地学雑誌
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120 巻 , 6 号
選択された号の論文の18件中1~18を表示しています
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論説
  • 金光 玄樹, 下條 将徳, 平田 岳史, 横山 隆臣, 大藤 茂
    120 巻 (2011) 6 号 p. 889-909
    公開日: 2012/03/05
    ジャーナル フリー
     We measured the LA-ICP-MS U-Pb age distribution of detrital zircons in three psammitic schist samples of Hitachi and Nishidohira medium P/T metamorphic rocks from the southern part of Abukuma Belt, Northeast Japan. It has been proposed that these medium P/T metamorphic rocks mark the eastern extension of the Triassic collisional suture between the North China and South China blocks. Therefore, we aim to obtain the age of sedimentation, stratigraphy, and provenance of their protolith from the measurements, and evaluate the above proposition. The psammitic schist sample (DIO-9) of Hitachi Metamorphic Rocks, originating from quartzose sandstone at the lowest part of the Daioin Formation, contains detrital zircons of the youngest age clustered around 410 Ma and the youngest zircon at 395 ± 20 Ma (206Pb/238U age; 2σ). Considering that the upper part of the Daioin Formation contains Visean (Lower Carboniferous) corals and that the formation intercalates abundant felsic tuff layers, the lowest part of the Daioin Formation is likely to be correlated with Devonian Nakasato or Lower Carboniferous Hikoroichi Formation of South Kitakami Belt, Northeast Japan. Nishidohira Metamorphic Rocks lie beneath ultramafic rocks along the base of Hitachi Metamorphic Rocks, and consist of mafic, siliceous, calcareous, pelitic, and psammitic schists or gneisses. Because the siliceous schist of Nishidohira Metamorphic Rocks is meta-pelagic chert, the metamorphic rocks presumably originated from an accretionary complex. The ages of detrital zircons in two psammitic schist samples (ND-12 and -13) of Nishidohira Metamorphic Rocks mostly fall between ca. 300 Ma and 200 Ma, with the youngest two zircons at 154 ± 6 Ma (ND-12) and 175 ± 3 Ma (ND-13) (206Pb/238U age, 2σ). The protolith age of the psammitic schists must be ca. 154 ± 6 Ma (Kimmeridgian of Late Jurassic) and ca. 175 ± 3 Ma (Aalenian of Middle Jurassic) or younger, suggesting that Nishidohira Metamorphic Rocks originated from a Jurassic accretionary complex. In the Hitachi area three tectonostratigraphic units superpose, i.e., in ascending order, (1) the Jurassic accretionary complex of Nishidohira Metamorphic Rocks, (2) ultramafic rocks, and (3) Hitachi Metamorphic Rocks that are at least partly correlated with the Paleozoic sequence of South Kitakami Belt. The tectonostratigraphy is similar to that of non-metamorphic rocks of Kitakami Mountains, Northeast Japan, where (1) the Jurassic accretionary complex of North Kitakami Belt is overlain by (2) the Hayachine mafic–ultramafic complex, which in turn is overlain by (3) the Paleo–Mesozoic succession of South Kitakami Belt.
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  • 楮原 京子, 小坂 英輝, 三輪 敦志, 今泉 俊文, 儘田 豊
    120 巻 (2011) 6 号 p. 910-925
    公開日: 2012/03/05
    ジャーナル フリー
     The Kitakami Lowland fault zone (KLFZ) is an active reverse fault zone that extends about 70 km eastward along the Quaternary volcanic front of the northeast Japan arc. The activity of this fault zone in the Quaternary is mostly recognized as a fault reactivation of Miocene normal faults in the area. We conducted a high-resolution seismic reflection profile survey, Nanshozan 09, to define the geological structure of the Nanshozan fault group, northern portion of the KLFZ. The Nanshozan fault group is composed by three reverse faults, F1, F2, and F3 faults, and deforms late Quaternary fluvial terraces and alluvial fans in the Kitakami Lowland. Several isolated andesite hills such as Jonaisan and Kitayachi-yama, line the eastern edge of the hanging-wall of the F3 fault. The high-resolution seismic reflection profile shows that the fault scarps and tilted fluvial terraces are traced down to a depth of 2 km along the Nanshozan fault group, formed by an west-dipping high-angle reverse fault (F1 fault) and along two of the thrusts that have a flat-ramp geometry (F2 and F3 faults). Furthermore, the results of K-Ar dating and field observations show that the Jonaisan hill is certainly composed of Pliocene andesitic rocks, not Quaternary intrusives. Cross-section restoring across the Nanshozan fault group implies that F2 fault activity occurred after the initiation of F3 fault activity. Hence, isolated hills near F3 fault, such as Jonaisan and Kitayachi-yama, are recognized as volcanic blocks that survived long periods of erosion while being deformed by F3 fault thrusting.
