[American Journal of Science, Vol. 312, February, 2012, P. 145–212, DOI 10.2475/02.2012.04]
LOW-TEMPERATURE THERMOCHRONOLOGY OF THE NORTHERN
ROCKY MOUNTAINS, WESTERN U.S.A.
S. LYNN PEYTON*, PETER W. REINERS, BARBARA CARRAPA,
and PETER G. DeCELLES
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
ABSTRACT. We dated 86 borehole and surface samples from basement-cored
Laramide uplifts of the northern Rocky Mountain foreland (Wind River, Beartooth,
Bighorn and Laramie Ranges) using the apatite (U-Th)/He system, and eleven samples
using the apatite fission-track system (Wind River and Bighorn Ranges). Apatite
(U-Th)/He ages generally decrease with increasing subsurface depth (decreasing
elevation), and typically range from ⬃100 to 50 Ma (Cretaceous to Eocene) within ⬃1
km of the surface, to ⬃20 Ma (Miocene) and younger ages at depths greater than ⬃2 to
2.5 km. Most samples display (U-Th)/He age dispersion ranging from tens to hundreds of Ma, and for some samples we find ages that are older than corresponding
fission-track ages. At least one sample per range shows a correlation between apatite
(U-Th)/He age and effective U concentration (eU ⴝ [U] ⴙ 0.235[Th]) of the crystal,
indicating that radiation damage has affected He diffusivity, and hence (U-Th)/He
age.
Forward modeling of simple Laramide-type thermal histories using a radiation
damage diffusion model predicts: 1) fossil apatite fission-track partial annealing and
apatite (U-Th)/He partial retention zones over similar elevation ranges, 2) (U-Th)/He
age dispersion within a fossil partial retention zone up to hundreds of Ma, and 3)
(U-Th)/He ages older than fission-track ages within a fossil partial retention zone if eU
ⲏ 20 ppm. We observe these features in our data from the Bighorn and Laramie
Ranges. Most of our samples, however, do not show the correlation between (UTh)/He age and eU predicted by radiation damage diffusion models. The age
dispersion of these samples could be due to the influence of both grain size and eU
content, or alternatively due to high U or Th secondary rims around the apatite
crystals. (U-Th)/He ages that are older than fission-track ages from Gannett Peak and
Fremont Peak in the Wind River Range, and some samples from the Beartooth Range,
are most likely the result of He implantation from high eU secondary rims.
Best-fit time-temperature paths from inverse modeling of (U-Th)/He age-eU
pairs, when extrapolated to other elevations to create model age-elevation plots,
reproduce the general distribution and dispersion of (U-Th)/He ages from the
Bighorn, Beartooth and Wind River Ranges and suggest that rapid exhumation within
the Laramide province likely began earlier in the Bighorn Range (before ⬃71 Ma) than
the Beartooth Range (before ⬃58 Ma). Inverse modeling of borehole data at the
northern end of the Laramie Range suggests that the well penetrated a fault sliver at
depth. The amount and timing of post-Laramide burial and exhumation cannot be
determined from these data.
Key words: Thermochronology, (U-Th)/He dating, apatite fission track, radiation
damage, Laramide orogeny, Rocky Mountains, exhumation
introduction
Advances in understanding the diffusion of He in apatite and other minerals over
the last decade have led to a proliferation of the use of (U-Th)/He dating as a
low-temperature thermochronometer. The technique elucidates shallow crustal processes in many areas, particularly those regions that have experienced rapid cooling
and simple thermal histories. There have, however, been many recent examples of
* Present address: Coal Creek Resources, Inc., 1590 South Arbutus Place, Lakewood, Colorado 80228;
speyton@email.arizona.edu
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S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
apatite (U-Th)/He (AHe) data that show anomalous results compared with corresponding apatite fission-track (AFT) data and other geological constraints (Crowley and
others, 2002; Hendriks and Redfield, 2005; Fitzgerald and others, 2006; Green and
others, 2006; Spiegel and others, 2009). Scatter of ages from multiple aliquots from a
single sample is common (Kohn and others, 2008b), and in some cases AHe ages may
be older than AFT ages (for example, Belton and others, 2004; Danisı́k and others,
2008). Typically, these data occur in continental areas that have been subjected to long
(hundreds of Ma), complex thermal histories involving reburial, slow cooling/
exhumation rates, and/or long residence time in the He partial retention zone (PRZ).
Possible causes of AHe age scatter and anomalously old AHe ages include the
presence of U- and Th-rich inclusions (for example, House and others, 1997), He
implantation from “bad neighbors” (Kohn and others, 2008a; Reiners and others,
2008), zonation of U and Th (Hourigan and others, 2005), the influence of alpha
ejection on He diffusion (Meesters and Dunai, 2002a), the effect of grain size (Reiners
and Farley, 2001), and the influence of radiation damage (Shuster and others, 2006;
Flowers and others, 2009; Gautheron and others, 2009). Recent work has led to a
better understanding of the influence of radiation damage on the diffusion of He in
apatite, which causes a decrease in He diffusivity (Green and others, 2006; Shuster and
others, 2006; Flowers and others, 2009; Gautheron and others, 2009; Shuster and
Farley, 2009). Shuster and others (2006) used He content, [4He], as a proxy for
radiation damage, whereas Flowers and others (2009) and Gautheron and others
(2009) used effective fission-track density. Effective fission-track density parameterizes
radiation damage as the accumulated alpha damage from both U and Th decay, but
with damage loss following the kinetic laws used for fission-track annealing. All of these
models assume that an apatite crystal lattice is damaged by recoil of a parent nuclide of
U, Th or Sm as it decays by ejecting an alpha particle (He nucleus), and that these
damage sites act as traps for He. An important feature of both the Shuster and others
(2006) and Flowers and others (2009) models is that they predict that for any group of
apatite crystals that have experienced the same thermal history (that is, those from the
same bedrock sample), AHe ages should be proportional to the amount of radiation
damage, which in turn will be proportional to the concentration of U, Th, and to a
lesser extent, Sm. This is typically represented by a quantity called effective uranium
(eU ⫽ [U] ⫹ 0.235[Th]). Thus, variation in parent nuclide concentration is a possible
explanation for age scatter observed in multiple grains from the same sample.
Radiation damage will have little effect on the age of a sample that has cooled
quickly from high temperatures above the PRZ to temperatures below the PRZ. In this
case, although radiation damage still accumulates at temperatures below the PRZ, all
He in both the undeformed crystal lattice and the radiation damage sites is effectively
trapped in the crystal. Apatite crystals that reside for a significant time relative to their
age at temperatures where He is trapped in radiation damage sites, but can still diffuse
out of the undeformed crystal lattice, will accumulate He, and thus display a range of
ages depending upon the amount of damage in each crystal. Under certain circumstances (high eU concentration and sufficient time at appropriate temperatures for He
to accumulate in damage sites) it is possible to produce AHe ages that are older than
AFT ages for the same sample. Previous studies have used radiation damage diffusion
models to successfully explain the scatter of AHe ages from the Canadian shield and
the Colorado Plateau, and then used this variation in AHe ages of multiple grains from
single samples to shed light on the thermal history of an area (Flowers and others,
2007; Flowers and others, 2008).
The goal of this study is to better understand the exhumation history of the
northern Rocky Mountain region (fig. 1) using low-temperature thermochronological
data, particularly the timing and amount of both Laramide and post-Laramide
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thermochronology of the northern Rocky Mountains, western U.S.A.
112
108
Crazy
Mountains
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Unit #1
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traverse
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front
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well
IDAHO
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traverse
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Texaco Gov’t
Rocky Mountain
#1
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traverse
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traverse
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Uinta Basin
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1 4
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Fig. 1. Location map of wells (Z) and surface samples (black triangles), Precambrian crystalline
outcrop (gray shading) and major faults in the Rocky Mountain region (modified from Blackstone, 1993,
and Grose, 1972). Abbreviations as follows: BL, Black Hills; BH, Bighorn Range; BT, Beartooth Range; CB,
Cheyenne Belt; FR, Front Range; GM, Granite Mountains; GR, Gore Range; LP, Lima Peaks; LR, Laramie
Range; MB, Medicine Bow Mountains; MR, Madison Range; PR, Park Range; SR, Sawatch Range; TR, Teton
Range; UM, Uinta Mountains; UU, Uncompahgre Uplift; WR, Wind River Range.
exhumation. Samples from the Beartooth, Bighorn, Wind River and Laramie ranges
likely reached maximum temperatures in the Late Cretaceous or early Paleocene,
when they were buried beneath a thick (⬃2 to 4 km) section of Paleozoic passive
margin and Mesozoic foreland basin sediments before the onset of the Laramide
orogeny (Roberts and Kirschbaum, 1995; DeCelles, 2004). With the exception of the
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S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
high peaks, most samples were probably reburied by lesser amounts (⬍1 km) of
Oligocene and Miocene strata before final exhumation to their present elevations
since the late Miocene (Love, 1960; McKenna and Love, 1972; McMillan and others,
2006). We might expect low-temperature thermochronology in this part of the Rocky
Mountain region to be challenging because many of the units examined are nearsurface Precambrian crystalline basement that has most likely experienced long
periods of time at temperatures within the AHe PRZ, along with more than one
episode of burial and exhumation resulting in incomplete resetting.
We interpret our results using radiation damage diffusion models and explicitly
consider the combined effect of grain size variation and radiation damage on AHe age.
We use both forward and inverse modeling with known geological constraints to
establish that radiation damage is indeed influencing our AHe results, but it cannot
explain all complexities of the data. Other factors that may be affecting these data,
such as He implantation, are also considered. We expand upon the inverse-modeling
approach of Flowers and others (2007) by recognizing that the thermal histories of
samples from the same vertical profile must be related to each other, and we should be
able to extrapolate any thermal history derived from a single sample to other samples
in the same profile by assuming a geothermal gradient. Similarly, other samples from
within the same profile should provide additional constraints on geothermal gradients
and thermal model viability. In this way we are able to test how well a best-fit
time-temperature path from inverse modeling fits our entire sample suite.
background
Geologic Background
The Beartooth, Bighorn, Laramie and Wind River Ranges (fig. 1) are part of a
series of basement-cored uplifts that formed within the Cretaceous foreland basin of
the western U.S.A. during the Laramide orogeny (Dickinson and Snyder, 1978). This
thick-skinned thrust belt was active between ⬃90 and 40 Ma, overlapping temporally
with the Cordilleran fold-thrust belt which initiated during the Late Jurassic (DeCelles,
2004). In general, shortening in each range is on the order of several km, and was
accommodated on major range-bounding thrusts that dip ⬃30° beneath the ranges
and extend at least into the mid-crust (Smithson and others, 1978) (fig. 2).
For this study we mainly measured AHe ages from Precambrian basement. It is
important to consider the geologic history of the region from Precambrian time
onwards because the AHe ages of some samples were probably not completely reset by
burial before exhumation during the Laramide orogeny. The Cheyenne Belt, which
runs NE-SW through south-central Wyoming (fig. 1), separates Archean basement of
the Wyoming Province to the north from Proterozoic basement rocks of the MazatzalYavapai Province to the south (Hoffman, 1989). The Mazatzal-Yavapai Province was
accreted to the Wyoming craton at ⬃1.7 Ga. The neo-Proterozoic breakup of the
supercontinent Rodinia is recorded by thick syn-rift sediments in the Uinta and
Wasatch Mountains; however, the Rocky Mountain region was sub-aerially exposed
until the Cambrian, when the Flathead sandstone was deposited unconformably on
basement throughout the region (Snoke, 1993). Platform/passive margin sedimentation during the Paleozoic was interrupted locally in the Rocky Mountain region by the
Pennsylvanian Ancestral Rockies orogeny. The plate-tectonic cause of this orogeny is
still controversial, but may have been related to subduction of the North American
plate beneath the South American plate during the Ouachita-Marathon orogeny
(Kluth and Coney, 1981; Ye and others, 1996). Uplift and erosion was widespread
throughout Colorado, but also reached as far north as the present-day Laramie Range
in southern Wyoming. Areas north and west of the Laramie Range experienced only
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thermochronology of the northern Rocky Mountains, western U.S.A.
A
W Bighorn
Range
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Elevation a.s.l.
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2
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Texaco
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Range
well
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Pz
-3
T
Tertiary rocks
Te
Eocene rocks
Tp
Paleocene rocks
Mz
Mesozoic rocks
Pz
Paleozoic rocks
Precambrian crystalline basement
Fig. 2. Schematic cross sections through wells used in this study. (A) Gulf Granite Ridge #1-9-2D well in
the Bighorn Range (modified from Brown, 1988); (B) Air Force well in the Wind River Range (modified
from Steidtmann and Middleton, 1991); (C) Amoco Beartooth Unit #1 well in the Beartooth Range (Wise,
1997); and (D) Texaco Government Rocky Mountain #1 well at the northern end of the Laramie Range
(Gries, 1983a). No vertical exaggeration, but note that scales are different for each cross section.
subdued tectonism with minor amounts of erosion (Miller and others, 1992; Maughan,
1993).
By Late Jurassic time a continuous subduction zone and associated magmatic arc
had formed along the western margin of North America, with the Farallon plate
subducting beneath the North American plate. This resulted in the formation of the
retroarc Sevier fold-thrust belt and its adjacent foreland basin, both of which propagated eastward until Eocene time (DeCelles, 2004). Foreland basin sediments at least 2
km thick, on top of Paleozoic passive margin sediments, buried the Rocky Mountain
region until the Late Cretaceous (Dickinson and others, 1988; Roberts and Kirschbaum, 1995; DeCelles, 2004). During the Late Cretaceous, flattening of the Farallon
slab caused an inboard sweep of magmatism coeval with the propagation of deformation into the foreland (Coney and Reynolds, 1977; Dickinson and Snyder, 1978;
150
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Constenius, 1996; Saleeby, 2003). Although thin-skinned thrusting continued in the
Sevier belt, deformation in the foreland had a thick-skinned style which dissected the
foreland into a series of smaller basins and ranges bound by an anastamosing array of
faults (Brown, 1988; Erslev, 1993). This thick-skinned deformation, known as the
Laramide orogeny, persisted until Eocene time when the subducting slab rolled back
and magmatism migrated westward. In southeastern Wyoming and Colorado, Laramide compression caused reactivation and/or overprinting of structures that formed
during the Ancestral Rockies orogenic event (Kluth and Coney, 1981; Maughan,
1993). Foreland basin sediments were eroded from the uplifting basement blocks
during the Laramide orogeny and were deposited in adjacent basins as the Fort Union,
Willwood, Wasatch, Wind River Formations, et cetera (Dickinson and others, 1988), as
well as basin-bounding conglomerates such as the Beartooth and Kingsbury conglomerates (DeCelles and others, 1991b; Hoy and Ridgway, 1998).
The amount of topographic relief between basins and ranges, and the amount by
which ranges were buried by synorogenic sediments at the end of the Laramide
orogeny, is controversial. McMillan and others (2006) suggested that at the end of the
Laramide orogeny the topographic relief of the Laramide ranges was similar to
present-day. This is supported by geomorphic modeling (Gregory and Chase, 1994)
and unroofing analysis of synorogenic conglomerates (DeCelles and others, 1991b;
Hoy and Ridgway, 1997). In contrast, however, Steidtmann and Middleton (1991)
suggested that by early Eocene time there was little topographic relief between the
crest of the Wind River Range and the adjacent Green River basin, and that by middle
Eocene time the range was almost buried by its own debris. Remnants of Oligocene
through Pliocene sediments are preserved at high elevations in some ranges, suggesting that during the middle of the Cenozoic the ranges were buried to high levels and
have since been exhumed (Mackin, 1947; Love, 1960; McKenna and Love, 1972;
McMillan and others, 2006; Riihimaki and others, 2007). The cause, timing and
amount of late Cenozoic exhumation are still debated.
Thermochronologic Background
At present the only published AHe study of a Laramide range is from the Bighorn
Range (Crowley and others, 2002). Profiles of AHe age versus elevation have very low
slopes, which Crowley and others (2002) suggest represent a fossil pre-Laramide He
PRZ. The authors acknowledge that these ages are problematic because burial depth
should have been great enough to reset AHe ages, and they suggest that either burial
was not as great as originally thought, that geothermal gradients were unusually low, or
that closure temperature of the AHe system was higher than expected based on He
diffusion kinetic data available at the time. The effect of radiation damage on AHe ages
and closure temperatures was not recognized until more recently (Shuster and others,
2006; Flowers and others, 2009).
Beland (ms, 2002) studied two boreholes at the northwestern and eastern edges of
the Wind River Basin by measuring AFT ages in both wells, and AHe ages in the
northwestern well. Unexpected results from the northwestern well show young (Miocene) AFT and AHe ages at depths less than ⬃2.5 km, and bimodal ages (AFT ⬍ 15
and ⬎ 300 Ma; AHe ⬍ 10 and ⬎ 175 Ma) below ⬃2.5 km. AFT ages from the eastern
well are similar, with young ages (⬍7 Ma) above ⬃3.5 km depth, and older ages (⬎300
Ma) below ⬃4 km. Beland (ms, 2002) explained these results by Miocene thrusting
and uplift followed by erosion. The older AFT and AHe ages at depth indicate that
these samples were not buried deeply enough by Phanerozoic sediments for ages to be
reset.
AFT studies of Laramide uplifts using vertical transects with both surface and
subsurface samples have been published from the Beartooth Range (Omar and others,
1994) and the Wind River Range (Shuster, ms, 1986; Cerveny, ms, 1990; Cerveny and
thermochronology of the northern Rocky Mountains, western U.S.A.
151
Steidtmann, 1993). Surface samples alone were used in studies of the Teton Range
(Roberts and Burbank, 1993), the Front Range (Bryant and Naeser, 1980; Kelley and
Chapin, 1997; Naeser and others, 2002), the Laramie, Medicine Bow and Park Ranges
(Kelley, 2005), the Gore Range (Naeser and others, 2002), the Sawatch Range (Bryant
and Naeser, 1980), and the Black Hills (Strecker, ms, 1996) (fig. 1). Typically, surface
samples from Precambrian basement from these ranges record either Laramide AFT
ages or older ages of a fossil partial annealing zone (PAZ). The spatial distribution of
the base of the fossil PAZ has been used to document faulting, folding and differential
exhumation of basement (Roberts and Burbank, 1993; Strecker, ms, 1996; Kelley and
Chapin, 1997; Kelley, 2005). Some Neogene AFT ages from the mountains of northcentral Colorado are related to extension of the Rio Grande Rift (Bryant and Naeser,
1980; Naeser and others, 2002).
Subsurface well samples have also been used to study the thermal evolution of
Laramide basins. Unlike the ranges, which reached their maximum burial immediately
before the onset of the Laramide orogeny, the basins reached maximum burial during
the Neogene, and have subsequently experienced erosional exhumation (Love, 1960;
Dickinson, 1986; Nuccio, 1994; Roberts and others, 2008). Thus, most basinal AFT
studies have documented the post-Laramide cooling history of the basins. AFT analysis
of samples from the northern Green River Basin suggests that the most recent phase of
cooling of at least 20 °C occurred during the Pliocene (Naeser, 1986; Naeser, 1989).
Analyses from multiple wells in the southwestern Powder River basin show cooling of
35 °C or more since 12 Ma (Naeser, 1992). AFT ages from a well in the Piceance basin
indicate rapid cooling of 10 to 15 °C/Ma since 10 Ma due to downcutting of the
Colorado River (Kelley and Blackwell, 1990). Kelley (2002) measured AFT ages of
detrital apatite grains from synorogenic sediments from two wells in the Denver basin.
These shallow (⬍1 km depth) samples were not buried deeply enough for AFT ages to
be reset during the Neogene, resulting in a predictable AFT age sequence representing unroofing of the southern Colorado Front Range during the Laramide orogeny.
methods
Samples
We acquired a total of 41 surface samples and 45 subsurface samples. Surface
samples were collected from traverses on Cloud Peak in the Bighorn Range, Gannett
Peak in the Wind River Range, Wapiti Mountain in the Beartooth Range, and Laramie
Peak in the Laramie Range. Samples were also collected from within ⬃1.6 km of the
Air Force well in the Wind River Range (fig. 1). Care was taken to remove the exposed
surface of any sample to remove any effects of wildfire on apatites within the outer few
centimeters of the rock (Mitchell and Reiners, 2003). Mineral separates for surface
samples from Fremont Peak in the Wind River Range (fig. 1) were obtained from the
University of Wyoming, and were the same samples used by Cerveny (ms, 1990).
We obtained subsurface samples by consolidating drill cuttings from wells through
the Precambrian crystalline hanging walls of major Laramide thrusts (figs. 1 and 2).
These wells were originally drilled to test the petroleum potential of sub-thrust
sedimentary rocks. Cuttings were obtained for: 1) the Gulf Granite Ridge #1-9-2D well
(section 9, T53N, R84W, Sheridan County, Wyoming) which drilled through the Piney
Creek thrust on the east side of the Bighorn Range; 2) the Amoco Beartooth Unit #1
well (section 19, T8S, R20E, Carbon County, Montana) which drilled through the
Beartooth thrust on the northeast side of the Beartooth Range; and 3) the Texaco
Government Rocky Mountain #1 well (section 12, T32N, R76W, Converse County,
Wyoming), which only penetrated Precambrian basement and did not reach the
Northern Laramie Range thrust. Mineral separates for subsurface samples from the Air
Force well in the Wind River Range (fig. 1) were obtained from the University of
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S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Wyoming. Apatite separation procedures for the Air Force well samples, as well as
those from Fremont Peak, are outlined in Cerveny (ms, 1990). Cuttings were originally
collected by the exploration companies at either ⬃3 m (10 ft) or ⬃10 m (30 ft)
intervals. Due to the limited amount of material available at each depth interval we
consolidated cuttings over an interval ranging between ⬃50 and 225 m, with most
samples spanning between 50 and 150 m. Consolidated samples generally weighed
between ⬃60 and 300 g. The depth interval between samples also varied and was based
on availability of material. The depth we report for a consolidated well sample
represents the central depth of the sampling interval.
By creating a composite sample over a particular depth range and using well
cuttings rather than solid core, we must consider possible effects on the resultant AHe
ages. Cuttings are not instantaneously transported away from the drill bit, resulting in
mixing. Crossing a thrust from crystalline basement into sedimentary rock can give an
estimate of the depth range over which mixing/contamination will take place. We
inspected cuttings from the Gulf Granite Ridge well in the Bighorn Range using a
microscope and found that contamination of crystalline material into sub-thrust
sedimentary cuttings decreased steadily from ⬃90 percent crystalline contaminants
15 m below the thrust, to less than 5 percent contaminants 160 m below the thrust. The
AHe or AFT age of contaminants will typically be older than the actual cooling age for
a particular depth because the contaminants are from shallower depths that cooled
earlier. Similarly, contamination from material which falls from, or is knocked off from
shallower in the well will also have older cooling ages.
