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Date of Award
Master of Science (MS)
Schermer, Elizabeth, 1959-
Stelling, Peter L.
The San Juan Thrust System represents the western elements of the Cascades orogen and preserves evidence for Cretaceous Cordilleran margin tectonics. The kinematics of deformation phases and their temporal relationship to accretionary wedge high-pressure low-temperature metamorphism remains uncertain. The structural and metamorphic evolution of the San Juan Thrust System was studied on Orcas and Shaw Islands in Western Washington. Detailed field mapping indicates that a widespread S1 flattening fabric (formed during D1) is subparallel to and cut by an S2 fabric found within brittle-ductile shear zones that bound the terranes (formed during D2). Post-cleavage brittle structures (formed during D3) offset terrane contacts and S2 fabrics and include cm- to m-scale slip on normal, strike-slip, and thrust faults that have mutual crosscutting relationships. Metamorphic investigations utilizing vein mineralogy and fluid inclusion analysis reveal the pressure and temperature relations during each stage of deformation. Maximum temperatures of ~200˚C and pressures of ≥5.5 kbar are indicated by both the preservation of aragonite within D1-D3 veins, and by a minor crystal-plastic component of quartz deformation within D2 shear zones. These conditions are interpreted to indicate deformation occurred in an accretionary wedge setting.
Using pressure-temperature constraints and kinematics of each event, I suggest a tectonic model that includes (1) D1 terrane accretion and fabric formation somewhere south along the continental margin, (2) NW-directed translation along the margin during oblique subduction as terranes were assembled on top of one another into their current nappe stack along D2 shear zones, and (3) subhorizontal extension and vertical thinning along brittle D3 structures. A predominance of margin-parallel extension in the forearc during D3 may be a response to “unbuttressed” collapse of an overthickened wedge, or from continued oblique subduction in the presence of a curved margin. This model supports previous interpretations of a NW-dominated thrust system, but high-pressure low-temperature constraints indicate that assembly of the nappe stack and later D3 brittle deformation represent continued structural evolution within an accretionary wedge. Results of this study indicate that the structures accountable for exhumation of the San Juan Thrust System and emplacement of terranes above the unmetamorphosed Wrangellia terrane are yet to be discovered.
Western Washington University
Orcas Island (Wash.); Shaw Island (Wash. : Island)
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Quillan, Kevin, "Structural Evolution of the San Juan Thrust System, Orcas and Shaw Islands, WA" (2017). WWU Graduate School Collection. 574.