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Volume 108, Issue 10
Displaying 1-7 of 7 articles from this issue
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Teruki Oikawa2002 Volume 108 Issue 10 Pages 615-632
Published: October 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSGeology, volcanic history and eruptive style of the Yakedake Volcano Group in the southern part of the Hida Mountain Range, were clarified using volcano-stratigraphy, K-Ar dating and 14C dating. The Yakedake Volcano Group consists of six volcanoes Odana, Iwatsuboyama, Warudaniyama, Shiratani-yama, Akandana, and Yakedake volcanoes. These volcanoes are grouped into the older volcanoes (120 to 70ka; Warudaniyama, Iwatsuboyama, and Odana) and the younger ones (26 ka to present; Yakedake, Akandana, and Shirataniyama). The youngest magmatic products are the Yakedake Dome Lava and the simultaneous Nakao pyroclastic flow deposit of the Yakedake Volcano at 2.3 ka (cal BP). All of these volcanoes are composed of lava and/or block-and-ash flow of andesite-dacite (SiO2 59-65 wt.%) with varying phenocrystal compositions. There is no evidence of explosive products such as pumice or scoria fall ever erupting throughout the history of these volcanoes. The long-term eruption rate of the Younger Yakedake Volcano Group is calculated to be 0.1-0.35km3/ky, and this rate is within the range of estimated average eruption rates of the late Quaternary volcanoes in Japan. But the volume of the Yakedake Volcano Group is still as small as about 8km3, smaller than the average volume of the late Quaternary volcanoes in Japan. Because the activity period and the total volume of the volcano group is short and small, it can be said that will continue in the future, taking the fact into consideration that the recent eruption rate has not lowered. Eruption style of these volcanoes in the near future is expected to be non-explosive, such as lava dome effusion and accompanied block-and-ash flow similar to that which has occurred in the past.View full abstractDownload PDF (2488K) -
Moriaki Yasuhara, Toshiaki Irizuki, Shusaku Yoshikawa, Futoshi Nanayam ...2002 Volume 108 Issue 10 Pages 633-643
Published: October 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSSeventy two species groups of Ostracoda were identified from 33 samples obtained from the drilling core 'A', excavated in the southern Osaka Plain, southwest Japan. Q-mode cluster analysis of ostracode faunas in this core reveals six biofacies (PS, SBm, LS, PL, CL and LC). This study provided ostracode evidence revealing temporal variation in depositional environment and relative sea level throughout the Holocene. In the southern Osaka Plain, the sea level rose rapidly from the period between ca. 9, 000 and ca. 6, 000 cal yr BP. The maximum sea-level highstand was at ca. 6, 000-5, 600 cal yr BP, after which the sea level has gradually fallen to its present one. This trend in sea-level change is well correlated to those of central Osaka Plain and Osaka Bay, suggesting that the relative sea-level changes were widespread throughout the Osaka sedimentary basin. Changes of salinity, distance from shoreline and influence by coastal current and waves are well correlated with the observed relative sea-level changes.View full abstractDownload PDF (1328K) -
Yasuo Kugimiya, Akira Takasu2002 Volume 108 Issue 10 Pages 644-662
Published: October 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSA tectonic melange zone containing serpentinites and edo-gite-bearing tectonic blocks (masses) of diverse size and lithology is developed within the high-grade zone of the Sambagawa metamorphic belt in central Shikoku, southwest Japan. Precise geological mapping of the eclogite-bearing tectonic blocks in the melange zone is required to reveal the tectonics of the deeper parts of the Sambagawa accretionary prism. However, such studies have not been published to date because of difficulties in fieldwork. We carried out precise geologic mapping of the Western Iratsu mass as a tectonic block, and of the surrounding metamorphic rocks which comprise the melange zone. The Western Iratsu mass consists of seven lithologies peridotite, clinopyroxene-hornblendite, garnet-epidote-amphibolite, garnet-white mica-amphibolite, marble, epidote-quartz