Diachronous evolution of systems tracts in a depositional sequence from the middle Pleistocene palaeo-Tokyo Bay, Japan     

MAKOTO ITO  SAKAE O'HARA 《Sedimentology》1994,41(4):677-697

Palaeo-Tokyo Bay is a relic of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. The sedimentary infill of palaeo-Tokyo Bay is characterized by shallow marine to paralic sediments of the middle to upper Pleistocene Shimosa Group. Sequence stratigraphical analysis has been used to describe spatial and temporal variations in the depositional systems of the lowest units of the Shimosa Group, deposited during the early stage of development of palaeo-Tokyo Bay. Three different type of depositional systems were recognized: sand ridge to shelf (SRS), shelf to delta (SDL) and shelf to non-deltaic nearshore (SNS) systems. They overlie early transgressive estuarine deposits infilling lowstand valleys incised in the south-eastern margin of palaeo-Tokyo Bay. These systems were developed during late transgressive through highstand stages of a relative sea level cycle, which may have been controlled by a glacio-eustatic sea level change at about 0·4 Ma. Spatial variation in depositional systems is largely identical to that in modern Tokyo Bay; environmental conditions similar to those prevailing at the present day probably characterized the early history of palaeo-Tokyo Bay. The timing of highstand systems tracts within a high frequency depositional sequence was analysed in terms of the effect of sedimentation rate, based on the mapping of a chronostratigraphical surface marked by the Hy4 volcanic ash layer. From spatial variations in sedimentation rate, it was possible to identify the diachronous evolution of highstand systems tracts from the SDL system, through the SNS system, to the SRS system. Time lag is indicated by major bounding surfaces, such as maximum flooding or downlap surfaces associated with a condensed section, which developed immediately above or below the Hy4 volcanic ash layer. The lag may be of the order of a few thousands to tens of thousands of years within a depositional sequence with a total of duration of about 100 000 years.  相似文献   

K–Ar ages from seamount chains in the back-arc region of the Izu–Ogasawara arc     

OSAMU  ISHIZUKA  KOZO  UTO  MAKOTO  YUASA & ALFREDG.  HOCHSTAEDTER 《Island Arc》1998,7(3):408-421

The Izu–Ogasawara arc contains, from east to west, a volcanic front, a back-arc extensional zone (back-arc knolls zone), and a series of across-arc seamount chains that cross the extensional zone in an east-northeast and west-southwest direction and extend into the Shikoku Basin. K–Ar ages of dredged volcanic rocks from these across-arc seamount chains and extension-related edifices in the back-arc region of the Izu–Ogasawara arc were measured to constrain the volcanic and tectonic history of the arc since the termination of spreading in the Shikoku Basin. K–Ar ages range between 12.5 and 1 Ma. Andesitic to dacitic rocks of 12.5–2.9 Ma occur mainly on the western part of the chains. The western part of the chains are the locus of volcanism behind the front which erupted mainly calc-alkaline andesitic lavas. The youngest rocks (< 2.8 Ma), characterized by cpx-ol basalt, occur along the western margin of the back-arc knolls zone. Basaltic rocks of 12.5–2.9 Ma have relatively high concentrations of Na₂O (> 2.0 wt%), Zr (> 50 p.p.m.) and Y (> 20 p.p.m.) and low CaO (< 12 wt%). On the other hand, basalts of 2.8–1 Ma have lower Na₂O (< 1.8 wt%), Zr (< 50 p.p.m.) and Y (< 20 p.p.m.), but significantly higher CaO (> 12 wt%). The age inferred for the initiation of back-arc rifting (∼ 2.35–2.9 Ma: Taylor 1992 ) behind the current volcanic arc coincides with the time that basalt chemistry changed drastically (eruption of the low-Na₂O and high-CaO basalt). This implies that post-2.8 Ma volcanism in the back-arc knolls zone is associated with rifting. Similarly, the change in chemical composition might be explained by a different type of source mantle following rift initiation. Volcanism in the western seamounts ceased after the onset of rifting at ∼ 2.8 Ma.  相似文献   