Another potential source of uncertainty in sample depth within wells arises from
the fact that wells may deviate from vertical as they are drilled. Sometimes this is
measured and recorded as a deviation survey, allowing for the depths measured in the
borehole to be corrected to true vertical depth. A deviation survey for the Amoco
Beartooth #1 well allowed us to correct sample depths to true depth. Unfortunately,
deviation surveys were not available for the Gulf Granite Ridge #1-9-2D well, the
Texaco Government Rocky Mountain #1 well, and the Air Force well, and therefore
they were assumed to be vertical boreholes. Assuming that a deviated borehole is
vertical will result in an overestimation of depth in the crust, and correspondingly an
underestimation of elevation. Throughout this paper we use the word “elevation” to
mean “elevation above sea level.”
Samples may also be contaminated by apatite from the drilling mud. Recent
studies have shown that modern drilling mud used in the Piceance Basin of Colorado
contains zircons that can skew zircon dating results (A. J. Vernon, personal communication, 2009). We assume that similar contamination of apatite is possible with our
subsurface samples.
Although there are several potential pitfalls to using well cuttings, we expect
meaningful ages because the depth range within samples is still small compared to the
entire sampling depth range of the well, typically ⬃3 km. Some small scatter in ages
should be expected, and it is recognized that older than expected AHe ages may be
from shallower in the borehole.
Apatites were separated by crushing and sieving, followed by magnetic and density
separations following the methodology outlined in Donelick and others (2005). Due to
the small sample size for well samples, these cuttings were crushed by hand using a
mortar and pestle. Larger surface samples were processed using a jaw crusher, roller
mill and Wilfley water table before magnetic and heavy liquid separation.
AHe Dating
We dated between one and ten aliquots for each sample. Forty-nine of our 357
dated aliquots contained multiple apatite grains; the remainder contained a single
grain. We used a binocular microscope to select clear, inclusion-free apatite crystals for
thermochronology of the northern Rocky Mountains, western U.S.A.
153
most samples. Even if crystals contain inclusions that are not readily visible using the
microscope, it is unlikely that they will have a large impact on the crystal age (Vermeesch
and others, 2007). Fremont Peak samples were analyzed at Caltech and Washington
State Universities using a furnace to degas the apatite crystals (Wolf and others, 1996).
Air Force well samples were analyzed at Yale University using laser-heating (House and
others, 2000). All other AHe analyses were performed at the University of Arizona
following standard procedures described in Reiners and others (2004). At Yale
University and the University of Arizona, each crystal was photographed and measured
before being wrapped in either a platinum or niobium tube. Analyses were made on
each sample using Nd:YAG and CO2 laser heating, cryogenic purification, quadrupole
mass spectrometry for 4He analysis, and isotope dilution and HR-ICP-MS for U, Th,
and Sm analysis. Alpha ejection corrections followed the method of Farley (2002).
All surface samples and most subsurface crystalline samples contained sufficient
apatite for us to pick several good crystals for AHe dating; however apatite was absent
from some subsurface sub-thrust sedimentary samples, and in others the detrital
apatites were too small for AHe dating without very large alpha-ejection corrections
(less than about 60 ␮m minimum dimension). If no inclusion-free crystals could be
found, crystals with small inclusions were selected, and after degassing they were
dissolved using more aggressive techniques typically used for zircon (Reiners and
others, 2004). Eight samples (10 aliquots) from the Beartooth Unit #1 well and 13
samples from the Fremont Peak traverse (39 aliquots) included multigrain aliquots.
AFT Dating
About 20 grains were analyzed for each of five new samples collected from a
traverse on Gannett Peak in the Wind River Range. Confined track-lengths were
measured together with the angle between the confined track and the C-crystallographic axis. Apatite etch pit diameter (Dpar) was determined by measuring four Dpar
for each grain. All samples passed the chi squared test; pooled ages were calculated
using the Trackkey program (Dunkl, 2002). Between six and 26 grains were analyzed
for six samples collected from a traverse on Cloud Peak in the Bighorn Range. Track
lengths were not measured for these samples due to poor sample quality. Five of the six
samples passed the chi-squared test. In this paper we also present previously-published
AFT data from Cerveny and Steidtmann (1993) for the Air Force well and from Omar
and others (1994) for the Amoco Beartooth Unit #1 well, as well as ages from Cerveny
(ms, 1990) from surface samples throughout the region.
data
We determined AHe ages on a total of 86 samples: 26 from the Wind River Range,
21 from the Bighorn Range, 24 from the Beartooth Range, and 15 from the Laramie
Range (table A1). For each of the ranges discussed below we include and display all of
our data. No data points were designated as “fliers” and removed from the dataset.
Note, however, that a few data points may not be displayed in the figures due to the
choice of age scale. We have chosen age scales to best represent the majority of our
data, rather than display every point. All AHe data are included in table A1; AFT data
are shown in table A2. Stated AHe 2␴ errors represent twice the formal analytical
precision propagated from uncertainties on He, U, Th, and Sm determinations. Unless
noted otherwise, all reported errors for both AHe and AFT ages are 2␴. Grain size is
reported as “equivalent spherical radius,” or Rs, which is the radius of a spherical grain
with the same surface area to volume ratio as our apatite crystal.
Mean Ages
Typically, arithmetic mean age and standard deviation, or a weighted mean are
used to represent multiple single-grain AHe ages for a sample (Fitzgerald and others,
154
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
2006); recently it has been suggested that a “central” age or geometric mean is more
appropriate (Vermeesch, 2008). None of these methods are appropriate for data that
show a large dispersion of ages for each sample. The weighted mean tends to be biased
towards young ages, as weights are based on actual analytical error. Thus, older ages
have lower weights than younger ages, although the percentage error may actually be
the same. The number of ages obtained per sample is quite small (between one and
ten grains were dated per sample for this study) and are likely a poor statistical
representation of the large age range, making any estimate of a mean a poor one.
Therefore, our results are simply displayed as single-grain ages (or aliquot ages if
multiple grains per aliquot).
Bighorn Range
We measured AHe ages of seven samples from a surface traverse on Cloud Peak
and 14 samples from the Gulf Granite Ridge #1-9-2D well (fig. 3A). In total, 36
single-crystal aliquots from the surface traverse and 70 single-crystal aliquots from the
well were dated using the AHe technique. Excluding three samples that have AHe ages
dispersed over a range greater than 100 Ma, surface and subsurface samples from the
Bighorn Range show an average of ⬃22 and ⬃20 Ma of age scatter respectively (fig.
3A). In order to interpret surface and well samples together we adopt the approach of
Crowley and others (2002) and plot the depth of a sample below the projected
Precambrian-Cambrian unconformity of Blackstone (1993) against age, rather than
elevation versus age (fig. 3B). This approach is appropriate where the fossil PRZ has
been warped or folded, and where widely spaced samples at different elevations may
have experienced a similar thermal history, such as in the Bighorn Range. It is not
appropriate for comparing samples from areas with different tectonic, and hence
different thermal histories. For example, samples from high elevations in the Wind
River Range experienced exhumation and cooling during the Oligocene, unlike
samples from lower elevations; making Wind River Range samples poor candidates for
this approach (Steidtmann and others, 1989; Steidtmann and Middleton, 1991). We
estimate the elevation of the Precambrian-Cambrian unconformity to be ⬃4.3 km for
the Cloud Peak traverse, and ⬃2.5 km at the Gulf Granite Ridge well location.
The deepest well sample, BH4549, is ⬃5 km below the Precambrian-Cambrian
unconformity at an elevation of ⬃ ⫺2.5 km and a corrected borehole temperature of
69 °C. AHe ages for this sample range from 3.0 ⫾ 0.3 to 13.5 ⫾ 0.4 Ma. Non-zero AHe
ages and a present-day temperature of 69 °C at a depth of ⬃4.5 km below the surface
indicate that the geothermal gradient in the Bighorn Range is very low. A least-squares
fit to the well temperature versus depth data gives a present-day geothermal gradient of
14 °C/km. Moving up the borehole, AHe ages are similar until ⬃4.1 km below the
unconformity. At this depth AHe ages show a wider scatter (3.1 ⫾ 1.1 to 31.6 ⫾ 3.3 Ma)
and the gradient of the age-depth trend decreases slightly above this point. The
shallowest well sample, BH225, is ⬃0.7 km below the Precambrian-Cambrian
unconformity at an elevation of ⬃1.8 km. AHe ages for BH225 range from 65.2 ⫾
3.4 to 103.5 ⫾ 5.1 Ma, not including a possible “flyer” of 351 ⫾ 14.9 Ma. Surface
samples from the Cloud Peak traverse have AHe ages ranging from 57.9 ⫾ 2.4 to
97.0 ⫾ 5 Ma. The lowest elevation surface sample has two “flyers” with AHe ages of
171.9 ⫾ 9.1 and 257.2 ⫾ 15.4 Ma.
AHe ages from this study do not show an obvious change in gradient to older ages
of a fossil PRZ; however when we include samples from Crowley and others (2002), a
distinct change to low gradient and older ages on the age-depth plot can be seen at
⬃0.25 km below the Precambrian unconformity (fig. 3B). We interpret this change in
gradient as the base of a fossil PRZ. The ages and scatter of the Crowley and others
(2002) data are consistent with our results.
155
thermochronology of the northern Rocky Mountains, western U.S.A.
4000
A
Surface samples
3000
2000
Subsurface samples
1000
20
30
0
40
-1000
BIGHORN RANGE
50
AHe
AFT
AHe Crowley et al. (2002)
AFT Cerveny (1990)
-2000
Temperature °C
Elevation (m)
10
60
70
-3000
0
100
200
300
400
Age (Ma)
B
-250
250
-250
250
1250
750
Depth below pC unconformity (m)
Depth below pC unconformity (m)
750
1750
2250
2750
3250
3750
1250
1750
2250
2750
3250
3750
4250
AHe
4250
AFT
4750
AHe Crowley et al. (2002)
AFT Cerveny (1990)
5250
4750
0
20
40
60
80
100
120
140
Age (Ma)
5250
0
100
200
300
400
Age (Ma)
Fig. 3. AHe and AFT ages from the Bighorn Range as a function of (A) elevation and (B) estimated
depth below the Precambrian-Cambrian unconformity of Blackstone (1993). As well as AHe and AFT results
from this study (dark blue triangles and green squares, respectively), we include AHe data from Crowley and
others (2002) (light blue triangles) and AFT ages from Cerveny (ms, 1990) (green circles). Dashed line
representing division between surface and subsurface samples is only for results of this study. All Crowley and
others (2002) and Cerveny (ms, 1990) data are from surface samples. Inset in (B) shows the same data with
an expanded time scale. All error bars are 2␴.
156
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
AFT ages from the Cloud Peak traverse vary between 45.8 ⫾ 14.8 and 133.6 ⫾ 17.8
Ma. Generally, AFT ages increase with increasing elevation, with the exception of the
lowest elevation sample of the traverse, BH090606-1, which has an AFT age of 131.1 ⫾
17.6 Ma (fig. 3, table A2). BH090606-1 is located ⬃10.3 km southwest of the summit of
Cloud Peak, and ⬃5.8 km southwest of its neighboring sample in the traverse,
BH090406-6. It is possible that BH090606-1 is separated from the rest of the traverse by
a fault, and therefore does not fit the trend of the Cloud Peak age-elevation profile.
Track lengths were not measured for the Cloud Peak samples due to very few confined
tracks resulting from poor sample quality. Without track lengths, we use ␹2 values as a
qualitative indicator of whether or not AFT ages represent a fossil PAZ. Samples
BH090406-6 and BH090406-5, with ages of 45.8 ⫾ 14.8 Ma and 56.7 ⫾ 10.2 Ma
respectively, have the highest ␹2 values and youngest ages of the Cloud Peak samples
(table A2) and likely represent a period of rapid cooling rather than a fossil PAZ. The
other samples from this traverse, with AFT ages ranging from 98.8 ⫾ 24 Ma to 133.6 ⫾
17.8 Ma, have lower ␹2 values (for example, BH090406-1 has a ␹2 value of zero) and
higher ages, and are interpreted to represent a fossil PAZ. The onset of rapid cooling,
or change in slope on the AFT age-elevation plot, probably falls between ⬃57 and 99
Ma. The two youngest samples, BH090406-6 and BH090406-5, have AHe ages that are
older than the AFT ages for the same sample. Cerveny (ms, 1990) measured thirteen
AFT ages from surface samples from the Bighorn Range (fig. 3A). Detailed surface
locations are not provided for these samples, making it impossible to calculate their
depth below the Precambrian-Cambrian unconformity. However, four samples are
from close to the unconformity itself and are included in figure 3B. These AFT ages
range from 186.7 ⫾ 18.7 to 340.6 ⫾ 38.6 Ma, with mean track lengths between 11.3 ⫾
2.7 and 12.2 ⫾ 2.4 ␮m (Cerveny, ms 1990). The large range in AFT age along with
bimodal track length distributions indicates that these samples lie within a fossil PAZ,
which seems to occur at a similar depth relative to the unconformity as the AHe PRZ.
Reiners and Farley (2001) documented a correlation between AHe age and grain
size for two surface samples from the Bighorn Range. They concluded that grain size
will affect sample age when the sample has resided at temperatures that are within the
He PRZ in apatite for long periods of time relative to the age of the grain. The variation
of AHe age with grain size for well samples with at least six dated apatite grains is shown
in figure 4. These samples do not show any clear correlation between age and grain
size. Three samples from the Bighorn Range show a positive correlation between age
and eU: BH761, BH1140 and BH1652 (fig. 4C). Other samples do not show any age-eU
correlation (fig. 4D). The two Cloud Peak samples with AHe ages older than AFT ages
(BH090406-5 and BH090406-6) have eU values ranging between 5 and 71 ppm, with an
average eU of 25 ppm.
Wind River Range
We measured AHe ages of six samples from the Air Force well, two surface samples
from within ⬃1.6 km of the Air Force well, thirteen samples from a traverse on
Fremont Peak and five samples from Gannett Peak. We also measured AFT ages of the
Gannett Peak samples. All samples are from Archean crystalline basement. We used
the same apatite separates from the Air Force well and Fremont Peak as Cerveny and
Steidtmann (1993). A total of 22, 10, 41 and 29 aliquots were dated for the Air Force
well, surface samples near the Air Force well, Fremont Peak traverse and Gannett Peak
traverse, respectively using the AHe technique. All aliquots contained a single apatite
crystal except for the Fremont Peak traverse, where 2 of the 41 aliquots contained a
single crystal and the remaining 39 aliquots contained between 2 and 14 apatite
crystals. Results, along with AFT ages from this study and Cerveny and Steidtmann
(1993), are plotted in figure 5. Each traverse is plotted individually against elevation,
rather than on a single chart of age versus depth below the Precambrian unconformity,
157
thermochronology of the northern Rocky Mountains, western U.S.A.
B
120
120
100
100
80
80
Age (Ma)
Age (Ma)
A
60
40
40
BH090406-1
Data are divided
BH090406-4
20 between (A) and (B)
BH454
for visual clarity
BH761
0
20
30
40
50
60
70
80
Equivalent spherical radius (µm)
BH953
BH1140
20
BH2897
BH3638
0
C
20
30
40
50
60
70
80
Equivalent spherical radius (µm)
D
120
120
100
100
Age (Ma)
Age (Ma)
60
80
60
40
BH090406-1
BH090406-4
BH454
BH2897
BH3638
BH953
80
60
40
BH1140
20
20
BH1652
BH761
0
0
10
20
30
eU (ppm)
40
50
0
0
10
20
30
40
eU (ppm)
50
60
Fig. 4. (A) and (B) AHe ages from the Bighorn Range as a function of grain size, which is the radius of a
spherical grain of equal surface area to volume ratio as the apatite crystal. Ages are from samples with at least
six grains. Data are divided between (A) and (B) for visual clarity. (C) Samples that show a positive
correlation between AHe age and eU. (D) Samples with at least six grains that do not show a correlation
between AHe age and eU. Error bars are 2␴.
because there is evidence that the high peaks in the Wind River Range were uplifted
during the Oligocene (Steidtmann and others, 1989; Steidtmann and Middleton,
1991).
Air Force well.—Two samples from the Air Force well, WY5289 and WY5389,
consisted of only one single-grain aliquot (table A1, fig. 5A). We could not find
sufficient inclusion-free crystals to obtain more ages for these samples. Three samples
consisted of four single-grain aliquots each, and one sample, WY5089, consisted of
eight single-grain aliquots. AHe ages for the well range from 7.9 ⫾ 0.3 to 82.2 ⫾ 3.4 Ma,
with the exception of one aliquot from sample WY5589 with an age of 163 ⫾ 8.4 Ma
(fig. 5A). A temperature of 63 °C was measured at a total depth of 3.05 km in the Air
Force well (Cerveny, ms, 1990). In general, AHe ages become older with increasing
elevation (decreasing depth in the well). The shallowest AHe ages from the Air Force
well (sample WY5089) range from 59.5 ⫾ 2.8 to 82.2 ⫾ 3.4 Ma. Between ⬃20 and 40
Ma of AHe age scatter is observed for those well samples with several single-grain
aliquots. Corresponding AFT ages from Cerveny and Steidtmann (1993) have a very
158
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
A
B
3000
4500
WIND RIVER RANGE
2500
Surface samples
1000
500
AHe
0
20
Temperature (°C)
1500
4000
10
Subsurface
samples
AIR FORCE
WELL
AFT Cerveny and
Steidtmann (1993)
30
Elevation (m)
2000
Elevation (m)
WIND RIVER RANGE
Gannett Peak
3500
40
3000
50
-500
AHe
AFT
60
-1000
2500
0
50
100
150
200
0
50
Age (Ma)
100
150
200
Age (Ma)
C
4500
WIND RIVER RANGE
Fremont Peak
Elevation (m)
4000
3500
3000
AHe
AFT Cerveny and Steidtmann (1993)
2500
0
50
100
150
200
Age (Ma)
Fig. 5. AHe and AFT ages from the Wind River Range as a function of elevation. (A) Air Force well and
nearby surface data (dashed line represents surface elevation), (B) Gannett Peak data and (C) Fremont Peak
data. Dark blue triangles are AHe results from this study; green circles are AFT data from Cerveny and
Steidtmann (1993) except for Gannett Peak, where AFT ages are part of this study. Note that vertical scales
are different for each graph. All error bars are 2␴.
thermochronology of the northern Rocky Mountains, western U.S.A.
159
steep slope below ⬃1100 m elevation, ranging from 37.9 ⫾ 5.8 Ma to 45.2 ⫾ 4.2 Ma,
with a lower slope and older ages above ⬃1100 m (fig. 5A). Track lengths vary from
10.5 ⫾ 2.2 to 13.5 ⫾ 1.2 ␮m, indicating some partial annealing, and generally decrease
with decreasing elevation below ⬃1100 m elevation (Cerveny and Steidtmann, 1993).
Although Cerveny and Steidtmann (1993) interpret a normal fault between ⬃1100
and 1300 m elevation, we prefer a simpler interpretation that ages above ⬃1100 m
represent an episode of slower erosion or cooling. The shallowest well sample has an
AFT age of 86.2 ⫾ 9.9 Ma. In general, at elevations below ⬃200 m AHe ages are
younger than AFT ages and are scattered between ⬃8 and 43 Ma. Above ⬃200 m AHe
ages show a lower gradient on the age-elevation plot, and vary between ⬃30 and 83 Ma,
with the majority of data points having AHe ages older than the corresponding AFT
age.
Individual grain AHe ages from sample WY5089 (elevation 1626 m) range from
59.5 ⫾ 2.8 to 82.2 ⫾ 3.4 Ma and show a positive correlation of age with eU (fig. 6A); eU
values vary between 39 and 113 ppm. The AHe ages from sample WY5089 are all close
to, or older than, the AFT age for the same sample (60.5 ⫾ 5.4 Ma). None of the other
well samples shows a positive correlation of AHe age with eU (fig. 6A). No well samples
show a correlation between age and grain size (fig. 6B).
Gannett Peak.—AFT ages from the Gannett Peak traverse are between 54.0 ⫾ 3.8
and 56.6 ⫾ 3.6 Ma; mean track lengths range between 13.7 and 14 ␮m, and Dpar values
range between 2.1 and 2.8 ␮m (table A2, fig. 5B). Except for three ages, all AHe ages
from the ⬃1 km Gannett Peak traverse are older than the corresponding AFT age (fig.
5B, tables A1 and A2). Sample GP1 (elevation 3573 m) shows the greatest scatter of
AHe ages, ranging from 41.8 ⫾ 0.9 to 143.2 ⫾ 2.6 Ma. Sample GP3 (elevation 4208 m)
shows the least, ranging from 76 ⫾ 5.5 to 79.9 ⫾ 3.3 Ma, although this sample only has
three ages. The average difference between the AHe and AFT ages for all samples from
Gannett Peak is 23 Ma (AHe older than AFT). Samples GP2, GP3, and GP5 all show a
general trend of AHe age increasing with eU content (fig. 6C). The four oldest ages
from sample GP1 range between 106.8 ⫾ 7.1 to 143.2 ⫾ 5.2 Ma and have moderate eU
values between 15 and 27 ppm (fig. 6C, table A1). The average eU value of all aliquots
from Gannett Peak is 47 ppm. None of the samples shows a correlation between age
and grain size (fig. 6D).
Fremont Peak.—AHe ages from the Fremont Peak traverse range from 61.3 ⫾ 3.7 to
94.5 ⫾ 5.7 Ma and form a vertical trend on the age-elevation plot (fig. 5C, table A1).
Sample WY8789 (elevation 3064 m) shows the greatest scatter of AHe ages, ranging
from 62.5 ⫾ 3.7 to 94.5 ⫾ 5.7 Ma. Excluding samples with less than three aliquots,
WSU-WY83 (elevation 3155 m) shows the least scatter, ranging from 75.7 ⫾ 4.5 to
78.2 ⫾ 4.7 Ma. All but two aliquots from Fremont Peak contained multiple apatite
grains (table A1) yet the scatter in sample AHe ages is very similar to Gannett Peak
samples (fig. 5). On average AHe ages are 14 Ma older than the corresponding AFT
age.
