Structure and properties of the Cascadia plate interface: Evidence from a newly described exhumed paleomegathrust in the Olympic Subduction Complex

Abstract The Olympic Subduction Complex (OSC) in the central Olympic Mountains is a deeply exhumed continuation of the offshore modern accretionary wedge of the Cascadia subduction zone. Metamorphic grade and thermochronology reveal that the OSC’s central core likely accreted by underplating at seismogenic depths. We characterize a previously unknown ~450-m-wide belt of block-in-matrix mélange containing an anastomosing system of fault strands, which in turn include discrete principal slip surfaces consistent with brittle-frictional, likely coseismic, deformation. The lithologies are deep marine turbiditic sandstones and mudstones without other elements of ocean plate stratigraphy. Raman spectroscopy of carbonaceous material refines peak paleotemperatures to 260–280 °C, consistent with exhumation from seismogenic zone conditions. We interpret these structures as a composite fault zone that records both fast slip and slow distributed deformation during the seismic cycle through coeval coseismic brittle-frictional and interseismic viscous deformation. The mélange formed by granular flow, pressure solution, alignment and growth of phyllosilicates, and crystal plastic deformation, while the fault strands are dominated by concentrated cataclasis, brecciation, and veining. We interpret this fault zone as an exhumed paleomegathrust interface, the first direct analog for the modern Cascadia subduction zone. The absence of basalt indicates that the megathrust fault was localized within the incoming plate stratigraphic sequence, facilitating sediment underthrusting, similar to offshore structures observed via seismic reflection imaging in Cascadia and elsewhere today. This exposure presents an exhumed model for plate interface systems hosted entirely in sedimentary rock even at mid-seismogenic depths. It is therefore a revealing potential analog useful in constructing models of seismogenic zone deformation and rupture processes.