Petrogenesis of the Tanzawa plutonic complex, central Japan: Exposed felsic middle crust of the Izu–Bonin – Mariana arc     

SHINICHI  KAWATE & MAKOTO  ARIMA 《Island Arc》1998,7(3):342-358

The Miocene Tanzawa plutonic complex, consisting mainly of tonalite intrusions, is exposed at the northern end of the Izu–Bonin – Mariana (IBM) arc system as a consequence of collision with the Honshu Arc. The Tanzawa plutonic rocks belong to the calc-alkaline series and exhibit a wide range of chemical variation, from 43 to 75 wt% SiO₂. They are characterized by relatively high Ba/Rb and Ce/Nb ratios, and low abundances of K₂O, LIL elements, and rare earth elements (REE). Their petrographic and geochemical features indicate derivation from an intermediate parental magma through crystal fractionation and accumulation processes, involving hornblende, plagioclase, and magnetite. The Tanzawa plutonic complex is interpreted to be the exposed middle crust of the IBM arc, which was uplifted during the collision. The mass balance calculations, combining data from melting experiments of hydrous basaltic compositions at lower-to-middle crustal levels, suggest that parental magma and ultramafic restite were generated by dehydration partial melting (∼ 45% melting) of amphibolite chemically similar to low-K tholeiitic basalt. Partial melting of hydrated mafic lower crust might play an important role in felsic middle-crust formation in the IBM arc.  相似文献   

Possible Miocene rifting of the Izu–Ogasawara (Bonin) arc deduced from magnetic anomalies     

TOSHITSUGU  YAMAZAKI & MAKOTO  YUASA 《Island Arc》1998,7(3):374-382

A magnetic anomaly map of the northern part of the Philippine Sea plate shows two conspicuous north–south rows of long-wavelength anomalies over the Izu–Ogasawara (Bonin) arc, which are slightly oblique to the present volcanic front. These anomalies are enhanced on reduced-to-pole and upward-continued anomaly maps. The east row is associated with frontal arc highs (the Shinkurose Ridge), and the west row is accompanied by the Nishi-Shichito Ridge. Another belt of long-wavelength anomalies very similar to the former two occurs over the Kyushu–Palau Ridge. To explain the similarity of the magnetic anomalies, it is proposed that after the spreading of the Shikoku Basin separated the Izu–Ogasawara arc from the Kyushu–Palau Ridge, another rifting event occurred in the Miocene, which divided the Izu–Ogasawara arc into the Nishi-Shichito and Shinkurose ridges. The occurrence of Miocene rifting has also been suggested from the geology of the collision zone of the Izu–Ogasawara arc against the Southwest Japan arc: the Misaka terrain yields peculiar volcanic rocks suggesting back-arc rifting at ～ 15 Ma. The magnetic anomaly belts over the Izu–Ogasawara arc do not extend south beyond the Sofugan Tectonic Line, suggesting a difference in tectonic history between the northern and southern parts of the Izu–Ogasawara arc. It is estimated that the Miocene extension was directed northeast–southwest, utilizing normal faults originally formed during Oligocene rifting. The direction is close to the final stage of the Shikoku Basin spreading. On a gravity anomaly relief map, northeast–southwest lineaments can be recognized in the Shikoku Basin as well as over the Nishi-Shichito Ridge. We thus consider that lines of structural weakness connected transform faults of the Shikoku Basin spreading system and the transfer faults of the Miocene Izu–Ogasawara arc rifting. Volcanism on the Nishi-Shichito Ridge has continued along the lines of weakness, which could have caused the en echelon arrangement of the volcanoes.  相似文献   

Nanometer‐scale anatomy of entire Stardust tracks     

Keiko NAKAMURA‐MESSENGER  Lindsay P. KELLER  Simon J. CLEMETT  Scott MESSENGER  Motoo ITO 《Meteoritics & planetary science》2011,46(7):1033-1051

