Redox front formation in an uplifting sedimentary rock sequence: An analogue for redox-controlling processes in the geosphere around deep geological repositories for radioactive waste     

H. Yoshida  R. Metcalfe  K. Yamamoto  Y. Murakami  D. Hoshii  A. Kanekiyo  T. Naganuma  T. Hayashi 《Applied Geochemistry》2008,23(8):2364-2381

Subsurface redox fronts control the mobilization and fixation of many trace elements, including potential pollutants such as certain radionuclides. Any safety assessment for a deep geological repository for radioactive wastes needs to take into account adequately the long-term redox processes in the geosphere surrounding the repository. To build confidence in understanding these processes, a redox front in a reduced siliceous sedimentary rock distributed in an uplifting area in Japan has been studied in detail. Geochemical analyses show increased concentrations of Fe and trace elements, including rare earth elements (REEs), at the redox front, even though concentrations of reduced rock matrix constituents show little change. Detailed SEM observations revealed that fossilized microorganisms composed of amorphous granules made exclusively of Fe and Si occur in the rock’s pore space. Microbial 16S rDNA analysis suggests that there is presently a zonation of different bacterial groups within the redox band, and bacterial zonation played an important role in the concentration of Fe-oxyhydroxides at the redox front. These water–rock–microbe interactions can be considered analogous to the processes occurring in the redox fronts that would develop around geological repositories for radioactive waste. Once formed, the Fe-oxyhydroxides within such a front would be preserved even after reducing conditions resume following repository closure.  相似文献   

Sequential processes in a landslide hazard at a slate quarry in Okayama,Japan   总被引：1，自引：0，他引：1  

Hiroshi Suwa  T. Mizuno  S. Suzuki  Y. Yamamoto  K. Ito 《Natural Hazards》2008,45(2):321-331

The 12 March 2001 landslide at a slate quarry in Okayama, Japan killed three workers. Composite studies based on field surveys of the landslide slope, interviews with local residents and quarry workers, and inspections of hydrological and seismological data have been used to clarify the causes of this slide and its movements. The results indicate that the landslide was enabled firstly by the steepness of the slope, which had been undercut by river; secondly, the structure was that of a dip-slope that was prone to deep-seated slides along bedding planes; thirdly, numerous joints and faults were present. Surprisingly, rainfall, earthquakes, and explosions do not appear to have played any role in the triggering of this slide. The interviews demonstrated that the frequency of precursory failures increased over a period of several hours before the 12 March 2001 landslide. Inspection of the seismograph records and the eyewitness evidence both indicate that the main part of the landslide consisted of two phases of slope failure within 23 s. After the slide, the frequency of the failures gradually decreased with time over a period of several days. Three new terms are proposed for landslides: foreslide, mainslide, and afterslide, following the terms foreshock, main shock, and aftershock used in seismology.  相似文献   

Factors influencing maintenance and decline of a diatom bloom in the Yatsushiro Sea,Japan     

Goh Onitsuka  Tomoyuki Shikata  Saho Kitatsuji  Kazuo Abe  Takeshi Yamamoto  Hiroki Ochiai  Hitoshi Matsuo 《Journal of Oceanography》2016,72(4):617-627

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Crustal structure and tectonics of the Hidaka Collision Zone, Hokkaido (Japan), revealed by vibroseis seismic reflection and gravity surveys     

Kazunori Arita  Takashi Ikawa  Tanio Ito  Akihiko Yamamoto  Matsuhiko Saito  Yasunori Nishida  Hideyuki Satoh  Gaku Kimura  Teruo Watanabe  Takeshi Ikawa  Toru Kuroda 《Tectonophysics》1998,290(3-4):197-210

