Insights into uppermost mantle deformation and tectonic evolution from the Horoman peridotite complex, Japan

The lithospheric mantle is crucial in influencing the dynamics at plate boundaries and in forming the geophysical signatures observed on the Earth's surface, although it is not directly observable. To comprehend its deformation history, it is necessary to employ methods that connect scales ranging from microstructures to orogenic belts. The Horoman peridotite complex in Hokkaido, Japan, provides an outstanding natural laboratory for such investigations, offering direct access to fragments of the upper mantle with well-preserved structures. 
In this study, we conducted quantitative microstructural and intracrystalline analyses using EBSD dataset (Matsuyama & Michibayashi, 2024), combined with rheological modeling. The complex exhibits systematic variations from mylonitic to equigranular textures and from E to A to AG type olivine crystallographic preferred orientations (CPOs) upward through the structural sequence. The microstructural parameters (grain size, aspect ratio, and shape factor) of olivine and orthopyroxene showed a fine-grained microstructure and high intracrystalline strain in the E type samples, consistent with deformation dominated by dislocation creep. In contrast, AG type samples displayed polygonal grain shapes and lower intracrystalline strain, suggesting deformation by diffusion creep.
Rheological modeling based on olivine flow laws (e.g., Hirth & Kohlstedt, 2003) indicates that E type CPOs develop through water-assisted dislocation creep involving the activation of specific slip systems, whereas AG type CPOs formed through melt-induced strain partitioning during diffusion creep. Integrating these results and the tectonics of the surrounding metamorphic belt (e.g., Toyosihma et al., 1997), we propose a three-stage deformation history: (1) early high-temperature, dry deformation producing A type CPOs; (2) syn-kinematic melt infiltration leading to AG type CPOs via diffusion creep; and (3) later water infiltration and thrusting generating E type CPOs through hydrous dislocation creep.
Comparison with other orogenic and ophiolitic peridotites suggests that the A–E and A–AG CPO transitions observed in the complex represent general upper-mantle processes involving water and melt. Our study highlights that the coexistence and transition of multiple deformation mechanisms—modulated by fluid and melt interactions—play a fundamental role in controlling mantle rheology and the evolution of lattice-preferred orientations.