地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
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125 巻 , 2 号
選択された号の論文の8件中1~8を表示しています
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  • 125 巻 (2016) 2 号 p. Cover02_01-Cover02_02
    公開日: 2016/05/12
    ジャーナル フリー
     かつて超大陸パンゲア西部の陸棚浅海や砂丘で堆積した古生代の地層がほぼ水平なまま隆起し,標高2,000 mの台地表面を飾っていることは驚きだ.隆起以前から存在したコロラド川は,大地の上昇に抗って,もともとの川筋を残したまま穿入蛇行を続けて,現在もV字谷を削っている.北米大陸直下にあるマントル上部の上昇流が急速隆起の原動力だが,この隆起が止まれば大峡谷の崖はすぐに浸食されはじめ,ただのつまらない緩斜面に急速に変わってしまうだろう.単独の急崖で多様な地質時代の地層を一目で見ることができるのは大きな魅力であるが,あの急な崖をつくる地層そのものが特異なわけでない.同じ地層がほとんど変形もせず延々と周辺地域の地下につながっているからである.
     日本人がグランドキャニオンを訪れる場合,米国西海岸南部からインターステーツ40号線(旧ルート66号線)を東に進み,途中で北に折れて,大峡谷の南側に出るコースが定番であろう.Mather Point展望台やBright Angel Trailheadに自動車横づけで,台地上からのあの絶景を眺めることができる.しかし,あえて崖沿いにさらに東に小一時間進むと,大峡谷東部のコロラド川上流部では谷幅が広がり,台地の上からも蛇行する川の流れが見える.夕闇迫る頃のDesert View展望台から,ちょっと趣の異なる光景を楽しむことができた.さらに,よく目をこらすと,水平な古生層最下部のカンブリア系と,その下の先カンブリア時代の岩石との間の明瞭な不整合(the Great Unconformityと呼ばれる)を目視できる.
    (写真と説明:磯﨑行雄)
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論説
  • 伊藤 陽晃, 上野 健一
    125 巻 (2016) 2 号 p. 185-199
    公開日: 2016/05/12
    ジャーナル フリー
     Changes in precipitation distribution associated with a surface temperature rise in the late 1980s, which accompanied a climatic regime shift, are investigated using Automated Meteorological Data Acquisition System (AMeDAS) data of the Japan Meteorological Agency, and their relations with the variability of surface pressure patterns are revealed. After the temperature rise, the frequency of winter monsoon pressure patterns did not decrease. According to the classification of pressure patterns, long-term variations of surface pressure pattern frequencies showed negative relations between the winter monsoon and high-pressure patterns before 1992, which subsequently changed to the negative relations between the winter monsoon and traveling low-pressure systems. After the temperature rise, in years when low-pressure patterns prevailed, precipitation increases were observed in western Japan due to low-pressure systems traveling over the Pacific coast, and in wider areas from central Japan to Kyusyu areas due to low-pressure systems traveling over the Sea of Japan. Recently, the frequency of stationary front patterns has been increasing with precipitation increasing in the Setouchi area of western Japan. In years with a warm winter, precipitation increased in wider areas of the Pacific coast to inland areas of central Japan. In cold winters, low-pressure systems traveling over the Pacific coast decreased precipitation in central to western Japan. However, after a temperature rise, low-pressure systems traveling over the Sea of Japan decreased precipitation at the coast of the sea of Japan from central Japan to Hokkaido.
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  • 末岡 茂, 梅田 浩司, 安江 健一, 丹羽 正和, 島田 耕史, 石丸 恒存, 檀原 徹, 岩野 英樹, 八木 公史
    125 巻 (2016) 2 号 p. 201-219
    公開日: 2016/05/12
    ジャーナル フリー
     In this study, multi-system thermochronology, i.e., fission-track (FT), K-Ar and U-Pb methods are used to identify the cooling and denudation history of the Tsuruga body of Kojaku granite, southwest Japan. Apatite FT age of 51.8 ± 6.5 Ma, zircon FT age of 70.4 ± 2.0 Ma, biotite K-Ar ages of 66.7–62.0 Ma, and zircon U-Pb age of 68.5 ± 0.7 Ma were obtained for granitic samples, whereas plagioclase K-Ar ages of 19.1–18.8 Ma and whole-rock K-Ar age of 19.0 ± 2.9 Ma were inferred for the basaltic dyke intruding into the granite. The zircon FT lengths are not significantly shorter than their initial lengths, implying rapid cooling at the zircon FT partial annealing zone (PAZ). On the other hand, the apatite FT length distribution shows a typical pattern for granitic pluton without reheating, indicating a slow cooling history at the apatite FT PAZ. Based on the results of these thermochronometric analyses, inverse thermal calculations using the FT data, and simple thermal conduction modeling of the granitic body, the cooling and denudation histories of the Tsuruga body are reconstructed: (1) the Tsuruga body intruded at ca. 68 Ma, late Cretaceous, at a depth of several kilometers, (2) rapidly cooled to below the zircon FT PAZ by heat conduction within a few million years or less, and (3) slowly cooled due to peneplanation during the past 50–60 million years. On the other hand, the whole-rock Rb-Sr age previously reported for the Kojaku body is younger than when the cooling curve of the Tsuruga body obtained by this study intersects with the closure temperature of the whole-rock Rb-Sr system. This may imply a time lag between the formation ages of these bodies, but more thermochronometric studies are required to draw a definitive conclusion. The K-Ar ages of the basaltic dyke are interpreted as its formation age, indicating that dyke intrusion was associated with the Green Tuff movement.
