Sediment incorporation into continental arc systems and paleogeography of the western North American Cordillera: Insights from the North Cascades, Washington
AuthorSauer, Kirsten B.
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Continental magmatic arcs are the main tectonic setting where new continental crust is formed. Metasedimentary rocks exposed at the deep crustal levels of exhumed arc systems provide evidence for the vertical and horizontal transfer of upper crustal rocks into arcs within convergent-margin settings. These incorporated sediments are proposed to play a significant role in the evolution of the arc by potentially triggering high-flux magmatism and driving newly formed crust towards more felsic compositions. Therefore, it is important to document the mechanisms by which sediment is transferred from the surface to between upper-mantle and mid-crustal depths, the rates and timing of sediment burial, and the contribution of partial melts of metasedimentary rocks to arc plutons. This dissertation focuses on understanding the tectonic history of metasedimentary units in the deep-crustal levels of continental magmatic arcs, using the exhumed roots of a former arc system represented by the crystalline core of the North Cascades, Washington, as a case study. This study uses the sedimentary-provenance information recorded in detrital-zircon U-Pb and Hf-isotope and whole-rock Nd-isotope analyses of (meta)sedimentary rocks from northwestern Washington and southern British Columbia, in conjunction with similar datasets from coeval (meta)sedimentary rocks to the south along the western continental margin to understand the source and mechanisms of sediment incorporation. Together, these results provide significant information about the North Cascades and the evolution of the Jura-Cretaceous North American Cordilleran margin. Analyses of samples collected from arc-related basins adjacent to the North Cascades crystalline core reveal different sources throughout their Jurassic to Late Cretaceous depositional history. Jurassic and Lower Cretaceous samples reveal unimodal U-Pb age peaks, near-depleted mantle Hf-isotope compositions, and εNdi values between +7 and +4. Mid-Cretaceous samples are characterized by multiple detrital-zircon age peaks with less radiogenic, but dominantly suprachondritic, Hf-isotope compositions and εNdi values between +4 and +1. Upper Cretaceous sediments are characterized by multiple Mesozoic peaks, unradiogenic Late Cretaceous zircons, distinct peaks ca. 1.38 and 1.8–1.6 Ga, and unradiogenic Nd-isotope compositions. These results indicate that the tectonic setting evolved from basins associated with oceanic arcs that were isolated from the continent, to a mid-Cretaceous, likely high-standing continental arc, and finally, to a collapsed continental magmatic arc. Analogous detrital-zircon patterns are observed along strike of the western North American continental margin. The distinct Proterozoic peaks in the Upper Cretaceous rocks may be indicative of a Mojave province sediment source. Zircon data was also collected from metasupracrustal units throughout the North Cascades crystalline core. The Cascade River Schist and Napeequa Schist have been interpreted to represent Triassic oceanic-arc related rocks that were accreted before arc magmatism initiated. Arc magmas intruded into these rocks, and some parts of these units were entrained into the arc system coincidently with the beginning of arc magmatism in the North Cascades (ca. 96–84 Ma). The samples of the Cascade River Schist analyzed in this study reveal mid-Cretaceous protolith ages and detrital-zircon and Nd-isotope signatures similar to coeval forearc and/or accretionary-wedge units. The Napeequa Schist samples reveal Triassic protolith ages and radiogenic Nd- and Hf-isotope compositions that reflect its oceanic arc provenance. The main bodies of incorporated sediment are represented by the Skagit and Swakane Gneisses. The protolith of these units was buried between ca. 74–66 Ma, coincident with a second, ca. 78–71 Ma magmatic pulse. Results from Skagit Gneiss Complex metasedimentary samples reveal either Triassic–Cretaceous detrital zircons with radiogenic Hf-isotope signatures or Proterozoic–Cretaceous populations with more varied detrital-zircon Hf-isotope signatures. Maximum depositional ages (MDAs) for all samples are between ca. 134 and 79 Ma and εNdi values are between +6 and -7, with no distinct pattern with MDA. In comparison, the Swakane Gneiss samples have a narrower range of MDAs, likely between ca. 93 and 81 Ma. Samples with ca. 93–88 Ma MDAs are characterized by only Mesozoic populations and εNdi values between +4 and +2, whereas the younger samples have Mesozoic peaks coupled with abundant ca. 1.38 and 1.8–1.6 Ga zircons and unradiogenic Nd-isotope compositions (i.e., εNdi = -1 to -7). Most initial Hf-isotope results from the Mesozoic and Proterozoic Swakane Gneiss detrital zircons are suprachondritic, with the exception of distinct unradiogenic ca. 100–81 Ma zircons. The unradiogenic grains are only found in samples deposited after ca. 86 Ma that also contain distinct Proterozoic peaks. A similar pattern is observed in the ca. 79 Ma Skagit Gneiss sample. The detrital-zircon patterns observed in the Swakane and Skagit Gneiss metasedimentary rocks are characteristic of forearc and accretionary-wedge units distributed along the continental margin between southern California and southern British Columbia. Thus, sediment was likely transferred to depth by either underthrusting or underplating of sediments that originated outboard of the arc system. The MDAs coupled with in situ peaks indicate vertical burial rates of ~4mm/yr, which are similar to rates observed in modern convergent-margin settings. Although both the Swakane Gneiss and Skagit Gneiss were emplaced during a pulse of magmatism, whole-rock Nd data from coeval plutons do not show obvious evidence of assimilation of sediment with a Swakane/Skagit isotopic signature. Thus, magmatism in the North Cascades was likely predominantly related to mantle-driven processes.The Pelona-Orocopia-Rand and related schists (PORS) are a well-documented example of a metasedimentary package that was underplated beneath the southern portion of the Sierra Nevada arc. Previous studies have noted many similarities between the PORS and the Swakane Gneiss. Thus, to better understand the relationship between these two units now separated by ~1600 km, new detrital-zircon U-Pb and Hf-isotope data from the PORS was collected. The PORS protoliths record an influx of Proterozoic zircons at the same time as in the Swakane protolith, and, coupled with similarities in lithology, structural setting, and timing of emplacement, indicate that these distinct metasedimentary rocks were likely deposited and buried as a part of the same underplating system. These results establish a piercing point for Late Cretaceous paleogeographic reconstructions and are consistent with paleomagnetic data that suggest rocks of, and surrounding, the North Cascades and southern Coast Plutonic Complex formed ~1600 km to the south. Overall, these findings provide new insight into the mechanisms that transfer sediment to the deep levels of continental magmatic arcs. The isotope and age signatures of metasedimentary rocks within the North Cascades arc indicate that these units likely originated in forearc and/or accretionary-wedge units and were incorporated through a combination of underthrusting and underplating mechanisms. These observations contrast with other studies that propose that underthrusting of retroarc crust is a major mechanism of sediment incorporation in Cordilleran arc systems. Together, these observations contribute to the overall understanding of the Jurassic–Late Cretaceous evolution of the continental margin and arc systems of the North American Cordillera.