We present observations from an exhumed subduction complex that resembles the environment of modern deep episodic tremor and slow slip (ETS). We focus on the Cycladic Blueschist Unit on Syros Island in Greece. Syros metabasites consist of blueschists and eclogites that record prograde deformation, with peak metamorphism of 1200–1600 MPa and 450–550 °C. Field observations reveal that coexistence of blueschist and eclogite sets up an important rheological contrast: blueschists show distributed viscous dislocation creep, whereas eclogites dispersed within the blueschist matrix show brittle shear fractures and veins. These observations are consistent with the inferred prominent role of high fluid pressures from geophysical studies, but are inconsistent with models of deep ETS that invoke changes in rate-and-state friction parameters along a narrow fault. Instead, we suggest deep ETS may be controlled by coupled brittle-viscous deformation in partially eclogitized oceanic crust embedded within high-fluid-pressure patches along the plate interface.

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