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  • 松山 洋
    120 巻 (2011) 6 号 p. 926-943
    公開日: 2012/03/05
    ジャーナル フリー
     Taroshi-taki, a waterfall located in a mountainous region at Hanamaki City, Iwate Prefecture, sometimes freezes in winter. The circumference of Taroshi-taki has been measured in mid-February every year since 1975. In this region, it is said that in a year when Taroshi-taki freezes (does not freeze) with a larger circumference, a good (bad) harvest can be expected; it will be a hot (cool) summer. This study investigated whether a hot (cool) summer follows a cold (warm) winter in relation to the seasonal march of a temperature anomaly. Because the temperature anomaly must change its sign between winter and summer, we clarified when and why this occurs through a composite analysis in the northern hemisphere, mainly using NCEP/NCAR reanalysis data. Data for 1975-2001 were analyzed because data on energy transport and its budget described below were available only for that period.
     During 1975-2001, Taroshi-taki froze (did not freeze) in 17 (10) years. We prepared the composite temperature anomaly for these 17 (10) years from the long-term mean (1971-2000) each month, and investigated its seasonal march. In these 17 (10) years, a negative (positive) temperature anomaly covering all of Japan in winter became positive (negative) in April; the temperature anomaly in winter disappeared in April in each case. Analyses of energy transport and its budget in the atmosphere (apparent heat source and moisture sink) revealed that, in April, when Taroshi-taki froze, the tropical Pacific released more energy to the west than the long-term mean. In April, when Taroshi-taki did not freeze, the spatial pattern of energy transport and its budget were almost opposite.
     In years when Taroshi-taki froze, the negative temperature anomaly in winter from eastern Asia to the North Pacific expanded the north-south temperature gradient (enhancement of baroclinity) including this area. During the subsequent April, this caused more atmospheric energy to be transported from the tropical Pacific to the middle latitudes, including Japan, compared to the long-term mean. This made the negative temperature anomaly in winter around Japan positive in April, which was maintained throughout the warm period. It is possible that the freezing of Taroshi-taki in winter represents negative feedback of north-south energy transport from the cold season to the warm season, which constitutes an important cause-and-effect sequence in the climate system.
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  • 小暮 哲也, 北村 圭吾, 山田 達也, 西澤 修, 薛 自求
    120 巻 (2011) 6 号 p. 944-959
    公開日: 2012/03/05
    ジャーナル フリー
     Relative permeability curves of water and supercritical CO2 (scCO2) were obtained for two porous sandstones under steady-state flow conditions. The present measurements provide accurate relative permeability values for water and scCO2, which is important to better understand the behavior of CO2 injected into the deep saline aquifer when carrying out carbon dioxide capture and storage (CCS).
     We have developed a new method for measuring the volume of scCO2 and water under the same pressure and temperature conditions as in the porous rock employing a high-pressure water-CO2 separator. Water saturation in the sandstone was determined with a mass balance analysis measuring changes of the surface boundary between water and scCO2. We measured Berea sandstone and Tako sandstone, which were cored and shaped into a cylinder form. The mixed fluid of water and scCO2 was injected into a rock sample that was previously saturated with CO2-saturated water. The flow volumes of the water and scCO2 were measured for different saturation conditions by changing the mixing ratio of the fluids at the sample inlet. All measurements were made at 40°C and 10 MPa, including those for the water-CO2 separator. The injection flow rate was 0.5 ml/min. By plotting the ratios of water and scCO2 of the inflow and outflow with respect to elapsed time, we can determine whether the flow is steady or unsteady.
