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Master of Science (MS)
Engebretson, David C.
Burmester, Russell F.
Babcock, R. Scott (Randall Scott)
Use of a small-diameter core drill has allowed the paleomagnetic sampling of the rims of fractured pillow basalts of the lower Crescent Formation in the northern Olympic Mountains. The pillows selected have spherical or oblate morphologies which typically develop on horizontal or mildly-dipping surfaces. Pillow keel structures and sedimentary interbeds were used to obtain bedding attitudes and top directions for use in structural corrections. All specimens were subjected to progressive thermal demagnetization. After removal of a low blocking-temperature recent overprint, stable endpoints were reached by 580°C in 11 of the 33 sites sampled (large within-site scatter was commonly observed in the remaining sites). Among the accepted sites, within-site scatter was small and correction for bedding tilt significantly reduced the scatter between sites. The mean paleomagnetic pole for this investigation is 86.4° north latitude, 170.0° east longitude, A95=16.5° which agrees with the expected early to middle Eocene pole for North America. When combined with previous work from subaerial basalt exposures of the upper Crescent Formation in and near the eastern Olympic Mountains, these results (80.7° north latitude, 192.0° east longitude, A95 =8.0°, N=46) show no significant rotation (0.8° ± 14.4°) or poleward displacement (-3.6 ± 8.5). Analysis of the magnetic mineralogy suggests that the remanence is early, if not primary. The pole, therefore, should be valid for tectonic interpretation of the region. A circular distribution of virtual geomagnetic poles after correction for bedding tilt supports the hypothesis that the northern Crescent Formation experienced deformation due to the rise, in a dome-like fashion, of the sediments of the Olympic Core terrane. Erosion of a partial dome open to the west could have produced the curvature seen in the outcrop pattern of the Crescent Formation. The lack of significant rotation of the northernmost Coast Ranges is in contrast with the net clockwise rotation seen to the south. A possible explanation for this difference may be that southern Vancouver Island acted as a backstop thereby restricting rotational deformation. In addition, the Olympic Mountains may have been north of the rotational influence of differential Basin and Range extension.
Tectonics, Crescent Formation
Western Washington University
Subject – LCSH
Paleomagnetism--Washington (State)--Olympic Mountains; Plate tectonics--Washington (State)--Olympic Mountains; Geology, Structural--Washington (State)--Olympic Mountains
Olympic Mountains (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.
Warnock, Andrew C. (Andrew Clyde), "Paleomagnetism and Tectonics of the Crescent Formation Northern Olympic Mountains, Washington" (1992). WWU Graduate School Collection. 816.