Seismic Anisotropy of Mafic Blueschists: EBSD-Based Constraints From the Exhumed Rock Record

Seismic anisotropy constitutes a useful tool for imaging the structure along the plate interface in subduction zones, but the seismic properties of mafic blueschists, a common rock type in subduction zones, remain poorly constrained. We applied the technique of electron backscatter diffraction (EBSD) based petrofabric analysis to calculate the seismic anisotropies of 14 naturally deformed mafic blueschists at dry, ambient conditions. The ductilely deformed blueschists were collected from terranes with inferred peak P-T conditions applicable to subducting slabs at or near the plate interface in active subduction zones. Epidote blueschists display the greatest P wave anisotropy range (AVp similar to 7%-20%), while lawsonite blueschist AVp ranges from similar to 2% to 10%. S wave anisotropies generate shear wave splitting delay times up to similar to 0.1 s over a thickness of 5 km. AVp magnitude increases with glaucophane abundance (from areal EBSD measurements), decreases with increasing epidote or lawsonite abundance, and is enhanced by glaucophane crystallographic preferred orientation (CPO) strength. Two-phase rock recipe models provide further evidence of the primary role of glaucophane, epidote, and lawsonite in generating blueschist seismic anisotropy. The symmetry of P wave velocity patterns reflects the deformation-induced CPO type in glaucophane-an effect previously observed for hornblende on amphibolite P wave anisotropy. The distinctive seismic properties that distinguish blueschist from other subduction zone rock types and the strong correlation between anisotropy magnitude/symmetry and glaucophane CPO suggest that seismic anisotropy may be a useful tool in mapping the extent and deformation of blueschists along the interface, and the blueschist-eclogite transition in active subduction zones. The directional dependence of seismic wave speeds in the subsurface, or seismic anisotropy, can allow us to map the Earth's structure in subduction zones. To improve the interpretation of seismic data collected in active subduction zones, we characterized the range of seismic anisotropy created by blueschists (a common subduction zone rock-type) that were returned to the surface after being deformed in ancient subduction margins. We calculate the anisotropy of each blueschist rock from mineral orientations collected in the lab combined with the elastic properties of these minerals. Trends in seismic anisotropy were compared to the changes in composition and preferred orientations of minerals (produced by deformation). We found that blueschists can generate a broad range of seismic anisotropy, and that this anisotropy is enhanced by increasing amounts of the mineral glaucophane. The seismic anisotropy is further increased when the glaucophane minerals are more uniformly oriented, as is typical in rocks that have experienced higher levels of deformation. The seismic anisotropy and seismic wave speeds of blueschists are distinctive from those of other common subduction zone rocks. Therefore, these results suggest blueschist seismic anisotropy can be used to improve our ability to map structure and deformation occurring in active subduction zones. Calculated blueschist Vp anisotropy up to similar to 20% with a plateau at similar to 10%, with lineation-parallel fast axis and foliation-normal slow axis The anisotropy magnitude increases with glaucophane modal abundance/crystallographic preferred orientation (CPO) strength and is diluted by epidote/lawsonite abundance The glaucophane CPO type correlates with the Vp pattern and increasing AVp magnitude in mafic blueschists