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Volume 118, Issue 8
Displaying 1-6 of 6 articles from this issue
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Articles
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Mitsuo Manaka, Keisuke Fukushi, Yukari Miyashita, Jun'ichi Itoh, Yoshi ...2012 Volume 118 Issue 8 Pages 459-475
Published: August 15, 2012
Released on J-STAGE: December 19, 2012
JOURNAL FREE ACCESSWe examined and compared mineralogical and geochemical properties of fault gouges in the aftershock and non-aftershock areas of the 2000 Tottori-ken Seibu earthquake, to establish a new method for evaluating the activity of low-activity faults. The gouges were examined using X-ray powder diffraction analysis, sequential selective extraction tests, and color measurements. Results show that the aftershock-area gouge is mainly composed of illite and chlorite, while the non-aftershock-area gouge is mainly composed of halloysite. Iron in the aftershock-area gouge is mainly contained in illite, while in the non-aftershock-area gouge, it is mainly in the form of amorphous and crystalline iron oxide. Results of color measurements (L∗a∗b∗ color space) show that differences in L∗ values represent differences in the presence/absence of illite and halloysite, negative a∗ values from the aftershock-area gouge represent the presence of chlorite, and positive a∗ values from the non-aftershock-area gouge represent the presence of crystalline iron oxide. These results indicate that mineralogical and geochemical characteristics can clearly distinguish fault gouges in aftershock and non-aftershock areas of the 2000 Tottori-ken Seibu earthquake, and that color measurements can be an effective and simple proxy method for the screening of gouges that require further analysis in the field.View full abstractDownload PDF (6008K) -
Katsura Ishida, Kazuhiro Yoshida, Atsushi Matsuoka2012 Volume 118 Issue 8 Pages 476-492
Published: August 15, 2012
Released on J-STAGE: December 19, 2012
JOURNAL FREE ACCESSEvidence of surface water freshening during the late Quaternary glacial period has been recognized in deep-sea cores from the Sea of Japan. However, no evidence of seawater freshening in shallow-water environments during the glacial period has been found in the shallow marine strata in the Sea of Japan and along its coasts. In this study, changes in fossil ostracod taxa from the upper Sawane Formation in Niigata Prefecture, central Japan were investigated to clarify environmental changes during the Middle Pleistocene. Based on the fossil ostracod taxa present and the proportions of each taxon, we estimate that relative sea level changed between the lower sublittoral and upper bathyal zones at water depth of 150-200 m. Taxa that favored colder temperatures and lower salinities were abundant during low-stand period; in contrast, despite the high sea level only one warm-water ostracod was found in the study sections. This suggests that the upper Sawane Formation underwent environmental changes between the interstadial and glacial periods. Low-salinity ostracod taxa were scarce or absent in Pliocene and lower Pleistocene strata along the coasts of the Sea of Japan, and were absent during the high-stand period in the study sections. The ostracod taxa from low-salinity environments in the study sections differ from those dominant in regional low-salinity environments, such as estuaries and river mouths, in the modern Sea of Japan. These low-salinity taxa and other ostracod species that survived the surface water freshening event in the Sea of Japan during the late Quaternary now inhabit similar temperature and salinity environments. These observations provide evidence for a desalination of surface waters in the Sea of Japan in the Middle Pleistocene, in contrast to the restriction of low-salinity water to regional shallow environments that occurred during the glacial period.View full abstractDownload PDF (1971K) -
Jun Hosoi, Kazuo Amano2012 Volume 118 Issue 8 Pages 493-498
Published: August 15, 2012
Released on J-STAGE: December 19, 2012
