FAULT DEFORMATION AT THE BASE OF AND WITHIN THE CRESCENT FORMATION ALONG THE DOSEWALLIPS RIVER, OLYMPIC MOUNTAINS PENINSULA, WASHINGTON, USA

The thickest part of the Cascadia Subduction Zone accretionary wedge is the Olympic Mountains Peninsula in Washington, USA. Accreted material in the Olympic Mountains includes two rock units: metasedimentary deposits and a basaltic terrane (i.e., the basaltic Crescent Formation and the basal Blue Mountain Unit [BMU] turbidite). Early in the geologic history, the Hurricane Ridge Fault (HRF) formed at the contact between the two rock units (Tabor & Cady, 1978). However, Eddy et al. (2017) states that there is an about 8 million year age difference between the BMU and the Crescent Fm., implying a thrust fault between them that is east of the HRF. Field observations, data, and samples for microstructural analysis were collected along the Dosewallips River ­­on the eastern side of the peninsula beginning just west of the HRF, across the fault and BMU, ending about 4.5 km into the Crescent Fm. to the east. Evidence for fault-related comminution and frictional melt is present at two locations along the transect: the base of Crescent Fm. and within the Crescent Fm. about 1.6 km from the base. At the contact between the BMU and the Crescent Fm., there is a damage zone with fault-related structures leading to an about 4 m wide fault core, including a progressive increase in fracture density and comminuted material, and the presence of cataclasite and pseudotachylyte. The structure frequency declines to the east until the second site within the Crescent Fm. where the rock is also notably cut by cataclasite and pseudotachylyte, although a fault core has not been identified there. The lack of a well-developed fault core may imply short-lived seismic activity that ended due to deformation localizing elsewhere. The mineral assemblage in these fault structures includes prehnite, pumpellyite, and epidote. The fault at the BMU and Crescent Fm. contact (proposed name: Ori Fault) is a well-developed fault with a mappable damage zone and fault core, supporting the existence of the thrust fault suggested by Eddy et al. (2017). The absence of fault-related deformation between the sites implies two separate fault structures. Overall, evidence from this study suggests fault deformation at the contact between the BMU and Crescent Fm. as well as higher into the Crescent Fm.