     The relative permeability curve is obtained by plotting the relative permeability values with respect to the degree of water saturation. The results for Berea sandstone suggest that the relationship between relative permeability and the relative saturation of water and scCO2 correspond well to those of the previous study. However, the results for Tako sandstone deviate from the predicted curve of the previous study. This is likely to be due to the heterogeneity of Tako sandstone.
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  • 椚座 圭太郎, 丸山 茂徳
    120 巻 (2011) 6 号 p. 960-980
    公開日: 2012/03/05
    ジャーナル フリー
     The geotectonic evolution of the 450-270 Ma old serpentinite mélange of the Itoigawa–Omi area of the Hida marginal belt has been reexamined from the viewpoint of reconstructing the oldest Pacific–type orogenic belt of Japan from a recently collected data set.
     Initiation of subduction to form a juvenile island arc followed by the accretionary complex of Southwest Japan took place at ca. 520 Ma as indicated by U–Pb zircon ages from metasomatic jadeitite formed by subduction zone fluids. Also, the 520 Ma zircons are indentified from volcaniclastic sediments of the eastern Hida marginal belt. Continued subduction proceeded with serpentinization and amphibolitization of the newly formed mantle wedge peridotite and crust, respectively, and high–P/T type metamorphism reaching eclogite facies older than ca. 380 Ma, as indicated by the EC unit of the Itoigawa–Omi area. This prolonged subduction zone metamorphism since the Cambrian formed essential members of the Oheyama–Renge belt of the Pacific–type orogenic belt of Japan, leading to tectonic erosion by subducting the oceanic plate, thereby the position of the subduction zone remained unchanged.
     The ca. 300 Ma Renge schists of the greenschist through epidote–amphibolite facies to eclogite facies of the Itoigawa–Omi area resulted from recrystallization probably by a subducting oceanic ridge. This ridge subduction caused exhumation of the Oheyama–Renge belt as a serpentinite belt including various hanging wall rocks with pre–existing subduction zone metamorphic rocks at a later stage. Remobilization of serpentinite mélange of the Hida marginal belt continued to act as a lubricant zone at the past Benioff thrust after the Permian, and defined the structure of the Hida marginal belt up to the present.
     The Itoigawa–Omi area is a type locality of the oldest Pacific–type orogenic belt of Japan. The Hida marginal belt is thus redefined as a composite tectonic belt including the Oheyama–Renge, and Akiyoshi and Maizuru belts, along with Paleozoic fore–arc basin deposits such as the Hitoegane formation of ca. 500 Ma. Note that the Unazuki metamorphic rocks are not members of the Hida marginal belt developed at the periphery of South China plate, but the Hida metamorphic belt as a collision zone between the North and South China plates.
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短報
  • 小山 拓志
    120 巻 (2011) 6 号 p. 981-992
    公開日: 2012/03/05
    ジャーナル フリー
     Turf scarps and patch-shaped barelands are clearly observed on windy alpine ridges and wind-swept slopes of Japanese mountains. This study investigates the formation and retreat of turf scarps at the Daisyoujidaira site (2700 m a.s.l.) and Maru-yama site (3025 m a.s.l.), Southern Japanese Alps.
     The average distances of scarp retreat are 2.2 cm/2 years (Daisyoujidaira site: 2006-2008), 0.5 cm/year (Maru-yama site: 2006-2007), and 3.6 cm/year (Maru-yama site: 2007-2008).
     At the Daisyoujidaira site, generation of turf scarps started on the walls of trails or gullies. The retreat processes of turf scarps are considered to be a combination of frost-thaw action (needle ice creep), deflation, rain-splash erosion, and slope wash. However, at the Daisyoujidaira site, turf scarps with curtain-like exposure of plant roots are protected from deflation and rain-splash erosion during summer.
     On the other hand, at the Maru-yama site, eave collapsing and turf plucking occur frequently, and turf scarps continue to retreat. As a result, patch-shaped barelands continue to expand on the wind-swept slope of Maru-yama.
