Transient and episodic slow slip accommodates a great deal of tectonic strain and may be mechanically linked with locked regions of seismically hazardous faults. Best documented in subduction zones and associated with nonvolcanic tremor, most proposed mechanisms for slow slip revolve around the transition between stable and unstable frictional sliding. The dilemma is that slow slip is generated at a wide range of crustal depths, including at pressure-temperature conditions where frictional deformation mechanisms give way to predominantly viscous ones. We present a model for how fracture and viscous flow within mid-crustal shear zones can produce episodic creep transients. Our model for such transients stems from geological examples of shear zones that formed at temperatures and pressures of >500 °C and >0.6 GPa during early orogenesis, following Late Cretaceous subduction of a backarc ocean basin. Within these shear zones, relatively strong lenses of metabasalt localized fluid-filled fractures that were subsequently deformed by viscous flow in surrounding quartzofeldspathic gneiss. The spatial and temporal characteristics of the modeled creep events are similar to those of slow-slip events observed in modern subduction zones. We therefore suggest that some episodic tremor and slip can originate through combined fracture and viscous flow across shear zones comprising mixtures of strong and weak materials.

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