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2026 Vol.135 No.1

Microstructure and Crystallographic Fabrics Recorded in Horoman Peridotite, Hokkaido, Japan

Although the mantle is solid in terms of human timescales, it deforms and flows over geological timescales. This figure illustrates the physical reality of a flowing mantle as recorded in peridotite (sample BSZ01) from the basal shear zone of the Horoman peridotite massif. (a) Cross-polarized light photomicrograph showing intense shear deformation, characterized by fractured and strained porphyroclasts and the development of localized banded structures associated with grain-size reduction. These microstructures provide direct evidence for the plastic deformation of mantle rocks (Matsuyama and Michibayashi, 2023, 2024b). (b) Electron backscatter diffraction (EBSD) phase and orientation map, in which mineral phases—dominated by olivine—are visualized together with crystallographic orientations in a common reference frame. The inverse pole figure (IPF-X) color scheme highlights spatial variations in crystal orientation in an intuitive manner. (c) Pole figures of olivine crystallographic preferred orientation (CPO), demonstrating the systematic alignment of specific crystallographic axes with shear direction as a result of crystal-plastic deformation. The observed CPO pattern indicates that dislocation creep was the dominant deformation mechanism responsible for mantle flow in this shear zone. The development of such deformation microstructures and CPOs provides direct constraints on seismic anisotropy and the spatial heterogeneity of mantle rheology (Matsuyama and Michibayashi, 2024a). This figure symbolically captures the essence of rheological studies, in which deformation and flow processes operating in the Earth’s interior are decoded from the geological record preserved in mantle rocks. (EBSD analyses were conducted using a CamScan X500FE CrystalProbe at Géoscience Montpellier, Université de Montpellier, France.)

(Kazuki MATSUYAMA)

References

Matsuyama, K. and Michibayashi, K. (2023): Variation in olivine crystal-fabrics and their seismic anisotropies in the Horoman peridotite complex, Hokkaido, Japan. Journal of Geodynamics, 158, doi.org/10.1016/j.jog.2023.102006.

Matsuyama, K. and Michibayashi, K. (2024a): Effects of mineral mode and textural variation on the seismic anisotropy in lithospheric mantle: an example from Horoman peridotites. Earth, Planets and Space, 76, doi.org/10.1186/s40623-024-02094-5.

Matsuyama, K. and Michibayashi, K. (2024b): Structural evolution of the Horoman peridotite complex in conjunction with the formation of the Hidaka Metamorphic Belt, Hokkaido. Tectonophysics, 892, doi.org/10.1016/j.tecto.2024.230535.