Shear displacements in crustal fault zones are accommodated by a range of seismic styles, including standard earthquakes, non-volcanic tremor, and continuous and transitory aseismic slip. Subduction channel shear zones, containing highly sheared, fluid-saturated trench-fill sediments intermingled with fragments of oceanic crust, are commonly inferred to occur along active subduction megathrusts. If this interpretation is correct, these plate boundary faults are not discrete planes, but may resemble the mélange shear zones commonly found in exhumed subduction-related rock assemblages. Mélange deformation appears to depend critically on the ratio of competent to incompetent material, with shear surfaces localized along lithological contacts or within competent domains, while matrix flow accommodates shearing by distributed strain. If the style of strain and/or displacement accommodation in a mélange reflects the partitioning between aseismic and seismic slip, the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones, and along comparable mixed-lithology fault zones.