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Date Permissions Signed
Date of Award
Master of Science (MS)
Housen, Bernard Arthur
DeBari, Susan M., 1962-
Brown, Edwin H.
A study of the 48 Ma Cooper Mountain pluton (CMP) in the North Cascade Mountains of Washington using anisotropy of magnetic susceptibility (AMS) defined the orientation of magnetic fabrics. Fabrics in limited areas at the margins of the CMP tend to be parallel to the pluton margin and are therefore interpreted to be emplacement-related. The fabrics in the interior of the body, throughout the bulk of the pluton, are discordant with respect to the NW pluton margin. The fabric is manifest by NW-striking, moderately to steeply dipping foliation and NW-SE trending, moderately to shallowly plunging lineation, approximately parallel to regional structural trends in the Cascade Crystalline Core. Discordance of the fabric to the pluton margin and near concordance with regional structures suggests a tectonic origin.
Remanent magnetization was measured to determine if the CMP has been reoriented since emplacement. The characteristic remanence was unblocked in some samples at 580°C and in others at 370°C. A variety of techniques were used to determine that magnetite and pyrrhotite are the remanence-carrying minerals. Magnetic directions obtained from both of these remanence carriers plot, within error, on the North American expected Eocene direction (Diehl et al. 1983) suggesting that there has been no reorientation of the pluton since remanence was acquired and that magnetic fabrics require no correction to obtain their Eocene orientation.
Results of fabric analysis indicate that the Cooper Mountain pluton is a syntectonic pluton rather than post-tectonic (Haugerud et al. 1991b) due to the tectonic development of magmatic fabrics. This fabric is slightly oblique to the length of the Cascade orogen and is thus best interpreted to have formed as a consequence of regional dextral-shear due to transpression.
Cooper Mountain pluton, Geomagnetism, Syntectonic pluton, Remancence
Western Washington University
Subject – LCSH
Intrusions (Geology)--Magnetic properties--Cascade Range; Intrusions (Geology)--Washington (State)--Okanogan County; Intrusions (Geology)--Washington (State)--Chelan County; Geology, Structural--Cascade Range; Petrology--Cascade Range
Cascade Range; Okanogan County (Wash.); Chelan County (Wash.)
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 thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.
Fawcett, Tammy C., "Geology of the Cooper Mountain Pluton, North Cascade Mountains, Washington Based on Magnetic Fabrics, Magnetic Remanence and Petrography" (2001). WWU Graduate School Collection. 833.