JOURNAL FREE ACCESSAnalysis of giant (>1 m in diameter) pumices is useful for reconstructing submarine volcanism, as the types of giant pumice that can form during submarine eruptions depend on the style of the volcanic eruption, as well as the eruption environment. We here present the first discovery of giant pumices in the Green Tuff of the Ou Backbone Range in northeast Japan. Such pumices are characterized by elongated vesicles, which are a typical feature of woody pumice, and which indicate eruption on the sea floor at depths of >1,000 m. Thus, the woody pumices in the Green Tuff provide information about the environment of submarine volcanism at the time of eruption of the tuff. The woody pumices and Kuroko deposits in the study area are very similar to those found in modern back-arc rift zones, such as in the back-arc rift zones of the Okinawa and Izu-Ogasawara arcs. Therefore, we infer that the tectonics and volcanism of the Green Tuff in the study area resemble those of modern back-arc rift zones.View full abstractDownload PDF (705K) -
Kuniaki Nishiki, Kou Takahashi2012 Volume 118 Issue 8 Pages 499-515
Published: August 15, 2012
Released on J-STAGE: December 19, 2012
JOURNAL FREE ACCESSThe Yabashira volcanoes, located in the Suwa–Yatsugatake Volcanic Province, Central Japan, consist of five volcanic bodies (Tokin-no-iwa, Ryugamine, Yabashirayama, Kengamine, and Byobuiwa) aligned WNW–ESE, that have produced a total eruption volume of ca. 172 km3. Based on detailed volcanological, chronological, and petrological studies, we reconstructed the geological history of the Yabashira volcanoes as follows. Volcanic activity started at ca. 1.2 Ma, forming the Tokin-no-iwa volcanic body, which is a large volcanic body with relatively gentle surface slopes, consisting mainly of thin flows of basaltic and basaltic andesitic lava, and pyroclastic rocks. At ca. 1.1 Ma, volcanic activity (basalt and basaltic andesite, with minor andesite) extended to the ESE, resulting in the formation of two large volcanic bodies (Yabashirayama and Kengamine) with relatively gentle surface slopes. Thick andesitic lavas effused from the western slope of the Tokin-no-iwa volcanic body to form the Ryugamine volcanic body. Activity ceased at ca. 0.95 Ma at the Tokin-no-iwa and Ryugamine volcanic bodies, and at ca. 0.9 Ma at the Yabashirayama volcanic body. Thick andesitic to dacitic lavas effused from the southeastern slope of the Tokin-no-iwa volcanic body at ca. 0.9 Ma, forming the Byobuiwa volcanic body. Overall, activity at the Yabashira volcanoes ceased at ca. 0.8 Ma. Geochemical data indicates that basaltic to basaltic–andesitic activity was dominant at the Yabashira volcanoes. Whole-rock geochemical data indicate that the dominant magma was not derived from the fractional crystallization of a single parental magma. We infer that the geochemical variation of the magma erupted at the Yabashira volcanoes was generated by various processes, including varying degrees of partial melting and crustal assimilation.View full abstractDownload PDF (3140K) -
Masayuki Ehiro, Osamu Nishikawa, Isao Nishikawa2012 Volume 118 Issue 8 Pages 516-520
Published: August 15, 2012
Released on J-STAGE: December 19, 2012
JOURNAL FREE ACCESSTwo Late Carboniferous (late Bashkirian–Moscovian) ammonoids, Diaboloceras sp. and Pseudoparalegoceratidae gen. et sp. indet., are described from the Kawai Limestone of the Akiyoshi Belt, eastern Hiroshima Prefecture. This is the first record of Carboniferous ammonoids from the central part of the Akiyoshi Belt. These ammonoids and previously reported foraminiferans and brachiopods show that the age of the Kawai Limestone ranges from Late Carboniferous to Middle Permian. Fossil data indicate that the Kawai Limestone consists of three repetitions of an Upper Carboniferous to Middle Permian stratigraphic sequence. The Carboniferous–Permian limestones, sporadically exposed as inliers, along the southern margin of the Akiyoshi Belt in Hiroshima Prefecture are possibly constituents of a large seamount-reef complex.View full abstractDownload PDF (617K)
Report
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Kenji Kashiwagi, Yoshitaka Senokuchi, Yuji Abe, Katsuji Yoshida2012 Volume 118 Issue 8 Pages 521-526
Published: August 15, 2012
Released on J-STAGE: December 19, 2012
JOURNAL FREE ACCESSThis paper describes Saru-ana Cave, located along Kurobe Gorge in the Kanetsuri area, eastern Toyama Prefecture, central Japan. Saru-ana Cave (total passage length, ca. 120 m; relative height, ca. 40 m) opens onto a steeply inclined cliff face, 75 m above the bed of the Kurobe River. The cave consists of horizontal to inclined rooms and vertical shafts. Speleothems in the cave, although not abundant, include cave corals, flowstones, soda straws, stalactites, and a stalagmite. Fossil specimens of six Japanese Macaque individuals (Macaca fuscata) were collected from the cave for paleontological investigation.View full abstractDownload PDF (6990K)
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