Rates and Kinematics of Strike-slip Faults bounding the Olympic Peninsula of Washington State

Colin Amos, Western Washington University
Elizabeth Schermer, Western Washington University
Jenna Chaffeur, Western Washington University
Tammy Rittenour, Utah State University
William C. Duckworth, BGC Engineering Inc.
Jaime Delano, University of Canterbury

Abstract

Active transcurrent faults bounding the northern and southern margins of the Olympic Mountains in Washington state accommodate permanent strain in the upper plate of the Cascadia subduction zone. Kinematic constraints for these structures come primarily from surficial geologic mapping and lidar analysis of displaced debris flow channels, as well as previous paleoseismic trenching studies. In the north, the WNW-striking North Olympic fault zone parallels the Strait of Juan de Fuca for 80 km and exhibits primarily dextral displacements between 4 – 30 m for landforms developed in post-last glacial sediment and underlying bedrock. In contrast, offset landforms along the NE-striking Canyon River and Frigid Creek faults display dominantly sinistral displacements in a segmented fault zone extending ~40 km along the southern margin of the Olympic Mountains. Individual offsets of debris flow channels on the Canyon River fault range up to 60 m in areas outside the limit of alpine last-glacial advance. Indirect dating of offset degradational landforms (i.e., debris-flow channels) comes from luminescence ages of outwash fills and fluvial terrace cover sediments bracketing the timing of ice sheet retreat and alpine glaciation. We argue that such events effectively removed geomorphic evidence of prior surface ruptures from the landscape and provide maximum bounds on the age of younger displaced landforms. On the North Olympic fault, luminescence ages limit the timing of retreat of the Juan de Fuca lobe of the Cordilleran ice sheet at ca. 14 ka and suggest rates of dextral strike slip upward of 2 mm/yr. Bracketing ages from strath terraces outside last-glacial limit on the Canyon River fault dated at ca. 45 ka indicate comparable rates of sinistral strike-slip in the south. Taken together, these rates suggest that conjugate strike slip displacement along the flanks of the Olympic Mountains (dextral in the north, sinistral in the south) reflects a significant portion of margin-parallel shortening in the Cascadia forearc. This style of deformation contrasts with structures within and east of Puget Sound, where margin-perpendicular reverse faults and associated folds dominate the response to active upper-plate shortening.

Session

Neotectonics
earthquake geology