Off-fault mélange formation accommodates only a small fraction of strain in the subduction seismogenic zone

Exhumed accretionary complexes provide a window into the architecture of the subduction megathrust, but coexistence and overprinting of brittle and viscous deformation structures makes it difficult to disentangle the specific mechanisms hosting various slip modes. Within these exposures, interseismic strain between megathrust earthquakes is accommodated by mélange zones up to kilometers in thickness. Because mélange dominates the rock record, it is often the focus of geological investigations, in contrast to the narrow fault zones that host localized seismic slip. Pressure solution is invoked as a significant mechanism of interseismic strain accommodation in mélange zones within the subduction seismogenic zone, and its efficiency is commonly said to control the downdip limit of seismogenic slip and amount of coupling on the plate interface. However, observations from modern highly-locked subduction zones show that megathrust earthquakes account for the vast majority of total plate displacement. Here, we apply chemical and magnetic methods to a recently described Cascadia subduction zone paleomegathrust exposure to constrain the spatial and structural partitioning of strain accommodation across a seismogenic zone megathrust among the mélange, discrete faults, and coherent wall rocks. Both volume loss and distributed frictional sliding occur in the mélange but accommodate orders of magnitude less shear strain than plate convergence requires. Our work suggests that, even though mélange fabrics are volumetrically dominant in the rock record, the majority of plate displacement accommodated by the subduction seismogenic zone is instead taken up on discrete faults therein.