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Mélange (or block-in-matrix structures) exerts a first-order control on both the mechanical and chemical evolution of subduction megathrusts. However, the timing and mechanisms that form mélanges are variable and debated. Field observations and (micro-) structural analyses from a metasedimentary mélange in the lawsonite blueschist unit of the Catalina Schist (Santa Catalina Island, California, USA) reveal that syn-subduction deformation and fluid-mediated processes led to mélange formation at the plate interface. Deposited as turbidites, early shear occurred parallel to bedding planes (S1 foliation). At near peak subduction conditions, at the base of the subduction seismogenic zone (∼1.0 GPa, 320 °C), the rocks were intensely deformed in recumbent open to tight folds (F2) with axial planar cleavages (S2). Fracturing, fluid flow, and quartz precipitation are preserved as extensional vein mesh networks in fold noses. Continued shearing led to boudinage of these strengthened noses and transformation into strong blocks within the weaker less-veined matrix composed of high-strain fold limbs (S1−2). Microstructures reveal viscous deformation in the high-strain fold limbs occurred by pressure-solution creep of fine-grained quartz ± albite. In contrast, the fold noses and/or blocks contain coarse-grained quartz veins with little evidence of deformation. These rocks record the development of syn-subduction block-in-matrix mélange structures through the interaction of deformation and mineral precipitation; pressure solution weakened fold limbs-turned-matrix and veining strengthened fold noses-turned-blocks. Although mélange structure is often invoked to explain tremor and slow slip, rheological analysis indicates that these metasedimentary rocks can host tectonic creeping but cannot accommodate slow-slip strain rates by the deformation mechanisms preserved in their microstructures. 

Abstract 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 greatestPwave anisotropy range (AVp ∼7%–20%), while lawsonite blueschist AVp ranges from ∼2% to 10%.Swave anisotropies generate shear wave splitting delay times up 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 ofPwave velocity patterns reflects the deformation‐induced CPO type in glaucophane—an effect previously observed for hornblende on amphibolitePwave 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.