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Date of Award

Winter 2024

Document Type

Masters Thesis

Department or Program Affiliation


Degree Name

Master of Science (MS)



First Advisor

Mulcahy, Sean,

Second Advisor

Schermer, Elizabeth, 1959-

Third Advisor

Roland, Emily Carlson,


The Easton metamorphic suite (EMS) of the Northwest Cascades is an exhumed Jurassic-Cretaceous subduction complex that can be used to test models for subduction zone deformation. Regionally extensive blueschist/greenschist within the EMS were accreted in a thermally evolving subduction zone (148 – 137 Ma). Two units, the Darrington phyllite and the Shuksan greenschist, were originally interpreted as coherent units that subducted in a zone of distributed deformation. New field mapping, microstructural analysis, and thermometry suggest the Darrington phyllite should be divided into two separate units, the Darrington phyllite and Silver phyllite. The Darrington phyllite, Silver phyllite, and Shuksan greenschist preserve different foliation and deformation histories. In addition, Raman spectroscopy of carbonaceous material (RSCM) thermometry in the Darrington and Silver phyllite and published thermometry from the greenschist suggests the units have different thermal histories. RSCM temperatures for the Darrington phyllite and Silver phyllite are 374 – 400 °C and 430 – 450 °C respectively. In contrast, the Shuksan greenschist formed at conditions of ~360 °C. New deformation and thermometry data combined with existing 40Ar/39Ar ages suggest the Darrington phyllite and Silver phyllite are separate units and that the Silver phyllite and Shuksan greenschist were subducted as tectonic slices rather than as a coherent unit. The style of deformation within the EMS supports models of localized deformation along shear zones that facilitate subduction accretion.




subduction zone processes, metamorphic petrology, structural geology, raman spectroscopy, petrography and microstructure


Western Washington University

OCLC Number


Subject – LCSH

Metamorphism (Geology)--Cascade Range; Geology, Structural--Cascade Range; Schists--Cascade Range; Phyllite--Cascade Range

Geographic Coverage

Cascadia Subduction Zone; Cascade Range




masters theses




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