INVESTIGATING SEISMOGENIC ZONE PROCESSES VIA A CANDIDATE EXHUMED CASCADIA PALEOMEGATHRUST FAULT IN THE OLYMPIC SUBDUCTION COMPLEX

At the Cascadia subduction zone, the Olympic Subduction Complex (OSC) in the central Olympic Mountains represents an exhumed analog to the modern Cascadia accretionary wedge offshore and to clastic sediment rich wedges globally. Based on published metamorphic grade and thermochronology-based exhumation history, the OSC’s central core is thought to have accreted by underplating at seismogenic zone depths of 15 km during the Eocene to Miocene. We hypothesize that if these rocks were indeed underplated, they should preserve evidence of discrete fault-bounded packages where faults are paleomegathrust interfaces that previously hosted earthquake slip. We characterize a newly identified 500m wide belt of scaly block-in-matrix mélange rocks—which contains an anastomosing system of 10 major fault strands, each hosting discrete principal slip surfaces within them— across a variety of scales using geologic field mapping, outcrop photogrammetry, and optical and scanning electron microscopy. The steeply-dipping fault strands consistently strike NW-SE, similar to the foliation orientation measured both in outcrop blocks and fault rocks, suggesting the mélange is tectonic in origin. The lithologies consist of sandstones and mudstones, interpreted as deformed turbidites, but we identified no basalt or other non-terrigenous elements of ocean plate stratigraphy. Grain-scale cataclasis, brecciation, pressure solution, and crystal plastic deformation observed in the fault zone rocks indicate interacting coeval brittle—potentially coseismic—and plastic processes recorded within macroscopically scaly fabrics. Raman spectroscopy of carbonaceous material refines peak paleo-temperatures of samples throughout the study area to 260 to 305°C, consistent with expected seismogenic zone temperatures. We thus interpret this fault zone as a newly discovered exhumed paleomegathrust interface—the first of its kind from the modern Cascadia subduction zone. We suggest that the absence of extraformational units such as basalt indicates the megathrust fault was localized within the incoming plate stratigraphic sequence in the past, facilitating sediment subduction, similar to the offshore structure observed in seismic reflection profiles at many locations in Cascadia today.