov Seamount represents the younger limit for the formation of the Japan Basin, it may have earlier evolution compared with that of the Yamato Basin. There is a possibility that some volcanic rocks dredged from the Yamato Bank region may have different characteristics from those recovered from the Japan Basin and the Yamato Basin. Although many granitic rocks were dredged from the western part of the Japan Sea, the youngest K-Ar age (60Ma) may suggest the older limit for the formation age of the Japan Sea, if the K-Ar age represents the formation of the rock. However volcanic rocks recovered from the fringing region of the Japanese Islands would give us no direct information on the evolution of the Japan Sea. More radiometric age data are necessary to infer the evolution of the Japan Sea precisely. The author thanks Drs. Y. Takigami and K. Tamaki for their information on some radiometric ages. He is also grateful to Dr. M. Torii and two anonymous reviewers for their constructive comments on the manuscript. REFERENCES DALRYMPLE, G. B. and J. G. MOORE, Argon-40: excess in submarine pillow basalts from Kilauea Volcano, Hawaii, Science, 161, , DALRYMPLE, G. B. and M. A. LANPHERE, 40Ar/39Ar technique of K-Ar dating: a comparison with the conventional technique, Earth Planet. Sci. Lett., 12, , DYMOND, J., Excess argon in submarine basalt pillows, Geol. Soc. Am. Bull., 81, , FLEISCHER, R. L., J. R. M. VIERTL, P. B. PRICE, and F. AUMENTO, Mid-Atlantic Ridge, age and spreading rates, Science, 161, , FUNKHOUSER, J. G., D. E. FISHER, and E. BONATTI, Excess argon in deep sea rocks, Earth Planet. Sci. Lett., 5, , GNIBIDENKO, H., The tectonics of the Japan Sea, Mar. Geol., 32, 71-87, HILDE, T. W. C. and J. W. WAGEMAN, Structure and origin of the Japan Sea, in The Western Pacific: Island Arc, Marginal Seas, Geochemistry, edited by P. J. Coleman, pp , W. Australian Univ. Press., INGLE, J. C., Jr., Pleistocene and Pliocene foraminifera from the Sea of Japan, Leg. 31, Deep Sea Drilling Project, in Initial Report of the Deep Sea Drilling Project, edited by D. E. Karig, J. C. Ingle Jr. et al., Vol. 31, pp , U. S. Government Printing Office, Washington, D. C., ISEZAKI, N. and S. UYEDA, Geomagnetic anomaly pattern of the Japan Sea, Marine Geophys. Res., 2, 51-59, 1973.

11 Constraints on the Time of the Evolution of the Japan Sea Floor 485 KANEOKA, I., K-Ar ages of seamounts along the Japan Trench and the effect of acid leaching on the K-Ar age of a dredged submarine rock, Geochem. J., 5, , KANEOKA, I., The effect of hydration on the K/ Ar ages of volcanic rocks, Earth Planet. Sci. Len., 14, , KANEOKA, I., On the radiometric ages of volcanic rocks from the northeastern part of the Honshu Island, Japan, Mining Geology, Spec. Issue, 11, 69-78, 1983 (in Japanese). KARIG, D. E., J. C. INGLE Jr. et al. (eds.), Initial Report of the Deep Sea Drilling Project, Vol. 31, 927 pp., U. S. Government Printing Office, Washington, D. C., KoNo, Y. and T. YOSHII, Numerical experiments on the thickening plate model, J. Phys. Earth, 23, 63-75, LELIKOV, E. P. and I. I. BERSENEV, Early Proterozoic gneissmigmatite complex of the Japan Sea, south-western part, Proc. Acad. Sci. U. S. S. R., 223, , 1975 (in Russian). LUDWIG, W. J., S. MURAUCHI, and R. E. HoUTZ, Sediments and structure of the Japan Sea, Geol. Soc. Am. Bull., 86, , MACDOUGALL, J. D., Fission track annealing and correction procedures for oceanic basalt glasses, Earth Planet. Sci. Lett., 30, 19-26, NOBLE, C. S. and J. J. NAUGHTON, Deep-ocean basalts: inert gas content and uncertainties in age dating, Science, 162, , OKAMURA, K., On the nature of the marine algae of Japan and the origin of the Japan Sea, Bet. Mag., 41, OTOFUJI, Y., A. HAYASHIDA, and M. TORn, When was the Japan Sea opened?: Paleomagnetic evidence for Southwest Japan, in Formation of Active Ocean Margins, edited by N. Nasu, S. Uyeda, I. Kushiro, K. Kobayashi, and H. Kagami, pp , Terrapub, Tokyo, OZIMA, M. and K. SAITO, 40Ar-39Ar stepwise degassing experiments on some submarine rocks, Earth Planet. Sci. Lett., 20, 77-87, OZIMA, M., I. KANEOKA, and S. ARAMAKI, K-Ar ages of submarine basalts dredged from seamounts in the Western Pacific area and discussion of oceanic crust, Earth Planet. Sci. Lett., 8, , PARKER, R. L. and D. W. OLDENBURG, Thermal model of oceanic ridges, Nature Phys. Sci., 242, , SAHNO, V. G. and B. J. VASILIEV, Basaltoids of the Japan Sea bottom, in Problems of Geology and Geophysics of the Marginal Seas of the NW Pacific, edited by N. P. Vasilkovsky and B. Ya. Karp, pp , Far East Sci. Center Publisher, Vladivostok, 1974 (in Russian). SHIMAZU, M., On the absolute age of a granite dredged from the Yamato Bank, Circular "NIHONKAI" (Japan Sea), No. 2, 55-56, 1968 (in Japanese). STEIGER, R. H. and E. JAGER, Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology, Earth Planet. Sci. Lett., 36, , TSUCHI, R. and H. KAGAMI, Discovery of Nerineid Gastropoda from seamounts Erimo Kuril-Kamchatka Trenches, Res. Ocean. Works Japan, 9, 1-5, UENO, N., I. KANEOKA, and M. OZIMA, Isotopic ages and strontium isotopic ratios of submarine rocks in the Japan Sea, Geochem. J., 8, , VASILIEV, B. I., New data on age and mechanism of formation of marginal-sea basins and deep-sea trenches in the northwestern Pacific, Doklady Akad. Nauk SSSR, 225, 60-62, 1975 (translated in English from Russian). YUASA, M., K. TAMAKI, K. NISHIMURA, and E. HoNZA, Welded tuff dredged from Musashi Bank, northern Japan Sea and its K-Ar age, J. Geol. Soc. Japan, 84, , 1978.