3. Methods and sampling strategy

Strike-slip faults often have a dip-slip component (e.g., Barth et al., 2014) and variations in strike and geometry can result in linked contractional structures (Sylvester 1988; Spotila et al., 1998). Thermochronology presents a means to record the path of a rock as the rock cools and approaches the surface (exhumation), hence a rock sample’s cooling trajectory can be used as a proxy for deformation and a means to unravel the vertical component of the partitioned slip history of strike-slip faults such as along the Alpine Fault of New Zealand and the Fairweather Fault (e.g., White and Green, 1986; Batt et al., 2004; Lease et al. 2021). Provided that the derived cooling ages and modelled cooling paths do not correspond to known regional magmatic ages, by applying thermochronology to samples collected along both sides of the Totschunda fault we can evaluate the cooling and exhumation histories of rocks along the structure and infer changes in slip rates through time.
Rocks in the field area are largely composed of Late-Paleozoic island-arc rocks of Wrangellia Composite Terrane affinity (MacKevett, 1978). These rocks are primarily Permian fossiliferous shallow marine clastic sedimentary rocks and limestones of the Hasen Creek Formation, and volcanoclastic rocks, lavas, and graywackes of the Station Creek Formation (Figure 3). Intrusive rocks in our field area are limited to a Cretaceous (previously mapped as Triassic) gabbro intrusive body on the east side of the Totschunda fault, and many small (<3 km2) Oligocene shallow intrusive andesitic and dacitic porphyry plugs associated with the Wrangell arc. The presence of these andesitic and dacitic porphyry plugs, which are interpreted as hypabyssal rocks (MacKevett, 1978), on the surface suggests 2-3 km of exhumation since their emplacement, making them a reasonable target for thermochronology. Thus, our primary targets for sampling and analyses were these intermediate hypabyssal plugs.

3.1 U-Pb Zircon

We undertook U-Pb dating on zircon to determine the timing of intrusion for magmatic rocks along the Totschunda fault as well as the age of the cobbles in the White River tillites. We performed Laser Ablation Induced Coupled Plasma Mass Spectrometry using an Agilent 7900 ICP-MS and a Photon Machines G2 with a Helix 2 sample cell at the University of Rochester (e.g., Trail et al., 2017). Zircon separates were obtained during the mineral separation processing for apatite using standard mineral separation procedures. Grains were mounted in epoxy and then polished. The mounts were then carbon coated and photographed on the Cameca SXFive Microprobe at Syracuse University using the cathodoluminescence detector. These photos were used to assess crystal zoning and heterogeneity within grains and between samples. Spot locations were chosen by identifying crystal regions visibly free of inclusions. Zircons were ablated with a spot size ranging from 10 to 35 μm (Trail et al., 2017). The ICP-MS signals were evaluated with the Iolite 3.x software package to select time intervals that reflected the bulk of the zircon grain and avoided sections where the laser ablated through the grain and into the epoxy below, encountered an inclusion, or encountered surface contamination (Paton et al., 2010, 2011). We used the R33 zircon standard (419 Ma) for age correction ablating one R33 zircon for every 10 unknowns with at least one shot between samples (Black et al., 2004). We attempted 20-25 grains or spots per sample. Drift was corrected for using the NIST standard at one shot for every 10 unknowns with at least one shot between samples. Almost all our samples appeared to have some surface lead contamination from processing (potentially from milling or removal of sulfides using nitric acid) and so our time series selections for uranium lead ratios largely excluded the first few seconds of measurements. Additionally, on some grains, we encountered apparent inclusions or would ablate through the grain, and so those shots and times were also excluded. Isotope ratios, dates, and associated uncertainties are calculated from the integrations using the U_Pb_geochronology3data reduction scheme (Paton et al., 2010). The results are summarized in Table 1, S1, & S2.

