Deformation Microstructures of Epidote Blueschist Experimentally Deformed in Simple Shear and Their Implications for Seismic Properties in Subduction Zone

To understand deformation microstructures and seismic properties at the top of subducting slab in a warm subduction zone, deformation experiments of epidote blueschist were conducted in simple shear by using a modified Griggs apparatus. Deformation experiments were performed under the pressures (0.9-1.5 GPa), temperatures (400-500 ºC), shear strain (γ = 0.4- 4.5) and shear strain rates (γ˙= 1.5×10^-5- 1.8×10^-4 s^-1). The experiments revealed that two different lattice preferred orientations (LPOs) of glaucophane (type-1 and -2) were developed depending on the magnitude of shear strain (γ). At the low shear strain (γ ≤ 1 ), type-1 LPO of glaucophane was developed which is characterized as the [001] axes aligned subparallel to the shear direction and the (010) poles aligned subnormal to the shear plane . At the high shear strain (γ > 2), type-2 LPO of glaucophane was developed which is characterized as the [001] axe s aligned subparallel to the shear direction and the [100] axes aligned subnormal to the shear plane. LPOs of epidote were also developed depending on the magnitude of shear strain. Under low shear strain (γ < 2), LPO of epidote was mostly weak. At the shear strain of 2 < γ < 4, however, the (010) poles of epidote were aligned subparallel to the shear direction and the [100] axes were aligned subnormal to the shear plane. At the high shear strain of γ > 4, on the other hand, alignment of the (010) poles of epidote was changed from subparallel to subnormal to the shear plane, while the [001] axes were aligned subparallel to the shear direction. Although type-1 LPO of glaucophane was hardly observed in nature, this glaucophane LPO can contribute to the formation of the trench-normal seismic anisotropy which is exceptionally observed in the fore-arc region in subduction zone. Type-2 LPO of glaucophane, on the other hand, has often been reported in natural blueschists and it could affect the trench-parallel seismic anisotropy in the fore-arc region in subducting oceanic crust with a high-dip angle ( θ > 45º) . In addition, seismic P- and S-wave velocities of deformed epidote blueschist were reduced about 8-9 % and 6-7 %, respectively, compared to the seismic wave velocity of other surrounding mantle rocks. This result indicates that deformed epidote blueschist can contribute to the formation of seismic low velocity layer which is observed at the top of the subducting slab in subduction zones.