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Date of Award
Master of Science (MS)
Housen, Bernard Arthur
Amos, Colin B.
In this paper I present the results of paleomagnetically derived vertical axis rotations (VARs) of sites in two different flows of the Columbia River Basalt (CRB) – the 16 Ma Sentinel Bluffs member of the Grande Ronde flow and the 12 Ma Pomona Member of the Packsack Lookout – near the Doty fault in southwestern Washington. In two field seasons, I collected 99 cores from 14 sites, 11 in the Grande Ronde flow and three in the Pomona member flow. Of the 227 specimens that I demagnetized, 212 had well-defined magnetic directions. Positive fold and reversal tests results confirm the primary magnetization in both flows of the CRB, which is carried by magnetite and titanomagnetite. Using virtual geomagnetic poles (VGPs), I compared the tectonically corrected site mean directions from this study against reference directions measured in the same lithologies in the stable eastern Washington which has had little rotation since the Miocene. The resulting VARs in the Grande Ronde flow show a large range of rotations, from 68° counterclockwise (CCW) to 9.6° clockwise (CW) (Table 4), with an average CCW rotation of 22.3°. The Pomona Member sites all show CCW rotation between 0.5° and 2.5°, with an average CCW rotation of 2.1°. Previous paleomagnetic regional studies have consistently found an approximately 14-16° CW rotation in CRBs located on the forearc block. The CCW rotations of the CRBs adjacent the Doty fault suggest that fault activity, specifically sinistral strike-slip motion, occurred between 16 and 12 Ma, and possibly in the time since then. I hypothesize that an escape structure or wrench zone may be present in the region due to the concentrated, homogeneous stress present as the rotating Oregon-Washington forearc block squeezes the Puget lowlands block against the Canadian coast mountains. I propose that significant additional paleomagnetic research is needed in the area to determine the extent of the area with CCW rotation. This will also help create a more resolved map of where the boundaries of the rotating blocks are, how they may be rotating in relation to one another, and to help map the network of smaller faults facilitating the rotations.
Paleomagnetism, Paleomag, Structural Geology, Fault, Doty Fault, Vertical Axis Rotation, Doty
Western Washington University
Subject – LCSH
Paleomagnetism--Washington (State); Geology, Structural--Washington (State); Faults (Geology)--Washington (State)
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Linneman, Charles, "Paleomagnetic Determination of Vertical Axis Block Rotation near the Doty Fault in southwestern Washington" (2023). WWU Graduate School Collection. 1159.