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 The magmatic arc represented by the crystalline core of the North Cascades (Cascades core) reached a crustal thickness of >55 km in the mid-Cretaceous. Eocene collapse of the arc was marked by migmatization, magmatism, and exhumation of deep-crustal (9-12 kb) rocks at the same time as subsidence and rapid deposition in nearby transtensional nonmarine basins. The largest region of deeply exhumed rocks, the migmatitic Skagit Gneiss Complex, consists primarily of leucocratic, biotite tonalite orthogneiss intruded between ca. 76-59 Ma and 50-45 Ma. Well-layered biotite gneiss is also widespread. U-Pb (isotope dilution-thermal ionization mass spectrometry) dating of zircon and monazite from trondhjemitic leucosome and biotite gneiss mesosome indicates that metamorphism and melt generation/crystallization occurred at least intermittently from ca. 71 to 47 Ma, and the youngest U-Pb dates overlap Ar/Ar (biotite, muscovite) dates, compatible with rapid cooling. Mesoscopic to map-scale, gently plunging, upright folds have hinge lines subparallel to orogen-parallel (NW-SE) lineations in the Skagit Gneiss Complex, and are as young as 48 Ma. Eocene top-to-northwest flow occurred in parts of the complex. The gently to moderately dipping foliation, subhorizontal lineation, and constrictional domains are compatible with ductile transtension linked to dextral-normal displacement on the Ross Lake fault system, the northeastern boundary of the Cascades core. On the south flank of the core, sediments were deposited in part at ca. 51 Ma in the Swauk basin and shortly afterward folded, and then intruded by 47 Ma Teanaway basaltic dikes. Extension taken up by these dikes ranges from ~10% to 43%. Extension directions from Teanaway and other Eocene dikes are arc-parallel to arc-oblique. The shallow-crustal extension direction is counterclockwise (mostly 10°-30°) to the ductile flow direction, implying decoupling of brittle and ductile crust; however, some coupling is supported by the temporal coincidence between basin formation and partial melting and ductile flow, and the upright folding of both the Skagit Gneiss Complex and Swauk basin. Arc-oblique to arc-parallel flow probably resulted in part from dextral shear along the plate margin, along-strike gradients in crustal thickness, and thermally controlled rheology.