This study is the first integrated geological and geophysical investigation of the Hidaka Collision Zone in southern Central Hokkaido, Japan, which shows complex collision tectonics with a westward vergence. The Hidaka Collision Zone consists of the Idon'nappu Belt (IB), the Poroshiri Ophiolite Belt (POB) and the Hidaka Metamorphic Belt (HMB) with the Hidaka Belt from west to east. The POB (metamorphosed ophiolites) is overthrust by the HMB (steeply eastward-dipping palaeo-arc crust) along the Hidaka Main Thrust (HMT), and in turn, thrusts over the Idon'nappu Belt (melanges) along the Hidaka Western Thrust (HWT). Seismic reflection and gravity surveys along a 20-km-long traverse across the southern Hidaka Mountains revealed hitherto unknown crustal structures of the collision zone such as listric thrusts, back thrusts, frontal thrust-and-fold structures, and duplex structures. The main findings are as follows. (1) The HMT, which dips steeply at the surface, is a listric fault dipping gently at a depth of 7 km beneath the eastern end of the HMB, and cutting across the lithological boundaries and schistosity of the Hidaka metamorphic rocks. (2) A second reflector is detected 1 km below the HMT reflector. The intervening part between these two reflectors is inferred to be the POB, which is only little exposed at the surface. This inference is supported by the high positive Bouguer anomalies along the Hidaka Mountains. (3) The shallow portion of the IB at the front of the collision zone has a number of NNE-dipping reflectors, indicative of imbricated fold-and-thrust structures. (4) Subhorizontal reflectors at a depth of 14 km are recognized intermittently at both sides of the seismic profile. These reflectors may correspond to the velocity boundary (5.9–6.6 km/s) previously obtained from seismic refraction profiling in the northern Hidaka Mountains. (5) These crustal structures as well as the back thrust found in the eastern end of the traverse represent characteristics of collisional tectonics resulting from the two collisional events since the Early Tertiary.  相似文献   

Spessartine Garnets in a Manganiferous Carbonate Formation from Nsuta, Ghana     

Frank K. Nyame  Katsuo Kase  Masahiro Yamamoto 《Resource Geology》1998,48(1):13-22

Abstract: Manganese carbonate ore beds and host rock manganiferous phyllites at the Nsuta mine, western Ghana, contain well developed garnet crystals. Individual crystals are idioblastic, sometimes porphyroblastic, and homogeneous, and are associated with rhodochrosite (with or without kutnahorite), quartz and muscovite. The conspicuous absence of chlorite in garnet-rich assemblages, and of garnet in chlorite-rich rocks, suggest chemical constraints may have been important in the formation of the two minerals. Gondite bands within carbonate ores are interpreted to have resulted from localised processes in which manganese carbonates, in environments rich in alumino-silicate minerals, may have been completely exhausted during metamorphic reactions.  相似文献   

The relationships between drastic changes in Sr isotope ratios of magma sources beneath the NE Japan arc and the spreading of the Japan sea back-arc basin   总被引：1，自引：0，他引：1  

Dr. K. Shuto  Dr. J. Ohki  Dr. H. Kagami  Dr. M. Yamamoto  Dr. N. Watanabe  Dr. K. Yamamoto  Dr. N. Anzai  Dr. T. Itaya 《Mineralogy and Petrology》1993,49(1-2):71-90

Summary Based on Sr isotopic data for Tertiary and Quaternary basaltic rocks from the NE Japan arc, relationships are discussed between the temporal variation of magma source characteristics and the opening of the Japan Sea. The basaltic rocks from the trench side and from the transitional zone show initial⁸⁷Sr/⁸⁶Sr ratios (Sr_i ratios) in the range of 0.70411–0.70546 but no temporal variation in Sr_i ratios. The back-arc side basaltic rocks with ages of 29.8 to 15 Ma have Sr_i ratios similar to those of the trench side and the transitional zone, and these values also show no temporal change. In contrast, the basaltic rocks from the back-arc side, with ages younger than 15 Ma, show significantly lower Sri ratios (0.70396 to 0.70290), which are slightly higher than those of N-type MORB. These Sr isotopic features may imply that at least before 15 Ma the magma source regions (the sub-continental mantle) beneath the NE Japan arc had an enriched chemical character and that after 15 Ma, the magma sources for volcanic rocks from the back-arc side show a drastic change in Sr isotopic character, from an enriched nature to a depleted one. The depleted magmas may have been formed as a result of injection of depleted asthenosphere (or of a depleted mantle diapir) into the subcontinental mantle under the back-arc side of the NE Japan arc, during the spreading of the Japan Sea back-arc basin. The middle Miocene basaltic rocks from the back-arc side are characterized by lower contents of LIL elements such as K₂O and Rb compared with those from the trench side, suggesting that during the middle Miocene (syn-opening stage of the Japan Sea) the degree of partial melting may have been higher in the back-arc side mantle than in the trench side mantle. High degree of partial melting in the back-arc side mantle can be attributed to an increasing geothermal gradient in the mantle due to the injection of hot asthenosphere. This injection might also have caused the melting of the lower crust from which the voluminous middle Miocene acidic volcanics in the back-arc side and transitional zone may have been produced.