Although almost all aliquots from the Fremont Peak traverse contain multiple
grains, we nonetheless plot AHe age versus eU (fig. 6E). If the AHe age and eU content
of a multigrain aliquot are a pooled average of the individual grains (Vermeesch,
2008), and if the AHe ages of the individual grains in the aliquot correlate with eU,
then we might expect the multigrain aliquot ages to correlate with eU. None of the
samples with three or more aliquots shows a positive correlation between age and eU,
although sample WY-87-89 does show a hint of a negative correlation (fig. 6E). The
lowest eU value of any aliquot is 12 ppm (table A1), and the average eU value of all
aliquots from Fremont Peak is 47 ppm. No samples exhibit a correlation between age
and grain size (which in this case is the mass-weighted average equivalent spherical
radius, fig. 6F). With the exception of the six lowest samples from the Fremont Peak
160
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
A
B
AIR FORCE WELL
160
WY-50-89
AIR FORCE WELL
160
WY-51-89
WY-51-89
WY-54-89
WY-54-89
WY-55-89
80
WY-55-89
120
Age (Ma)
Age (Ma)
120
40
80
40
0
0
50
100
150
eU (ppm)
200
0
20
C
30
40
50
60
70
Equivalent spherical radius (µm)
D
GANNETT PEAK
160
GP1
GANNETT PEAK
160
GP2
Age (Ma)
Age (Ma)
GP3
GP4
120
GP1
GP2
GP3
GP5
80
40
GP4
120
GP5
80
40
0
0
50
100
eU (ppm)
0
150
E
20
100
100
80
80
60
WY-70-89
40
WY-71-89
Age (Ma)
Age (Ma)
WY-50-89
30
40
50
60
70
80
Equivalent spherical radius (µm)
F
60
WY-70-89
40
WY-71-89
WY-79-89
WY-81-89
20
WY-79-89
WY-81-89
20
WY-83-89
FREMONT PEAK
0
0
50
100
eU (ppm)
WY-83-89
FREMONT PEAK
WY-87-89
150
0
20
WY-87-89
40
60
80
100
120
Equivalent spherical radius (µm)
Fig. 6. (A) AHe ages from the Air Force well as a function of eU. Only sample WY5089 shows a positive
correlation between age and eU. (B) AHe ages from the Air Force well as a function of grain size. No samples
show a positive correlation between age and grain size. (C) AHe ages from Gannett Peak as a function of eU
and (D) grain size. Samples GP2, GP3 and GP5 show a positive correlation between age and eU. No samples
show a correlation between age and grain size. (E) AHe ages from Fremont Peak as a function of eU and (F)
grain size. Most aliquots contain multiple apatite grains, so we plot mass-weighted equivalent spherical radius
in (F). No Fremont Peak samples show a positive correlation between age and eU or grain size. Error bars are
2␴.
thermochronology of the northern Rocky Mountains, western U.S.A.
161
traverse, which were sampled over a 19 km horizontal traverse rather than from the
peak itself (Cerveny, ms, 1990), AFT ages from Fremont Peak are slightly older than
those from Gannett Peak. Fremont Peak AFT ages vary from 57.8 ⫾ 4.8 Ma at 3215 m
elevation, to 61.8 ⫾ 5.1 Ma at 4191 m elevation (Cerveny and Steidtmann, 1993),
whereas the oldest AFT age from Gannett Peak is 56.6 ⫾ 1.8 Ma. Long mean track
lengths greater than 14 ␮m suggest rapid cooling of Fremont Peak samples (Cerveny
and Steidtmann, 1993).
Beartooth Range
AHe ages were measured for 16 samples from the Amoco Beartooth #1 well at the
northeast corner of the Beartooth Range, and 8 surface samples. Seven of the surface
samples were from a traverse on Wapiti Mountain, within 3.5 km of the well location,
with the highest sample within 8.4 km of the well location. In total we dated 89 aliquots,
ten of which contained multiple apatite crystals. Subsurface samples were consolidated
from Precambrian crystalline basement above the Piney Creek thrust as well as from
sedimentary rocks below it. These AHe data are shown in figure 7 along with AFT data
from Omar and others (1994) and Cerveny (ms, 1990). Here we simply plot age versus
elevation, rather than depth below the Precambrian unconformity, because the
majority of our surface samples are so close to the well location. It should be noted,
however, that the AFT data of Omar and others (1994) were collected along the
Beartooth Highway over a horizontal distance of ⬃30 km.
The deepest three samples from the Beartooth well, BT3088, BT3766 and BT3851
(elevations ⫺1143 m, ⫺1805 m and ⫺1874 m; depths 3047 m, 3709 m, and 3778 m
below the surface; present-day temperatures ⬃80 °C, ⬃97 °C and ⬃100 °C respectively) give AHe ages that are zero or near zero, indicating that they are below or near
the base of the present-day PRZ. Ages for these samples range from 0 to 2.1 ⫾ 0.6 Ma,
with one aliquot having an age of 20.2 ⫾ 1.1 Ma (fig. 7). Moving uphole, between
⫺1000 m and ⫹700 m elevation most AHe ages are scattered between ⬃0 and 30 Ma;
above 1100 m elevation both surface and subsurface ages are scattered between ⬃30
and 73 Ma, with transitional ages of 15 to 57 Ma between 700 m and 1100 m.
At elevations below 3130 m, AFT ages of Omar and others (1994) range from 48 ⫾
4 to 57 ⫾ 6 Ma, and have a steep slope on the age-elevation plot (fig. 7). Track lengths
are narrow and unimodal, with means decreasing from 13.9 ␮m at 2905 m to 11.8 ␮m
at ⫺457 m. At elevations above 3130 m AFT ages range from 101 ⫾ 14 to 282 ⫾ 32 Ma,
have a low slope on the age-elevation plot and broad bimodal track length distributions
(means between 9.6 and 12.2 ␮m), thus representing a fossil PAZ. Cerveny’s (ms,
1990) AFT ages are slightly older but still demonstrate the presence of the fossil PAZ
with ages of 319 ⫾ 27 and 348 ⫾ 31 Ma (mean track lengths of 11.0 ⫾ 2.0 and 11.1 ⫾
2.2 ␮m, respectively) above 3200 m, and a range from 56.2 ⫾ 8 to 67.7 ⫾ 6.5 Ma below
3130 m (mean track lengths between 11.0 ⫾ 2.0 and 14.8 ⫾ 1.4 ␮m). At elevations
between ⬃900 and 3200 m many AHe ages are older than the AFT ages, although not
by as much or as consistently as with samples from Gannett and Fremont Peaks in the
Wind River range (figs. 5 and 7). Below ⬃900 m almost all AHe ages are younger than
corresponding AFT ages.
The six single-crystal aliquots from sample BT072007-2 (elevation 2868 m) have
AHe ages ranging from 46.6 ⫾ 16 to 137.5 ⫾ 19 Ma, and give the appearance that this
sample is within a fossil PRZ (fig. 7). This sample was taken from an intrusive porphyry
forming a prominent cliff near the summit of Wapiti Mountain that was mapped by
Lopez (2001) as Cretaceous/Tertiary. To better understand this intrusive event and
how it affected our AHe ages, we determined a zircon U-Pb crystallization age for
sample BT072007-2 using laser ablation–multicollector–inductively coupled plasma–
mass spectrometry at the University of Arizona LaserChron Center (analytical techniques are outlined in Gehrels and others, 2008). Thirty two locations on 20 grains
162
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Age (Ma)
0
100
200
300
400
3000
Surface samples
2000
Elevation (m)
Subsurface samples
1000
3000
0
Surface samples
2000
-2000
1000
20
AHe - single grain age
AHe - m ultiple grain age
40
AFT Om ar et al. (1994)
AFT Cerveny (1990)
60
0
80
BEARTOOTH
RANGE
-1000
Temperature (°C)
-1000
Elevation (m)
Subsurface samples
100
AHe - single grain age
AHe - m ultiple grain age
AFT Om ar et al. (1994)
AFT Cerveny (1990)
120
-2000
0
50
100
150
Age (Ma)
Fig. 7. AHe and AFT ages from the Beartooth Range as a function of elevation. As well as AHe data from
this study (dark blue triangles) we include AFT data from Omar and others (1994) (green circles) and
Cerveny (ms, 1990) (green squares). Dashed line represents surface elevation. All error bars are 2␴.
163
thermochronology of the northern Rocky Mountains, western U.S.A.
100
A
ΒΤ072007−6
ΒΤ927
ΒΤ072007−6
ΒΤ927
ΒΤ072007−7
ΒΤ1250
ΒΤ072007−7
ΒΤ1250
ΒΤ072107−2
ΒΤ1966
ΒΤ072107−2
ΒΤ1966
80
Age (Ma)
Age (Ma)
80
60
40
0
50
100
eU (ppm)
150
40
0
200
20
C
D
ΒΤ072007−1
ΒΤ072007−1
ΒΤ183
ΒΤ183
ΒΤ2344
ΒΤ2344
ΒΤ2524
ΒΤ2524
ΒΤ072007−2
ΒΤ072007−2
100
0
100
0
0
30
40
50
60
70
Equivalent spherical radius (µm)
200
Age (Ma)
200
Age (Ma)
60
20
20
0
B
100
20
40
eU (ppm)
60
80
20
30
40
50
60
70
80
90
Equivalent spherical radius (µm)
Fig. 8. (A) AHe ages from the Beartooth Range that show a positive correlation with eU; (B) same
samples as (A) with AHe age plotted as a function of grain size; (C) Beartooth samples with four or more
aliquots that do not show a correlation of AHe age with eU; (D) same samples as (C) with AHe age plotted as
a function of grain size. Error bars are 2␴.
were analyzed. Ages from the tips of the zircon crystals, rather than the inherited cores,
gave a Cenomanian formation age of 98.3 ⫹0.3/⫺1.0 Ma, younger than the three
oldest AHe ages for the sample (table A1). Thus, these aliquots provide evidence that
some apatite crystals contain He that probably did not originate from within the apatite
crystal. The low eU concentrations of 4 ppm for these aliquots mean that non-zero He
concentrations at the grain boundaries and He implantation could significantly impact
AHe ages. Therefore, none of our samples from the Beartooth Range represents an
AHe fossil PRZ.
For the nine Beartooth samples with four or more aliquots and non-zero ages we
have plotted AHe age versus eU (fig. 8). Subsurface samples BT1250 and BT1966 are
also included because they show a positive correlation between age and eU, even
though they only have two single-crystal aliquots, and one multi-crystal aliquot per
sample. Three surface samples, BT072007-6, BT072007-7, BT072107-2, and one subsurface sample, BT927, display a positive correlation between age and eU (fig. 8A, table
A1). The three surface samples are very close to each other in elevation, and each has a
different range of eU values (table A1); yet when grouped together on an age-eU plot
they appear as one sample with a positive age-eU correlation (all solid black data points
164
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
in fig. 8A). For this reason we merge these three samples together and refer to them in
later sections as BT⫹2122. Samples BT183 and BT2344 also seem to show a trend of
increasing age with increasing eU, although one out of the four aliquots in each
sample does not fit the trend very well (fig. 8C). Two samples, BT183 and BT1966,
show a positive correlation between age and grain size, and two more, BT927 and
BT072007-6, show a negative correlation (figs. 8B and 8D).
Laramie Range
Nine subsurface samples from the Texaco Government Rocky Mountain #1 well at
the northern edge of the Laramie Range, along with six surface samples from a traverse
on Laramie Peak, were dated using the AHe technique. The Laramie Peak traverse is
located ⬃60 km southeast of the well location (fig. 1). All samples were from
Precambrian basement. In total we measured AHe ages of 42 single-crystal aliquots
from the subsurface, and 20 single-crystal aliquots from Laramie Peak. The only AFT
data available to us are three ages from Cerveny (ms, 1990). Similar to the Bighorn
Range, we plot the age data against both elevation and estimated depth below the
Precambrian unconformity of Blackstone (1993) (fig. 9).
AHe ages from Laramie Peak are very widely scattered and increase with elevation.
They range from 68.8 ⫾ 3.6 to 247.9 ⫾ 21.9 Ma at 2359 m elevation, to 138.3 ⫾ 2.2 to
326.9 ⫾ 12.6 Ma at 3131 m elevation. Cerveny’s (ms, 1990) three AFT ages also increase
with elevation, from 63.6 ⫾ 6.8 Ma at 2293 m, to 104.3 ⫾ 12.0 Ma at 3104 m, suggesting
that the upper sample is in the lower part of a fossil PAZ. Track length data agree with
this interpretation, with the highest sample having the shortest mean track length of
12.8 ⫾ 1.9 ␮m (Cerveny, ms 1990). All of the surface AHe ages are older than the age
trend suggested by the AFT data (fig. 9), and show significantly more scatter than the
AHe ages from the Gannett Peak and Fremont Peak traverses in the Wind River Range
that are also older than AFT ages. There is no obvious correlation between old AHe
ages (⬎AFT ages) and low eU values, with the majority of aliquots having eU values
between ⬃20 and 40 ppm (fig. 10A, table A1). This suggests that He implantation may
not be the cause of the old AHe ages at Laramie Peak. None of the three Laramie Peak
samples that have multiple aliquots show an age-eU correlation, although one,
02PRLP2, shows a negative correlation between age and grain size (figs. 10A and 10B).
AHe ages from the well samples are less scattered than those from Laramie Peak,
the amount of scatter varying between ⬃13 and 57 Ma, if we ignore three possible
“flyers” with ages greater than 300 Ma (fig. 9, table A1). These “flyers” have eU values of
9, 50 and 66 ppm (table A1). For elevations below 1100 m, the youngest AHe ages of
each sample vary between 3.3 ⫾ 1.7 Ma at ⫺581 m and 44.1 ⫾ 2.4 Ma at 155 m and
show no systematic trend. The oldest AHe ages of each sample do seem to display
trends, however, with the four deepest samples showing a different trend to the
shallower samples. Below 0 m the maximum AHe age increases with elevation,
changing from 54.3 ⫾ 2.8 Ma at ⫺734 m to 90.9 ⫾ 5.0 Ma at ⫺270 m (fig. 9A). Above
0 m the maximum AHe age drops to 56.9 ⫾ 2.6 Ma at 155 m and remains fairly
constant to 1026 m. At elevations above 1026 m the maximum AHe age increases to
97.1 ⫾ 3.5 Ma at 1931 m (fig. 9A).
AHe ages from the deepest well sample, NLR2761 (elevation ⫺734 m, present-day
temperature ⬃60 °C), correlate clearly with eU (fig. 10C). None of the well samples
show an obvious correlation between age and grain size (fig. 10D).
Summary
Key observations from our data are: 1) In general, our AHe data show age trends
that we might expect, with older ages at high elevations and younger ages at lower
elevations. There are no dramatic age changes or trend reversals such as those found
by Beland (ms, 2002). 2) AHe ages for all of our samples show a large amount of
thermochronology of the northern Rocky Mountains, western U.S.A.
A
165
3500
3000
2500
Surface samples
Subsruface samples
10
1500
20
1000
30
500
40
0
LARAMIE
RANGE
-500
AHe
AFT Cerveny (1990)
Temperature °C
Elevation (m)
2000
50
60
-1000
0
100
200
300
Age (Ma)
Depth below pC unconformity (m)
B
0
1000
2000
3000
AHe
AFT Cerveny (1990)
0
100
200
300
Age (Ma)
Fig. 9. AHe ages (dark blue triangles) and AFT ages (green circles, from Cerveny, ms, 1990) from the
Laramie Range as a function of (A) elevation (dashed line represents elevation of the surface), and (B)
depth below the Precambrian unconformity. All error bars are 2␴.
166
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
400
A
LARAMIE PEAK
400
B
LARAMIE PEAK
02PRLP2
02PRLP5
02PRLP5
300
02PRLP7
Age (Ma)
Age (Ma)
300
200
02PRLP2
100
02PRLP7
200
100
0
0
0
120
50
100
eU (ppm)
150
20
C
120
GOV’T ROCKY MTN WELL
100
100
80
80
30
40
50
60
70
80
Equivalent spherical radius (µm)
D
GOV’T ROCKY MTN WELL
NLR96
NLR508
60
NLR96
40
NLR508
Age (Ma)
Age (Ma)
NLR2297
NLR2608
NLR2761
60
40
NLR2297
20
NLR2608
20
NLR2761
0
0
50
100
eU (ppm)
150
0
20
30
40
50
60
70
80
Equivalent spherical radius (µm)
Fig. 10. (A) AHe ages from Laramie Peak as a function of eU and (B) as a function of grain size. No
samples show a positive correlation between age and eU or grain size. (C) AHe ages from the Texaco
Government Rocky Mountain #1 well in the Laramie Range as a function of eU and (D) as a function of grain
size. Only sample NLR2761 shows a positive correlation between age and eU. No well samples show a
correlation between age and grain size. Error bars are 2␴.
scatter. With the exception of the Laramie Range the amount of scatter is often similar
for both surface and subsurface samples, indicating that consolidation of cuttings, or
contamination from caving of the borehole, are not serious issues. 3) At higher
elevations in the Wind River, Beartooth, Bighorn and Laramie ranges AHe ages are
often older than AFT ages. The presence of AHe ages that are older than the rock
formation age in the Beartooth range suggests that He implantation may be a problem.
4) In both the Bighorn and Laramie ranges there is evidence that the AHe fossil PRZ
and AFT fossil PAZ occur at similar elevations. 5) Several samples, but not all, show a
positive correlation between AHe age and apatite eU content. 6) We see no evidence
for a consistent relationship between grain size and AHe age.
modeling
We have taken three approaches to interpret the AHe data from these Laramide
ranges. Firstly, we used forward modeling to clarify the relationship between eU
concentration, grain size and AHe age. Secondly, we used forward modeling of generic
Laramide-type exhumation histories to illustrate the possible effects of radiation
thermochronology of the northern Rocky Mountains, western U.S.A.
167
damage on AHe age-elevation profiles. Thirdly, we have inverted AHe age-eU pairs
from one sample from each range for time-temperature history and then extrapolated
this time-temperature history up and down a wellbore to create a model age-elevation
plot. Known geologic data were used to constrain the range of possible inversion
results.
Forward Modeling
Reiners and Farley (2001) demonstrated that grain size influences the AHe age of
an apatite crystal when the crystal has spent a long time relative to the age of the crystal
at temperatures within the PRZ of He in apatite. They illustrated this phenomenon
with two samples from the Bighorn Range. We expect that the other ranges in our
study have experienced a somewhat similar thermal history to the Bighorn Range, and
that many of our samples have probably resided within the AHe PRZ for a significant
duration of the Phanerozoic. It also seems likely that radiation damage has accumulated in many of our samples during their residence in the PRZ, especially as several
samples show a correlation between AHe age and eU content. It is possible that many
of our samples do not show a clear correlation of age with eU, or age with grain size,
because of the combination of both effects. Therefore, we used forward modeling to
investigate the relationship between AHe age, radiation damage and grain size using
the radiation damage accumulation and annealing model (RDAAM) of Flowers and
others (2009) and program HeFTy (Ketcham, 2005). We initially investigated the
general nature of these relationships and the relative significance of grain size and
radiation damage using an end member isothermal hold model. We then forward
modeled a time-temperature path more appropriate to our samples (“generic Laramide exhumation model”) to see how the combined effects of radiation damage and
grain size might effect our real AHe ages and to better understand our real AHe
age-elevation plots.
Isothermal hold model.—Details of the isothermal hold model are discussed in the
Appendix. Results show that radiation damage increases the temperatures of the PRZ
compared to the Durango diffusion model (also shown by Flowers and others, 2009),
and that the effect of radiation damage on AHe age is more significant than that of
grain size (fig. A1).
Generic Laramide exhumation model.—To gain a better understanding of the effect
that radiation damage may have had on the AHe age variation in our samples, and to
see how relevant it may be to our larger dataset, we forward modeled a simplified
Laramide-type thermal history and calculated profiles of age versus elevation for three
different model “boreholes,” each of which experienced a different maximum burial
temperature. The thermal models for a surface sample from each borehole included
cooling from temperatures higher than the AHe PRZ and the AFT PAZ at the end of
the Precambrian (200 °C at 610 Ma) to near-surface temperatures (10 °C at 600 Ma).
To simulate burial by Paleozoic and Mesozoic sediments, uniform heating from 600 to
65 Ma resulted in maximum temperatures of 70 °C, 80 °C and 90 °C for the three
model boreholes. To simulate the Laramide orogeny, between 65 and 60 Ma the
samples cooled to a surface temperature of 5 °C, where they remained to 0 Ma (fig.
11A). To keep the models simple and facilitate interpretation of results, we assumed
no post-Laramide burial and exhumation, an oversimplification for much of our study
area. These thermal histories were then extrapolated to greater depths using a
geothermal gradient of 20 °C/km, up to a maximum of 4 km, to simulate the deeper
borehole samples. At various depths we calculated AHe age for eU contents between
10 and 150 ppm using the RDAAM, AHe age using the Durango diffusion model, and
AFT age using the annealing model of Ketcham and others (2007) and a Dpar value of
1.65 ␮m. Results are shown in figure 11.
168
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
A
B
Max. Temp. 70 °C
Time (Ma)
700
600
500
400
Age (Ma)
300
200
100
0
0
100
200
400
500
600
Max. Temp. 80 °C
10 pp
m
Max. Temp. 70 °C
1.5
2
DUR
100
Depth (km)
1
Age (Ma)
50
0
100
0
0.5
1
2.5
Depth (km)
Max. Temp. 90 °C
200
m
50 pp
pm
100 p
m
150 pp
AFT
0.5
Temp. (oC)
300
0
0
3
3.5
4
pp
1.5
10
2
DU
2.5
50
m
R
m
pp
m
pp
0 m
10 pp
0
15
3
3.5
oC
T
AF
4
D
Max. Temp. 80 °C
Max. Temp. 90 °C
Age (Ma)
URU
DD
R
2.5
o
pm
pRDAAM
- max temp. 80 C
50
m
m
p
p
pp
000 pm
101 50 p m
1 pp
4
3
3.5
4
0
1T5 FT
A
AF
600
0.5
1
1.5
2
Age (Ma)
50
0
100
0
0.5
1
2.5
1.5
3
3.5
50
3.5
2
m
3
m
pm
0ppp
110
pp
Depth (km)
1.5
500
4
2
2.5
3
3.5
pmm
ppp
1100
UR R
DDU
m
pp
50
RDAAM
- max temp. 90C
pm
0p m
10 pp m
0
15 pp
50
1
2.5
400
pp pp
m
m
0.5
300
0
0
0
200
Depth (km)
m
10 pp
100
100
150 ppm
2
DUR
Depth (km)
1.5
Age (Ma)
50
0
0
600
100 ppm
pm
150 p
0.5
1
500
m
50 ppm
AFT
Age (Ma)
400
10 pp
300
DUR
0
200
AFT
100
Depth (km)
0
10
C
0
15
T
AF
T
AF
4
Fig. 11. (A) Simple Laramide-type thermal histories used as input into forward modeling for three
hypothetical boreholes whose surface samples reached maximum temperatures of 70 °C, 80 °C and 90 °C
respectively at 65 Ma. The input thermal histories are extrapolated to other elevations using a geothermal
gradient of 20 °C/km. Age is plotted as a function of elevation for each borehole with a maximum surface
sample temperature of (B) 70 °C, (C) 80 °C, and (D) 90 °C. Solid lines represent AHe ages calculated for
different eU values using the RDAAM, assuming 0 ppm Sm and a grain radius of 45 ␮m (the average
equivalent spherical radius of our samples). Short-dashed line represents AHe age calculated using the
Durango He diffusion model. Long-dashed line represents AFT age. Diagonal shading shows where AHe
ages are greater than AFT ages. Gray shading shows region where AHe ages may be widely dispersed, and
where small changes in depth or eU concentration can result in large changes in AHe age. Insets show
expanded time scales.
thermochronology of the northern Rocky Mountains, western U.S.A.
