I. Motivation IV. Results & Discussion Jason N. Ott1*, Cailey B. Condit1 and Vera Schulte-Pelkum2 1University of Washington, Earth and Space Sciences, Seattle WA 2University of Colorado Boulder, Boulder CO *jasonott@uw.edu Seismic Anisotropy of Mafic Blueschists: Constraints from Exhumed Glaucophane-Rich Blueschists with Implications for the Subduction Interface Diagram of a typical subduction zone illustrating major structural features and the evolution of pressure and temperature with increasing depth. II. Materials • Samples selected from naturally deformed, mafic blueschists with variable mineralogies spanning a broad range of the blueschist facies P-T conditions. o Bandon OR: BAN09, BAN15, LAB34 o Cycladic Islands: CY106, CY107 o Diablo Range CA: DR221, DR265 o New Caledonia: LAB555, LAB570, LAB572 o Catalina Schist CA: GB15-02A • Kinematically oriented thin sections prepared (foliation normal/lineation parallel) for EBSD analysis. Blueschist sample P-T estimates from the literature overlain on metamorphic facies diagram with slab-top paths of van Keken et al. (2018). VI. Future Work • The remaining mafic blueschist samples targeted for this study will be mapped, analyzed, and integrated into the framework of this compilation study. • Synthetic receiver functions will be applied to subduction zone models using the results from this study to develop constraints applicable to imaging of blueschist-metamorphosed oceanic crust. • Deformation experiments on glaucophane samples at varying temperatures, strain rates, and initial grain sizes in combination with microstructural analysis of recovered samples will be used to link observations of naturally deformed blueschists to an experimentally determined flow law. III. Methods • We can better understand subduction zone dynamics by using exhumed subduction exposures to constrain remote geophysical observations. • Seismic anisotropy can illuminate subduction zone structure and link deep processes to their expression as geological hazards at the surface. • Understanding the range of seismic anisotropy in mafic blueschists, a key constituent of subducting slabs, will improve imaging of the subduction zone interface. The seismic anisotropy of the blueschist samples (in AVp %) as a function of fabric strength of the [100], [010], and [001] pfJ-indices of glaucophane in the samples—corresponding to the a-, b-, and c-axes respectively—displays a general increase in magnitude of anisotropy with glaucophane fabric strength. The strongest correlation between pfJ-index and AVp % occurs with the [100] direction. Misorientation to mean orientation maps of glaucophane in samples CY107 (top) and GB15-02A (bottom), representative of samples near the deepest and shallowest extents of warm slab-top paths under blueschist facies conditions. Deformation microstructures show evidence of intragranular misorientation gradients and subgrain boundary formation that suggest deformation may be facilitated by dislocation creep. The seismic anisotropy (in AVp %) of the blueschist samples displays a general increase in magnitude with increasing volumes of glaucophane in the sample. The seismic wave velocity orientations (AVp patterns) can be divided into 2 main types: 1) Fast-velocity girdle in the foliation plane with slow-velocities normal to the foliation that corresponds to a glaucophane CPO with point-maxima in the a-, b-, and c-axes, and 2) Fast-velocity direction parallel to the sample lineation with a slow-velocity girdle normal to the lineation direction corresponding to a glaucophane CPO with the c-axis oriented along the lineation and a girdle formed by the a- and b-axes. VII. Abstract and References Blueschists show strong seismic anisotropy, scaling with glaucophane volume and fabric strength. This is likely a useful tool for imaging subducting slabs with receiver functions. V. Key Takeaways • The seismic anisotropy of mafic blueschists is largely controlled by the modal percentage of the amphibol