rock and albite-white mica-quartz schist, in ascending structural order. The protoliths of these rock types are considered to be ultramafic rock, gabbro, basic volcaniclastic rock, limestone, siliceous sediment with intercalated limestone, and pelitic rock, respectively. The Western Iratsu mass contains abundant marbles, rather than the metacherts which occur in surrounding Sambagawa schists. Based on reconstruction of the succession of the original lithologies of the Western Iratsu mass, the protolith is considered to have been a topographic high on an oceanic plate such as a seamount, oceanic plateau or island arc with capping reef limestones.View full abstractDownload PDF (2944K) -
Megumi Ichise, Hitoshi Tanaka, Tsutomu Takahashi, Takami Miyamoto, Yos ...2002 Volume 108 Issue 10 Pages 663-670
Published: October 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Lower Cretaceous strata exposed in the Sanchu Graben in the Kanto Mountains have been correlated with the Monobe-gawa Group in Shikoku on the basis of fossil evidence. It is well-known that they yield the bivalve assemblage which is called the Northern-Tethyan fauna. Recently, we obtained the abundant bivalve fossils from Nakanosawa and Hinatazawa at the eastern part of the Sanchu graben. These assemblages bear a close resemblance to the Tethyan fauna in Albian Age. On the other hand, they show little affinity to the Northern-tethyan fauna which was previously reported from the Sanchu Graben. Those lines of fossil evidence suggest that the Cretaceous System of the Sanchu Graben yields not only the Northern-tethyan fauna but also the Tethyan fauna in Albian Age. In addition, Inoceramus cf. hobetsensis was reported from Numa-sato located to southeast of Hinatazawa. The strata yielding Inoceramus hobetsensis (Turonian species) probably overlie those yielding the Albian Tethyan fauna.View full abstractDownload PDF (1164K) -
Akira Ishiwatari, Mie Imasaka2002 Volume 108 Issue 10 Pages 671-684
Published: October 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Yoka Formation in the central part of the Tango Peninsula consists of early Miocene lavas and pyroclastic rocks of basaltic and andesitic composition, which extruded during the Japan Sea opening. These volcanic rocks are grouped into five types on the basis of phenocryst assemblages; Type 1 is olivine (Ol)-clinopyroxene (Cpx)-plagioclase (Pl), Type 2 is Ol- orthopyro-xene (Opx)-Pl, Type 3 is Ol-Pl, Type 4 is only Pl (large phenocrysts), and Type 5 is Cpx-Opx-Pl±Ol. The bulk-rock FeO*/MgO (*total iron as FeO) increases from Type 1 to Type 4 with almost constant SiO2, indicating a tholeiitic fractionation trend, al-though Type 5 shows relatively high SiO2. All these rocks are characterized by enrichment of large ion lithophile elements (LILEs) and light rare earth elements (LREEs), high Zr and Zr/Y, and depletion of Nb, indicating continental island-arc basalt signature. The newly found picritic basalt is classified into Type 1, and has more MgO (12 wt.%), Ni (300 ppm) and Cr (1100 ppm) than other Type 1 basalts. It is depleted in HFSEs but is slightly enriched in LILEs compared to other Type 1 basalts, al-though they are chemically very close (e.g. equal in SiO2). It also shows the same mineral chemistry as other Type 1 basalts except for its chromian spinel, which shows distinctly higher Cr/(Cr±Al) (0.64-0.84) than that of other Type 1 basalts (0.49-0.59). These facts suggest that the picritic basalt was formed through a higher degree of partial melting under slightly hydrous condition from almost the same source mantle as for other Type 1 basalts. A closely resembling picrite has been reported from the Okinawa Trough area, where active back-arc rifting is taking place, suggesting upheaval of the high-temperature mantle in the early stage of the Japan Sea opening.View full abstractDownload PDF (1842K) -
Shunichi Suzuki, Yoshitaka Sekine2002 Volume 108 Issue 10 Pages 685-688
Published: October 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSDownload PDF (670K) -
Ritsuo Nomura, Hiroshi Nishi, Leg 199 Scientific Party2002 Volume 108 Issue 10 Pages XVII-XVIII
Published: 2002
Released on J-STAGE: December 14, 2010
JOURNAL FREE ACCESS
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