The northern Cascade Mountains in Washington (USA) preserve an exceptional shallow to mid-crustal record of Eocene transtension marked by dextral strike-slip faulting, intrusion of dike swarms and plutons, rapid non-marine sedimentation, and ductile flow and rapid cooling in parts of the North Cascades crystalline core. Transtension occurred during ridge-trench interaction with the formation of a slab window, and slab rollback and break-off occurred shortly after collision of the Siletzia oceanic plateau at ca. 50 Ma. Dike swarms intruded a ≥1250 km2 region between ca. 49.3 Ma and 44.9 Ma, and orientations of more than 1500 measured dikes coupled with geochronologic data provide important snapshots of the regional strain field. The mafic Teanaway dikes are the southernmost and most voluminous of the swarms. They strike NE (mean = 036°) and average ~15 m in thickness. To the north, rhyolitic to basaltic dikes overlap spatially with 49.3–46.5 Ma, mainly granodioritic plutons, but they typically predate the nearby plutons by ca. 500 k.y. The average orientations of five of the six dike domains range from 010° to 058°; W-NW– to NW–striking dikes characterize one domain and are found in lesser amounts in a few other domains. Overall, the mean strike for all Eocene dikes is 035°, and the average extension direction (305°–125°) is oblique to the strike (~320°) of the North Cascades orogen. Extension by diking reached ~45% in one >7-km-long transect through the Teanaway swarm and ranged from ~5% to locally ~79% in shorter transects across other swarms, which corresponds to a minimum of ~12 km of extension. The dominant NE-striking dikes are compatible with the dextral motion on the N- to NW-striking (~355–320°) regional strike-slip faults. Some of the W–NW- to NW-striking dikes were arguably influenced by pre-existing faults, shear fractures, and foliations, and potentially in one swarm where both NE- and lesser W-NW–striking dikes are present, by a switch in principal stress axes induced by dike emplacement. Alternatively, the W-NW– to NW-striking dikes may reflect a younger regional strain field, as ca. 49.3–47.5 Ma U-Pb zircon ages of the NE-striking dikes are older than those of the few dated W-NW– to NW-trending dikes. In one scenario, NE-striking dikes intruded during an interval when strain mainly reflected dextral strike-slip faulting, and the younger dikes record a switch to more arc-normal extension. Diking ended as magmatism migrated into a N-S–trending belt west of the North Cascade core that marks the initiation of the ancestral Cascade arc.

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 km 2 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 subdivided 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. v ACKNOWLEDGMENTS I would like to thank some very important people in my life, whom without, this thesis would have never been possible. First, I would like to thank my Mom and Dad. It is because of your love, support, and encouragement, that I could explore my dreams and become the person I am today. I want to thank Christina Pecota, for putting up with my craziness during my "thesising time". Thank you for all your support during this time, it has helped me more than you realize. Kelly Dustin, Callie Sendek, and Monika Leopold; your friendship and support has meant the world to me, especially Monika who started this adventure with me as a roommate and quickly turned into a best friend. Thank you for always being there for me. Without your Illustrator help my figures probably would be a lot different. To Leslie, Alphonse and Ginny, thank you for always taking care of any situation in the department. To my North Cascades colleagues and field assistants (Kelly Dustin, Neil Fenning, Nick Sylva, Gregg Armstrong, Mark Christsten), thank you for your help. Mike Eddy at MIT for taking time to date some of my rocks. I would like to especially thank my thesis committee members, Ellen Metzger and Jonathan Miller: Your doors were always open, and guidance and encouragement were always plentiful. Last but certainly not least; I would like to thank my thesis advisor, Robert Miller for his constant support of my work. The patience and mentorship you showed me were essential in getting me through this project. You will always be my geology D...