3 Methods
We compiled bedrock U-Pb zircon and detrital U-Pb zircon watershed populations across southern Alaska and combined them with known plate boundary constraints (subduction vs presence of a slab window) to determine the spatial patterns of arc magmatism since ca. 100 Ma (Figures 2 and 3). We choose this age cut off based on when large segments of the Wrangellia composite terrane would have interacted with the North American plate (e.g. Tikoff et al., 2022), but acknowledge ocean-basin suturing was diachronous in both time and space (Trop et al., 2020; Waldien et al., 2021). We bolstered this dataset with40Ar/39Ar and K-Ar ages on mafic dikes and volcanics and detrital40Ar/39Ar lithic grains. We did not compile 40Ar/39Ar and K-Ar ages on hornblende, muscovite, biotite, or potassium feldspar to avoid dates that reflect exhumation or metamorphic related events—not true magmatic events except from magmatic products from the Wrangell Arc. We generally avoided ashfall samples, but some thick welded tuffs are included in the dataset. The detrital datasets may include some recycled grains from sedimentary or metamorphic lithologies (Wilson et al., 2015), but the detrital datasets mimic the igneous bedrock datasets, except for the Wrangell Arc. We do not include detrital ages from Cretaceous-Cenozoic strata to avoid potentially far-traveled sources (Finzel et al., 2019).
Locations were all plotted and assumed to be in the geodetic datum, WGS84. For the purposes of this study the potential 500- to 1000-meter difference from NAD27 is not germane, but we recommend attempting to figuring out (often not reported) the original datum applied, if using this compiled database for other purposes. Detrital geochronology datasets for individual watersheds from the Western and Central Alaska Range Arc and the Wrangell Arc are presented individually in the supplement, but compiled for figure presentation. The watershed outlines are presented in the references noted in the supplemental files. Data and further references are available in supplemental tables S1, S2, S3, and S4.
We overlayed our geochronology dataset on a digital elevation model and a simplified terrane map of Alaska (Figures 4 and 5). To highlight arc localization, time slices were created for 100-60 Ma, 60-50 Ma, 50-30 Ma, and 30 Ma to Recent for the western and Central Alaska Range Arcs (northern continental Aleutian since ca. 48 Ma; Bezard et al., 2021) and the Talkeetna Mountains. For the Western and Central Alaska Range Arcs, time slices are 100-60 Ma, 60-50 Ma, 50-25 Ma, and 25 Ma to Recent to reflect the continuation of arc magmatism after the initnal arrival of the Yakutat slab at ca. 30 Ma (Brueseke et al., 2019). These divisions are based on known periods of arc magmatism, pulses of arc magmatism, and significant changes in plate boundary conditions (Herriott et al., 2014; Lease et al., 2016; Finzel et al., 2019; Trop et al., 2019; Terhune et al., 2019; Fasulo et al., 2021; Jones et al., 2021, Regan et al., 2021, 2022; Benowitz et al., 2022, Trop et al., 2022). We also compare bedrock and detrital data with a map of crustal thickness variations (Figure 6) (Yang et al., this issue ).