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Master of Science (MS)
Beck, Myrl E.
Burmester, Russell F.
Engebretson, David C.
Paleomagnetic directions of samples from the Ten Peak and Sulphur Mtn. plutons, Hidden Lake stock, and Oval Peak batholith in the North Cascades reveal multi-component magnetization and instability. In the Hidden Lake stock and Ten Peak pluton, a strong viscous overprint parallels the present day field. The Sulphur Mountain pluton shows complete magnetic instability; making it impossible to compute a meaningful mean direction. The rocks of the Oval Peak batholith are believed to have been magnetically reset during the Eocene, although the observed declination of 167° and inclination of -67° is significantly different from the expected Eocene direction. The mean direction for the Oval Peak batholith is very similar to directions obtained by Strickler (1982) for the Black Peak batholith, and by Stauss (1982) for dikes in the Corbaley Canyon area. These three units with similar paleomagnetic discordance are found within a panel of rock bound on the east by the Ross Lake fault zone and on the west by the Entiat fault. The rock of the Corbaley Canyon area is Eocene, while the Black Peak batholith has been dated using K/Ar methods at 88.4 Ma and 103 Ma (hornblende), and 73 Ma (biotite). It seems likely that the magnetization of the Black Peak batholith was thermally reset during the Eocene, when it was intruded by the Golden Horn batholith. The inclination for the observed direction from the three units is similar to that expected for the Eocene, but the declination is discordant by approximately 170°. Acquisition of both normal and reverse aberrant directions during anomalous states of the magnetic field is improbable. Bias by an uncleaned overprint might explain the imperfection of antiparallelism, but cannot account for discordance of both normal and reverse magnetizations. Models involving large scale clockwise rotation or tilt down to the northwest could account for the observed discordance. However, there is no geologic evidence to support either model. A better explanation may be that relatively small blocks have individually rotated clockwise or have undergone large tilts down to the northwest, but again, geologic evidence for such displacement has yet to be recognized.
Petrology and magnetic characteristics of rocks from this study were compared to those of magnetically stable intrusive rocks of the Mt. Stuart and Chilliwack batholiths and Fawn Peak stock. Comparison of natural remanent magnetization (NRM) with saturation isothermal remanent magnetization (IRM) for magnetically stable and unstable samples indicates that the magnetization of the unstable samples from this study resides in multi-domain magnetic grains, whereas the magnetically stable samples show single-domain grain magnetic behavior. Tiny grains of magnetite are seen in the plagioclase of the magnetically stable rocks whereas none appear in the magnetically unstable rocks. These tiny magnetite grains may be responsible for magnetic stability in the stable rocks. Lack of magnetite in the plagioclase of the unstable rocks may explain why they are magnetically unstable. Initial magmatic compositions, cooling conditions, or subsequent alteration of plagioclase may be considered as possibilities explaining the lack of magnetite.
Petrographic studies show that the magnetically unstable rocks of this study are more altered (i.e., sericitization of plagioclase) and strained (recognized by undulatory extinction and recrystallized quartz) than the magnetically stable rocks from the Mt. Stuart and Chilliwack batholiths and Fawn Peak stock. Magnetically unstable rocks of this study show a persistent VRM overprint stable to approximately lOOoe and parallel to the present day field. Stott and Stacey (I960) have shown that stress may aid a rock in acquiring a VRM. It is not known when these rocks experienced deformation in their history, but, as the field direction has changed little since the Mid-Tertiary, it appears a possibility that stress may have aided in the acquisition of a VRM overprint since that time.
Western Washington University
Subject – LCSH
Paleomagnetism--Cascade Range; Geology, Stratigraphic--Cretaceous; Geology--Cascade Range
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.
Harrison, William J. (William James), "Paleomagnetism of Four Late Cretaceous Plutons North Cascades, Washington" (1984). WWU Graduate School Collection. 830.