Abstract– We have developed new sample preparation and analytical techniques tailored for entire aerogel tracks of Wild 2 sample analyses both on “carrot” and “bulbous” tracks. We have successfully ultramicrotomed an entire track along its axis while preserving its original shape. This innovation allowed us to examine the distribution of fragments along the entire track from the entrance hole all the way to the terminal particle. The crystalline silicates we measured have Mg‐rich compositions and O isotopic compositions in the range of meteoritic materials, implying that they originated in the inner solar system. The terminal particle of the carrot track is a ¹⁶O‐rich forsteritic grain that may have formed in a similar environment as Ca‐, Al‐rich inclusions and amoeboid olivine aggregates in primitive carbonaceous chondrites. The track also contains submicron‐sized diamond grains likely formed in the solar system. Complex aromatic hydrocarbons distributed along aerogel tracks and in terminal particles. These organics are likely cometary but affected by shock heating.  相似文献   

Techniques and methods of arsenic contaminated groundwater exploration in the Hetao plain of Inner Mongolia,China          下载免费PDF全文

Toshiaki ITO  Fumio KANAI  Kazuro BANDO  Research group for arsenic contamination of groundwater in Inner Mongoli  Asia Arsenic Network  Research Group for Applied Geology 《中国地质》2010,37(3):730-740

1990年报道了在中国内蒙古河套平原由于饮用污染的地下水而患砷中毒的病例.估计目前内蒙受到砷中毒影响的居民已超过41万人.从1997年至2004年,在长达8年的时间里,我们进行了水文地质调查、地质调查和医学调查,并装置了管道输送的供水系统.这些调查揭示了河套平原地下水受砷污染的机理:由于被吸收的砷释放进入地下水,蓄水层中不断加强的还原环境造成铁氢氧化物的溶解.为了防止砷中毒病人数量的进一步增加,停止饮用污染过的水和供应安全的饮用水是重要的.我们需要根据当地的地质和水文地质条件来了解砷污染的机理.  相似文献   

A global-scale simulation of the CO₂ exchange between the atmosphere and the terrestrial biosphere with a mechanistic model including stable carbon isotopes, 1953–1999     

AKIHIKO ITO 《Tellus. Series B, Chemical and physical meteorology》2003,55(2):596-612

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Osumilite-bearing Granulites in the Eastern Grenville Province, Eastern Labrador, Canada: Mineral Parageneses and Metamorphic Conditions   总被引：1，自引：1，他引：0  

ARIMA  MAKOTO; GOWER  CHARLES F. 《Journal of Petrology》1991,32(1):29-61

Osumilite-bearing metasedimentary gneiss occurs in the contactaureole of the Sand Hill Big Pond gabbronorite complex of Labradorianage in eastern Grenville Province, eastern Labrador, Canada.The osumilite (Os) occurs in association with cordierite (Cd),orthopyroxene (Opx), sillimanite (Sil), sapphirine (Sa), spinel(Sp), K-feldspar (Kf), plagioclase (PI), phlogopite (Ph), hematite(Hm),magnetite (Mt), corundum (Co), and quartz (Q) in zones adjacentto the gabbronorite intrusion. The osumilite-in isogard is delineatedat a distance of 1-3km from the intrusive contact. The osumiliteis characterized by enhanced Mg/(Mg + Fe) ratios (0.89-0.92)and relatively high K₂O contents (4. 13-4.78 wt. % ) The compositionalvariation of the osumilite is best understood by the substitutions(Mg+Fe+Mn)+St=2A1 and (K+Na)+(Mg +Fe+Mn)=Al+vacancy. Symplectiticintergrowth of Opx-Cd-Kf-Q, which partly of completely replacedosumilite, is ubiquitously present. The stable osumilite-bearingassemblages (all with Hm, Mt, and P1) deduced from petrographicfeatures and from the phase relations in the KMAS system areOs-Sa-Cd, Os-Cd-Sil-Q, Os-Cd-Kf-Q, Os-Cd-Opx-Q, and Os-Opx-Kf-Q.The saphirine-bearing assemblages are restricted to silica-deficient(quartz-absent) zones of the gnesis, which include Sa-Os-Cd,Sa-Sil-Cd, Sa-Opx-Sp, Sa-Sp-Cd, and Sa-Sp-Co. Orthopyroxenecoronas mantling phologite reflect the breakdown reaction Ph+ Q= Opx+Kf+vapor under extremely low water activity in thevapour. Petrogenetic grids in MAS, KMAS, and KMAS-H₂O-CO₂appropriateto the mineral assemblages in the metasedimentary gneisses suggestthat the changes of the mineral assemblages in the area studiedreflect dehydration reactions Ph+Sil+Q=Cd+Kf+V and Ph+Q=Opx+Kf+V, and osumilite and sapphirine-forming reactions Opx+Cd+Kf+Q=Os,Cd+Kf+Q=Os+Sil+, Opx+Sil+Kf=Os+Sa, and Opx+Sil =Sa+ Cd. Relativelyhigh Mg/(Mg+Fe²⁺) (0. 64-0.88) in the whole-rock compositionand high oxygen fugacity (near hematite-magnetite buffer), togetherwith exceptionally high temperature ( 1000?C) and extremelylow water activity (0.2) at peak meta-morphic conditions mightstabilize the osumilite- and sapphirine-bearing assemblagesat middle or lower crustal levels. Relatively low water activityis probably caused by the relatively dry precursor, which hadbeen metamorphosed at upper amphibolite facies metamorphic conditionsbefore gabbronorite emplacement.  相似文献   