169
For boreholes with maximum temperatures of 70 °C and 80 °C, a fossil AHe PRZ is
preserved in the upper 1 km and 0.5 km of the age-depth profiles respectively. A fossil
PRZ is not observed in ages from the model borehole with a maximum temperature of
90 °C (fig. 11D), or from any of the model boreholes when the Durango diffusion
model is used. The age-depth plots for maximum temperatures of 70 °C and 80 °C
show that AHe age is predicted to be older than the corresponding AFT age within the
fossil PRZ if eU ⲏ 20 ppm (diagonally shaded zones in figs. 11B and 11C). The rollover
of AHe ages into the fossil PRZ is extremely rapid for samples with eU contents ⬎ 10
ppm. Interestingly, the fossil PRZs occur at similar depths to the AFT fossil PAZs for eU
ⲏ 20 ppm and hence both AFT and AHe ages change very rapidly with depth within
the PRZ; very slight changes in depth or eU can result in very dramatic changes in AHe
age. Whereas conventional Durango He diffusion kinetics predict that an AHe PRZ
occurs at shallower depth (higher elevation) than an AFT PAZ, the RDAAM predicts
that with increasing eU concentration, the AHe PRZ will occur at increasing depths.
For the model time-temperature paths tested here and eU ⲏ 20 ppm, the fossil AHe
PRZ occurs at similar depths to the fossil AFT PAZ.
The other main feature of the synthetic age-elevation plots for these model
boreholes is the present-day PRZ, which is also affected by radiation damage. For a
model borehole with a maximum surface-sample temperature of 70 °C, the presentday PRZ for an eU⫽10 ppm apatite is between ⬃0.7 and 3 km depth (low radiation
damage ⫽ low temperature and depth of PRZ), whereas it is between ⬃2.2 and 4 km
depth for an eU⫽150 ppm apatite (high radiation damage ⫽ high temperature and
depth of PRZ; fig. 11B). Very similar results were found for the present-day PRZ in the
other two model boreholes because they have the same present-day temperature
profile. Therefore, AHe ages of near-surface samples are affected by the maximum
temperature reached, although below ⬃1 km depth the age-elevation curves are
similar, regardless of the maximum temperature.
The age-depth curves show that low eU samples will have younger AHe ages than
higher eU samples, and that the amount of age dispersion will change depending on
the sample depth. A sample at 2.5 km depth in our models shows ⬃50 Ma age
difference between grains with eU⫽10 ppm and eU⫽150 ppm, whereas at shallower
depths, within the fossil PRZ, this age dispersion can be hundreds of Ma. AHe ages of 0
Ma are not reached until ⬃4 km depth (present-day temperature 85 °C) for apatites
with eU ⲏ 50 ppm using the RDAAM model, in contrast to ⬃3 km (65 °C) for the
Durango diffusion model.
Samples in these model boreholes experienced 65 °C, 75 °C and 85 °C of cooling
between 65 and 60 Ma. Using a 20 °C/km geothermal gradient, this corresponds to
3.25 km, 3.75 km and 4.25 km of exhumation, cooling rates of 13 °C/Ma, 15 °C/Ma
and 17 °C/Ma, and exhumation rates of 0.65 km/Ma, 0.75 km/Ma and 0.85 km/Ma
respectively. Typically, steep age-elevation gradients are interpreted to be the result of
rapid exhumation and cooling, with the ages representing the time of rapid cooling
and the gradient giving the exhumation rate (for example, Ehlers, 2005). The amount
of exhumation can be estimated from the elevation difference between the base of the
fossil PRZ and the base of the present-day PRZ (that is, zero age). Examining the AHe
age-elevation plots in figure 11, we see that each eU curve provides a reasonable
estimate of the amount of exhumation; however, this would be very difficult to
determine from real data with dispersion. Similarly, using the gradient of the AHe
age-elevation plot to estimate the exhumation rate will also be difficult. Thus, radiation
damage can lead to misinterpretation of an age-elevation plot.
Grain size.—To investigate the effect of grain size further, we examined a hypothetical sample from 0.5 km depth in the model borehole of figure 11B. This hypothetical
sample reached a maximum temperature of 80 °C (surface sample reached 70 °C).
170
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
B
500
400
400
Age (Ma)
Age (Ma)
A
500
300
200
300
200
100
100
0
0
0
20
40
60
80
0
20
40
60
80
Grain size R s ( µm)
eU (ppm)
C
600
500
400
Age (Ma)
300
200
100
0
140
120
100
40
Gr
50
ain
60
siz
eR
80
s(
µm
)
40
80
)
pm
60
70
eU
(p
20
90
Fig. 12. AHe age plotted against (A) eU and (B) grain size for a hypothetical sample from 0.5 km depth
in borehole from figure 11B. The combined effect of eU and grain size can result in distributions that are not
positively correlated. (C) Surface representing the variation of AHe age with both grain size and eU for the
same sample. Black dots are data points from (A) and (B), illustrating that although these points do not show
good age-eU or age-grain size correlations, they fall on a single surface in eU-grain size-age space. Color
gradient shows AHe age and is meant as a visual aid.
thermochronology of the northern Rocky Mountains, western U.S.A.
171
Inputting this thermal history into HeFTy, we calculated AHe ages for various
combinations of eU concentration and grain size (Rs). Figure 12 shows age-eU (fig.
12A) and age-Rs (fig. 12B) plots for five grains from our hypothetical sample, which
were selected from the surface in eU-Rs-age space defined by this thermal history (fig.
12C). Although both Flowers and others (2009) and our isothermal hold model show
that typical variations of grain size have a smaller effect on AHe age than typical
variations of eU, these grains were chosen to show that grain size is still important, and
can significantly obscure age-eU correlations. While there is a general trend of
increasing age with increasing eU, a positive age-eU correlation is not obvious for these
grains, especially if the youngest grain is removed. The age-Rs plot does not show any
correlation. However, these points were all calculated from the same thermal history,
and all lie on the same surface in eU-Rs-age space (fig. 12C).
While we have shown that some apparently random scatter in AHe ages may be
due to a combination of influences from both grain size and eU variation, and that
these points should fall on a surface in eU-Rs-age space, we speculate that this surface
would be difficult to resolve with real data. With so few aliquots for each sample it is
difficult to tell if ages fall on a surface in eU-Rs-age space, or if age scatter arises from
some other factor.
Comparison to real data.—Comparison of our forward modeling results (fig. 11) to
real data (figs. 3, 5, 7, and 9) shows some striking similarities. The general distribution
of our data is similar to that predicted by the models, with AHe ages much younger
than AFT ages deep in the boreholes, becoming more similar to, and often overlapping AFT ages shallower in the boreholes. Almost all of our samples also show
pronounced scatter in AHe ages, typically on the order of a few tens of Ma. The
Bighorn Range data, for example, when plotted with respect to the PrecambrianCambrian unconformity elevation, and including data from Crowley and others
(2002), show a pronounced rollover to older AHe ages close to the unconformity (fig.
3B). AFT ages from Cerveny (ms, 1990) also show a wide range (187 to 341 Ma) at the
unconformity. This break in slope to older AHe ages represents the base of the fossil
PRZ, which occurs at a similar elevation to the widely scattered AFT ages, which are
likely within a fossil PAZ. Similarly, samples from Laramie Peak show a wide scatter of
AHe ages, all of which are older than corresponding AFT ages, the highest elevation of
which seems to be at the base of a fossil PAZ (Cerveny, ms, 1990) (fig. 9). Samples from
the Beartooth Range (fig. 7) resemble a situation intermediate between figures 11C
and 11D, where we see an AFT fossil PAZ, but do not sample the AHe fossil PRZ,
although our AHe samples are not from as high elevation as the AFT ages of Omar and
others (1994). AFT and AHe ages from the Air Force well in the Wind River Range (fig.
5A) (Cerveny and Steidtmann, 1993) resemble figure 11C, where the shallowest AHe
sample has AHe ages older than the AFT age.
There are, however, differences between the forward modeling results and the
real data, which are hardly surprising given the simple nature of our models. Not all of
the observed AHe age scatter can be explained by a combination of radiation damage
and grain size variation. For example, sample GP1 from Gannett Peak in the Wind
River Range consists of ten dated grains. Excluding the smallest and largest grain,
grain sizes range from ⬃33 to 41 ␮m. With such a small grain size variation, the impact
of grain size is essentially removed and AHe ages should show a correlation with eU.
However, no such correlation is observed (fig. 6C), indicating that something else is
causing the AHe age scatter of sample GP1. Also, our models cannot explain AHe ages
from Fremont Peak and Gannett Peak in the Wind River Range, which are older than
corresponding AFT ages. Both AHe and AFT ages in these areas do not show the large
variation with elevation that is predicted if the older AHe ages are due to their
presence in a fossil PRZ.
172
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Although radiation damage of apatite cannot explain all the features of our data,
the fact that: 1) many samples show a correlation between eU and AHe age, and 2)
AHe fossil PRZs are at similar elevations to AFT fossil PAZs, shows that radiation
damage has affected the AHe ages of our samples. For this reason we used the RDAAM
(Flowers and others, 2009), which offers the most complete explanation of the effect of
radiation damage on He diffusion in apatite to date, to perform inverse modeling of
the thermal histories from our data (Ketcham, 2005).
Inverse Modeling
The RDAAM predicts that the AHe age-eU distribution of aliquots from a single
sample is dependent upon the thermal history experienced by that sample. Therefore,
inverse modeling can be used to determine the range of possible thermal histories that
can produce the observed age-eU distribution. We used the Monte-Carlo inversion
routine in HeFTy (Ketcham, 2005) with the RDAAM for He diffusion in apatite
(Flowers and others, 2009) to find a best-fit thermal history as well as the range of good
and acceptable histories (see Ketcham, 2005, for more information and definitions of
good and acceptable). We then generated an age-elevation profile by extrapolating the
best-fit thermal history to other elevations using the present-day geothermal gradient.
While it is unlikely that the geothermal gradient has remained constant throughout
geological time, this simplifying assumption makes the modeling much more tractable.
At each elevation we forward modeled an AFT age using the annealing model of
Ketcham and others (2007) and a Dpar value of 1.65 ␮m, as well as AHe ages for low and
high eU concentrations appropriate for each range. If the maximum temperature was
poorly constrained by the inversion, we iteratively adjusted it to match the ages of
samples within a fossil PRZ. If necessary, we iteratively adjusted the past geothermal
gradient to effectively stretch or squeeze the age-elevation profile to match the
observed data. The present-day geothermal gradient was fixed from borehole temperatures.
Ideally, we should compare age-elevation profiles calculated from the inversion of
AHe data with age-elevation profiles from the inversion of AFT age and track length
data. Unfortunately, detailed track length data are not available for either the AFT ages
of Cerveny (ms, 1990), Cerveny and Steidtmann (1993) or Omar and others (1994),
making inversion impossible. Instead, this paper focuses on the interpretation of the
AHe ages.
Borehole temperatures from well logs were used to estimate present-day geothermal gradients. In general, these provide an underestimate of the modern geothermal
gradient as there is insufficient time for temperatures to equilibrate before logging. To
correct bottom-hole temperatures for this underestimation we used the method of
Horner (Barker, 1996) if we had three or more temperature measurements, or Waples
and others (2004) if we had two or less. Surface temperatures were assumed to be 5 °C.
Using these data we calculated modern geothermal gradients of 14 °C/km for the
Bighorn Range, 19 °C/km for the Wind River Range, 25 °C/km for the Beartooth
Range and 20 °C/km for the Laramie Range.
Details of input data for inverse thermal history modeling are discussed in the
Appendix to this paper. We ran inversions for two samples from the Bighorn Range,
BH761 and BH1652, sample WY5089 from the Air Force well in the Wind River Range,
sample BT⫹2122 (combined sample) from the Beartooth Range, and sample NLR2761
from the Laramie Range.
Inversion results—Bighorn Range.—Results from inversion of three AHe age-eU
pairs for BH761 are shown in figure 13. In order to show sufficient detail, thermal
histories are only shown for the last 200 Ma for all of our inversions; before this time
thermal histories are very poorly constrained. Results from the inversion of BH1652
(not shown) are very similar. This is reassuring because samples from the same
173
thermochronology of the northern Rocky Mountains, western U.S.A.
A
0
BH761
Temperature (°C)
20
sample elevation 1293 m
40
60
80
100
120
30 good solutions
159 acceptable solutions
10,000 time-temperature paths
140
200
180
160
140
120
100
80
Time (Ma)
60
40
20
0
120
Age (Ma)
100
B
BH761
80
60
40
20
0
0
10
20
30
eU (ppm)
120
60
80
60
40
40
20
20
0
10
20
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eU (ppm)
40
50
BH1652
D
100
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0
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120
BH1140
C
Age (Ma)
Age (Ma)
100
40
0
60µ
m
30µ
0
10
m
20
30
40
50
eU (ppm)
Fig. 13. (A) Inverse modeling results for sample BH761 from the Bighorn Range. 10,000 timetemperature paths were tested. 30 good and 159 acceptable solutions were found. Results are shown as
envelopes encompassing good (dark gray) and acceptable (light gray) paths. Black rectangles represent
input time-temperature constraints on the inversions. The best-fit solution is shown as a black line. The
dashed black line is the best-fit solution after modification of the maximum temperature. (B) AHe age-eU
correlations predicted from the best-fit inversion solution from sample BH761, calculated for grain sizes of
30 ␮m (long-dashed gray line), 45 ␮m (solid gray line) and 60 ␮m (short-dashed gray line). Actual AHe ages
(black triangles) and ages calculated from the best-fit inversion solution using actual values for eU, Sm and
grain size (open squares) are also shown. (C) and (D) Results from extrapolating the best-fit solution from
BH761 to the elevation of (C) BH1140 and (D) BH1652 using a geothermal gradient of 14 °C/km. Error
bars are 2␴.
174
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
borehole likely experienced similar thermal histories, and supports the use of both the
inversion technique and the RDAAM for the interpretation of our data.
The maximum peak temperature reached by sample BH761 before Laramide
exhumation is poorly constrained; however this sample does provide constraints on
the minimum peak temperature reached before exhumation. The lowest peak temperature of an acceptable solution is ⬃78 °C, and for a good solution is ⬃85 °C. These
values are determined from the nodal points of the inversion solutions (fig. A2) rather
than the solution envelopes in figure 13. This sample is from a depth of 761 m below
the surface at a temperature of ⬃16 °C, indicating that it has cooled by at least ⬃60 °C
since maximum burial before the Laramide orogeny. Assuming that the present-day
geothermal gradient (14 °C/km) is valid in the past, this corresponds to at least 4 km
of exhumation. The timing of the exhumation is not well constrained, although nodal
points of the acceptable inversion solutions indicate that exhumation had started by
⬃71 Ma (fig. A2). Nodal points of the good solutions indicate that exhumation had
started by ⬃85 Ma. Some good and acceptable solutions show exhumation starting
⬃110 Ma, which is the upper age limit on our input constraint box, indicating the
oldest possible age for onset of Laramide exhumation cannot be resolved with our
data. Good and acceptable solutions also indicate that post-Laramide burial and
exhumation is not necessary to explain our data.
The best-fit time-temperature path from the inversion of sample BH761 was
extrapolated to the elevations of samples BH1140 and BH1652, which also show a
positive correlation between AHe age and eU, using a geothermal gradient of
14 °C/km. Predicted age-eU correlations were calculated for different grain sizes and
compared to real data (figs. 13C and 13D). For all three samples the predicted age-eU
correlations match the real age-eU distributions quite well, further confirming that
radiation damage of apatite is having a significant effect on our samples, and
supporting the use of RDAAM as an appropriate diffusion model for interpreting our
AHe ages. We also calculated the predicted age for individual grains of each sample,
each of which has a specific grain size, eU and Sm content (fig. 13).
Extrapolation of the best-fit time-temperature path over the entire elevation range
of our samples allowed us to create “model” age-elevation profiles of AHe and AFT
ages and compare them to our actual data (fig. 14). We calculated AHe profiles for a
low eU value of 5 ppm and a high eU value of 90 ppm (based on actual sample values),
an average Sm concentration of 169 ppm and an average grain size of 45 ␮m. The peak
temperature, which was not well constrained by the inversion results, was adjusted to
match the change in gradient of the AHe ages to a fossil PRZ at shallow depths
(Crowley and others, 2002). This resulted in a maximum temperature of 95.6 °C for
sample BH761, which would imply ⬃80 °C of cooling since maximum burial or ⬃5.7
km of exhumation using a geothermal gradient of 14 °C/km. Recalculations confirmed that this adjustment of maximum temperature did not affect the predicted AHe
age-eU distributions in figure 13. The model age profiles provide a reasonable match
to the real age distributions over a very large depth/elevation range of 5 km (fig. 14),
again confirming both the validity of using the RDAAM and our modeling approach.
Inversion results—Wind River Range.—One sample from the Air Force well, WY5089,
shows a correlation between AHe age and eU. Results from the inversion of four AHe
age-eU pairs from this sample are shown in figure 15A. As with the Bighorn data, the
maximum peak temperature attained by WY5089 before the Laramide orogeny is
poorly constrained, although the inversion results indicate that it was at least ⬃85 °C.
This sample is from a depth of 569 m below the surface, at a temperature of ⬃16 °C,
indicating that it has cooled at least 69 °C since pre-Laramide maximum burial. Using
the present-day geothermal gradient of 19 °C/km, this corresponds to at least 3.6 km
of exhumation. The distribution of nodal points from the inversion solutions shows
thermochronology of the northern Rocky Mountains, western U.S.A.
175
Fig. 14. Thermochronometer age plotted against depth below the Precambrian unconformity for the
Bighorn Range, showing AHe and AFT ages from this study (dark blue triangles and green squares,
respectively), AHe ages from Crowley and others (2002) (light blue triangles), and AFT ages from Cerveny
(ms, 1990) (green circles). Solid lines represent ages calculated from extrapolating the best-fit timetemperature path from the inversion of BH761 age-eU data to other elevations. Green line is modeled AFT
age; light blue line is modeled AHe age for an eU of 5 ppm, an average grain size of 45 ␮m, and an average
Sm content of 169 ppm; dark blue line is modeled AHe age for an eU of 90 ppm, an average grain size of 45
␮m and an average Sm content of 169 ppm; black dashed line is AHe age calculated using conventional
Durango diffusion kinetics. Inset shows same profile but with an expanded time scale to show more detail.
Red arrows indicate samples shown in figure 13. Error bars are 2␴.
that exhumation began before ⬃66 Ma, although the upper age limit on the start of
Laramide exhumation is not well constrained by our data (fig. A3). The narrowness of
the path envelope (fig. 15A) and the distribution of path nodal points imply that there
was at least 20 °C of cooling between ⬃75 and 59 Ma. The best-fit thermal history also
includes a post-Laramide heating event of ⬃20 °C and cooling from late Miocene to
present (fig. 15A), although the envelope of acceptable fits indicates that the postLaramide thermal history is very poorly constrained by our data.
The age-eU distribution predicted by the best-fit time-temperature path matches
the distribution of the actual AHe ages for sample WY5089 quite well, with the
exception of one grain with an age of 82 Ma (fig. 15B). This best-fit thermal history was
then extrapolated to other elevations using the present-day geothermal gradient, and
176
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Fig. 15. (A) Inverse modeling results for sample WY5089 from the Air Force well in the Wind River
Range. 500,000 time-temperature paths were tested, finding 462 acceptable solutions. No good solutions
were found. Results are shown as a gray envelope of acceptable paths. Black rectangles represent input
time-temperature constraints on the inversion. The best-fit solution is shown as a black line. (B) AHe age-eU
correlations predicted from the best-fit solution from sample WY5089, calculated for grain sizes of 30 ␮m
(long-dashed gray line), 45 ␮m (solid gray line) and 60 ␮m (short-dashed gray line). Actual AHe ages (black
triangles) and ages calculated from the best-fit inversion solution using actual values for eU, Sm and grain
size (open squares) are also shown. (C) Thermochronometer age plotted against elevation for samples from
thermochronology of the northern Rocky Mountains, western U.S.A.
177
AFT and AHe age-elevation profiles constructed. For AHe ages we use a low eU value of
10 ppm and a high eU value of 130 ppm, based on actual sample values, and an average
Sm value of 685 ppm (fig. 15C). We repeated this procedure for an inversion that
included the 82 Ma grain as input, but excluded the grain with an eU of 113 ppm
(table A1, fig. 15B), and compared age-elevation profiles. Here we show results of the
inversion that included the 82 Ma grain and excluded the 113 ppm grain, as it provides
a better visual match to actual data on the age-elevation profile. The model AHe age
profiles in figure 15C encompass the majority of the age dispersion of our AHe data.
Inversion results—Beartooth Range.—Three individual well samples, BT927, BT1250
and BT1966, and one composite surface sample, BT⫹2122, show AHe age-eU correlations. Results from the inversion of three AHe age-eU pairs from sample BT⫹2122 (fig.
16A) show that the peak temperature attained by BT⫹2122 was at least 83 °C before
exhumation during the Laramide orogeny, but the maximum peak temperature is
again poorly constrained. The present-day temperature of the sample is assumed to be
5 °C, the average surface temperature. Therefore this sample has cooled by at least
78 °C since maximum burial, which translates to ⬃3.1 km of exhumation using the
present-day geothermal gradient of 25 °C/km. Results also show that exhumation
began before ⬃58 Ma (although the maximum onset age for exhumation is not well
constrained), with at least 30 °C of cooling between ⬃74 and 41 Ma (fig. A4).
Post-Laramide burial and exhumation is not necessary to explain our data.
