地質学雑誌
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
126 巻 , 11 号
選択された号の論文の6件中1~6を表示しています
論説
  • 七山 太, 渡辺 真人, 山崎 徹, 岩野 英樹, 檀原 徹, 平田 岳史
    2020 年 126 巻 11 号 p. 605-620
    発行日: 2020/11/15
    公開日: 2021/02/15
    ジャーナル フリー

    北海道中央部,トムラウシ地域の日高累層群中には,チカプペツ層と呼ばれる周囲の地層とは固結度の大きく異なる泥岩主体の地層の存在が知られていた.今回,チカプペツ層中の酸性凝灰岩とシイ十勝川層(新称)のタービダイト砂岩からジルコン粒子を分離し,ジルコンU-Pb年代を測定した結果,19.5±0.1Maと22.5±0.7Ma(前期中新世)を示す年代値が得られた.また,前者の石灰質ノジュール1試料から前期中新世を示す珪藻化石群集が得られたことから,その堆積年代は23-19Maと理解された.これまでも日高帯分布域には,後期漸新世の堆積年代を示す襟裳層や立牛層の挟在が明らかにされており,この時期に大規模な右横ずれ変位が開始されていたことが判明している.チカプペツ層やシイ十勝川層は,前期中新世に,同様の右横ずれ変位に伴って発生した海成層と考えられる.

  • 羽地 俊樹, 佐藤 活志
    2020 年 126 巻 11 号 p. 621-630
    発行日: 2020/11/15
    公開日: 2021/02/15
    ジャーナル フリー

    室戸岬に分布する中期中新世の貫入岩群の方位から,貫入時の古応力の推定を試みた.ビシャゴ碆の整然層にみられるシート群に混合ビンガム分布法を適用し,単一のNW-SE方向に最小圧縮主応力軸を持つ応力を得た.室戸岬地域の地層は貫入岩の定置後に傾動を伴う変形があったことが明らかである.変形史を仮定して復元した応力の古方位は,最小主応力軸が鉛直に近い逆断層型応力であった.混在岩中に見られる同時期の貫入岩群の方位もまた,逆断層型応力での貫入を示唆している.中期中新世の西南日本前弧域は,浮揚性沈み込みの影響で圧縮場であったと想定されており,室戸岬地域の貫入岩の方位はこれと整合的である.

レター
  • 牛丸 健太郎, 山路 敦
    2020 年 126 巻 11 号 p. 631-638
    発行日: 2020/11/15
    公開日: 2021/02/15
    ジャーナル フリー
    電子付録

    The orientations of 76 sheet intrusions in northern Amakusa-Shimoshima, western Kyushu, were inverted to understand paleostress condition(s) in the backarc behind the junction of the SW Japan and Ryukyu arcs. The radiometric ages of intrusive rocks in the Amakusa area suggest that the sheet intrusions are 14-17 Ma, contemporaneous with the formation of the Japan Sea. The Eocene host formations are folded about a NNE-trending axis with an interlimb angle of ~140°, but the relative timing of the folding and magmatism is not constrained. The sheet intrusions have various orientations, but E-W-trending dikes are dominant. The tilt correction did not significantly affect the E-W-trending intrusions, because they meet the fold axis at high angles. Stress inversion using the mixed Bingham distribution yielded two stresses from in-situ and tilt-corrected orientations. The stresses calculated from the in-situ set of orientations appeared to fit better than those from the tilt-corrected set of orientations, because the stresses of the normal faulting and strike-slip faulting regimes from the in-situ set of orientations had nearly vertical stress axes. We thus suggest that magmatism postdated the folding.

  • 磯﨑 行雄, 長谷川 遼, 益田 晴恵, 堤 之恭
    2020 年 126 巻 11 号 p. 639-644
    発行日: 2020/11/15
    公開日: 2021/02/15
    ジャーナル フリー
    電子付録

    Detrital zircon U-Pb ages were measured for nine non-fossiliferous sandstones from the uppermost Izumi Group, a “Late Cretaceous” forearc clastic sequence in the eastern Izumi Mountains of western Kii Peninsula, SW Japan. Seven out of nine sandstones yielded Paleocene grains. These results confirm that the uppermost Izumi Group was deposited in the Paleogene (Selandian-Thanetian or younger), extending the total depositional duration of the group to ca. 27 myr, almost double the previous estimate. The new age data raise the possibility that a stratigraphic interval across the Cretaceous-Paleogene extinction boundary may be preserved within the group. The occurrence of Paleogene strata further constrains the onset of the low-angle Median Tectonic Line to post-Paleocene.

報告
  • 図子田 和典, 岡村 太路, 二村 康平, 束田 和弘, 竹内 誠
    2020 年 126 巻 11 号 p. 645-654
    発行日: 2020/11/15
    公開日: 2021/02/15
    ジャーナル フリー
    電子付録

    Miocene dikes and sills are widely exposed in the eastern Shitara area, Aichi Prefecture, central Japan. Whole-rock chemical composition, grain size, lightness, and the alteration ratio of the intrusive rocks from three outcrops in the Shitara area are described in this paper. The central part of the intrusive rock bodies, commonly dark gray, gradually changes into grayish white toward its marginal part. The lightness of the polished surface of the rocks was measured using the hue, saturation, lightness (HSL) tool in the Inkscape graphics editor. At the white part, plagioclase crystals are fine-grained and severely replaced by secondary minerals such as calcite, whereas plagioclase, at the dark gray part, is much larger than that in the white part and less altered. Turning to chemical composition, common feature cannot be found among three outcrops.

ノート
  • Shunya Kaneki, Hiroyuki Noda
    2020 年 126 巻 11 号 p. 655-661
    発行日: 2020/11/15
    公開日: 2021/02/15
    ジャーナル フリー

    Vitrinite reflectance Ro is commonly used as a paleotemperature proxy for rocks. Among various models used to reconstruct paleothermal histories, EASY%Ro, a set of ordinary differential equations to describe the maturation of vitrinite, has been widely used since 1990. Approximations of EASY%Ro solutions employing specific paleothermal histories have been used in the field of structural geology to estimate maximum paleotemperatures Tmax. However, comparisons of those approximations with results obtained with EASY%Ro have not been reported in scientific literatures, and the errors on the approximations and the limitations of their use to determine Tmax remain undocumented. Here, we performed such comparisons and found that the use of those approximations can lead to considerable deviations from the results obtained with EASY%Ro. We then derived new approximations of EASY%Ro solutions for four representative paleothermal histories that provided estimates of Tmax within 3 °C of those obtained with EASY%Ro. We also describe the Jacobian of EASY%Ro, which can be used to propagate errors from Ro to Tmax. We expect that the four approximations provided herein will be useful, although we recommend using EASY%Ro itself if greater precision or particular paleothermal histories other than those assumed in this study are required.

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