ARC ACCRETION AND DEFORMATION IN THE NORTHWEST CASCADES, WASHINGTON

A record of Permian to Neogene polyphase deformation, metamorphism and magmatism is preserved in the Cascades River region of the North Cascades, recording deep to shallow crustal processes at an evolving convergent margin. The accreted Cascade River island arc comprises greenschist-amphibolite facies wall rocks intruded by Jurassic-Cretaceous plutons and deformed in the Mesozoic continental margin magmatic arc. Our mapping and U-Pb zircon ages define a protolith stratigraphy with plutonic and volcanic rocks as old as Permian (~260 Ma) overlain by Jurassic (~180 Ma) metasediments.

New structural and geochronologic data combined with prior work document at least four ductile deformation events (D1-D4) followed by brittle strike-slip faulting along the Entiat fault. D1 created greenschist to lower amphibolite facies S1 foliation. D2 is a composite event that includes large-scale NW and SE-vergent recumbent isoclines and refolded small-scale folds northeast of the Entiat fault and a SW-vergent thrust southwest of the Entiat fault. D3 created a NW-striking dextral shear zone roughly along the Entiat fault. D4 produced km-scale gently NW- and SE plunging upright folds. New U-Pb zircon ages that bracket deformation include an orthogneiss with the S1 fabric dated at 168.6 ± 4.8 Ma and an undeformed (post-D2) dike at 89.8 ± 2.2 Ma. New ⁴⁰Ar-³⁹Ar ages on metamorphic hornblende and muscovite include S1 hornblende with a cooling age of 144.6 ± 6.5 Ma and S2 muscovite with a cooling age of 87.58 ± 0.72 Ma. Sericite in D3 mylonite yielded an age of 46.13 ± 0.31 Ma.

The data bracket D1, interpreted as accretion of the island arc, to between 169 and 145 Ma. The initiation of a prolonged and complex D2 during burial and intrusion is recorded by metamorphic and magmatic ages ranging from ~95 Ma to 76 Ma. This may reflect differing kinematics at different crustal levels during transpression and NW translation or the local effects of pluton emplacement and rheological contrasts. Lower grade overprint and D3 dextral shear suggest initiation of the Entiat fault in the Eocene, followed by D4 NE-SW shortening consistent with regional trantension and transpression during Paleogene time. The complex deformation and metamorphism of the NW Cascades reflects long-lived oblique subduction and strain partioning along the western edge of the magmatic arc.