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Date Permissions Signed
Date of Award
Master of Science (MS)
Schermer, Elizabeth, 1959-
Brown, Edwin H.
Hirsch, David M., 1969-
Deformation events associated with crustal thickening are preserved in the Mt. Formidable region of the North Cascades, a Cretaceous orogen and continental arc at the southern end of the Coast Plutonic Complex. The kinematics and timing of deformation in this area during burial, magmatism and metamorphism are the focus of this study. Rocks in the Mt. Formidable region span from at least the Late Triassic to the Early Cretaceous and include supracrustal units and associated metaplutonic rocks that have been interpreted as an island arc sequence. Rocks within the study region include the sub-arc Marblemount Meta-Quartz Diorite, the supra-arc metaclastic and metavolcanic Cascade River unit, interlayered greenschist wallrock and leucogneiss intrusions of the Magic Mountain Gneiss and the ocean-floor Napeequa Schist. Based on new geochronologic data from this study, the name "Magic Mountain Gneiss" is abandoned and the unit is herein referred to as the Magic Mountain Complex. New results indicate that rocks in the Mt. Formidable region experienced at least three generations of ductile deformation and two phases of metamorphism. F1 folds are isoclinal; the axial-planar, regional S1 foliation formed as a result of upper greenschist to lower amphibolite-facies metamorphism (M1). D2 formed tight to isoclinal F2 passive shear folds and a local, incipient crenulation foliation (S2) at greenschist-facies metamorphic conditions (M2). Restored F2 fold orientations are scattered, but suggest that D2 involved sub-vertical shortening and possibly NW or SE sub-horizontal shearing. Open to close F3 folds occur in Type 1, 2 and 3 fold interference patterns with F2 folds. Post-metamorphic D3 is characterized by NE-SW shortening and upright buckling. Variation in foliation, restored S2 surfaces and F3 axial planes suggest post-D3 deformation by a map-scale F4 structure. New dates from the Magic Mountain Complex provide important age constraints for D1 deformation and offer insight into the thermal history of the Mt. Formidable region. Two new 40Ar/39Ar cooling ages, 82.7 ± 2.3 Ma for hornblende and 46 ± 16 Ma for muscovite, are consistent with Late Cretaceous and Eocene cooling ages from the northern Cascades core. A 238U/206Pb zircon age of a foliated, pre-D1 orthogneiss sill is ~155 Ma. A late to post-D1 pegmatite dike, which is folded by F2, yields a 238U/206Pb zircon age of 137.5±5.9 Ma. These ages bracket D1 deformation and regional metamorphism between the latest Jurassic to earliest Cretaceous. Dates from this study are consistent with deformation being older than previously estimated in this portion of the Cascades, which has been inferred as mid to Late Cretaceous. Correlation of rocks in the Mt. Formidable region to similar Triassic arc units and Late Jurassic to Early Cretaceous intrusions in southern British Columbia suggests that D1 may record a major deformational event related to terrane accretion.
Western Washington University
Formidable, Mount (Wash.)
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Labadie, Julia E. (Julia Eileen), "The structural and tectonic history of the Mt. Formidable region, North Cascades, Washington" (2010). WWU Graduate School Collection. 33.