Deformation microstructures and seismic properties of experimentally deformed epidote blueschist and implications for seismic velocity and anisotropy in warm subduction zones

To understand the deformation microstructures and seismic properties at the top of a subducting slab in warm subduction zones, deformation experiments of epidote blueschist were conducted in simple shear by using a modified Griggs apparatus. Deformation experiments were performed under high pressure (0.9–1.5 GPa), temperature (400–500 °C), shear strain (γ) in the range of 0.4–4.5, and shear strain rate of 1.5×10^-5–1.8×10^-4 s^-1. After experiments, crystallographic preferred orientations (CPOs) of minerals were determined by electron backscattered diffraction (EBSD) technique, and microstructures of deformed minerals were observed by transmission electron microscopy (TEM). Seismic velocity and anisotropy of constituent minerals and whole rocks were calculated using the CPOs and elastic constants of each mineral. The CPO of glaucophane showed the [001] axes aligned subparallel to shear direction and the (010) poles aligned subnormal to the shear plane at low shear strain (γ ≤1), while the [100] axes aligned subnormal to the shear plane at high shear strain (γ >2). The CPO of epidote showed non-systematic fabric at a shear strain of γ <2, but it showed the (010) poles aligned subparallel to shear direction and the [100] axes aligned subnormal to the shear plane at a shear strain between 2< γ <4. At a high shear strain of γ =4.5, the alignment of the (010) epidote poles had altered from subparallel to subnormal to the shear plane, while the [001] axes were aligned subparallel to shear direction. TEM observations and EBSD mapping revealed that the CPO of glaucophane was developed by dislocation creep, somewhat affected by the cataclastic flow at high shear strain. On the other hand, the CPO development of epidote was considered to have been affected by dislocation creep under a shear strain of 2< γ <4, but the CPO was highly affected by cataclastic flow with rigid body rotation under a high shear strain (γ >4). The average seismic velocity of P-wave (Vp_aver) and S-wave (Vs_aver) of whole rocks (epidote blueschists) were in the range of 7.19–7.63 km/s and 4.22–4.47 km/s, respectively, and the AVp (seismic anisotropy of P-wave) and Max.AVs (maximum seismic anisotropy of S-wave) were in the range of 4.5–11.0% and 3.91–6.40%, respectively. The Vp_aver and Vs_aver of experimentally deformed epidote blueschist were reduced about 8–13% and 6–11%, respectively, compared to the seismic velocities of lithospheric mantle surrounding the slab. The delay time of S-wave calculated from subducting oceanic crust composed of epidote blueschist was generally increased with increasing the subducting angle of the slab and volume proportion of glaucophane. Our experimental results indicate that the magnitude of shear strain and rheological contrast between component minerals plays an important role on the formation of CPOs of glaucophane and epidote. In addition, our calculational results of seismic properties suggest that volume proportion of constituent minerals, CPO types of glaucophane and epidote, and the subducting angle of the slab are important factors to control seismic velocity and anisotropy observed in warm subduction zones.