Figure 6: Photo Collage. Note that facing directions are given in
black text at top of each figure where applicable. a) Sample location
for 19SOLO02 showing heavily oxidized quasi-gossanous material
downstream of the Cretaceous gabbro suggestive of local hydrothermal
fluid flow. b) Sample location for 19SOLO02 showing where rocks were
sampled (directly to the left of the bag) and heavily oxidized and
weathered material to the left of the red line in the image. Red dashed
square highlights hematite precipitation on fracture surfaces suggesting
hydrothermal fluids at some point in the geologic past. ZHe, AFT, and
AHe ages are displayed on the figure to document age ‘inversion’. c)
Photo of the unnamed creek bed with the White River “Tillites” in the
background. Small camera denotes location where photo 6D was taken,
looking towards the location of this photo. Approximate cobble sample
locations shown with sample name and red stars. d) Photo from ridge-top
of White River ‘Tillites’ outcrop extent. Camera denotes image location
of photo 6C that looks uphill towards this location. Red arrow denotes a
geologist for scale. Black arrows highlight a Holocene (?) scarp on a
splay of the Totschunda fault. Black dotted lines denote intersection of
bedding planes with topography and approximate strike. Note the strong
change in ‘strike’ of beds across the fault scarp. Lime Creek labeled in
the background. e) Upthrown fault blocks along the Totschunda fault in
Lime Creek. Mapped fault trace continues out of frame along the creek
and up through the mountainside. Fault blocks are dominantly comprised
of Paleozoic metasediments and thus were not good targets for
thermochronologic analyses but do make an impressive landscape feature.
These upthrown blocks are probably < ~25,000
years old as otherwise they would have been removed by major glaciation
in the valley (last glacial maximum; Kaufman et al., 2011). f)
Sample locations for 19SOLO06 and 19SOLO07 shown as red stars. Lime
Creek and Lime Glacier ice-cored moraine shown in the foreground.
ZHe dating was undertaken on the Cretaceous Gabbro to better resolve the
thermal history of the rocks on the east side of the Totschunda fault
(19SOLO02, Table 3; Figures 6, S3, S5, S6). 19SOLO02 single-grain ZHe
ages from our analyses were the same regardless of [eU] or grain
size. ZHe ages for sample 19SOLO02 yielded a mean age of 8.0 ± 0.5 Ma
(±1s; single-grain ages = 3). This age is younger than both the AFT age
(87.3 Ma) and the mean AHe age (27.9 Ma) although similar to the
youngest AHe single-grain age (9 Ma) from this sample. High [eU]
zircons dated using (U-Th)/He can have lower closure temperatures than
apatite from the same sample due to radiation damage from high uranium
concentrations facilitating diffusion of He from the grain (e.g.,
Johnson et al., 2017), but the [eU] for the three 19SOLO02 grains
analyzed only ranges from ~100 to ~300
ppm (Table 3). However, sample 19SOLO02 was collected in an area of
extensive penetrative fluid flow (approximately 200 meters wide)
(Figures 6a, 6b). This 8.0 ± 0.5 Ma ZHe age from this sample is also
similar in age to some of the ‘White River Tillites’ dated lava flows
(8.4 ± 0.7 Ma; 8.7 ± 0.9 Ma; Denton and Armstrong, 1969) lying within
the basin that is immediately adjacent to 19SOLO02 (Figure 3). We
therefore infer the young ZHe age (relative to the AFT and AHe ages from
the same sample 19SOLO02) of ca. 8 Ma is related to hydrothermal fluid
flow along the Totschunda fault coeval with a dated magmatic event in
the ‘White River Tillites’ area (Denton and Armstrong, 1969). Although
the low [eU] zircons selected for (U-Th)/He dating had no evident
cracks nor defects (S3), we suggest that He loss due to hydrothermal
fluid flow in these zircons may be a physical process (such as leaching)
and not exclusively due to volume diffusion (e.g., Johnson et al.,
2017).
4.4 HeFTy Modeling
Multi-kinetic inverse thermal models for samples from the Cretaceous
Gabbro and Oligocene porphyry samples were generated using HeFTy v.
1.9.3 (Ketcham, 2005; S7). Depicted constraint boxes were used in a
later stage of modelling iterations to optimize the ratio between tried
paths and good paths (S7). For the Cretaceous gabbro samples just east
of the Totschunda fault (19SOLO01 & 19SOLO02; S8, S9) rapid cooling
occurred around ca. 95 Ma to temperatures of ~100°C by
~85 Ma followed by very slow cooling (~
0.8°C/Ma) to 60°C until ca. 25 Ma when more rapid cooling
(~ 2.5°C/Ma) resumed (Figure 5). Overall, with little
variation, the Oligocene hypabyssal porphyry plug samples (samples
19SOLO06; 07; 10 and 11; S10 - S13) preserve nearly identical thermal
histories (Figure 5). Given the
Oligocene crystallization ages for these magmatic bodies, we associate
Oligocene rapid cooling with thermal relaxation after intrusion,
followed by near isothermal shallow crustal residence until the Late
Miocene when the hypabyssal rocks experienced rapid cooling due to
inferred exhumation at ~13°C/Ma from ca. 6 Ma until
present. The abrupt change in AHe ages across the Totschunda fault (ca.
28 Ma east side; ca. 2 Ma west side), the preserved upthrown blocks
(Figures 4, 6) and the intrusive lithology of these Oligocene rocks
support exhumation as the mechanism responsible for the recorded
cooling.