Deformation microstructures of blueschists in Alpine Corsica, France, and implications for seismic anisotropy and the low-velocity layer in subducting oceanic crust

Abstract

Understanding the deformation microstructures and seismic properties of blueschists is important for interpreting the seismic anisotropy at the top of subducting oceanic crust. The deformation microstructures and seismic properties of epidote blueschist, epidote-lawsonite blueschist, and lawsonite blueschist from Alpine Corsica, France, were studied using electron backscatter diffraction mapping and transmission electron microscopy. The crystal-preferred orientations (CPOs) of glaucophane show a strong maximum of the [001] axes aligned sub-parallel to the lineation. The maximum of the (010) poles of epidote is aligned sub-parallel to the lineation. The [001] axes of lawsonite are strongly aligned sub-normal to the foliation. Intracrystalline misorientation, dislocation, and occasional fragmentation of glaucophane indicate that glaucophane CPO was developed mainly by dislocation creep. The intracrystalline deformation features of epidote, such as subgrain boundaries and dislocations, indicate that the CPO of the epidote was formed by dislocation creep. In contrast, our data indicate that lawsonite CPO was developed by a combination of rigid-body rotation and dislocation creep with a minor effect of twinning. The P-wave anisotropy (AVp) and maximum S-wave anisotropy (max.AVs) of epidote blueschists (AVp = 12.3–16.9% and max.AVs = 6.53–11.75%) are higher than those of lawsonite blueschists. The seismic velocities of the blueschists are lower than those of mantle peridotites. The coexistence of epidote and lawsonite in the blueschist results in variations in seismic anisotropy and P-wave velocity in the whole rock, depending on the modal content of epidote and lawsonite. A high content ratio of epidote to lawsonite in blueschist produces strong P- and S-wave anisotropies of blueschist when the glaucophane content is more than ∼50%. The P-wave velocity of blueschist decreases proportionally with increasing epidote content. Our results indicate that the seismic properties of deformed blueschists contribute to the seismic anisotropy and the low-velocity layer in subducting oceanic crusts in subduction zones.