The best-fit time-temperature path from the inversion of BT⫹2122 was extrapolated over the entire elevation range of our samples and “model” age-elevation profiles
of AHe and AFT ages calculated and compared to actual data. The peak temperature
of the samples and the past geothermal gradient were iteratively adjusted to better
match the distribution of real data. The past geothermal gradient was reduced from
the present-day value of 25 °C/km to 20 °C/km, although 25 °C/km was retained at 0
Ma. The maximum temperature was reduced to 102.7 °C to match the change in
gradient of the AFT ages to a fossil PAZ at high elevations (Omar and others, 1994).
Using this maximum temperature, a present-day surface sample temperature of 5 °C,
and a geothermal gradient of 20 °C/km, we calculate ⬃5 km of exhumation from the
Beartooths. Predicted age-eU distributions were calculated for all samples with an
age-eU correlation and compared to real data (figs. 16B-E). For all four samples the
predicted age-eU correlations match the real age-eU distributions very well.
AHe age-elevation profiles were calculated for a low eU value of 5 ppm and a high
eU value of 150 ppm, based on actual sample values, and an average Sm value of 99
ppm (fig. 17). The model AHe age profiles match the general distribution of the real
data, although the real data show more dispersion than predicted by the model. The
model AFT age-elevation profile shows a lower gradient than the real AFT profile of
Omar and others (1994), which is almost vertical below ⬃3100 m elevation (fig. 17).
Inversion results—Laramie Range.—Inverse modeling results from three AHe age-eU
pairs from sample NLR2761 are shown in figure 18A. As with the other ranges,
time-temperature paths were not allowed to be cooler in the past than the present-day
temperature of the sample, which for NLR2761 is 60 °C. The acceptable-fit nodal
points show that that the maximum temperature attained by sample NLR2761 was less
the Air Force well in the Wind River Range, showing AHe ages from this study (dark blue triangles) and AFT
ages from Cerveny (ms, 1990) (green circles). Solid lines represent ages calculated from extrapolating the
best-fit time-temperature path from the inversion of WY5089 age-eU data to other elevations. Green line is
modeled AFT age; light blue line is modeled AHe age for an eU of 10 ppm, an average grain size of 45 ␮m,
and an average Sm content of 685 ppm; dark blue line is modeled AHe age for an eU of 130 ppm, an average
grain size of 45 ␮m and an average Sm content of 685 ppm; black dashed line is AHe age calculated using
conventional Durango diffusion kinetics. Red arrow indicates sample WY5089 used in inversion modeling.
Error bars are 2␴.
178
Age (Ma)
Age (Ma)
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
eU (ppm)
Age (Ma)
Age (Ma)
eU (ppm)
eU (ppm)
eU (ppm)
Fig. 16. (A) Inverse modeling results for sample BT⫹2122 from the Beartooth Range. 100,000
time-temperature paths were tested, finding 140 good and 650 acceptable solutions. Results are shown as
envelopes encompassing good (dark gray) and acceptable (light gray) paths. Black rectangles represent
input time-temperature constraints on the inversions. The best-fit solution is shown as a black line. The
dashed black line is the best-fit solution after modification of the maximum temperature. (B) AHe age-eU
correlations predicted from the modified best-fit solution from sample BT⫹2122, calculated for grain sizes
of 30 ␮m (long-dashed gray line), 45 ␮m (solid gray line) and 60 ␮m (short-dashed gray line). Actual AHe
ages (black triangles) and ages calculated from the modified best-fit inversion solution using actual values for
eU, Sm and grain size (open squares) are also shown. Error bars are 2␴. (C) to (E) results from extrapolating
modified best-fit solution from BT⫹2122 to elevation of (C) BT927, (D) BT1250 and (E) BT1966 using a
geothermal gradient of 20 °C/km, except at 0 Ma when 25 °C/km is used.
thermochronology of the northern Rocky Mountains, western U.S.A.
179
Fig. 17. Thermochronometer age plotted against elevation for samples from the Beartooth Range,
showing AHe ages from this study (dark blue triangles), AFT ages from Omar and others (1994) (green
circles), and AFT ages from Cerveny (ms, 1990) (green squares). Solid lines represent ages calculated from
extrapolating the modified best-fit time-temperature path from the inversion of BT⫹2122 age-eU data to
other elevations. Green line is modeled AFT age; light blue line is modeled AHe age for an eU of 5 ppm, an
average grain size of 45 ␮m, and an average Sm content of 99 ppm; dark blue line is modeled AHe age for an
eU of 150 ppm, an average grain size of 45 ␮m and an average Sm content of 99 ppm; black dashed line is
AHe age calculated using conventional Durango diffusion kinetics. Inset shows same profile but with an
expanded time scale to show more detail. Red arrows indicate samples shown in figure 16. Error bars are 2␴.
180
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Fig. 18. (A) Inverse modeling results for sample NLR2761 from the Laramie Range. 500,000 timetemperature paths were tested, finding 2 good and 15 acceptable solutions. Results are shown as envelopes
encompassing good (dark gray) and acceptable (light gray) paths. Black rectangles represent input
time-temperature constraints on the inversions. The best-fit solution is shown as a black line. (B) AHe age-eU
correlations predicted from the best-fit solution from sample NLR2761, calculated for grain sizes of 30 ␮m
(long-dashed gray line), 45 ␮m (solid gray line) and 60 ␮m (short-dashed gray line). Actual AHe ages (black
triangles) and ages calculated from the best-fit inversion solution using actual values for eU, Sm and grain
size (open squares) are also shown. (C) Thermochronometer age plotted against depth below the Precambrian
thermochronology of the northern Rocky Mountains, western U.S.A.
181
than 95 °C (fig. A5), although with so few solutions it is impossible to say how well this
temperature is constrained. A maximum temperature of ⱕ95 °C implies that cooling
of this sample was less than 35 °C, approximately half that of the other ranges discussed
above. Given its geological history, we might expect samples from the Laramie Range
to have experienced a cooler thermal history than those from other ranges; there was
less sediment deposited in southeast Wyoming than the rest of the region during the
Paleozoic due to its proximity to the Transcontinental arch (Boyd, 1993), and the area
was exhumed during the Pennsylvanian Ancestral Rockies orogeny (Maughan, 1993).
Comparison of the AHe age-eU relationship predicted by the best-fit time-temperature
path to actual data shows a good correlation (fig. 18B).
The best-fit time-temperature path from the inversion of NLR2761 was extrapolated over the entire elevation range of our samples and “model” age-elevation profiles
of AHe and AFT ages calculated and compared to actual data (fig. 18C). Unlike the
Bighorn, Wind River and Beartooth results described above, the ages predicted for the
Laramie Range based on the age-eU correlation of this deep sample are very different
to actual ages. While the predicted ages fit the trend of the four deepest well samples
(dark blue triangles in fig. 18C), they do not fit the age trends of shallower samples.
Model ages become rapidly older with increasing elevation, indicating a fossil PRZ,
whereas actual ages do not. To match the ages of the shallower well samples the
maximum temperature of the best-fit time-temperature path for NLR2761 would have
to be higher, but then it would not match the age-eU distribution. With no other
samples displaying a positive AHe age-eU correlation it is not possible to check this
result using inverse modeling; however, we can use forward modeling to find a possible
thermal history. A simple Laramide-type forward model similar to figure 11, with 75 °C
of cooling between 60 and 70 Ma, provides a reasonable match to our AHe data but a
poor match to Cerveny’s (ms, 1990) AFT data (fig. 19A). Increasing the maximum
burial temperature and the amount of cooling (to 85.8 °C of cooling between 60 and
70 Ma) provides a good match to the AFT data but a poor match to the AHe data (fig.
19B).
We propose that the four deepest Laramie Range samples (greater than ⬃2500 m
below the Precambrian unconformity) have experienced a different thermal history to
the shallower samples (less than ⬃2500 m elevation below the Precambrian unconformity, fig. 18C). The cooler peak temperatures required to explain NLR2761 imply that
it could not have resided at the deeper crustal levels implied by its present-day depth
and burial by Mesozoic sediments before the Laramide orogeny. We suggest that the
four deepest well samples are separated from the shallower ones by a Laramide thrust
fault. The competing effects of heating due to thrusting and cooling due to erosion
could result in a thermal history quite different to those of shallower samples, which
have only experienced cooling due to erosion. It is also possible that the minimum
temperature constraint of 60 °C in the inverse modeling is invalid, if the sample was
shallower before thrusting. While the Texaco Government Rocky Mountain well did
not penetrate the main Laramide thrust to reach sub-thrust sedimentary rocks, it likely
penetrated a subsidiary fault sliver, many of which have been documented throughout
unconformity for samples from the Laramie Range, showing AHe ages from this study (dark blue and gray
triangles) and AFT ages from Cerveny (ms, 1990) (green circles). Solid lines represent ages calculated from
extrapolating the best-fit time-temperature path from the inversion of NLR2761 age-eU data to other
elevations. Green line is modeled AFT age; light blue line is modeled AHe age for an eU of 5 ppm, an
average grain size of 45 ␮m, and an average Sm content of 113 ppm; dark blue line is modeled AHe age for
an eU of 100 ppm, an average grain size of 45 ␮m and an average Sm content of 113 ppm; black dashed line
is AHe age calculated using conventional Durango diffusion kinetics. Inversion result provides a reasonable
match to AHe data shown by dark blue triangles, but a poor match to AHe ages shown by gray triangles. Red
arrow indicates sample NLR2761 used in inversion modeling. Error bars are 2␴.
182
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Fig. 19. Thermochronometer age plotted against depth below the Precambrian unconformity for
samples from the Laramie Range, showing AHe ages from this study (dark blue and gray triangles) and AFT
ages from Cerveny (ms, 1990) (green circles). Solid lines represent ages calculated from forward modeling.
Green line is modeled AFT age; light blue line is modeled AHe age for an eU of 5 ppm, an average grain size
of 45 ␮m, and an average Sm content of 113 ppm; dark blue line is modeled AHe age for an eU of 100 ppm,
an average grain size of 45 ␮m and an average Sm content of 113 ppm; black dashed line is AHe age
calculated using conventional Durango diffusion kinetics. (A) Forward model chosen to provide reasonable
match to AHe data, (B) forward model chosen to provide reasonable match to AFT data.
thermochronology of the northern Rocky Mountains, western U.S.A.
183
the Laramide foreland (for example, Gries, 1983a; Gries, 1983b). During sample
collection and processing we noted that cuttings from the lowest four samples
contained fragments of pink feldspar indicative of a granitoid, whereas shallower
samples comprised mostly gray/green amphibolite, consistent with the presence of a
major fault between them.
discussion
In this section we expand on whether the effect of radiation damage on the
diffusion of He in apatite can explain features of our data such as the age dispersion of
samples and the presence of anomalously old AHe ages that are older than corresponding AFT ages. We also discuss other possible causes of anomalously old ages. Lastly, we
examine what we can learn about the thermal and geological history of the region
from these data.
Radiation Damage: The Smoking Gun?
eU, grain size and age.—Positive correlations between AHe age and eU in many of
our samples are consistent with the strong effect of radiation damage on He diffusivity
and age. However, this raises the question of why many samples do not show such
correlations. Although Flowers and others (2009) noted that the influence of crystal
size on AHe age is small compared to the influence of eU, our forward modeling
showed that it is nonetheless important (for example, fig. 12). Model AHe ages for any
time-temperature path fall on a surface in age-eU-grain size space, even though they
may not show a clear positive correlation in either two-dimensional age-eU or age-grain
size space. Identifying these surfaces in real data is difficult due to the small number of
aliquots per sample, but the lesson here is that we should not expect all samples to
show simple age-eU (or age-Rs) correlations. As discussed earlier, it is possible to
recognize a few samples (for example GP1) whose AHe age scatter is clearly not
controlled by a combination of eU and grain size alone.
Elevation of fossil PRZ and PAZs.—Forward modeling of simple Laramide-type
time-temperature paths produced age-elevation plots that are similar to our observed
data (compare fig. 11C to fig. 14). These results also show that if apatite crystals are
affected by sufficient radiation damage, a fossil AHe PRZ will span similar elevations to
the fossil AFT PAZ. Radiation damage increases He retentivity proportionally with eU
(Flowers and others, 2009), so for samples with a wide range of eU, AHe ages within a
fossil PRZ on an age-elevation plot will fall within a region (gray shading in fig. 11),
rather than upon a single line.
We see evidence of fossil PAZ and PRZs spanning similar elevations in both the
Bighorn Range and Laramie Range. AFT ages from thirteen surface samples from the
Bighorn Range vary between 94.1 ⫾ 10.1 Ma and 340.6 ⫾ 38.6 Ma, with the exception
of one sample with an AFT age of 75.5 ⫾ 6.2 Ma, suggesting that these samples are
from a fossil PAZ (Cerveny, ms, 1990). Similarly, AHe ages display a distinct gradient
change on an age-depth plot as depths shallow towards the Precambrian unconformity, indicating the base of a fossil PRZ (fig. 3B). In the Laramie Range, surface
samples from Laramie Peak have widely scattered AHe ages that are older than
Cerveny’s (ms, 1990) corresponding AFT ages (fig. 9). The highest elevation sample
has an AFT age of 104.3 ⫾ 12.0 Ma, ⬃38 Ma older than the sample 406 m below it,
indicating that it is located near the base of a fossil PAZ. A broad track length
distribution supports this interpretation (Cerveny, ms, 1990). Forward modeling
showed that when samples are from a fossil PRZ, AHe ages may display a very large
range of ages and can be much older than AFT ages from the same elevation (fig. 11B),
an observation consistent with the Laramie Peak data.
Inverse modeling.—Inverse modeling was successful in finding time-temperature
paths to match actual AHe age-eU data using the RDAAM, and provides additional
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S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
evidence that radiation damage is affecting our AHe ages. Particularly convincing is
that when these time-temperature paths are extrapolated to other sample elevations in
the Bighorn and Beartooth profiles where there are age-eU correlations, the predicted
age-eU distributions match the actual data very well (figs. 13C, 13D, 16C, 16D and
16E). For the Bighorn, Beartooth and Air Force well (Wind River Range) data, the
model age-elevation profiles encompass the AHe data, and predict a systematic age-eU
variation that is observed in only a few samples (figs. 14, 15 and 17). At first it seems
surprising that the predicted AHe age-elevation profiles encompass our data so well,
especially when so many of the individual samples do not show age-eU correlations,
and age dispersion is influenced by other factors such as grain size. However, the
thermal histories that produce the largest predicted range of AHe age with eU will also
result in the largest variation of age with grain size, and probably also with any
unknown factors that are impacted by long residence time in the He PRZ. Since
radiation damage has a larger effect on AHe age than grain size, it will predict the
largest range of possible ages and thus encompass the data.
Our work confirms that the surface AHe ages reported by Crowley and others
(2002) and Reiners and Farley (2001) from the Bighorn Range represent a fossil PRZ,
and suggests that these older-than-expected AHe ages, and the scatter in these ages,
result from the combination of radiation damage of apatite along with a low geothermal gradient in the Bighorn Range. Using well data we calculated a present-day
geothermal gradient in the Bighorn Range of 14 °C/km, lower than the 22 to
32 °C/km gradients in the adjacent Bighorn and Powder River basins (Heasler and
Hinckley, 1985; Naeser, 1992). Model age-elevation profiles, which were calculated
from the best-fit time-temperature path from inversion using the RDAAM and assumed
a constant geothermal gradient of 14 °C/km throughout geological time, provide a
good match to the Bighorn AHe ages. Thus, radiation damage has increased the
closure temperature of the AHe system, and, in combination with a low geothermal
gradient, has resulted in the preservation of a fossil PRZ at lower elevations than
predicted from conventional Durango diffusion kinetics.
It should be noted that the concept of a single “true” AHe sample age from which
our actual ages are scattered due to the influence of various factors and ultimately that
we would like to correct our ages towards (Fitzgerald and others, 2006), becomes
largely irrelevant once the influence of radiation damage is recognized. All AHe ages
become equally valid and a mean age or true age has little geological significance.
While the best-fit time-temperature paths from inversion using RDAAM adequately predict AHe ages, the predicted AFT age profiles, while similar in general
shape to actual ages, are not as satisfying. In the Beartooth Range the model AFT
age-elevation profile has a much lower gradient than the actual profile from Omar and
others (1994) (fig. 17). In the Air Force well the model predicts that deep samples
should be in the present-day PAZ, whereas actual AFT ages have a very steep profile
and maintain the trend of the shallower samples (fig. 15C). Similarly, the predicted
AFT age profile in the Bighorn Range does not fit new AFT ages from Cloud Peak or
data of Cerveny (ms, 1990) (fig. 14). Also noteworthy is that inversions that included
both AHe and AFT ages never found any acceptable time-temperature paths. Although
the RDAAM of Flowers and others (2009) is clearly relevant and provides the best
explanation of features of our data to date, it seems that further refinements are
needed to reconcile AHe ages with AFT ages.
Possible Causes of Anomalously Old AHe Ages
While radiation damage can explain many features of our data, some data cannot
be so readily explained. Data from Gannett Peak and Fremont Peak span over a
kilometer of elevation with AHe ages that are similar to, or older than, corresponding
thermochronology of the northern Rocky Mountains, western U.S.A.
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AFT ages (fig. 5). The RDAAM predicts that AHe ages can be older than AFT ages
when samples are from the overlap region of the fossil PRZ and PAZ (striped region of
figs. 11B and 11C). Within this region AFT ages will increase rapidly with small
increases of elevation, and AHe ages may show large scatter (hundreds of Ma); we
observe neither in the Gannett Peak and Fremont Peak samples. Instead AFT ages are
very consistent with elevation, and AHe ages, with the exception of sample GP1, show
⬃20 Ma of dispersion or less. In the Beartooth Range, three out of six dated grains
from an intrusive porphyry (sample BT072007-2) gave AHe ages at least 10 Ma older
than the U-Pb formation age, which also cannot be explained using the RDAAM. In
this discussion we consider possible causes of these anomalously old AHe ages (see
Fitzgerald and others, 2006, for a detailed listing of possible causes). The effect of grain
size is not included here because it was discussed earlier.
U- and Th-rich inclusions.—At least in so far as our microscopic inspection permits,
clear, inclusion-free apatites were chosen for analysis. It is unlikely that very tiny
inclusions (smaller than a few ␮m) contribute significant He and unrecovered U-Th
(Vermeesch and others, 2007). Three single-grain aliquots from sample GP1 (GP1aJ,
GP1aK and GP1aL, table A1) from the Gannett Peak traverse contained inclusions and
after degassing were dissolved using more aggressive techniques typically used for
zircon (Reiners and others, 2004). AHe ages for these aliquots range from 48.1 ⫾ 2.1 to
143 ⫾ 5.2 Ma. AHe ages for the six aliquots that were dissolved using conventional
apatite techniques (Reiners and others, 2004) range from 41.8 ⫾ 1.7 to 126 ⫾ 11 Ma.
The similarity of the age ranges regardless of the dissolution method (and hence
regardless of whether we recover U and Th from inclusions) indicates that inclusions
are unlikely to be the cause of the large age range and anomalously old ages for
samples from Gannett Peak.
Zonation of U and Th.—Zonation of U and Th is unlikely to cause errors in
(U-Th)/He age greater than about 30 percent (Wolf and others, 1996; Hourigan and
others, 2005). AFT analysis of Gannett Peak apatites revealed many grains with rims
that are depleted in tracks relative to the cores, indicating that this may be an issue;
however, ion probe analyses of apatite from Fremont Peak showed that zonation,
although present, was not systematic (A. Kent, personal communication, 1999).
Additionally, ages for samples GP3 and GP4 show that AHe ages are often more than
30 percent older than the AFT age. Correcting for zoning would still result in AHe ages
older than AFT ages. Thus, zoning is unlikely the cause of the anomalously old AHe
ages.
Interaction of diffusion and alpha ejection.—He in the outer ⬃20 ␮m of an apatite
crystal is depleted due to alpha particle ejection. When an apatite is at temperatures
where He can diffuse out of the crystal (that is, when it resides within the PRZ), He
depletion due to alpha ejection will result in decreased diffusive He loss. Applying a
standard alpha-ejection correction to the measured age will result in an overcorrection
of the AHe age by as much as ⬃20 percent (Farley, 2000; Meesters and Dunai, 2002a),
depending on the thermal history. Whereas this could be affecting the AHe ages of
many of our samples, it is probably not the cause of the anomalously old ages from
Fremont and Gannett Peaks. AFT ages from both Gannett and Fremont Peaks have
steep age-elevation gradients and narrow, unimodal track length distributions indicating rapid cooling from temperatures above to temperatures below the AFT PAZ. Thus,
it is unlikely that these samples spent a significant amount of time in the AHe PRZ.
Most of our AHe ages, however, are either from a fossil PRZ, or are from wellbore
samples that reside in the present-day PRZ, and have probably been influenced by this
effect to some degree. Thus, many of our AHe ages may be too old relative to quickly
cooled samples or other thermochronometers, and it is important to interpret these
ages using a modeling rather than a closure-temperature approach (that is, rather than
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S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
interpreting the AHe age to be the time the sample cooled through the closure
temperature). We interpreted our data using forward and inverse modeling in
program HeFTy, which accounts for alpha ejection effects continuously in the production-diffusion calculations (Ketcham, 2005). AHe ages calculated from a timetemperature path using HeFTy include the effect of decreased diffusion due to alpha
ejection, and can, therefore, be compared to actual ages.
Implantation of He.—We suspect that implantation of He from U- and Th-rich
phases external to, but within ⬃20 ␮m of, the apatite crystal may be a significant cause
of the anomalously old AHe ages. These U- and Th-rich phases may be in the form of
weathering products (Reiners and others, 2008) or “bad [mineralogic] neighbors”
(Spencer and others, 2004; Kohn and others, 2008b). The influence of He implantation on AHe age is not restricted to rocks that have experienced a certain type of
thermal history, such as slowly-cooled cratonic rocks. The effect has also been documented in rapidly-cooled volcanogenic sediments (Spiegel and others, 2009). Spiegel
and others (2009) also noted that the effect of He implantation on AHe age will be
most pronounced for low-eU apatites, which could also explain why we do not observe
age-eU-grain size correlations in many of our samples.
Thin section analysis of several samples from the Beartooth, Wind River and
Bighorn Ranges commonly shows brown material along apatite grain boundaries that
may be secondary phases formed by weathering. SEM elemental abundance maps from
samples from the Beartooth Range show that this material is often high in Fe, Ce and
Th (fig. 20). Similar results were found in samples from the Bighorn Range. Depending upon when these high-Th or high-U phases formed, and whether they are
preserved at the grain boundary during mineral separation, they can either cause AHe
ages to be too young or too old (Reiners and others, 2008). Archean basement rocks in
the Beartooth, Bighorn and Wind River Range contain radiation-damaged (metamict)
zircon that may be a source of the U and Th in these secondary phases. Given the wide
range of variables associated with these secondary phases, such as thickness, eU
concentration, percentage of apatite grain boundary in contact with these phases,
timing of formation, amount removed during mineral separation et cetera, it seems that
this effect could be responsible for much of the age scatter observed in our samples. In
particular, three anomalously old ages from sample BT072007-2 (elevation 2868 m) in
the Beartooth Range all have very low eU values (4 ppm) and AHe ages older than
their formation age, suggesting that old ages are likely caused by He implantation.
