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

Abstract

To understand the causes of seismic low-velocity layers and seismic anisotropy of the subducting oceanic crust in warm subduction zones, the seismic properties of glaucophane, epidote, and albite in epidote blueschist were studied. The epidote blueschists were experimentally deformed in simple shear at high pressure (0.9-1.5 GPa), temperature (400-500 degrees C), and shear strain up to gamma = 4.5. The seismic velocity and anisotropy of polycrystalline glaucophanes, epidotes, albites, and whole rocks were calculated using the lattice preferred orientation (LPO) of grains and elastic constants of each mineral. The average P -and S-wave velocity (Vpaver and Vsaver) of epidote blueschist (whole rock) were in the range of 7.19-7.63 km/s and 4.22-4.47 km/s, respectively, and the anisotropy of P-wave and the maximum anisotropy of S-wave were in the range of 4.5-11.0% and 3.91-6.40%, respectively. The Vpaver and Vsaver values of experimentally deformed epidote blueschists were reduced to 8-13% and 6-11%, respectively, compared to those of lithospheric mantle rocks surrounding the subducting slab. This result indicates that the low-velocity layers can be attributed to the LPOs of the minerals in the epidote blueschist. The delay time of the S-wave, which was calculated using the subducting oceanic crust composed of epidote blueschist, was generally increased with increasing subducting angle of the slab and volume proportion of glaucophane. The calculation of the seismic anisotropy of the subducting slab showed that the trench -normal and trench-parallel polarization directions of the fast S-wave can be caused by different LPO types of glaucophane and epidote with different subducting angles of the slab in the subduction zone. These results suggest that the volume proportion and LPO types of glaucophane and epidote and the subducting angle of the slab are important factors in controlling the seismic velocity and anisotropy of subducting oceanic crust in warm subduction zones.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).