3.2 Apatite Fission Track Thermochronology

Apatite fission track (AFT) thermochronology has a thermal sensitivity from ca. 120˚C to 60˚C (e.g., Gleadow et al., 1983; Reiners and Brandon 2007). Above these temperatures fission tracks anneal instantaneously over geologic time and below these temperatures, fission tracks essentially cease annealing over geologic time. The zone between the 120-60˚C temperature range where tracks are annealed progressively more slowly as temperature decreases is known as the partial annealing zone (Gleadow and Fitzgerald, 1987). The relative proportions of confined fission track lengths provide information on the thermal history of the sample such as the timing and rate of cooling through the partial annealing zone or partial annealing due to reheating (Gleadow et al. 1986). For example, a track length distribution where almost all tracks are >14-16 µm in length reflects rapid cooling with little to no residence time in the partial annealing zone. More complex track length distributions containing both long and short tracks reflect slow cooling through the partial annealing zone or partial annealing due to reheating with later rapid cooling (e.g., Gleadow et al., 1983; 1986).
The density of spontaneous fission tracks in etched apatite crystals relative to the [U] concentration of individual grains measured using induced fission tracks following irradiation (the external detector method) is used to determine an AFT age (e.g., Hurford and Green, 1983). The composition of apatite plays a role in the annealing of fission tracks; in general tracks in Cl-rich apatite are more resistant to annealing compared to those in Fl-rich apatite (e.g., Gallagher et al., 1998; Ketcham 2007). We collected age and confined track length data including a kinetic parameter that approximates chemical composition (Dpar) and angle of each track with respect to the C-axis of each grain (Donelick et al., 2005). We applied AFT thermochronology to bedrock samples along the Totschunda Fault and to detrital cobbles from the White River ‘Tillites.’ Thermochronology applied to bedrock bounding the Totschunda fault provides potential insight into fault slip related exhumation. In contrast, thermochronology applied to cobbles can provide information on their provenance and pre-deposition cooling (exhumation of the source region) or the burial and subsequent exhumation history of the basin provided the sediments were sufficiently buried to reset the tracks (e.g., Beamud et al., 2011; Fitzgerald et al., 2019). AFT analyses (Table 2) were undertaken at Syracuse University with details given in S14.

3.3 Apatite and Zircon (U-Th)/He dating

Helium analyses were conducted at the University of Colorado in the Thermochronology Research and Advanced Instrumentation Laboratory using an ASI Alphachron for He extraction and measurement (e.g., Flowers et al., 2023a). Individual grains are placed in Nb-tubules, lasered to heat the grain and extract gas, which is spiked with 3He, purified with SAES getters, and analyzed on a Pfeiffer Balzers QMS quadrupole mass spectrometer. The process is then repeated to evaluate if there was complete extraction of 4He from the crystal (e.g., Farley, 2002; Flowers et al., 2023a). Grains are retrieved and then dissolved in nitric acid before being analyzed for U, Th, and Sm using an Agilent 7900 Quadrupole ICP-MS. Grains were picked to avoid broken ends, zoning, inclusions, fractures, and obvious radiation damage to limit the possibility of excessive He from zircon inclusions or He loss from radiation damage and avoid diffusion properties that may result in overdispersion of single grain ages (e.g., Fitzgerald et al., 2006; Flowers et al., 2023b). Grains of adequate size (> ~70 microns a-axis) are selected to minimize the FT correction. See Supplementary Files 4, 5, and 6 for more detailed information on zircon and apatite (U-Th)/He dating.

3.4 Multi Kinetic Modelling

Thermochronology data from individual samples were modelled using HeFTy v. 1.9.3 (Ketcham 2005). Inputs to HeFTy are AFT single grain ages, c-axis projected confined lengths, Dpar-, single grain zircon (U-Th)/He (ZHe) and apatite (U-Th)/He ages, and zircon (U-Pb) age constraints when available. HeFTy uses a Monte Carlo approach, generating model AFT age and track length distributions and modeled ZHe and apatite AHe diffusion profiles that are then compared to the measured data using a GOF criteria. Good and acceptable fits are (0.05 and 0.50 respectively). Model paths are allowed to explore temperature-time space (with both cooling and heating path segments) at random intervals. See Supplementary File 7 for more details about the criteria used during thermal modeling.