Figure 5: HeFTy modeled “good-fit” and “acceptable-fit” T-t
envelopes for Oligocene hypabyssal intrusive rocks from the west side of
the Totschunda fault (left) and Cretaceous gabbroic rocks from the east
side of the Totschunda fault (right). AHe, AFT, and zircon U-Pb ages are
plotted on the models where applicable with 2σ error bars for AHe and
U-Pb dates and 1σ error bars for AFT dates. Tectonic and plate boundary
events are labeled as colored squares with explanation in the figure
legend. Corresponding AFT length distributions are plotted to the right
or left of each sample. The “mean” cooling path for all the Oligocene
hypabyssal rocks is presented as a composite cooling envelope (shown are
median paths for all hypabyssal samples). Annotated U-Pb, AFT, and AHe
ages were not used as constraint boxes during HeFTy modeling.
Two bedrock samples (19SOLO01, 19SOLO02) from the Cretaceous gabbro
immediately east of the Totschunda fault have AFT ages of 84 ± 8 Ma (±
1σ) (19SOLO01) and 87 ± 8 Ma (19SOLO02). Small plutonic bodies cool
rapidly after emplacement (ca. <1 Ma) (Nabelek et al., 2012)
so we infer these ages reflect exhumation related cooling and not post
emplacement thermal relaxation given the ca. 100 Ma U-Pb zircon
crystallization age of these gabbro samples.
On the west side of the Totschunda fault, four samples of Oligocene
hypabyssal andesitic and dacitic porphyry plugs (19SOLO06, 07; 10; and
11) yield AFT ages of 20 ± 3 (±1σ) (19SOLO06), 24 ± 5 (19SOLO07), 28 ± 5
(19SOLO10), and 21 ± 5 Ma (19SOLO11). The track length distributions of
all six bedrock samples comprised a mix of long and shorter tracks
indicative of complex thermal histories with significant residence time
in a partial annealing zone (e.g., Gleadow et al., 1986) (Figure 5,
Tables 2, S15).
The detrital cobble AFT ages can be divided into two groups: 30-35 Ma
(19SOLO18 a, b, c) from three cobbles at one sample station and 8-13 Ma
(19SOLO03 a, b, c and 19SOLO12 a, b) from five cobbles at two additional
sample stations (Figures 3, 6; Tables 2, S16, S17). The uranium
concentration in apatite crystals was low (4-7 pm) which resulted in
relatively large uncertainties in AFT ages (± 3-6 Ma, ± 1σ). Low [U]
plus relatively young ages mean confined fission tracks suitable for
track length measurement were rare. Without sufficient length
measurements (> 25) we did not model these samples.
However, AFT ages overlap within uncertainty of the U-Pb ages for the
two groups (ca. 26 to 28 Ma and ca. 8.9 to 10 Ma; Figure 6). We
interpret the cobble AFT data to indicate rapid cooling associated with
post intrusion thermal relaxation of the original hypabyssal and
volcanic units followed by exhumation, erosion, transportation and
deposition in the basin.
4.3 Apatite and Zircon (U-Th)/He
Data
We report inverse variance weighted mean AHe ages for five Lime Creek
samples total (30 single-grain ages in total; Table 3 and Figures 4, 5,
6, S3-S6); one from the Cretaceous gabbro east of the Totschunda fault
(single grain ages = 6; 53.4 - 9.0 Ma single grain ages) and four from
Oligocene hypabyssal rocks west of the fault (single grain ages = 24;
4.7 - 0.5 Ma single grain ages). Single-grain AHe ages were not included
in sample mean age calculations if they exceeded a z-score of two (two
standard deviations from the overall sample population mean). For
included single-grain age data, there was no strong correlation of
[eU] vs. age or grain size (Rs) vs. age for any of
these samples (see S6). The young AHe ages and low uranium
concentrations in apatite from the Lime Creek drainage
(~2.4 U pm for the hypabyssal intrusive rocks and
~4.7 U pm for the Cretaceous gabbro) reduced any
possible radiation damage related kinetic effects (e.g., Flowers et al.,
2009; Flowers et al., 2022a, b).