Deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust

Daeyeong Kim, Ikuo Katayama, Katsuyoshi Michibayashi, Tatsuki Tsujimori

    Research output: Contribution to journalArticle

    16 Citations (Scopus)

    Abstract

    Investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust, and here we report on our systematic fabric analyses of glaucophane, lawsonite, and epidote in naturally deformed blueschists from the Diablo Range and Franciscan Complex in California, and the Hida Mountains in Japan. Glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations (CPOs) characterized by a (100)[001] pattern. These characteristics, together with a bimodal distribution of grain sizes from some samples, possibly indicate the occurrence of dynamic recrystallization for glaucophane. Although lawsonite and epidote display high aspect ratios and a strong CPO of (001)[010], the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism. The P-wave (AVP) and S-wave (AVS) seismic anisotropies of glaucophane (AVP=20.4%, AVS=11.5%) and epidote (AVP=9.0%, AVS=8.0%) are typical of the crust; consequently, the fastest propagation of P-waves is parallel to the [001] maxima, and the polarization of S-waves parallel to the foliation can form a trench-parallel seismic anisotropy owing to the slowest VS polarization being normal to the subducting slab. The seismic anisotropy of lawsonite (AVP=9.6%, AVS=19.9%) is characterized by the fast propagation of P-waves subnormal to the lawsonite [001] maxima and polarization of S-waves perpendicular to the foliation and lineation, which can generate a trench-normal anisotropy. The AVS of lawsonite blueschist (5.6-9.2%) is weak compared with that of epidote blueschist (8.4-11.1%). Calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench-parallel and trench-normal seismic anisotropy beneath NE Japan, but not to that beneath the Ryukyu arc. Our results demonstrate, therefore, that lawsonite has a strong influence on seismic velocities in the oceanic crust, and that lawsonite might be the cause of complex anisotropic patterns in subduction zones.

    Original languageEnglish
    Pages (from-to)8-21
    Number of pages14
    JournalPhysics of the Earth and Planetary Interiors
    Volume222
    DOIs
    Publication statusPublished - Sep 2013

    Fingerprint

    lawsonite
    blueschist
    seismic anisotropy
    oceanic crust
    S waves
    crusts
    glaucophane
    S-wave
    anisotropy
    epidote
    trench
    P waves
    foliation
    P-wave
    polarization
    high aspect ratio
    preferred orientation
    crystal
    Japan
    occurrences

    Keywords

    • Blueschist
    • Crystal preferred orientations
    • Fabric analyses
    • Seismic anisotropies
    • Subducting oceanic crust

    ASJC Scopus subject areas

    • Geophysics
    • Space and Planetary Science
    • Physics and Astronomy (miscellaneous)
    • Astronomy and Astrophysics

    Cite this

    Deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust. / Kim, Daeyeong; Katayama, Ikuo; Michibayashi, Katsuyoshi; Tsujimori, Tatsuki.

    In: Physics of the Earth and Planetary Interiors, Vol. 222, 09.2013, p. 8-21.

    Research output: Contribution to journalArticle

    Kim, Daeyeong ; Katayama, Ikuo ; Michibayashi, Katsuyoshi ; Tsujimori, Tatsuki. / Deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust. In: Physics of the Earth and Planetary Interiors. 2013 ; Vol. 222. pp. 8-21.
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