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Date of Award
Fall 2025
Document Type
Masters Thesis
Department or Program Affiliation
Geology
Degree Name
Master of Science (MS)
Department
Geology
First Advisor
Mulcahy, Sean
Second Advisor
Boujibar, Asmaa
Third Advisor
Betka, Paul Michael
Abstract
Exhumed subduction complexes are critical for understanding the tectonic history of convergent margins and the thermal evolution of early subduction processes. Thin slivers of high-grade rocks, often called metamorphic soles, preserved in many subduction complexes are formed during subduction initiation when the down going slab is heated and accreted to the overlying mantle wedge. Two primary mechanisms have been proposed to preserve these high-grade assemblages during continued subduction: 1) subduction refrigeration that cools and preserves the high-grade rocks at depth and 2) rapid exhumation which brings the high-grade assemblages to cooler and shallower levels. The Easton metamorphic suite of the Northwest Cascades is a Jurassic-Cretaceous subduction complex with an inverted metamorphic gradient of older accreted high-grade rocks above younger regional blueschist assemblages. Conventional thermometry and phase equilibrium modeling, combined with published geochronology, show that the peak temperatures of the high-grade assemblages decrease with age structurally downward, but at approximately constant pressure for ~20 Ma. The oldest rocks consist of blocks of Grt ± Cpx amphibolite that were metamorphosed at 700-760 °C (9-12 kbar) circa 203 Ma. These blocks are contained within foliated amphibolite that records peak metamorphic conditions of 630-700 °C (8-11 kbar) at 183 Ma. The foliated amphibolite is underlain by a highgrade blueschist that was metamorphosed at 500-550 °C (9-10 kbar) at ~ 173 Ma. Previous studies on the structurally lower regional Shuksan Greenschist indicate the unit was metamorphosed at 350-400 °C (7-8 kbar) between 140-136 Ma. The combined P-T-t history within the metamorphic sole of Easton metamorphic suite experienced a two stage cooling history that consisted of early subduction refrigeration at 10 kbar from ~183 Ma to 162 Ma that was followed exhumation to approximately 7-8 kbar by 148 Ma, followed by underplating of v regional blueschist units. The results from the Easton metamorphic suite suggest that both subduction refrigeration and exhumation-driven cooling were critical but temporally distinct mechanisms in the thermal evolution of early subduction complexes.
Type
Text
Keywords
Subduction initiation, metamorphism, metamorphic sole, amphibolite, blueschist, metasomatism, subduction refrigeration, exhumation
Publisher
Western Washington University
OCLC Number
1558597240
Subject – LCSH
Subduction zones--North Cascades (B.C. and Wash.); Metamorphism (Geology)--North Cascades (B.C. and Wash.); Amphibolite--North Cascades (B.C. and Wash.); Metamorphic rocks--Inclusions--North Cascades (B.C. and Wash.); Metasomatism (Mineralogy); Geology, Structural--North Cascades (B.C. and Wash.)
Geographic Coverage
North Cascades (B.C. and Wash.)
Format
application/pdf
Genre/Form
masters theses
Language
English
Rights
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.
Rights Statement
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Creative Commons License
This work is licensed under a Creative Commons Attribution-Share Alike 4.0 License.
Recommended Citation
Keirs, Austin M., "Investigating Models of Metamorphic Sole Preservation through Thermometry and Phase Equilibrium Modeling of the Easton Metamorphic Suite, Northwest Cascades, Washington" (2025). WWU Graduate School Collection. 1451.
https://cedar.wwu.edu/wwuet/1451