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 ).