Structural Analysis of Eocene Dike Swarms in and Near the Duncan Hill Pluton, North Cascades, Washington

Abstract

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...

Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, northwestern Cordillera (USA)

“…Granite and rhyolite dikes next to the shallow part of the ca. 46 Ma Duncan Hill pluton commonly strike northeast (Tabor et al, 1987a;Dellinger, 1996;Bryant and Miller, 2014). Northwest-striking dikes have also been reported from southeast of the pluton (Waters, 1926;Tabor et al, 1987a;Sylva and Miller, 2014).…”

Eocene dike orientations across the Washington Cascades in response to a major strike-slip faulting episode and ridge-trench interaction

“…Orientations of dikes were measured as part of mapping and structural analysis concentrating on metamorphic and plutonic rocks in the North Cascades (e.g., Cooper Mountain and western Golden Horn dikes; Raviola, 1988) and as more focused studies of dikes (Teanaway, Corbaley Canyon, and Duncan Hill dikes; Doran, 2009;Bryant, 2017). Dike strikes were also summarized from other workers (Tabor et al, 1968;Mendoza, 2008).…”

“…A much smaller maxima strikes WNW (301°) and dips steeply NNE. Dike orientations were subdivided on the basis of distance from the Duncan Hill pluton by Bryant (2017). Dikes (n = 111) within 3 km of the pluton have a mean strike of 027°, whereas those farther from the pluton have a mean of 021°.…”

“…The potential influence of host rock lithology, combined with distance from the Duncan Hill pluton, was also considered (Bryant, 2017). The average strikes of dikes intruding the strongly foliated Twentyfive Mile Creek schist are NW (~310°; n = 57 dikes) within 5 km of the Duncan Hill pluton and swing to E-W (087°; n = 66 dikes) at distances of 5-10 km from the pluton (Bryant, 2017). In contrast, dikes intruding the weakly to moderately foliated Chelan Complex consistently strike NE (~038°; n = 136) within 10 km of the pluton (Fig.…”

“…In contrast, dikes intruding the weakly to moderately foliated Chelan Complex consistently strike NE (~038°; n = 136) within 10 km of the pluton (Fig. 10) (Bryant, 2017).…”