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Drastische Veränderungen der Sr-Isotopenverhältnisse von Magmenquellen unterhalb des NE japanischen Vulkanbogens und ihre Beziehung zum Spreading des japanischen Back-Arc-Beckens

Zusammenfassung Die Beziehungen zwischen der zeitlichen Veränderung der Charakteristika von Magmenquellen und der Öffnung des Japanischen Meeres werden anhand von Sr-Isotopendaten tertiärer und quartärer basaltischer Gesteine diskutiert. Basaltische Gesteine von der Grabenseite und aus der Übergangszone ergaben initiale⁸⁷Sr/⁸⁶Sr Verhältnisse (Sr_i-Verhältnisse) von 0.70411–0.70546 und lassen keine zeitabhängige Änderung erkennen. Basaltische Besteine aus dem Back-Arc-Bereich mit Altern zwischen 29.8 und ca. 15 Ma zeigen ähnliche Sr_i-Verhältnisse und ebenfalls keine zeitliche Veränderung. Im Gegensatz dazu sind basaltische Gesteine aus dem Back-Arc-Bereich, die jünger als ca. 15 Ma sind, signifikant in ihren Sr_i-Verhältnissen (0.70396–0.70290) erniedrigt. Diese Verhältnisse liegen etwas höher als die von N-MORB. Die Sr-Isotopenergebnisse lassen vermuten, daß zumindest vor ca. 15 Ma der Herkunftsbereich der Magmen (subkontinentaler Mantel) unter dem NE japanischen Vulkanbogen chemisch angereichert war, während die Magmenquellen der jüngeren vulkanischen Gesteine des Back-Arc-Bereiches durch eine drastische Abreicherung charakterisiert sind. Die abgereicherten Magmen könnten, während der Öffnung des japanischen Back-Arc-Beckens, als Folge der Injektion abgereicherter Asthenosphäre (oder eines abgereicherten Manteldiapirs) in subkontinentalen Mantel unterhalb des Back-Arcs des NE japanischen Vulkanbogens, gebildet worden sein. Die miozänen basaltischen Gesteine des Back-Arc-Bereiches sind außerdem durch niedrigere Gehalte an LIL-Elementen, wie z.B. K₂O und Rb charakterisiert. Dies wird als Hinweis auf eine erhöhte Aufschmelzungsrate in diesem Bereich im mittleren Miozän (im Zuge der gleichzeitigen Öffnung des japanischen Meeres) verstanden. Die erhöhte Aufschmelzrate im Mantel des Back-Arc-Breiches wird auf einer Erhöhung des geothermischen Gradienten infolge der Injektion von heißer Asthenosphäre zurückgeführt. Diese Injektion von Asthenosphäre könnte auch der Grund für die Aufschmelzung von Unterkruste und für die Produktion weitverbreiteter saurer miozäner Vulkanite im Back-Arc-Bereich und der Übergangszone sein.

