Publication Date

Spring 2017

Degree Type


Degree Name

Master of Science (MS)




Robert B. Miller


Dikes, Duncan Hill pluton, Eocene, North Cascades, Structural Geology, Washington

Subject Areas



Voluminous Eocene dikes in the North Cascades are thought to provide insights into the regional strain field during postulated ridge–trench interaction in the Pacific Northwest. One understudied area with a large number of dikes is spatially associated with the elongate (NW-SE), ~46 Ma granodioritic Duncan Hill pluton. The ~80 km2 study area contains the shallow part of this pluton and the host schist of the Twentyfive Mile Creek unit and tonalite and migmatite of the Cretaceous Chelan Complex. Field observations and petrographic analyses indicate that the dikes in the study area can be sub-divided compositionally and texturally into three types: intermediate-mafic, granite porphyry, and rhyolite. Extension magnitudes from the dikes (n=438) are ~4% to 7%. The dikes have predominantly steep dips, but vary widely in strike and define a broadly bimodal pattern of NW (301°) and NE (026°) strikes. Orientations indicate that the Duncan Hill pluton did not modify the strain field, whereas the strong foliation and anisotropy in the Twentyfive Mile Creek unit may be responsible for some of the NW strikes. The NE-striking dikes likely record the regional NW-SE extension that was active when they intruded. From the data collected in this study, it cannot be ruled out that the NW-striking dikes intruded at different times and under a different regional strain field. This work has shown that dike orientations in the study area are much more complex than those documented elsewhere in the North Cascades.