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資料
  • 藁谷 哲也, 梶山 貴弘
    120 巻 (2011) 6 号 p. 993-1002
    公開日: 2012/03/05
    ジャーナル フリー
     At Atabad, northern Pakistan, steep valley slopes on a right side of the Hunza River collapsed on January 4, 2010 (Atabad landslide) and a river blockade occurred. There have been catastrophic landslides with 107-108 m3 scales as well as river blockades in 1858 and 1962 in the surrounding region, indicating that the region frequently experiences landslides. The Atabad landslide has an area of 1,500 m long by 1,000 m wide, and originates from the main scarp located at approximately 3,200 m a.s.l. The released debris, estimated to be ca. 4.5 × 107 m3 in volume, accumulated in a mound and mud flow deposit, which spread over an area of approximately 1,500 m long by 500 m wide.
     The water level of the landslide-dammed lake (Atabad lake) formed by the river blockade rose rapidly to approximately 30 m within ten days from the collapse and reached a maximum water depth of 120 m in July. On July 7, 2010, the lake reached a maximum length of 21 km and a maximum area of 12 million m2, with a volume of storage of 440 × 106 m3, as revealed by monitoring lake growth using multi-temporal satellite imagery. It is considered that water inflow from melting glaciers with rising air temperature caused the expansion of the lake area and the rise of lake level. The excavation of a spillway decreased the risk of outburst flooding and extreme rise of water level since July when the river discharge was at the maximum.
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報告
  • 末岡 茂, Barry P. KOHN, 池田 安隆, 狩野 謙一, 堤 浩之, 田上 高広
    120 巻 (2011) 6 号 p. 1003-1012
    公開日: 2012/03/05
    ジャーナル フリー
     The applicability of low-temperature thermochronology has been extended considerably over the past decade and the successful application of (U-Th)/He (He) thermochronometry is one of the most noteworthy advances. In this study, a zircon He analysis is used to identify the denudational history and pattern of the Akaishi Range, which has been uplifted since the late Pliocene. Zircon He grain ages from nine samples range from 21.5 to 3.0 Ma, while the ages are systematically younger to the east. These ages are interpreted to reflect the uplifting of the Akaishi Range because the youngest ages are consistent with the age at which uplifting was initiated according to the depositional ages of gravel. The decreasing ages to the east can be explained by subsequent denudation of the uplifted Akaishi Range, assuming a westerly tilting uplift of the region west of the Itoigawa-Shizuoka Tectonic Line (ISTL). Although denudation cannot be identified exactly because of a lack of precise estimates of the paleo-geothermal gradient of the study area, it is certain that the entire area between the Median Tectonic Line and ISTL has been denuded by a few kilometers since the onset of the range uplift. This implies that the topography of the Akaishi Range reflects post-uplift factors, e.g., spatial distribution of bedrock uplift rates and various denudation processes, rather than inherited geometry from the pre-uplift topography.
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東京地学協会第14回海外巡検報告
  • 大山 正雄
    120 巻 (2011) 6 号 p. 1013-1025
    公開日: 2012/03/05
    ジャーナル フリー
     Iceland is the northernmost country of the world. It is a volcanic island located immediately south of the Arctic Circle. Currently, about 11% of the island is covered with glacial ice. The Mid-Atlantic Ridge, which forms the spreading boundary between the Eurasian and the North American tectonic plates, passes through the middle of the island. It may be said that the natural phenomena in Iceland are formed by three elements: volcanoes, glaciers, and the plate boundary. We visited this geographically fascinating land in August 16-27, 2010.
     The volcanos and thermal springs are very active along the plate boundary. Most of the lavas are basalts. The thermal water is generally alkaline. The main rivers are braided meltwater rivers flowing from glacial areas. Because the meltwater rivers contain suspended sediments in abundance, the water is gray. A subglacial eruption caused a colossal flood and formed a vast outwash plain. Thermal water is used by geothermal power plants as well as heating systems because even the summers are cold. During this trip, we also observed fractures of the plate boundary, known as “gja” in Iceland.
