Investigating Seismogenic Zone Processes via a Candidate Exhumed Cascadia Paleomegathrust Fault in the Olympic Subduction Complex

Anna Ledeczi, University of Washington
Harold Tobin, University of Washington
Tsai-Wei Chen, University of Washington
Sean Mulcahy, Western Washington University
Peter Lindquist, University of Washington

Abstract

At the Cascadia subduction zone, the Olympic Subduction Complex (OSC) exposed in the central Olympic Mountains represents an exhumed analog to the modern Cascadia accretionary wedge offshore and to clastic sediment dominated wedges globally. Based on published metamorphic grade and thermochronology-based exhumation history, the central core of the OSC is thought to have accreted by underplating at seismogenic zone depths of approximately 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 in which the faults represent paleomegathrust interfaces that likely hosted earthquake slip in the past. 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 methods including geologic field mapping, outcrop photogrammetry, optical microscopy, and scanning electron microscopy. The steeply-dipping fault strands consistently strike NW-SE, similar to the foliation orientation measured both within outcrop blocks and fault rocks, suggesting a tectonic origin to the mélange. The lithologies consist of sandstones and mudstones, interpreted as deformed turbidites originally deposited on the downgoing plate, but we identified no basalt, chert, or other non-terrigenous elements of ocean plate stratigraphy. Documented S-C fabrics and long axes of sandstone phacoids consistently indicate top-to-the-SW directed thrust displacement, now dipping at an average angle of 65°, perhaps rotated during exhumation. 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. Based on these characteristics, we interpret this fault zone as a newly discovered example of an exhumed paleomegathrust interface within the Olympic Mountains, 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.

Session

Subduction zone geology