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Date of Award

Summer 2023

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

Roeske, Sarah Melissa


The Easton metamorphic suite of the Northwest Cascades Thrust System (NWCS) is a well-preserved subduction accretion complex in Washington State. The regional blueschist units of the Easton metamorphic suite include the Mt. Josephine semi-schist, Darrington Phyllite, and Shuksan greenschist/blueschist and all are interpreted to have accreted after the onset of Jurassic subduction beneath North America. This study uses zircon U-Pb geochronology, structure, and field observations to test the regional correlations between units in the Easton metamorphic suite and address models for the timing of subduction accretion along the North American margin in the Late Jurassic – Early Cretaceous. The results suggest that previously correlative units in the Easton metamorphic suite are instead structurally distinct litho-tectonic units. The Mt. Josephine semi-schist has at least two distinct groups of maximum depositional ages at ~144 Ma and ~122 Ma and suggests different sources within the same unit or that the Mt Josephine semi-schist consists of units accreted at separate times. The Lummi Formation in the NWCS yields similar maximum depositional ages as the older portions of the Mt Josephine semi-schist and may be correlative. The oldest portions of the Mt. Josephine appear to be ~8 m.y. younger than the Darrington phyllite and the youngest Mt. Josephine maximum depositional ages are younger than previously published white mica 40Ar/39Ar ages of the Darrington. The Shuksan greenschist has a protolith age of ~166 Ma but the structural and metamorphic history is different from the Darrington phyllite and the two units should be considered separately. The geochronology presented in this study documents an almost complete continuum of zircon ages from the Mid-Jurassic up to ~120 Ma. Deposition and subsequent quick subduction accretion in the Easton metamorphic suite occurred ~30 m.y. earlier than the main accretionary phase in the Franciscan of California and suggests that proposed changes in relative Pacific - North American plate motion did not solely drive changes from subduction erosion to subduction accretion.




geology, subduction zone, geochronology, NWCS, structural geology, U-Pb dating, Easton metamorphic suite


Western Washington University

OCLC Number


Subject – LCSH

Geological time; Subduction zones--Cascade Range; Metamorphism (Geology)--Cascade Range; Geology, Structural--Cascade Range

Geographic Coverage

Cascade Range




masters theses




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