Anomalously old apatites from Fremont Peak and Gannett Peak in the Wind River
Range do not display such low eU concentrations (eU ⬎ 12 ppm and 15 ppm,
respectively), and, with the exception of sample GP-1, do not display such a wide range
of ages as BT072007-2. These samples show evidence of rapid cooling, which leads us to
infer that the anomalously old AHe ages may be due to He implantation, which has
had less effect on most aliquots than on sample BT072007-2 due to their higher eU
concentrations. We recommend that in order to avoid problems that may arise from
helium implantation in this region, future workers investigate the use of grain abrasion
to remove the outer 20 ␮m of apatite crystal (for example, Kohn and others, 2008b;
Spiegel and others, 2009).
Implications for Geologic History
Our forward and inverse modeling of AHe ages provide insights into what our
data can and cannot tell us about the geological history of the region. Model
time-temperature paths and age-elevation profiles provide reasonable matches to data
from the Bighorn Range, the Air Force well in the Wind River Range, and the
Beartooth Range. Inversion results allow us to estimate the amount of cooling that
occurred from the time of maximum burial before the Laramide orogeny to the
present day, and the minimum age for onset of cooling. Constraints on the minimum
thermochronology of the northern Rocky Mountains, western U.S.A.
A
187
B
Ap
M
C
D
E
F
Fig. 20. (A) Backscattered electron image of a sample from the Beartooth Range showing apatite crystal
(Ap) with secondary monazite corona (M). SEM elemental abundance maps for (B) calcium and (C)
phosphorus confirm identification of apatite; (D) elemental abundance map showing secondary iron at
apatite grain boundaries and in cracks; (E) cerium and (F) thorium elemental maps identify potentially
secondary monazite adjacent to apatite.
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S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
peak temperature attained before Laramide exhumation allow us to estimate the total
amount of cooling and exhumation since maximum burial during the Cretaceous.
Estimates of at least 60 °C of cooling for the Bighorn Range, 78 °C for the Beartooth
Range, and 69 °C for the Wind River Range correspond to ⬃3 to 4 km of exhumation if
we use present-day geothermal gradients; this amount of exhumation falls within
estimates of pre-Cenozoic sedimentary cover for these ranges (Mallory and others,
1972; Blackstone, 1981; DeCelles and others, 1991b; Roberts and Kirschbaum, 1995;
DeCelles, 2004). The maximum peak temperature of samples was not well constrained
from inversion results. However, the change in age-elevation gradient at the base of the
fossil AHe PRZ was used to constrain the maximum temperature of the Bighorn
sample, and similarly the base of the AFT PAZ was used to constrain the Beartooth
sample. These maximum temperature constraints give cooling estimates of 80 °C and
98 °C for the Bighorn and Beartooth ranges respectively, resulting in larger exhumation estimates of ⬃5.7 km and ⬃5 km, respectively. Hoy and Ridgeway (1997) used
sequential restorations of cross sections to estimate that ⬃5.5 km of exhumation
occurred across the east-central flank of the Bighorn Range during the Laramide
orogeny, in good agreement with our estimates. Their cross sections show that
Precambrian basement has also been eroded from the Bighorn Range, and that
thickness estimates of pre-Cenozoic sedimentary rocks only give a minimum estimate
for the amount of exhumation in the Bighorns.
The maximum possible age for onset of Laramide exhumation is unconstrained
by our data, but inverse modeling of AHe ages shows that exhumation and/or cooling
started before (and possibly significantly before) ⬃71 Ma in the Bighorn Range.
Cerveny (ms, 1990) suggested from AFT data that cooling in the Bighorn Range began
⬃75 Ma. Using our AFT data from Cloud Peak, we interpret the change in slope on the
AFT age-elevation plot, which represents the onset of rapid exhumation, to be between
⬃99 and 57 Ma, but with such large error, no track length data, and so few data points,
it is difficult to refine the onset of cooling and/or exhumation further using these AFT
ages.
In the Wind River Range, inverse modeling suggests exhumation started before
⬃66 Ma (figs. 15A and A3). Cerveny and Steidtmann (1993) used AFT data to find that
cooling in the Wind River Range began by at least ⬃85 Ma, but that most rapid cooling
in the core of the range occurred between 62 and 57 Ma, in agreement with our new
AFT data from Gannett Peak.
Inverse modeling shows that exhumation in the Beartooth Range started before
⬃58 Ma. Omar and others (1994) proposed that uplift of the northeast corner of the
Beartooth Range began at 61 ⫾ 3 Ma (1␴ error) from the interpretation of AFT ages.
Cerveny (ms, 1990) concluded from AFT ages that a major uplift event began in the
Beartooth Range at ⬃68 Ma and increased in rate at ⬃57 Ma. Thus, all of these AFT
studies are consistent with our results; however, it should be noted that the “minimum
age for onset of exhumation” that we have determined from AHe ages does not
exclude the possibility, for example, that exhumation started at the same time in all
ranges sometime before ⬃71 Ma, or that exhumation could have started earlier in the
Beartooth Range than in the Bighorn Range. Although our AFT results from the
Bighorn Range are ambiguous regarding the timing of onset of cooling, results of
Cerveny (ms, 1990) and Omar and others (1994) seem to suggest that the onset of
cooling occurred earlier in the Bighorn Range than in the Beartooth Range. Envelopes of the good-fit time-temperature paths from AHe age inversion also show that
rapid cooling and exhumation likely occurred earlier in the Bighorn Range;
however, it should be noted that there is slight overlap in the acceptable-fit
solutions (fig. 21).
189
thermochronology of the northern Rocky Mountains, western U.S.A.
A
BH
BT
B
WR
BH
BT
Fig. 21. (A) Envelopes of good-fit time-temperature paths from the inversion of AHe age-eU data from
sample BH761 in the Bighorn Range (light gray shading) and sample BT⫹2122 in the Beartooth Range
(dark gray shading). No good-fit paths were found from the inversion of sample WY5089 in the Wind River
Range. (B) Envelopes of acceptable-fit time-temperature paths from the same inversions as part (A). Dashed
line shows envelope of acceptable-fit paths from the inversion of sample WY5089 in the Wind River Range.
BH, Bighorn Range; BT, Beartooth Range; WR, Wind River Range.
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S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Another approach to assessing the timing and regional pattern of exhumation
and inferred uplift of Laramide ranges is to use patterns of coarse-grained, proximal,
syntectonic sedimentation. Many of the Laramide uplifts are flanked by coarse
conglomeratic strata that were derived directly from their adjacent ranges. The
presence of contractional growth structures in some of these conglomerate units,
together with the broad range of clast types derived from the Precambrian through
Mesozoic pre-orogenic rock column, indicates deposition during deformation and
surface uplift. Many workers have used these locally-derived conglomerates to establish
a regional west-to-east progression of Laramide basement deformation. For example,
the Lima Conglomerate, in the Lima Peaks area of southwestern Montana (fig. 1) was
derived from the Laramide Blacktail-Snowcrest uplift, and is Coniacian-Santonian in
age (⬃88-84 Ma) (Perry and others, 1988). To the east, the Sphinx Conglomerate in
the Madison Range (fig. 1) was deposited between the Maastrichtian and the late
Paleocene (⬃75-58 Ma) (DeCelles and others, 1987). The Beartooth Conglomerate on
the east side of the Beartooth Range is late Paleocene in age (DeCelles and others,
1991a), and the Tongue River, Kingsbury and Moncrief Conglomerates along the
east-central flank of the Bighorn Range span late Paleocene (Tongue River) to Eocene
ages (Kingsbury and Moncrief) (Hoy and Ridgway, 1997). These conglomerates do not
document the onset of erosion in their respective ranges, because finer grained
Mesozoic sediments that were eroded first were unable to generate coarse detritus, but
instead represent the exposure of more resistant Paleozoic strata in the hanging walls
of the Laramide structures (Hoy and Ridgway, 1997). The depositional ages of these
conglomerates appear to conflict with evidence from AHe ages regarding the regional
pattern of exhumation, insofar as the thermochronological data seem to indicate
earlier cooling in the Bighorn Range relative to the Beartooth Range. However, most of
these conglomerate units rest on highly angular basal unconformities on top of upper
Cretaceous rocks, which themselves were strongly deformed before any conglomerate was
deposited and preserved. This implies that a significant amount of deformation, surface
uplift, sedimentary bypassing, and hence cooling, took place before many of the Laramide
conglomerate units began to accumulate (DeCelles and others, 1991b).
Dickinson and others (1988) used a regional approach to date the onset of
deformation across the Laramide province. They documented the oldest horizon in
each basin for which isopachs indicate a Laramide depocenter, the oldest horizon
containing locally derived clasts, and the youngest horizon that was once contiguous
with strata in adjacent basins (and therefore predates Laramide deformation). They
concluded, in contrast to both the evidence from synorogenic conglomerates discussed above, and to our results from the inversion of AHe ages, that the onset of
Laramide deformation was approximately synchronous across the central Rocky
Mountain region during the Maastrichtian, 65 to 75 Ma, and that there are no
systematic areal trends in the timing of inception of deformation.
In contrast to the sedimentological studies, oxygen isotope paleoelevation studies
support our AHe inversion results showing earlier cooling in the Bighorn Range
relative to the Beartooth Range. Fan and Dettman (2009) used oxygen isotope analysis
of bivalve shells in Laramide basins to interpret that high elevation (⬃4.5 ⫾ 1.3 km)
was attained in the Bighorn region by the late Paleocene, but that local relief in the
Beartooth Range and Sevier thrust belt to the west of the Bighorns did not exceed 1 to
2 km during late Paleocene to early Eocene time. Although these ages are younger
than the ⬃Late Cretaceous cooling suggested by our inversion results in the Bighorn
Range, they are consistent with an earlier start of uplift and exhumation in the
Bighorns with respect to the Beartooths.
Reconciling the thermochronologic results presented here, which suggest that
the Bighorn Range experienced rapid cooling and/or exhumation earlier than the
thermochronology of the northern Rocky Mountains, western U.S.A.
191
Beartooth Range, with sedimentologic studies suggesting either an eastward progression of Laramide deformation or a synchronous onset of deformation, is difficult in the
context of a simple relationship between cooling, exhumation and deformation.
Fundamental assumptions in all of these studies are that 1) the timing of tectonics,
exhumation/erosion and cooling are approximately synchronous, and 2) the lag time
between exhumation/erosion and deposition of the conglomerates preserved along
the flanks of the ranges, along with the sediment transportation distance, is short, and
3) sedimentary bypassing (that is, exhumation that is not preserved in the sedimentary
record) is negligible. It is certainly possible that deformation began in both areas at
approximately the same time, but that differences in erosional exhumation rates led to
different cooling ages. It is also plausible that the timing of deposition of conglomerates along the flanks of Laramide ranges may not accurately, or completely, track the
timing of major exhumation. Durable conglomerate-prone rock types must be present
at the surface in order to generate coarse-grained sediments, and the Laramide ranges
in general were buried by highly variable lithologies ranging from Cretaceous shales to
resistant Paleozoic dolostones. If erosion was initially not as pronounced in the
Beartooth Range as it was in the Bighorn Range, perhaps due to different paleogeography or climate (Roberts and Kirschbaum, 1995), then we might expect older cooling
ages from Bighorn rocks.
It is also possible that the cooling recorded in the AHe ages was not related to
exhumation in the Bighorn Range but to changes in the geothermal gradient. For a
sample at a depth of 4 km, a decrease in the geothermal gradient from 30 °C/km to
15 °C/km (approximate present-day geothermal gradient in the Bighorn Range)
would result in cooling of 60 °C. Dumitru and others (1991) suggested that low-angle
subduction of the Farallon slab during the Laramide orogeny resulted in refrigeration
of the overlying lithosphere and a corresponding decrease in geothermal gradient;
however, they point out that in the Rocky Mountains, where the lithosphere was ⬎100
km thick during the Laramide orogeny, cooling would be so slow that there would be
no obvious signal in isotopic ages. An increase in hydrologic flow in the area of the
nascent Bighorn Range, but not in the Beartooth Range, could also have decreased the
geothermal gradient resulting in older Bighorn cooling ages. Increased hydrologic
flow is generally associated with increased topography (for example, Ehlers, 2005),
making it difficult to argue that increased hydrologic flow was the cause of cooling,
rather than tectonics. More thermochronologic data from ranges throughout the
Laramide foreland, and especially more AFT ages from the Bighorn and Beartooth
ranges, using more recent and standardized analytical protocols, may provide a clearer
picture of the exhumation history of these ranges, and help to reconcile the regional
pattern of cooling with the regional kinematic history.
Unfortunately, our data do not provide any rigorous constraints on the timing and
amount of post-Laramide burial and exhumation in the Rocky Mountain region.
Inversion constraints always allowed for the possibility of post-Laramide burial and/or
exhumation, but the models did not require it. Inverse modeling results show that
post-Laramide heating due to burial by Oligocene to Pliocene sediments and cooling
due to Miocene exhumation is unconstrained and cannot be resolved from the AHe
data. Applying a similar thermochronologic approach to subsurface samples from
within the Laramide basins, which were not exhumed during the Laramide orogeny
and thus reached maximum burial in the Neogene, may be a more appropriate way to
resolve late-Cenozoic exhumation.
Inverse modeling of the Laramie Range well data led to the interpretation of a
fault sliver in the Precambrian basement at the northern end of the Laramie Range
that has experienced a different thermal history to the samples in the overriding
hanging wall. The Texaco Government Rocky Mountain #1 well penetrates this fault
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S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
between samples NLR1872 and NLR2297, which are at elevations of 155 and ⫺270 m
respectively. By comparing the maximum temperature predicted for a sample by the
best-fit time-temperature path from inversion with that predicted by the forward model
that fits the shallow AHe data, we can estimate the amount of vertical throw on this
fault. The difference in maximum temperatures between the two models is 42.5 °C,
which corresponds to ⬃2 km of vertical throw using a geothermal gradient of
20 °C/km. Given the non-uniqueness of time-temperature histories from forwardmodeling, along with uncertainties in the inversion constraints and a lack of knowledge of past geothermal gradients, this number should be regarded as a rough
approximation.
conclusions
Simple forward modeling using HeFTy and the RDAAM of Flowers and others
(2009) is a valuable tool for understanding the age distribution of our data. Using
thermal histories appropriate to the Laramide foreland province we have shown that
radiation damage of apatite, and its effect on He diffusion, can explain many of the
features of our AHe data. These include: 1) the general distribution of ages on an
age-elevation profile, 2) the age dispersion of some individual samples, 3) the
extremely wide dispersion of ages from within a fossil PRZ, 4) the similar elevation
ranges of fossil PAZ and PRZs, and 5) why AHe ages are often older than AFT ages
within a fossil PRZ. We have also demonstrated that grain size, whilst not as large an
effect on AHe age as eU concentration, can nonetheless cause the distribution of ages
on an age-eU or age-grain size plot to appear random.
Radiation damage cannot explain AHe ages that are older than AFT ages from
Gannett Peak and Fremont Peak in the Wind River Range, and some samples from the
Beartooth Range. We interpret these anomalous ages to be the result of He implantation, which could be responsible for much of the scatter that we observe in our data.
Inversion of AHe age-eU pairs from a single sample using HeFTy, followed by
extrapolation of the resulting best-fit time-temperature path to other elevations, is a
reasonable method for modeling the thermal history of a vertical suite of samples and
creating model age-elevation plots. In both the Bighorn and Beartooth ranges,
extrapolated thermal histories predicted AHe age-eU distributions of several samples
that were not used in the inversion. Predicted age-elevation curves for appropriate lowand high-eU values encompass our data from the Air Force well in the Wind River
Range, the Beartooth Range, and the Bighorn Range. Observed AFT ages are not
matched by the corresponding model AFT age-elevation curves as well as AHe ages,
suggesting that further refinement of the diffusion model may be needed.
Inversion modeling suggests that rapid exhumation and cooling began earlier in
the Bighorn Range (before ⬃71 Ma) than in the Beartooth Range (before ⬃58 Ma), in
contrast to evidence from synorogenic sedimentary rocks that deformation of the
foreland at this latitude propagated from west to east. More low-temperature thermochronologic data may help resolve this issue. Results indicate that there has been
between 60 and 80 °C of cooling in the Bighorn Range, at least 69 °C of cooling in the
Wind River Range, and between ⬃78 and 98 °C of cooling in the Beartooth Range
since maximum burial during the Late Cretaceous. Inverse modeling of AHe age-eU
pairs also led to the interpretation of a fault sliver at the northern end of the Laramie
Range.
The presence or absence of post-Laramide burial and exhumation cannot be
resolved from our AHe data. AHe and AFT dating of samples from wells within the
Laramide basins may be a more suitable approach to resolve late Cenozoic exhumation.
thermochronology of the northern Rocky Mountains, western U.S.A.
193
acknowledgments
Well cuttings were provided by Anadarko Petroleum, ExxonMobil, Chevron, the
Texas Bureau of Economic Geology and the U.S. Geological Survey. John Byrd, Mike
McGroder, Jerry Kendall, Julie Gibbs, Steve Decker, Ed Donovan, Randy McDonald,
Bev DeJarnett and John Rhoades facilitated these donations. Jim Steidtmann at the
University of Wyoming kindly shared apatite separates from the Wind River Range.
Becky Flowers allowed us to use, and provided help with the RDAAM. Rich Ketcham
contributed extensively and tirelessly to our understanding of HeFTy. We thank Ken
Farley at Caltech for allowing us to use his lab to analyze several of the Fremont Peak
samples. Jackie Bott, Rich Bottjer, Kelley Stair and Ross Waldrip provided assistance in
the field. Stefan Nicolescu provided training, assistance and support with all aspects of
AHe dating. George Gehrels and Scott Johnston enabled the U-Pb dating. Jen
McGraw, Derek Hoffman, and James McNabb helped with mineral separations and
apatite picking. We thank Becky Flowers and Andy Gleadow for constructive reviews.
Funding was provided by scholarships from the Geological Society of America, the
Arizona Geological Society, the Colorado Scientific Society, the University of Arizona
Galileo Circle, ExxonMobil and Chevron.
Appendix
Forward Modeling
Isothermal hold model.—We calculated the AHe age of a spherical apatite grain held at a fixed
temperature of 30 °C, 50 °C, 70 °C and 90 °C for 1000 Ma. Grain radius (Rs) was varied between 30 and 90
␮m, and eU between 5 and 150 ppm, which are reasonable end members from our dataset (table A1). Sm
content was assumed to be zero. Results are shown in figure A1, with each surface representing one
isothermal hold temperature. Apatite held at 30 °C near the top of the PRZ, with an eU of 20 ppm or higher,
records an age up to 4% older than the hold time. This is due to the age overcorrection that results from the
interaction of alpha ejection with He diffusion (Meesters and Dunai, 2002a). Apatite held at 90 °C, below the
PRZ, records a zero age. Apatite held within the PRZ, at 50 °C and 70 °C, shows variation of age with eU and
to a lesser extent with grain radius. For an eU range of 5 to 150 ppm, apatite held at 70 °C has an age range of
0 to 834 Ma for a 30␮m radius grain, and 3 to 954 Ma for a 90 ␮m radius grain. Similarly, apatite held at 50 °C
has an age range of 252 to 1032 Ma for a 30 ␮m radius grain, and 576 to 1013 Ma for a 90 ␮m radius grain.
For comparison, AHe ages calculated using a Durango diffusion model (Farley, 2000) for grain radii
between 30 and 90 ␮m show an age range of 239 to 707 Ma for a hold temperature of 30 °C, 8 to 70 Ma for a
hold temperature of 50 °C, 0 to 4 Ma for a hold temperature of 70 °C, and 0 Ma for a hold temperature of
90 °C. An isothermal hold temperature of 30 °C for 1000 Ma is within the PRZ for the Durango diffusion
model, but essentially above it for the RDAAM. Similarly, 70 °C is below the PRZ for the Durango diffusion
model, but within it for the RDAAM.
Inverse Modeling
Inversion input.—Six time-temperature constraint boxes were input into HeFTy for each inversion.
These were based on geologic control, other published ages (such as zircon fission-track) and present-day
temperature of the sample. With the exception of the present-day temperature constraint, these boxes were,
in general, quite broad in their age and temperature ranges. The oldest constraint box, box 1, is
Precambrian and represents the last known or estimated time the sample was above the AFT and AHe
closure temperature. Box 2 constrains temperatures when the Precambrian basement was exposed at the
surface before deposition of the Cambrian Flathead sandstone (Snoke, 1993). Box 3 constrains time and
temperature during maximum burial before the Laramide orogeny. Boxes 4 and 5 constrain post-Laramide
thermal history, and allow HeFTy to test time-temperature paths that might range from a single cooling
episode during the Laramide orogeny, to multiple cooling events, to Laramide cooling followed by
reheating and cooling, thus allowing for all possible scenarios that have been proposed for the region. With
the exception of the Wind River Range, samples from high peaks likely experienced only one episode of
exhumation during the Laramide orogeny, whereas surface and well samples from lower elevations closer to
the edges of the ranges may have experienced some amount of reburial during the middle Cenozoic and
exhumation during the late Cenozoic. Constraint 6 is the present-day sample temperature.
