剪断センス指標としての斜方輝石ポーフィロクラストと幌満かんらん岩体の運動履歴 Elongated orthopyroxene porphyroclasts as a shear sense indicator and kinematic history of the Horoman Peridotite Complex, Hokkaido, Japan

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変形したカンラン岩中の伸長した斜方輝石ポーフィロクラストに発達した(100)離溶ラメラの内部非対称性は, 独立した剪断センスの指標となる可能性がある.統計学的にこの有用性を検証するために, 北海道・幌満カンラン岩体の異なる構造ユニットにおいて, XZ平面上での3つの幾何学的パラメータ(R : 伸長比, θ : 伸長方向, φ : (100)離溶ラメラ方向)を測定した.R-φおよび伸長比で階級分けしたθ-φダイアグラムに認められる非対称性は, 斜方輝石ポーフィロクラストの初期方位の違いによる(100)面上の転位すべりと剛体粒子回転の違いによって形成されたと考えられる.さらに, 信頼できる別の剪断センス指標であるカンラン石格子定向配列と比較することによって, 斜方輝石の剪断センス指標としての有用性が示され, より複雑な運動学的な履歴を読みとることが可能となる.

Elongated orthopyroxene porphyroclasts commonly form stretching lineations in deformed peridotites or metabasic rocks. Internal obliquity of (100) exsolution lamellae with respect to elongation direction is a candidate for a shear sense indicator in these rocks. However, it requires statistical analyses of shape and lattice preferred orientation of orthopyroxene porphyroclasts with reference to another reliable shear-sense indicator such as lattice preferred orientation (LPO) of olivine. We measured three geometrical parameters ; R : aspect ratio, θ : long axis orientation, φ : (100) exsolution lamellae orientation on the XZ (foliation normal, lineation parallel) plane, for 1100 porphyroclasts in 160 thin sections selected from 5 structural zones in the Horoman Peridotite Complex, Hokkaido, Japan.Bimodal distributions of internal obliquity are shown in R-φ and θ-φ diagrams of all structural zones. The equigranular zone and the internal shear zone (ISZ) show high contrast of aspect ratio between opposite internal obliquity. This asymmetry is explained by a higher R being attained by synthetic slip on the (100) slip plane, whereas the other peak consists of grains which were deformed by antithetic slip on (100) and rotated towards an unfavorable orientation for slip. The asymmetric features of geometry of elongated orthopyroxene porphyroclasts and LPO of olivine consistently indicate a top-to-the-south sense of shear. On the other hand, high R symmetric bimodal distribution of internal obliquity in the basal shear zone (BSZ), where LPO and shape preferred orientation (SPO) of olivine show apparent asymmetry (a top-to-the-north sense of shear), can not be explained by a single progressive shear deformation. But it implies inverted shear movement in the basal shear zone.Statistical analyses of internal obliquity and aspect ratio of elongated orthopyroxene porphyroclasts can be used as an independent shear-sense indicator for deformed peridotites. Furthermore, these analyses compared with the LPO of olivine can decipher a more complex kinematic history which can not be revealed only by olivine fabrics.

収録刊行物

  • 地質學雜誌

    地質學雜誌 107(2), 165-178, 2001-02-15

    日本地質学会

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各種コード

  • NII論文ID(NAID)
    110003013173
  • NII書誌ID(NCID)
    AN00141768
  • 本文言語コード
    ENG
  • 資料種別
    ART
  • ISSN
    00167630
  • NDL 記事登録ID
    5681556
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-174
  • データ提供元
    CJP書誌  NDL  NII-ELS  J-STAGE 
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