人工冻结法协同竖井淋洗原位修复污染黏性土的可行性试验     

陈雄志  毋文涛  张军  芮大虎  李国玉  伊藤譲 《冰川冻土》2021,43(5):1489-1497

针对黏性土淋洗效率低下问题,提出了人工冻结法协同竖井淋洗原位修复污染土壤的方法.通过垂直布设冻结板,水平冻结土体的竖井淋洗模型试验,研究冻结过程中土中温度场、水分场分布及融解期的排水方式,探讨了人工冻结法协同竖井淋洗原位修复污染黏性土的可行性.研究表明:利用人工冻结法,使未冻土侧水分或淋洗液在"冻吸力"作用下向冻结锋面迁移,然后利用塑料排水板完成融化水(淋出液)与土壤分离的方案可行,能够解决抽液过程中抽液井和注液井之间土体容易形成渗流通道出现优先流,导致淋洗效率低下的问题.在分步冻结模式下,未冻土侧水分在"冻吸力"作用下向冻土侧迁移,冻结-吸水量为22.78 L,融化-排水量为24.60 L;经历一次冻融循环后,相同冻结模式下冻结-吸水量达到30.40 L,在负压抽吸模式下排水量达到44.21 L.该研究结果为今后原位修复污染黏性土提供了新的思路.  相似文献   

Application of Image Analysis to Observe Microstructure in Sandstone and Granite   总被引：1，自引：0，他引：1  

Youqing CHEN  Takashi NISHIYAMA  Toshihide ITO 《Resource Geology》2001,51(3):249-258

Abstract: Advanced techniques are examined to observe microstructure of rocks using image analysis combined with methods such as the fluorescent approach and the application of optical characteristics of minerals. Analyzed are discrimination of grains in rocks, distribution patterns of grain orientation in sandstone, changes of grain shape as weathering advances and distribution patterns of microcracks in granite. In Shirahama sandstone, relatively large and flat grains are orientated parallel to the bedding on the plane perpendicular to the bedding, while grains on the plane parallel to the bedding show random patterns. In weathered granite, it is clarified that the grain surface becomes complex as weathering advances and differences among three major mineral species are identified. In Inada granite, intracrystalline cracks predominate over intercrystalline cracks and grain boundary cracks both in total length and number. Furthermore, three types of microcracks show different orientations; the intercrystalline cracks show a dominant orientation which coincides with the orientation of the rift plane, the easiest plane to split, while the intracrystalline cracks and grain boundary cracks show no preferred orientation.  相似文献