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.