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  • 楮原 京子, 田代 佑徳, 小坂 英輝, 阿部 恒平, 中山 英二, 三輪 敦志, 今泉 俊文
    125 巻 (2016) 2 号 p. 221-241
    公開日: 2016/05/12
    ジャーナル フリー
     We should understand the earthquake potential in and around Quaternary fault zones, in view of recent destructive inland earthquakes at previously unknown active fault zones in Japan. The Senpoku Plain and its surrounding areas are characterized by high seismic activity in northeast Japan, highlighted by four destructive earthquakes, M 6.8 in 2008, M 6.4 in 2003, M 6.5 in 1962, and M 7.0 in 1900, which occurred during the past 100 years, although few geomorphic features indicate active faulting. A comprehensive survey was conducted on the tectonic geomorphology in the area to understand the structural and geomorphic expression of the Ichinoseki–Ishikoshi Flexure Line (IIFL), which suggests Quaternary activity. Geological and geomorphical mapping shows that the IIFL is located between the Kitakami Lowland Fault Zone and the Senpoku Plain. The IIFL extends about 30 km from Isawa to Ishikoshi with a slightly sinuous trace. A high-resolution seismic reflection profile and a gravity profile define the subsurface geometry of the IIFL. The IIFL is interpreted to be a steeply west-dipping reverse fault. The Pliocene Kazawa and Yushima Formations typically dip 40° to 20°E along the IIFL, and are overlain by the Pleistocene Mataki Formation, which becomes thinner toward the fold axis of the IIFL, and their dips decrease progressively upward. This suggests that the Mataki Formation was deposited concurrently with fault activity of the IIFL. Fission-track dating of a tuff layer within the uppermost section of the Kazawa Formation indicates that active reverse faulting of the IIFL began at about 2 Ma. At least 280 m of the tectonic uplift is consumed by active faulting and the average uplift rates are estimated to be 0.14–0.08 mm/yr. Vertical separations of Hh surface are about 15 to 40 m. Heights of fold scarps on L1 surface are about 2 m. Their ages are determined to be 0.4–0.5 Ma for Hh and 24–12 ka for L1, respectively. Therefore, the Quaternary average uplift rates of the IIFL are estimated to be 0.03–0.17 mm/yr. Quaternary activity of the IIFL is weak, but there are differences in the magnitude of dissection in the Iwai Hills between the hanging-wall and the footwall of the IIFL.
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短報
  • 石村 大輔, 宮内 崇裕, 早瀬 亮介, 小原 圭一, 山市 剛
    125 巻 (2016) 2 号 p. 243-256
    公開日: 2016/05/12
    ジャーナル フリー
     Rradiocarbon dates of organic bulk sediment samples, as well as animal and plant fossil samples, from Holocene coastal lowland sediments are compared to clarify characteristic differences between the dates of both samples and estimate the true age of sedimentation. The samples were collected from drilled cores obtained from two sites. An OSN-Br core was drilled at a river mouth in Kuji City, Iwate Prefecture, and consisted of marine sediments showing sea-level rise during the Holocene. Two KYD-Br cores were drilled at a small valley in Yamada Town, Iwate Prefecture, and consisted of peaty sediment and interbedded event deposits (e.g., tephra and tsunami deposits). Based on a comparison with the dates of samples collected from the same horizon or ages of tephras, radiocarbon dates of organic bulk sediment samples of the OSN-Br core showed a systematic age difference between the dates of animal and plant fossil samples and tephras. These organic bulk sediment samples were interpreted to include old carbon derived from the hinterland, particularly from Paleogene to Cretaceous sedimentary rocks. On the other hand, radiocarbon dates of organic bulk sediment samples of the KYD-Br cores agree with dates of in-situ plant fossil samples (seed and leaf) and are consistent with the age of tephras. Therefore, it can be assumed that the dates of organic bulk sediment samples represent the true age of sedimentation.
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