ABSTRACT The incorporation of metasedimentary rocks into the mid- to deep crust of continental magmatic arcs has significant mechanical and geochemical consequences for arc systems. The Late Cretaceous–Eocene North Cascades arc is one of the few continental magmatic arcs in the world that exposes a large amount of exhumed deep-crustal metasedimentary rocks. Here, we investigate a range of processes that may have been important in transferring sediment into the arc by combining field mapping with bulk-rock Nd analyses, U-Pb and Hf-isotopic study of detrital zircons, and U-Pb dating of zircon and monazite to determine the timing of metamorphism and melt crystallization from metasedimentary samples collected in two deep-crustal domains of the North Cascades (the Skagit Gneiss and Swakane Gneiss). We also use these data to examine provenance links between the metasedimentary rocks and potential sediment sources in the accretionary wedge (western mélange belt), the forearc (Nooksack Formation), and the present-day backarc (Methow terrane) to the North Cascades arc. Jurassic strata of the Methow terrane and the Nooksack Formation have unimodal detrital zircon age peaks and near-depleted mantle ε Ηfi values, whereas zircons from the middle Cretaceous strata of the Methow terrane have a bimodal age distribution and less radiogenic ε Ηfi values. In comparison, the accretionary western mélange belt (WMB) has Jurassic to Upper Cretaceous sandstones characterized by multiple Mesozoic age peaks, and the Upper Cretaceous sandstones also reveal distinct Proterozoic zircon populations and unradiogenic Late Cretaceous zircons. The Skagit metasedimentary rocks yield zircon-age signatures that fall into two groups: (1) a wide range of zircon dates from Proterozoic to latest Cretaceous and (2) a more limited range of Late Triassic to latest Cretaceous grains with no Proterozoic zircons. Both groups reveal a mix of ε Ηfi values. The Swakane metasedimentary rocks have similar detrital zircon age signatures to Group 1 Skagit metasediments. For Swakane rocks, >100 Ma zircons have radiogenic ε Ηfi values, whereas younger zircons plot between near-depleted mantle to unradiogenic values. Overall, the data are most consistent with some metasedimentary rocks of the Swakane and Skagit Gneisses being sourced from either the forearc or the accretionary wedge. This sedimentary material was buried to mid-crustal depths by ca. 75–65 Ma, coeval with major magmatism within the North Cascades arc. Moreover, the distinct combination of unradiogenic Late Cretaceous detrital zircons and ca. 1.4–1.3 and 1.8–1.6 Ga Proterozoic peaks is documented in many of the forearc and accretionary-wedge units exposed along western North America. The Proterozoic peaks likely reflect zircon derived from southwestern Laurentian crust, equivalent to the latitude of the present-day Mojave Desert. Therefore, the detrital-zircon results from both the Swakane and Skagit Gneisses, as well as parts of the accretionary wedge, support at least moderate translation of sedimentary material along the margin of western North America during the Late Cretaceous.

ABSTRACT This guide describes a three-day field trip to the Paleogene sedimentary and volcanic rocks exposed between the Straight Creek–Fraser River and Entiat faults in the central Washington Cascades. These rocks record a history of deposition, deformation, and magmatism that can be linked to tectonic events along the North American margin using a robust chronology coupled with detailed sedimentological, stratigraphic, and structural studies. These events include deposition in a large sedimentary basin (Swauk basin) that formed in the forearc from <59.9–50 Ma; disruption and deformation of this basin related to the accretion of the Siletzia oceanic plateau between 51 and 49 Ma; the initiation, or acceleration of right-lateral, strike-slip faulting and the development of at least one strike-slip sedimentary basin (Chumstick basin) starting ca. 49 Ma; and the re-establishment of a regional depositional system after ca. 45–44 Ma (Roslyn basin) as strike-slip faulting was localized on the Straight Creek–Fraser River fault. These events are compatible with the presence of the Kula-Farallon ridge near the latitude of Washington ca. 50 Ma and its southward movement, or jump, following the accretion of Siletzia. This trip visits key outcrops that highlight this history and links them to regional studies of sedimentation, faulting, and magmatism to better understand the geologic record of this tectonic setting.