We allowed HeFTy to try 10,000, 100,000, or 500,000 thermal paths in the Monte Carlo simulation,
depending upon the number of solutions found. The upper limit of 500,000 paths was determined by the
amount of time the inversions took to run on a laptop computer (several hours). HeFTy allows a maximum
of five input data points (age-eU pairs in this case), although testing showed that no solutions were ever
found if five age-eU pairs were used as input. Solutions were found from the inversion of four age-eU pairs
sample name
WIND RIVER RANGE
Air Force well
WY5089aA
3.32
WY5089aC
1.39
WY5089aD
1.96
WY5089aE
3.74
WY5089aF
1.14
WY5089aG
3.15
WY5089aH
2.52
WY5089aJ
2.00
WY5189aA
1.64
WY5189aB
0.79
WY5189aD
3.69
WY5189aE
1.42
WY5289aA
0.50
WY5389aB
0.90
WY5489aA
1.88
WY5489aB
2.14
WY5489aC
0.91
WY5489aD
2.33
WY5589aA
1.95
WY5589aB
2.83
WY5589aC
2.99
WY5589aD
1.11
Surface samples near Air Force well
WR070906-7aA
4.38
WR070906-7aB
1.36
WR070906-7aC
3.79
WR070906-7aD
1.11
WR070906-7aE
1.42
WR070906-7aF
1.94
WR070906-5aA
0.75
WR070906-5aB
0.39
WR070906-5aD
1.52
WR070906-5aE
1.39
mass
(ȝg)
Rs
(ȝm)
58.3
39.1
46.7
57.0
36.7
51.5
47.4
46.4
45.7
35.5
51.3
43.9
27.2
37.2
46.8
47.6
34.6
49.6
46.3
48.1
51.4
37.3
56.4
40.3
50.8
34.8
38.3
41.9
32.2
24.6
43.6
36.2
halfwidth
(ȝm)
55.8
39.3
42.5
51.3
32.0
45.0
41.3
41.8
43.3
33.3
47.5
38.5
26.3
35.0
43.3
43.0
30.5
45.3
42.0
45.8
45.3
33.0
48.8
36.0
46.3
32.8
36.5
39.3
28.3
23.0
40.3
32.8
35
42
50
43
40
21
66
39
53
55
48
55
55
43
38
78
40
77
113
20
160
62
117
54
38
29
39
37
12
96
63
46
U
(ppm)
22
34
79
134
42
26
38
59
28
91
19
40
18
36
8
151
18
21
64
26
167
25
6
24
21
22
16
11
0
18
39
89
Th
(ppm)
770
878
873
267
836
550
779
737
671
939
751
743
642
588
596
277
877
732
773
295
929
563
551
871
615
568
575
723
616
1253
736
795
Sm (ppm)
40
50
68
74
50
27
75
53
60
76
52
64
59
51
39
113
44
82
128
26
199
67
118
60
43
34
43
39
13
101
73
67
eU
(ppm)
10.88
12.47
19.39
27.65
12.30
7.28
16.39
8.43
14.32
17.52
13.67
15.13
14.94
13.42
8.02
28.88
10.52
25.51
31.99
4.15
41.83
7.40
11.55
2.00
2.43
4.23
1.31
1.72
8.16
3.06
2.71
9.69
4He
(nmol/g)
Apatite (U-Th)/He results
Table A1
48.6
45.0
51.6
67.6
44.4
47.6
39.9
28.7
43.5
41.9
47.9
42.9
46.0
47.8
37.0
46.8
42.7
56.9
45.7
29.1
38.7
20.1
18.0
6.1
10.3
22.3
5.6
7.9
113.2
5.6
6.8
26.4
raw age
(Ma)
0.74
0.65
0.71
0.59
0.63
0.66
0.57
0.46
0.67
0.61
0.75
0.64
0.69
0.74
0.62
0.71
0.70
0.69
0.69
0.60
0.71
0.68
0.51
0.62
0.69
0.70
0.60
0.71
0.69
0.70
0.72
0.61
Ft
65.7
69.6
72.7
115.5
70.6
72.6
69.7
62.9
64.6
69.1
63.8
67.3
66.3
64.4
59.5
65.8
61.3
82.2
66.7
48.4
54.1
29.8
35.1
9.7
14.9
32.1
9.3
11.1
163.0
7.9
9.5
43.0
corr. age
(Ma)
2.9
3.3
2.9
4.6
3.4
3.7
4.2
10.6
3.2
2.8
2.5
2.6
2.7
2.4
2.8
2.3
2.4
3.4
2.6
4.2
2.0
1.2
1.6
0.4
0.6
1.3
0.5
3.1
8.4
0.3
0.4
1.6
±2ı
(Ma)
2231
2231
2231
2231
2231
2231
2539
2539
2539
2539
1626
1626
1626
1626
1626
1626
1626
1626
858
858
858
858
678
7
-210
-210
-210
-210
-623
-623
-623
-623
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Depth subSample elev. pC unconf.
(m)
(m)
# grains
194
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
sample name
WIND RIVER RANGE
Fremont Peak samples
WSU-WY70Ma
WSU-WY70Sa
WY7089.2
WY7089.2b
WY7089.3
WY7089.3b
WY-71-89
WY7189.2
WY7189.3
WY7189.4
WY7889.3
WY7889.4
WSU-WY7989La
WSU-WY7989Ma
WSU-WY7989Sa
WY-79-89
WY7989.2
WY8089.2
WY-81-89
WY8189.2
WY8189.3
WY8189.4
WY8189.5
WY8289.2
WY8289.3
WY-86-89
WY8689.2
WSU-WY83La
WSU-WY83Ma
WY-83-89
WY8489.2b
WY8489.3
WY8789
WY8789.2
WY8789.3
halfwidth
(ȝm)
44.8
43.5
48.8
73.7
49.6
60.3
79.1
47.7
43.7
84.3
46.6
47.5
108.3
76.7
62.3
51.5
61.2
83.1
82.8
53.3
45.4
55.9
60.8
56.6
49.5
65.4
113.1
93.0
86.4
65.3
54.1
49.6
48.9
53.8
42.5
mass
(ȝg)
9.87
10.50
35.96
23.18
13.49
24.77
28.29
16.28
9.05
37.88
17.19
11.90
27.80
30.24
26.50
47.51
17.02
35.70
96.32
21.86
14.25
20.77
27.68
20.46
19.71
59.25
81.75
27.80
46.00
30.16
9.53
9.89
9.05
11.98
11.56
51.8
46.7
55.8
85.3
55.7
66.3
89.6
54.3
50.3
96.6
54.7
55.1
117.0
82.5
67.3
59.5
67.6
88.7
90.9
61.8
52.9
61.9
69.4
62.8
56.5
70.2
123.5
106.5
94.0
72.5
60.9
55.2
54.5
59.2
48.2
Rs
(ȝm)
41
42
49
62
61
68
22
74
78
47
66
78
10
12
11
20
20
38
49
44
26
43
39
26
34
20
11
10
23
27
26
25
64
31
26
U
(ppm)
28
28
27
88
55
108
34
124
137
89
53
45
17
25
16
35
17
32
48
45
25
33
31
32
25
110
43
7
45
41
30
23
43
11
10
Th
(ppm)
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
Sm (ppm)
47
49
56
83
74
93
30
103
110
68
79
88
14
18
15
28
24
46
61
54
32
51
46
34
40
46
21
12
33
36
33
30
74
34
29
eU
(ppm)
13.56
14.35
18.80
28.54
23.61
32.24
10.88
31.98
33.67
26.17
22.63
25.64
4.71
6.76
3.96
8.41
6.24
15.67
22.22
15.80
10.93
15.55
16.50
9.93
12.47
17.94
9.11
4.08
11.55
12.10
9.97
8.80
20.56
13.11
9.38
4He
(nmol/g)
(continued)
Table A1
52.8
54.6
62.3
63.3
58.7
63.7
67.3
57.1
56.3
71.2
53.3
53.8
63.3
68.5
49.7
54.6
47.6
63.5
67.6
53.8
62.9
56.8
66.3
54.5
57.7
71.4
78.2
64.9
64.2
61.7
55.9
53.7
51.7
71.3
60.4
raw age
(Ma)
0.72
0.69
0.73
0.82
0.73
0.77
0.83
0.72
0.70
0.84
0.73
0.73
0.87
0.81
0.77
0.74
0.78
0.83
0.83
0.76
0.72
0.76
0.78
0.76
0.74
0.77
0.86
0.86
0.84
0.79
0.76
0.73
0.73
0.75
0.70
Ft
73.6
79.1
84.9
77.5
80.1
83.1
81.6
78.9
79.9
85.0
72.9
73.3
73.0
84.2
64.2
73.3
61.3
76.7
81.6
71.0
87.3
75.0
85.5
71.5
78.2
92.6
90.8
75.7
76.9
78.2
74.1
73.3
71.1
94.5
86.6
corr. age
(Ma)
4.4
4.7
5.1
4.7
4.8
5.0
4.9
4.7
4.8
5.1
4.4
4.4
4.4
5.1
3.9
4.4
3.7
4.6
4.9
4.3
5.2
4.5
5.1
4.3
4.7
5.6
5.4
4.5
4.6
4.7
4.4
4.4
4.3
5.7
5.2
4191
4191
4191
4191
4191
4191
4012
4012
4012
4012
3860
3860
3686
3686
3686
3686
3686
3537
3362
3362
3362
3362
3362
3215
3215
3158
3158
3155
3155
3155
3128
3128
3064
3064
3064
Depth subSample elev. pC unconf.
±2ı (Ma)
(m)
(m)
3
6
10
2
4
5
2
5
3
2
4
3
1
3
5
10
3
3
8
4
4
5
5
4
5
11
4
1
3
4
2
3
3
3
5
# grains
thermochronology of the northern Rocky Mountains, western U.S.A.
195
sample name
WIND RIVER RANGE
Fremont Peak samples
WY8789.4
WY8789.5
WY8589.2
WY8589.3
WY-88-89
WY8889.3
Gannett Peak samples
GP3aA
GP3aB
GP3aC
GP4aA
GP4aC
GP4aD
GP4aE
GP4aF
GP1aB
GP1aC
GP1aD
GP1aE
GP1aF
GP1aG
GP1aH
GP1aJ
GP1aK
GP1aL
GP2aA
GP2aB
GP2aC
GP2aD
GP2aE
GP2aF
GP5aA
GP5aB
GP5aC
GP5aD
GP5aF
halfwidth
( m)
47.9
48.9
75.0
59.3
59.7
72.0
72.8
51.3
34.5
49.8
40.3
39.5
46.0
48.8
33.0
38.0
23.0
28.8
37.8
38.5
31.0
45.5
35.5
31.0
29.0
34.8
27.3
37.8
39.8
40.8
70.8
60.0
66.0
70.5
63.3
mass
( g)
9.73
18.03
23.81
20.70
84.61
43.76
7.01
2.40
0.82
2.90
1.45
1.64
2.38
3.03
1.32
1.44
0.42
0.79
1.25
1.27
1.01
1.93
1.21
0.88
0.78
1.06
0.60
1.13
1.40
1.46
8.37
4.83
4.68
6.12
4.43
75.3
52.8
36.3
49.9
43.2
40.6
50.2
50.0
36.0
39.0
24.9
32.8
40.8
38.1
33.4
44.1
36.6
32.6
30.9
38.1
31.7
40.1
42.7
43.5
76.8
67.4
70.6
72.3
68.5
53.4
56.5
83.7
66.6
67.5
82.9
Rs
( m)
53
41
20
36
29
32
18
30
27
10
12
12
13
46
11
18
15
19
132
45
18
51
58
51
32
64
54
82
83
74
40
23
23
10
22
U
(ppm)
64
53
22
47
33
38
26
33
36
23
32
24
28
90
21
40
48
9
65
21
15
24
24
24
14
62
17
38
86
14
16
37
43
8
12
Th
(ppm)
64
50
48
460
300
327
355
280
592
202
291
306
199
370
157
232
331
329
250
103
117
117
88
95
174
381
352
280
330
NA
NA
NA
NA
NA
NA
Sm (ppm)
68
53
25
47
36
41
24
38
35
15
20
17
19
67
16
27
26
21
147
49
22
56
63
56
35
78
58
91
103
78
44
32
33
12
24
eU
(ppm)
23.70
16.33
6.34
14.12
11.32
11.07
6.73
12.67
8.50
5.71
3.81
6.99
4.48
9.52
4.94
14.37
4.16
7.21
32.18
10.49
3.23
11.45
13.94
12.80
10.81
24.36
19.47
29.75
34.24
18.94
14.47
10.67
11.21
3.90
8.33
4He
(nmol/g)
(continued)
Table A1
63.9
56.4
46.5
54.5
56.6
49.0
50.1
61.5
43.5
67.1
34.8
71.8
42.4
26.0
56.4
95.4
29.1
62.7
40.4
39.1
27.4
37.5
40.7
41.9
55.9
57.0
61.6
60.1
60.8
45.1
60.7
62.0
62.1
61.7
62.8
raw age
(Ma)
0.80
0.72
0.61
0.71
0.67
0.65
0.71
0.71
0.61
0.63
0.46
0.57
0.64
0.62
0.58
0.67
0.60
0.58
0.56
0.63
0.56
0.65
0.67
0.67
0.81
0.78
0.79
0.80
0.78
0.72
0.74
0.81
0.77
0.78
0.82
Ft
79.9
78.2
76.0
77.3
85.0
75.7
70.9
87.0
71.7
106.8
76.4
126.3
66.0
41.8
97.8
143.2
48.2
108.5
72.3
61.9
48.5
57.8
61.0
62.3
69.4
73.2
77.9
75.5
77.9
62.5
82.5
76.8
81.0
79.5
77.1
corr. age
(Ma)
3.3
3.3
5.5
3.3
4.1
2.8
2.7
3.0
3.4
7.1
12.8
11.2
4.5
1.7
8.6
5.2
2.1
6.8
3.0
3.1
5.8
3.0
2.4
2.5
3.1
3.2
3.5
3.3
3.2
3.7
5.0
4.6
4.9
4.8
4.6
4208
4208
4208
3978
3978
3978
3978
3978
3573
3573
3573
3573
3573
3573
3573
3573
3573
3573
3298
3298
3298
3298
3298
3298
3146
3146
3146
3146
3146
3064
3064
3043
3043
2847
2847
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
4
5
3
4
14
4
Depth subSample elev. pC unconf.
±2 (Ma)
(m)
(m)
# grains
196
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
sample name
BEARTOOTH RANGE
Surface samples
BT072207-2aA
BT072207-2aB
BT072007-1aA
BT072007-1aB
BT072007-1aC
BT072007-1aD
BT072007-1aE
BT072007-1aF
BT072007-2aA
BT072007-2aB
BT072007-2aC
BT072007-2aD
BT072007-2aF
BT072007-2aG
BT072007-4aB
BT072007-4aC
BT072007-6aA
BT072007-6aB
BT072007-6aC
BT072007-6aE
BT072007-7aA
BT072007-7aB
BT072007-7aC
BT072007-7aD
BT072007-7aE
BT072007-7aF
BT072107-2aB
BT072107-2aC
BT072107-2aD
BT072107-2aE
BT072107-2aF
BT072107-1aA
BT072107-1aB
BT072107-1aC
BT072107-1aD
BT072107-1aE
BT072107-1aF
halfwidth
(m)
45.3
33.5
48.3
29.3
55.3
30.0
69.5
42.5
29.0
30.8
47.5
38.8
38.8
31.5
34.5
44.3
44.0
43.0
52.5
38.0
45.5
41.8
43.8
49.0
40.3
26.3
29.5
44.5
52.3
32.3
41.5
29.0
28.5
26.0
26.8
32.0
29.5
mass
(g)
2.17
1.44
3.58
0.76
4.13
0.82
3.00
2.27
0.45
1.18
1.81
1.81
1.28
0.80
0.65
1.69
2.52
3.25
2.63
1.18
3.36
1.39
1.70
1.93
2.53
1.07
0.96
2.01
1.78
1.06
0.94
0.59
1.68
0.71
0.65
0.77
0.69
49.0
38.8
51.4
32.9
60.3
33.8
59.3
44.7
30.1
34.1
48.4
41.1
38.2
32.1
34.6
46.4
46.2
47.7
54.3
40.6
49.4
43.6
42.3
52.5
44.3
30.5
32.3
44.3
49.4
36.5
39.9
31.6
34.8
28.8
28.8
34.7
32.7
Rs
(m)
4
9
16
17
16
19
16
10
7
4
3
3
6
7
22
26
44
52
29
74
125
110
71
135
86
62
33
15
16
19
28
12
39
11
11
11
19
U
(ppm)
3
23
12
22
11
11
12
10
12
2
5
2
4
7
16
13
46
46
13
26
46
61
49
71
54
17
9
4
5
5
9
5
10
5
5
10
5
Th
(ppm)
119
91
110
134
224
302
163
139
30
22
16
17
24
32
22
24
291
325
163
285
123
132
86
160
122
60
28
41
16
17
139
80
101
75
85
72
80
Sm (ppm)
5
15
19
23
18
21
19
13
10
4
4
4
7
9
26
29
54
63
32
81
136
125
82
152
98
67
35
16
17
20
30
13
41
12
12
13
20
eU
(ppm)
0.97
1.50
4.79
4.32
4.78
3.43
4.69
3.06
1.36
1.72
1.85
1.88
1.34
1.94
3.21
4.48
11.43
12.77
6.51
17.38
33.21
26.06
15.68
36.47
22.12
10.37
5.93
2.83
3.48
3.40
6.36
8.74
6.01
2.09
1.14
1.65
4.19
4He
(nmol/g)
(continued)
Table A1
36.1
18.7
45.9
35.2
47.5
29.0
45.6
43.7
25.1
76.7
76.1
90.2
35.6
39.8
23.1
28.6
38.5
37.5
38.0
39.7
45.1
38.6
35.3
44.3
41.6
28.8
31.6
33.6
37.2
31.5
38.6
124.2
26.9
33.1
17.4
22.5
38.8
raw age
(Ma)
0.70
0.63
0.72
0.58
0.76
0.59
0.75
0.68
0.54
0.59
0.70
0.66
0.63
0.57
0.60
0.69
0.69
0.70
0.73
0.65
0.71
0.67
0.66
0.72
0.68
0.56
0.58
0.68
0.71
0.62
0.65
0.57
0.60
0.53
0.53
0.60
0.58
Ft
51.3
29.9
64.0
61.3
62.8
49.2
60.7
64.6
46.6
129.2
109.3
137.5
56.5
70.0
38.6
41.5
56.1
53.9
51.8
60.9
63.7
57.3
53.3
61.3
61.4
51.9
54.9
49.5
52.4
50.8
59.7
219.1
44.6
62.3
32.7
37.8
66.7
corr. age
(Ma)
8.5
4.3
3.2
6.3
3.4
5.6
3.0
4.8
16.3
23.8
12.6
18.6
6.0
8.9
5.5
2.8
2.2
2.3
2.6
2.9
2.7
3.5
3.2
3.6
2.9
3.0
4.0
3.2
3.4
7.4
5.4
32.5
2.8
11.0
12.4
7.7
7.0
2963
2963
2868
2868
2868
2868
2868
2868
2868
2868
2868
2868
2868
2868
2551
2551
2266
2266
2266
2266
2128
2128
2128
2128
2128
2128
1972
1972
1972
1972
1972
1878
1878
1878
1878
1878
1878
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Depth subSample elev. pC unconf.
±2 (Ma)
(m)
(m)
# grains
thermochronology of the northern Rocky Mountains, western U.S.A.
197
sample name
BEARTOOTH RANGE
Amoco Beartooth #A1 well
BT183aB
BT183aC
BT183aE
BT183aG
BT622aC
BT799aC
BT799aD
BT927aA
BT927aB
BT927aC
BT927aD
BT927aE
BT927aF
BT1067aE
BT1250aA
BT1250aB
BT1420aB
BT1420aE
BT1524aB
BT1966aB
BT1966aG
BT2179aA
BT2344aC
BT2344aF
BT2344aG
BT2344aH
BT2524aA
BT2524aB
BT2524aG
BT2622aA
BT2622aB
BT3088aA
BT3088aB
BT3088aC
BT3088aD
halfwidth
(ȝm)
32.0
25.0
33.0
25.3
80.0
49.3
35.0
36.3
40.5
27.5
30.3
24.0
27.8
53.5
43.5
42.5
51.5
29.5
43.8
41.3
30.0
35.8
32.3
75.3
35.0
36.5
33.5
36.5
37.5
29.8
37.8
28.5
33.3
38.3
30.0
mass
(ȝg)
0.61
0.37
1.82
0.60
7.89
2.08
0.99
1.19
1.61
0.47
0.88
0.63
0.67
5.66
2.03
1.87
3.90
0.43
2.24
3.23
0.70
1.57
1.03
10.85
0.99
1.34
0.71
1.38
1.33
1.18
0.90
0.94
0.48
0.70
0.84
33.3
27.1
37.7
30.0
79.8
53.3
37.8
39.7
44.1
30.9
32.2
28.3
32.4
58.5
47.6
46.3
57.5
30.0
48.4
48.7
33.0
40.9
36.4
82.6
37.8
37.8
31.8
40.7
38.2
35.0
38.3
31.6
34.1
36.2
31.8
Rs
(ȝm)
70
28
48
34
5
37
7
4
4
122
88
28
38
68
8
5
6
18
24
44
15
44
6
43
11
30
17
8
7
17
4
16
8
12
12
U
(ppm)
16
17
13
7
1
11
5
7
6
38
31
21
23
35
12
6
4
8
21
14
13
4
10
7
5
5
10
3
10
13
3
63
21
43
35
Th
(ppm)
137
75
98
115
5
31
36
18
27
164
87
60
97
203
31
17
22
23.06
123
41
125
74
17
34
65
77
76
21
53
158
55
142
158
144
258
Sm
(ppm)
73
32
51
35
5
39
8
6
5
131
95
33
44
76
11
6
7
20
29
47
18
45
8
45
12
31
19
8
10
20
4
31
13
22
20
eU
(ppm)
15.29
4.59
12.87
6.08
1.31
9.34
1.90
0.31
0.42
22.59
14.76
4.55
5.36
14.68
0.83
0.19
0.81
1.54
3.10
3.52
0.52
1.28
0.25
3.32
0.10
0.63
1.42
0.30
6.26
1.94
0.20
0.18
0.00
0.05
0.13
4He
(nmol/g)
(continued)
Table A1
38.5
26.7
46.1
31.7
46.0
44.0
43.7
10.1
15.9
31.8
28.8
25.7
22.6
35.7
14.4
5.5
21.9
14.4
19.7
13.8
5.5
5.3
5.6
13.7
1.5
3.8
13.7
6.8
117.6
17.8
8.5
1.1
0.0
0.4
1.2
raw age
(Ma)
0.59
0.51
0.63
0.55
0.82
0.73
0.63
0.64
0.67
0.56
0.57
0.52
0.58
0.75
0.69
0.69
0.75
0.55
0.70
0.71
0.58
0.66
0.61
0.82
0.63
0.63
0.57
0.65
0.63
0.60
0.63
0.55
0.58
0.60
0.55
Ft
65.5
52.8
73.2
57.7
56.4
60.3
69.6
15.8
23.8
56.7
50.1
49.3
39.3
47.6
20.8
8.0
29.4
26.2
28.1
19.6
9.5
8.1
9.3
16.6
2.4
6.0
24.1
10.4
188.2
29.7
13.5
1.9
0.0
0.7
2.1
corr. age
(Ma)
5.5
7.3
3.4
5.3
4.0
3.6
19.6
4.6
4.2
2.7
2.4
5.7
5.3
2.4
2.2
5.1
2.4
3.0
1.3
0.8
6.3
0.5
2.2
0.7
1.1
0.4
5.5
4.4
15.9
3.2
6.0
0.5
NA
1.0
0.6
1730
1730
1730
1730
1291
1115
1115
986
986
986
986
986
986
849
665
665
493
493
392
-48
-48
-260
-399
-399
-399
-399
-578
-578
-578
-682
-682
-1143
-1143
-1143
-1143
Sample
±2ı (Ma) elev. (m)
Depth subpC unconf.