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世界露頭百選(No. 4-5)
  • 石川 晃
    120 巻 (2011) 6 号 p. 1026-1034
    公開日: 2012/03/05
    ジャーナル フリー
     西太平洋に現存するオントンジャワ海台は,白亜紀に生じた地球史上最大規模の火成活動の産物であるが,その成因についてはいまだ不明な点が多い。海台縁辺部が水上に露出しているソロモン諸島・マライタ島には,海台形成後に生じたキンバーライト類似のマグマ活動により,海台直下に由来する多様なマントル捕獲岩が表層にもたらされており,大規模融解現象の発生機構やその起源物質を解明するための有用な情報源となる。地質温度圧力計の適用によると,各種捕獲岩の由来深度はモホ面直下(約60km)からリソスフェア最下部(約120km)に至ることがわかる。また由来深度に対応して,卓越する岩相や化学組成が系統的に変化する観察事実から,海台下リソスフェアの構造としてジュラ紀の海洋プレート直下に海台玄武岩を抽出した融け残りマントルが底付けされている様子が復元される。このことは,マライタ島に産する一部の捕獲岩はかつて地球深部に存在しており,温度・組成異常をあわせもつ「マントルプルーム」の構成物質として地表近傍へ上昇してきたことを示唆している。
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  • 丸山 茂徳, 宇都宮 敦, 石川 晃
    120 巻 (2011) 6 号 p. 1035-1044
    公開日: 2012/03/05
    ジャーナル フリー
     オントンジャワ海台は太平洋中央部の太平洋スーパープルームによって白亜紀(122 Maと90 Ma)につくられた巨大な海台である。形成後,西方に移動し,オーストラリアプレートが北上することによってプレート相対運動が変化して,20-25 Maごろから海溝に沈み込みはじめた。その結果,ソロモン諸島には,それらの海台の破片が付加体として露出している。付加しはじめた時期については異論があり,これまでの多くの研究者は2 Ma 以降であると考えてきた。
     しかしながら,われわれは地質調査と文献に基づいた証拠,すなわち一連の逆断層の形成と構造浸食,および20-25 Ma以降の沈み込み帯火山活動の停止から以下の解釈を行った。海台が25-20 Maに海溝に沈み込みを開始すると,海台のその浮力のために、沈み込み角度が低角度になり島弧火山活動が停止した。そして始新世以降の付加体を構造浸食して,崩壊物をマントルへと運んだ。また中新世におけるオントンジャワ海台の衝突と沈み込みが背後のウッドラーク海盆の拡大の開始のトリガーになったと思われる。
     過去25-20 Maにわたるオントンジャワ海台の沈み込みは,付加体としては海台のごく一部(< 1%)のみを上盤側のプレートに付加したのみで,主要部のほとんどはマントル深部へ沈み込んだと思われる。これは顕生代の環太平洋造山帯に普通にみられる現象である。
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  • 120 巻 (2011) 6 号 p. 1045-1046
    公開日: 2012/03/05
    ジャーナル フリー
     日本の露頭百選シリーズを企画する。本邦に産する学術的に重要な露頭の様子とその意義を紹介することを目的とする。とくに日本列島の地質・地理を理解する上で不可欠な露頭を中心に選び,それらを地質年代の順番に通覧すると,日本列島の地質構造発達史,地理地形発達史,および文明史が読みとれるものにする。各露頭の記述・記載のフォーマットを以下のように統一する。必須の構成要素として,(1)露頭のカラー写真,(2)その意義解説のための図あるいは説明入りの写真,(3)その露頭が語る地学的意味の説明文,(4)参考文献,そして(5)英文の要約を揃える。
     『地学雑誌』は,2010-2011年に3つの号を使って「新しい日本列島像」を理解するための特集号を企画した。本シリーズは,基本的にその内容を参考に新しい視点で書かれた原稿の応募を期待する。さらに,地学的意義が大きい陸上地形,海底峡谷,日本人がつくった歴史的建造物(遺跡から近代の建築まで),代表的地質構造,化石,鉱床,などについての原稿も含める。なお応募論文の採用の可否は編集委員会で判断される。
     以上の趣旨を理解された上,多くの原稿を応募する。参考のため,以下に想定される重要露頭の具体列を並べるが,これらはあくまで暫定的な候補である。
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