We use structural and microstructural observations from exhumed subduction-​related rocks exposed on Syros Island (Cyclades, Greece) to provide constraints on the length scales and types of heterogeneities that occupy the deep subduction interface, with possible implications for episodic tremor and slow slip. We selected three Syros localities that represent different oceanic protoliths and deformation conditions within a subduction interface shear zone, including: (1) prograde subduction of oceanic crust to eclogite facies; (2) exhumation of oceanic crust from eclogite through blueschist-greenschist facies; and (3) exhumation of mixed mafic crust and sediments from eclogite through blueschist-greenschist facies. All three localities preserve rheological heterogeneities that reflect metamorphism of primary lithological, geochemical, and/or textural variations in the subducted protoliths and that take the form of brittle pods and lenses within a viscous matrix. Microstructural observations indicate that the matrix lithologies (blueschists and quartz-rich metasediments) deformed by distributed power-law viscous flow accommodated by dislocation creep in multiple mineral phases. We estimate bulk shear zone viscosities ranging from ~1018 to 1020 Pa-s, depending on the relative proportion of sediments to (partially eclogitized) oceanic crust. Eclogite and coarse-grained blueschist heterogeneities within the matrix preserve multiple generations of dilational shear fractures and veins formed under high-pressure conditions. The veins commonly show coeval or overprinting viscous shear, suggesting repeated cycles of frictional and viscous strain. These geologic observations are consistent with a mechanical model of episodic tremor and slow slip (ETS), in which the deep subduction interface is a rheologically heterogeneous distributed shear zone comprising transiently brittle (potentially tremor-genic) sub-patches within a larger, viscously creeping interface patch. Based on our observations of outcrop and map areas of heterogeneous patches and the sizes, distributions, and amounts of brittle offset recorded by heterogeneities, we estimate that simultaneous brittle failure of heterogeneities could produce tremor bursts with equivalent seismic moments of 4.5 × 109–4.7 × 1014 N m, consistent with seismic moments estimated from geophysical data at active subduction zones.

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