Structure and properties of the Cascadia plate interface: evidence from a newly-described exhumed paleomegathrust in the Olympic subduction complex

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. Duplexes of underplated sediments bounded above and below by abandoned paleomegathrust interfaces could therefore be preserved during exhumation. We characterize a previously unknown ~500 m wide belt of block-in-matrix mélange containing an anastomosing system of 9 major fault strands, which in turn include mm to cm wide discrete principal slip surfaces consistent with brittle-frictional, likely coseismic, slip. The lithologies are turbiditic sandstones and mudstones; other elements of ocean plate stratigraphy are absent. Raman spectroscopy of carbonaceous material refines peak paleotemperatures to 260-280°C, consistent with the seismogenic zone. We interpret these intermingled structures as a composite fault zone that records both slow and fast slip of the seismic cycle through coeval coseismic brittle-frictional and interseismic viscousdeformation. We show that the mélange forms by cataclasis, pressure solution, and development and abandonment of localized shear surfaces, while the fault strands are dominated by concentrated cataclasis and brecciation. We calculate the degree of pressure solution experienced by the mélange at the thin section scale using scanning electron microscopy and compare the accumulated strain across the fault zone through anisotropy of magnetic susceptibility. 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 in the past, facilitating sediment underthrusting, similar to offshore structures observed via seismic reflection imaging in Cascadia and elsewhere today.