Structural History of the Formidable-Le Conte Mountain Area, North Cascades, WA

A record of Permian to Tertiary polyphase deformation, metamorphism and magmatism is preserved in the Cascades River region of the North Cascades Crystalline Core, Washington. The tectonic history recorded here can be used to understand the deep crustal processes occurring within active convergent plate margins around the world. New structural and geochronologic data from this study are used to advance our understanding of terrane origin, accretion, and processes that altered and thickened the crust of the North Cascades. The study focuses on several rock units within an accreted Permian-Jurassic island arc, the Chelan Mountains terrane, that was tectonized in Cretaceous time. Rock units include the Magic Mountain Gneiss (Kmm), the Marblemount Meta-Quartz Diorite (Kmqd), and the Cascade River Schist (Kcrs). Local dikes and sills and their relationship to regional tectonism are also discussed. Observations of ductile structures revealed at least four deformation events in the study area. The first deformation event (D1) created a penetrative greenschist to lower-amphibolite facies S1 foliation roughly parallel to unit contacts. The second deformation event (D2) formed abundant tight to isoclinal F2 folds of various size. A majority of F2 fold axial planes are upright and trend E-W, and the largest observed folds are gently inclined, gently NE plunging, and NW and SE vergent. The third deformation event (D3) created an S3 shear fabric during dextral shearing on steep NW-striking planes. The fourth deformation event (D4) created upright folds that plunge to the SE. New radiometric ages refine the age of Chelan Mountains Terrane protoliths and timing of metamorphic and deformation events. Zircon U-Pb dating provided ages for protoliths and dikes that bracket deformation events. Sampling of different compositional layers in the Kmm provided two age populations that represent separate pulses of magmatism at ~260 Ma and ~214 Ma. Two samples of Spider Mountain Schist (Ksms) yielded two maximum depositional ages of 181.6 ± 4.9 Ma and 179.6 ± 4.5 Ma. A granodioritic orthogneiss sill, the youngest rock with the S1 fabric, yielded a 168.6 ± 4.8 Ma age. An undeformed, post-D2, tonalite dike yielded an age of 89.8 ± 2.2 Ma. Metamorphic hornblende and muscovite dated by 40Ar-39Ar geochronology provided several new dates to constrain metamorphism. A hornblende sample aligned within S1 yielded a cooling age of 144.6 ± 6.5 Ma. A muscovite sample aligned within S2 yielded a cooling age of 87.58 ± 0.72 Ma. Unoriented hornblende in an amphibolite sample yielded a cooling age of 81.4 ± 1.4 Ma. A sericite lens aligned with S3 in mylonite yielded a cooling age of 46.13 ± 0.31 Ma. The structural and geochronology data in this thesis helps bracket D1 to between 168.6 ± 4.8 Ma and 144.6 ± 6.5 Ma, interpreted as accretion of the Chelan Mountains terrane. The D2 event occurred by 89.8 ± 2.2 Ma and is interpreted to reflect NW translation and deformation of the Chelan Mountains terrane. The 87.58 ± 0.72 Ma muscovite dates cooling after D2. The 81.4 ± 1.4 Ma hornblende dates cooling after Late Cretaceous plutonism, and the 46.13 ± 0.31 Ma sericite lens dates cooling during and after D3 dextral shearing. D4 then folded all preexisting ductile structures. The results of this thesis are significant because they clarify previous research regarding the origin of protoliths, refine the timing of accretion of the Chelan Mountains Terrane, and clarify the relationship between plutonism, metamorphism, and the timing of orogen-parallel vs orogen-perpendicular deformation events