Portland State University. Department of Geology
Date of Award
Master of Science (M.S.) in Geology
1 online resource (xii, 118 pages)
The Olympic Mountains, in the cost ranges of northwest Washington, expose a Cenozoic accretionary complex east of the Cascadia Subduction Zone. Accreted material consists of metasedimentary deposits thrust eastward beneath a basaltic terrane (i.e., the basaltic Crescent Formation and the basal Blue Mountain Unit [BMU] turbidite) along a major fault, the Hurricane Ridge Fault (HRF). Recent isotopic dating of zircons from the BMU indicate that it is about 8 my younger than the basalt, implying another major fault may exist east of the HRF, between the BMU and the Crescent Fm. Field observations, data, and samples for microstructural analysis were collected along the Dosewallips River on the eastern side of the mountains beginning just west of the HRF, across the fault and BMU, ending about 4.5 km to the east in Crescent Fm. 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 that encloses a 4 m wide fault, consisting of an increase then drop in fracture density, progression of comminuted material, and the presence of cataclasite and pseudotachylyte. The structure frequency declines to the east for 0.8 km until our second study site within the Crescent Fm. This outcrop is also notably cut by cataclasite and pseudotachylyte and has a similar increase then drop in fracture density leading to a fault structure, although a fault core has not been identified there. The mineral assemblage in these fault structures includes prehnite, pumpellyite, and epidote, implying that fault deformation occurred at depth along the boundary between the prehnite-pumpellyite and greenschist facies. The fault at the BMU and Crescent Fm. contact, which we name the Ori fault, is a well-developed fault with a mappable damage zone and fault core. This supports isotopic age determined hypothesized thrust fault between the BMU and Crescent Fm. The absence of fault-related deformation between the west and east sites implies two separate fault structures. Overall, evidence from this study suggests primary fault deformation at the contact between the BMU and Crescent Fm. and also within the Crescent Fm. The fault structures documented in this study reveal that along the eastern side of the Olympic Mountains, the Crescent Fm. is more deformed than previously described and that the Siletzia terrane was once seismically active, potentially linked to accretionary processes.
Maher, Christine, "Fault Deformation at the Base of and within the Crescent Formation along the Dosewallips River, Olympic Mountains Peninsula, Washington, USA" (2019). Dissertations and Theses. Paper 5177.