Abstract

The presence of early Eocene near-trench magmatism in western Washington and southern British Columbia has led to speculation that this area experienced ridge-trench interaction during that time. However, the effects of this process as they are preserved in other parts of the geologic record are poorly known. We present high-precision U-Pb zircon geochronology from Paleogene nonmarine sedimentary and volcanic sequences in central and western Washington that preserve a record of tectonic events between ca. 60 and 45 Ma. The data reveal that the Swauk, Chuckanut, and Manastash Formations formed a nonmarine sedimentary basin along the North American margin between ≤59.9 and 51.3 Ma. This basin experienced significant disruption that culminated in basinwide deformation, uplift, and partial erosion during accretion of the Siletzia terrane between 51.3 and 49.9 Ma. Immediately following accretion, dextral strike-slip faulting began, or accelerated, on the Darrington–Devil’s Mountain, Entiat, Leavenworth, Eagle Creek, and Straight Creek–Fraser fault zones between 50 and 46 Ma. During this time, the Chumstick Formation was deposited in a strike-slip basin coeval with near-trench magmatism. Faulting continued on the Entiat, Eagle Creek, and Leavenworth faults until a regional sedimentary basin was reestablished ≤45.9 Ma, and may have continued on the Straight Creek–Fraser fault until 35–30 Ma. This record of basin disruption, volcanism, and strike-slip faulting is consistent with ridge-trench interaction and supports the presence of an oceanic spreading ridge at this latitude along the North American margin during the early Eocene.

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