Orientation Analysis and Formation Ages of Fractures Filled with Clay Minerals (Clay Veins) in Hiroshima and Shimane Prefectures, Southwest Japan.

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  • 広島県と島根県に分布している粘土鉱物をうめた割れ目 (粘土細脈) の配行解析と形成年代
  • 広島県と島根県に分布している粘土鉱物をうめた割れ目(粘土細脈)の配行解析と形成年代〔英文〕
  • ヒロシマケン ト シマネケン ニ ブンプシテ イル ネンド コウブツ オ ウメ

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Fractures filled with clay minerals (clay veins) occur extensively in granitic andrhyolitic rocks throughout Hiroshima and Shimane Prefectures, southwest Japan. The main constituent clay minerals are mica clay mineral, smectite and kaolin mineral. Based on K-Ar dating of mica clay mineral, the formation age of the clay veins is almost the same or slightly predates the emplacement age of the host or surrounding granitic rocks. Therefore, it is suggested that most of the clay veins were formed immediately after solidification of the host or surrounding granitic rocks.<BR>The orientations of the clay veins in 13 specific regions in Hiroshima and Shimane Prefectures were systematically analyzed, and 20 K-Ar ages of constituent clay minerals (mainly mica clay mineral) within the veins of each region were determined, i.e. 31-080 Ma.<BR>The orientation analysis of fractures suggests that most of the fractures were formed under a regional stress field. The azimuth of maximum principal stress axis appears to have varied at least several times during period from the Cretaceous to Palaeogene in Hiroshima and Shimane Prefectures. The maximum compression stress directions vary from WNW-ESE to NE-SW, E-W, NW-SE or NNE-SSW, E-W and finally N-S or NWSE, over a ca. 50 million year period. Comparing these results with those obtained prebiously from dyke orientations, the azimuthes of maximum compression stress axis are breafly resemble.<BR>Thus, orientation analysis of clay veins is a useful technique to determine paleostress fields in granitic terrain.

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