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With 6 Figures  相似文献   

Palaeointensity study of the Hawaiian 1960 lava: implications for possible causes of erroneously high intensities     

Y. Yamamoto  H. Tsunakawa  H. Shibuya 《Geophysical Journal International》2003,153(1):263-276

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Study on wall-type gravel drains as liquefaction countermeasure for underground structures   总被引：1，自引：0，他引：1  

R. P. Orense  I. Morimoto  Y. Yamamoto  T. Yumiyama  H. Yamamoto  K. Sugawara 《Soil Dynamics and Earthquake Engineering》2003,23(1):114

One of the most dramatic causes of damage to engineering structures during earthquakes has been the development of soil liquefaction beneath and around the structures. In order to dissipate the excess pore water pressures near structures, gravel drains are usually employed. In this study, the use of recycled concrete crushed stones as gravel drain materials is addressed. In order to investigate the performance of wall-type gravel drains, two series of shaking table tests were performed. The test results showed that gravel drains, when appropriate grain size distribution is considered, effectively dissipate the excess pore water pressure underneath the structure, and consequently reduce the magnitude of uplift. To supplement the laboratory tests, finite element analyses were also performed. For specified structure, ground and earthquake conditions, there is a critical width of gravel drain at which no uplift of structure will occur. The results of the model tests and the finite element analyses were then employed in developing design charts for determining the critical width of gravel drain to prevent buoyant rise of structure when the surrounding soil mass liquefies.  相似文献   

The 1989 submarine eruption off eastern Izu Peninsula,Japan: ejecta and eruption mechanisms     

Takahiro Yamamoto  Tatsunori Soya  Shigeru Suto  Kozo Uto  Akira Takada  Keiichi Sakaguchi  Koji Ono 《Bulletin of Volcanology》1991,53(4):301-308

The submarine eruption of a new small knoll, which was named Teishi knoll, off eastern Izu Peninsula behind the Izu-Mariana arc occurred in the evening of 13 July 1989. This is the first historic eruption of the Higashi-Izu monogenetic volcano group. The eruption of 13 July followed an earthquake swarm near Ito city starting on 30 June. There were subsequent volcanic tremors on 11 and 12 July, and the formation of the Teishi knoll on the 100 m deep insular shelf 4 km northeast of Ito city. There were five submarine explosions, which were characterized by intermittent domelike bulges of water and black tephra-jets, which occurred within 10 min on 13 July. Ejecta of the eruption was small in volume and composed of highly crystalline basalt scoria, highly vesiculated pumice, and lithic material. Petrographical features suggest that the pumice was produced by vesiculation of reheated wet felsic tuff of an older formation. The Teishi knoll, before the eruption, was a circular dome, 450 m across and 25 m high, with steep sides and a flat summit. Considerations of submarine topographic change indicate the knoll was raised by sill-like intrusion of 10⁶ m³ of magma beneath a 30 m thick sediment blanket. This shallow intrusion is assumed to have started on 11 July when volcanic tremors were observed for the first time, but there was no indications of violent interaction between wet host sediments and intruding magma. The submarine eruption of 13 July appears to have been Friggered by a major lowering of the magma-column. The basalt scoria, having crystal-contents of more than 60%, is assumed to be derived from the cooled plastic margin of the shallow intrusive body. However, glassy scoria, which would indicate the interaction between hot fluidal magma and external water, was not observed. A scenario for the 1989 submarine eruption is as follows. When rapid subsidence of the hot interior of the intrusive magma occurred, reduced pressure caused the implosion of cooled plastic magma, adjacent pressurized, hot host material, and wet sediment. The mixing of these materials triggered the vigorous vapor explosions.  相似文献   

Processes causing the chlorophylla maximum in the tidal front in Iyo-nada,japan     

Hidetaka Takeoka  Osamu Matsuda  Tamiji Yamamoto 《Journal of Oceanography》1993,49(1):57-70

A field survey was undertaken to reveal the structure of the tidal front in Iyo-nada. An obvious tidal front was found between the mixed region around Hayasui Straits and the stratified region in Iyo-Nada. Its structure was typical in the eastern part and was influenced by the river discharge in the western part. An intense chlorophylla maximum was found in the subsurface layer of the eastern typical front. Analysis using TS diagram suggested that, around the eastern front, there was an intrusion from the mixed water to the middle layer of the stratified water. This intrusion was supposed to be an important process supplying nutrients from the mixed water to the subsurface of the frontal region and causing the intense chlorophylla maximum.  相似文献