(m)
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
# grains
198
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
80.1
63.3
63.2
52.6
52.6
55.4
33.1
34.8
36.0
50.7
37.3
31.4
52.6
57.5
33.8
35
18
21
33
33
23
12
24
12
16
26
50
7
10
10
10
4
6
9
6
6
9
26
28
26
25
48
1
2
3
218
115
129
216
120
137
125
308
163
204
299
106
258
364
272
38
19
23
35
34
24
14
30
18
22
32
61
8
10
11
61.8
52.9
56.0
52.6
68.0
52.9
50.5
48.7
44.4
51.0
52.0
39.7
61.8
54.4
41.3
12.66
5.50
6.90
10.06
12.72
6.87
3.76
7.88
4.44
6.10
9.03
13.13
2.64
3.15
2.45
73.8
65.8
56.0
52.8
47.8
49.3
29.3
32.0
34.3
45.3
32.5
30.5
48.0
60.5
30.8
BIGHORN RANGE
Cloud Peak surface samples
BH090406-1aA
BH090406-1aB
BH090406-1aC
BH090406-1aD
BH090406-1aE
BH090406-1aF
BH090406-2aA
BH090406-2aB
BH090406-2aC
BH090406-2aD
BH090406-2aE
BH090406-3aA
BH090406-3aB
BH090406-3aC
BH090406-3aE
9.51
5.68
5.40
1.69
4.22
3.58
1.19
0.78
1.19
2.72
1.22
0.38
2.77
2.80
0.75
48.7
24.4
32.5
27.6
20.8
22.7
28.3
14.3
8.3
7.3
eU
(ppm)
11.06
2.88
8.75
4.89
2.75
4.99
8.05
2.93
1.02
2.51
Sm
(ppm)
0.2
0.2
0.0
0.0
12.3
Th
(ppm)
0.06
0.05
0.00
0.00
1.50
U
(ppm)
sample name
BEARTOOTH RANGE
Amoco Beartooth #A1 well
BT3766aA
0.47
27.3
29.51
21
99
286
44
BT3766aB
0.38
26.3
28.0
21
66
55
36
BT3766aC
0.60
30.3
32.4
23
78
305
42
BT3851aA
0.63
31.3
33.1
29
93
186
51
BT3851aC
1.07
33.5
37.4
12
44
181
22
Amoco Beartooth Unit #1 well - multigrain aliquots (half-width and Rs are mass-weighted averages)
BT622aAB
4.73
54.0
50.3
39
14
60
42
BT799aAB
2.78
41.7
41.7
18
16
95
22
BT1067aAF
2.90
41.4
44.8
45
22
95
50
BT1067aBCD
2.74
31.9
35.3
25
30
169
32
BT1250aCDEF
3.30
36.7
36.6
22
9
53
24
BT1420aACD
3.85
35.1
38.1
32
38
127
41
BT1524aAD
1.14
27.3
29.4
46
27
70
52
BT1524aCEF
1.50
28.99
31.2
33
22
84
38
BT1966aDFHI
5.94
44.6
43.8
21
6
30
23
BT2179aCFGH
3.10
31.6
35.2
61
8
132
63
Rs
(ȝm)
raw age
(Ma)
halfwidth
(ȝm)
4He
(nmol/g)
mass
(ȝg)
(continued)
Table A1
0.81
0.77
0.77
0.73
0.73
0.74
0.58
0.60
0.60
0.71
0.62
0.56
0.73
0.75
0.59
0.71
0.63
0.68
0.62
0.58
0.59
0.53
0.54
0.65
0.59
0.52
0.50
0.56
0.57
0.61
Ft
75.8
68.8
72.9
72.5
93.6
71.6
86.8
81.7
74.0
72.0
83.8
70.8
85.0
72.7
69.6
68.9
38.4
48.0
44.9
35.6
38.4
53.6
26.2
12.9
12.5
0.5
0.5
0.0
0.0
20.2
corr. age
(Ma)
3.7
3.4
3.6
3.8
4.7
3.5
4.9
3.8
3.4
3.1
3.8
3.4
4.7
3.8
4.4
3.9
2.4
2.9
2.6
1.6
1.5
2.6
1.4
0.6
0.6
0.6
0.7
NA
NA
1.1
±2ı
(Ma)
4011
4011
4011
4011
4011
4011
3810
3810
3810
3810
3810
3620
3620
3620
3620
1291
1115
849
849
665
493
392
392
-48
-260
-1805
-1805
-1805
-1874
-1874
289
289
289
289
289
289
490
490
490
490
490
680
680
680
680
Depth subSample elev. pC unconf.
(m)
(m)
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
3
4
3
2
3
4
4
1
1
1
1
1
# grains
thermochronology of the northern Rocky Mountains, western U.S.A.
199
sample name
BIGHORN RANGE
Cloud Peak surface samples
BH090406-3aF
0.59
BH090406-4aA
1.35
BH090406-4aB
1.84
BH090406-4aC
0.99
BH090406-4aD
1.73
BH090406-4aE
1.34
BH090406-4aF
1.71
BH090406-5aA
20.27
BH090406-5aB
0.55
BH090406-5aC
6.06
BH090406-5aD
4.90
BH090406-5aE
1.56
BH090406-6aA
2.68
BH090406-6aB
1.31
BH090406-6aC
1.73
BH090406-6aD
0.80
BH090406-6aE
1.45
BH090606-1aA
4.23
BH090606-1aB
2.96
BH090606-1aC
1.20
BH090606-1aD
2.06
Gulf Granite Ridge well
BH225aA
2.09
BH225aB
1.07
BH225aC
2.19
BH225aD
0.71
BH454aA
3.23
BH454aB
2.01
BH454aC
1.88
BH454aD
1.03
BH454aE
1.08
BH454aF
1.68
BH761aA
0.66
BH761aB
0.78
BH761aC
2.02
mass
(ȝg)
Rs
(ȝm)
33.0
44.4
47.3
39.3
45.1
37.8
42.1
102.0
31.5
70.7
58.4
45.1
50.8
39.8
48.4
35.7
43.0
49.7
51.2
36.8
42.6
43.4
38.2
45.9
33.5
50.6
42.0
46.5
37.8
41.3
50.0
29.4
30.0
50.4
halfwidth
(ȝm)
31.8
45.0
44.5
38.3
41.3
36.5
43.0
93.0
29.5
67.0
56.5
39.5
45.5
35.5
49.3
33.5
39.8
43.5
54.3
36.3
39.8
41.3
34.8
40.3
30.8
48.8
35.8
42.8
34.5
37.0
46.5
27.8
27.3
49.8
10
23
14
19
6
13
9
8
53
13
15
35
11
7
20
31
19
22
32
21
9
5
13
5
63
25
25
31
19
23
16
2
6
10
U
(ppm)
1
8
11
16
1
3
1
2
11
3
13
18
3
2
15
12
13
17
14
20
2
2
6
2
35
4
27
13
13
18
1
1
1
1
Th
(ppm)
61
174
389
105
83
208
55
59
183
171
151
91
73
343
110
148
106
134
150
88
102
86
76
67
180
456
570
493
444
303
87
210
144
92
Sm
(ppm)
Table A1
11
25
17
22
6
14
9
9
55
14
18
39
11
7
24
34
22
26
35
26
10
6
14
5
71
26
31
34
22
27
17
3
6
10
eU
(ppm)
2.55
6.20
23.09
7.38
1.74
3.67
2.70
2.10
14.73
3.54
3.54
8.98
2.41
1.85
5.74
8.83
5.74
6.21
8.75
5.40
3.18
1.05
4.16
2.14
19.76
7.90
7.73
9.52
5.04
7.33
11.11
0.66
5.68
3.15
4He
(nmol/g)
(continued)
44.0
45.7
240.9
60.5
49.6
48.3
55.7
44.1
49.1
46.8
35.6
42.1
39.0
44.6
44.8
47.7
48.1
43.1
45.9
38.2
58.8
34.3
53.1
72.8
51.2
55.3
44.6
50.6
41.4
49.2
122.6
43.9
160.4
55.6
raw age
(Ma)
0.67
0.63
0.69
0.58
0.72
0.66
0.69
0.63
0.66
0.71
0.54
0.55
0.71
0.58
0.68
0.70
0.64
0.68
0.63
0.66
0.85
0.57
0.79
0.75
0.68
0.72
0.64
0.70
0.61
0.67
0.71
0.72
0.62
0.67
Ft
65.2
72.1
351.0
103.5
69.2
72.7
80.1
69.8
74.4
65.6
66.5
77.1
54.5
76.4
66.2
68.5
75.1
63.3
72.9
57.9
69.0
60.5
67.2
97.0
75.0
77.1
69.6
72.0
68.0
73.7
171.9
61.2
257.2
83.0
corr. age
(Ma)
3.4
3.5
14.9
5.1
3.6
3.5
4.8
5.2
3.4
3.3
5.6
4.0
3.1
6.5
3.2
2.7
4.1
2.5
3.3
2.4
3.4
8.1
3.2
5.0
3.5
3.2
2.7
2.8
4.3
3.0
9.1
4.6
15.4
4.5
±2ı
(Ma)
1829
1829
1829
1829
1600
1600
1600
1600
1600
1600
1293
1293
1293
3620
3452
3452
3452
3452
3452
3452
3272
3272
3272
3272
3272
3097
3097
3097
3097
3097
2941
2941
2941
2941
671
671
671
671
900
900
900
900
900
900
1207
1207
1207
680
848
848
848
848
848
848
1028
1028
1028
1028
1028
1203
1203
1203
1203
1203
1359
1359
1359
1359
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Depth subSample elev. pC unconf.
(m)
(m)
# grains
200
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
sample name
BIGHORN RANGE
Gulf Granite Ridge well
BH761aD
BH761aE
BH761aF
BH953aA
BH953aB
BH953aC
BH953aD
BH953aE
BH953aF
BH1140aA
BH1140aB
BH1140aC
BH1140aD
BT1140aE
BT1140aF
BH1263aA
BH1263aB
BH1263aC
BH1263aD
BH1263aE
BH1652aA
BH1652aC
BH1652aD
BH2281aA
BH2281aB
BH2281aC
BH2281aD
BH2281aE
BH2630aA
BH2630aB
BH2630aC
BH2630aD
BH2630aF
BH2897aA
BH2897aB
halfwidth
(ȝm)
32.0
33.5
32.8
50.8
30.8
50.5
56.8
33.3
32.0
27.8
30.0
33.8
51.5
38.8
38.3
31.8
41.5
27.8
28.8
44.8
36.8
31.0
36.3
33.0
41.3
35.5
49.8
62.0
45.3
40.3
52.0
44.3
33.5
34.8
35.0
mass
(ȝg)
0.93
0.90
0.91
2.44
0.71
5.24
3.90
1.35
0.94
0.75
0.60
0.62
3.13
1.65
1.69
0.77
1.55
0.70
0.93
1.48
0.97
0.74
0.99
1.34
1.75
1.09
2.60
4.09
1.54
2.09
2.61
1.51
0.58
2.03
1.79
33.4
36.5
36.1
55.5
31.0
56.0
60.5
36.3
33.5
31.8
32.2
36.0
55.6
43.2
43.1
34.5
44.4
29.7
31.7
45.4
38.6
33.9
38.5
36.1
45.1
35.9
53.0
66.9
45.9
42.9
56.9
45.3
35.5
41.2
40.9
Rs
(ȝm)
3
17
18
11
18
18
12
14
10
9
13
5
17
17
13
20
9
20
14
6
19
3
20
26
7
9
9
10
45
16
31
12
20
6
7
U
(ppm)
8
4
23
1
15
10
16
21
18
8
17
5
19
21
19
40
1
24
22
2
14
3
24
3
30
10
5
6
30
47
57
19
62
22
47
Th
(ppm)
196
87
193
170
79
200
107
43
68
140
82
35
85
98
81
199
94
89
165
118
123
43
147
97
191
173
37
125
362
140
321
149
272
182
237
Sm
(ppm)
5
18
23
12
22
20
15
19
14
10
17
6
22
22
17
30
9
26
19
7
22
4
25
26
14
12
10
11
53
27
45
16
35
11
18
eU
(ppm)
0.99
4.41
4.75
3.09
4.22
5.63
4.56
4.62
3.19
1.75
4.04
0.98
5.45
8.02
3.75
5.34
2.13
4.89
3.26
1.30
4.65
0.40
5.53
25.67
2.35
1.54
0.89
4.24
7.73
2.55
2.16
0.68
1.85
0.78
1.56
4He
(nmol/g)
(continued)
Table A1
34.7
44.6
37.2
47.6
36.0
51.6
54.2
45.5
40.7
30.4
42.5
28.3
46.5
67.2
40.1
32.7
44.5
34.5
31.7
35.5
38.3
17.2
40.1
177.9
30.0
23.9
15.9
70.3
27.0
17.4
8.9
7.6
9.8
12.5
15.6
raw age
(Ma)
0.57
0.62
0.61
0.74
0.56
0.74
0.75
0.61
0.58
0.57
0.57
0.61
0.73
0.67
0.66
0.59
0.68
0.54
0.56
0.69
0.63
0.59
0.63
0.62
0.67
0.61
0.73
0.78
0.69
0.66
0.74
0.68
0.59
0.64
0.63
Ft
60.5
72.0
61.2
64.3
64.7
69.8
71.9
74.9
70.4
53.8
75.0
46.5
63.3
100.9
60.6
55.6
65.3
64.2
56.7
51.7
60.5
29.3
63.6
288.0
44.9
39.3
21.9
90.3
39.3
26.5
12.0
11.3
16.5
19.6
24.7
11.4
4.9
3.4
3.1
3.6
2.9
3.0
3.5
3.9
4.0
4.7
5.9
2.5
4.7
4.1
3.1
5.0
4.1
3.7
5.0
3.9
9.1
3.1
15.7
2.7
3.7
1.4
4.6
1.8
1.2
0.6
1.1
1.5
1.4
1.4
corr. age
(Ma)
±2ı (Ma)
1293
1293
1293
1101
1101
1101
1101
1101
1101
914
914
914
914
914
914
791
791
791
791
791
402
402
402
-227
-227
-227
-227
-227
-576
-576
-576
-576
-576
-843
-843
Sample
elev. (m)
1207
1207
1207
1399
1399
1399
1399
1399
1399
1586
1586
1586
1586
1586
1586
1709
1709
1709
1709
1709
2098
2098
2098
2727
2727
2727
2727
2727
3076
3076
3076
3076
3076
3343
3343
Depth subpC unconf.
(m)
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
# grains
thermochronology of the northern Rocky Mountains, western U.S.A.
201
sample name
BIGHORN RANGE
Gulf Granite Ridge well
BH2897aC
1.07
BH2897aD
1.61
BH2897aE
1.68
BH2897aF
1.41
BH3174aA
3.12
BH3174aB
1.25
BH3174aC
5.86
BH3174aD
1.82
BH3638aA
1.23
BH3638aB
1.11
BH3638aC
1.01
BH3638aD
2.27
BH3638aE
0.77
BH3638aF
0.90
BH4080aA
0.74
BH4080aB
1.60
BH4080aC
2.04
BH4080aD
1.51
BH4549aA
1.15
BH4549aB
2.76
BH4549aC
2.18
BH4549aD
0.97
LARAMIE RANGE
Texaco Gov't Rocky Mountain #1
NLR96aA
1.66
NLR96aB
0.89
NLR96aC
0.80
NLR96aD
0.65
NLR96aE
0.94
NLR508aA
0.94
NLR508aB
0.94
NLR508aC
0.77
NLR508aD
0.81
NLR508aE
0.70
NLR1001aA
0.57
mass
(ȝg)
Rs
(ȝm)
35.1
43.7
41.2
39.1
49.2
36.8
71.6
41.4
44.5
35.1
35.4
52.6
35.7
37.6
31.0
38.7
42.5
38.7
34.4
51.3
50.3
34.4
46.5
33.6
31.8
32.6
39.3
37.5
31.5
34.5
31.7
33.8
29.3
half-width
(ȝm)
33.8
39.8
40.3
34.0
46.3
35.3
70.3
39.3
41.0
33.3
36.0
47.0
34.0
33.0
30.0
35.8
39.8
36.5
31.5
46.0
47.8
34.0
44.8
33.5
30.8
30.0
39.5
35.0
28.3
31.8
30.3
31.5
29.8
25
27
21
130
33
50
45
46
54
28
43
1
15
13
9
3
8
3
15
14
10
14
7
22
49
41
15
8
4
6
43
16
7
U
(ppm)
47
24
24
145
39
81
87
91
70
24
5
12
46
17
10
16
16
13
39
11
19
10
10
12
24
133
32
19
7
12
135
46
12
Th
(ppm)
199
142
160
491
148
250
260
291
267
205
41
148
207
187
156
134
200
138
262
352
127
406
108
132
336
229
92
211
217
419
330
191
2096
Sm
(ppm)
36
33
27
164
43
69
66
67
71
33
45
4
26
17
11
7
11
6
24
16
14
16
10
25
55
72
22
12
6
9
75
26
10
eU
(ppm)
12.72
6.57
7.11
49.75
9.70
16.84
90.78
18.20
17.28
6.33
7.79
0.44
1.42
1.53
1.32
0.48
0.48
0.63
1.02
1.12
1.04
1.69
0.14
0.26
4.99
0.54
0.40
0.26
0.29
0.16
3.91
0.30
0.14
4He
(nmol/g)
(continued)
Table A1
65.3
36.8
49.1
55.7
42.0
44.7
249.9
49.5
45.0
35.0
32.2
18.4
9.9
16.0
21.5
12.2
7.7
18.5
7.8
12.4
13.2
19.1
2.7
1.9
16.7
1.4
3.3
3.8
8.6
3.2
9.5
2.1
2.2
raw age
(Ma)
0.68
0.59
0.56
0.57
0.64
0.62
0.55
0.59
0.56
0.59
0.54
0.58
0.66
0.65
0.64
0.69
0.61
0.78
0.65
0.68
0.59
0.61
0.72
0.61
0.63
0.54
0.63
0.66
0.63
0.59
0.71
0.70
0.59
Ft
95.6
62.8
87.1
97.1
65.9
72.3
451.0
84.2
80.2
59.5
59.5
32.0
15.0
24.6
33.9
17.6
12.6
23.7
12.0
18.3
22.2
31.6
3.7
3.1
26.6
2.6
5.3
5.8
13.7
5.4
13.5
3.0
3.7
3.2
2.7
3.8
3.5
3.0
2.5
14.8
2.8
2.8
2.9
3.2
7.7
0.9
1.8
3.0
1.3
1.9
1.2
0.7
1.8
2.9
3.3
0.9
1.1
1.4
0.5
0.7
0.9
2.6
2.2
0.4
0.3
1.9
corr. age
(Ma)
±2ı (Ma)
1931
1931
1931
1931
1931
1519
1519
1519
1519
1519
1026
-843
-843
-843
-843
-1120
-1120
-1120
-1120
-1584
-1584
-1584
-1584
-1584
-1584
-2026
-2026
-2026
-2026
-2495
-2495
-2495
-2495
Sample
elev. (m)
391
391
391
391
391
803
803
803
803
803
1296
3343
3343
3343
3343
3620
3620
3620
3620
4084
4084
4084
4084
4084
4084
4526
4526
4526
4526
4995
4995
4995
4995
Depth subpC unconf.
(m)
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
# grains
202
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
sample name
LARAMIE RANGE
Texaco Gov't Rocky Mountain #1
NLR1001aB
1.72
NLR1001aC
0.83
NLR1001aD
1.02
NLR1501aA
0.79
NLR1501aB
1.75
NLR1501aC
1.21
NLR1501aD
1.99
NLR1501aE
1.32
NLR1872aC
0.95
NLR1872aD
2.93
NLR1872aE
0.88
NLR1872aF
0.63
NLR2297ajm1
2.11
NLR2297ajm2
1.24
NLR2297ajm4
0.54
NLR2297ajm5
0.79
NLR2297ajm6
1.85
NLR2467aA
1.97
NLR2467aB
1.10
NLR2467aC
0.67
NLR2608aA
1.25
NLR2608aB
1.29
NLR2608aC
0.84
NLR2608aD
0.73
NLR2608aE
5.64
NLR2608aF
1.39
NLR2761aA
0.83
NLR2761aB
1.05
NLR2761aC
1.70
NLR2761aD
1.26
NLR2761aE
0.83
Laramie Peak surface samples
02PRLP2aA
1.49
02PRLP2aB
1.67
02PRLP2aC
0.66
mass
(ȝg)
Rs
(ȝm)
40.5
36.2
35.1
32.2
39.8
37.2
44.8
35.9
37.2
56.4
33.7
33.6
45.9
46.9
31.3
38.9
54.0
43.5
39.7
37.7
42.5
42.9
38.7
34.7
62.0
40.8
35.3
42.2
47.7
43.4
37.1
40.5
46.1
30.8
half-width
(ȝm)
38.3
34.0
34.8
28.0
36.8
37.0
39.5
32.8
34.3
54.8
29.5
32.3
49.5
46.0
29.3
36.3
53.8
37.8
37.3
36.3
41.3
41.5
35.3
32.8
61.5
36.5
32.3
38.8
47.5
44.0
36.0
35.5
43.8
31.5
16
12
14
41
19
6
103
16
72
8
35
33
38
33
48
16
36
76
40
14
32
37
55
11
8
10
9
13
7
58
37
11
16
18
U
(ppm)
21
10
23
37
39
14
13
69
19
5
2
17
24
11
14
2
5
19
11
3
3
4
5
4
2
9
3
8
6
12
20
7
10
13
Th
(ppm)
268
211
263
203
35
12
28
51
29
54
65
120
246
16
57
68
56
158
63
30
26
45
33
50
56
66
59
72
30
92
177
70
75
148
Sm
(ppm)
21
14
20
50
28
9
106
32
76
9
36
37
44
36
52
16
37
81
42
15
33
38
56
12
8
12
10
15
8
61
41
13
18
21
eU
(ppm)
17.01
10.66
19.56
55.89
0.87
0.30
17.90
1.82
11.22
55.97
5.89
6.85
10.11
5.09
8.60
2.08
4.82
16.30
12.04
5.33
8.69
6.37
11.91
0.15
0.10
1.00
1.74
2.10
0.72
10.92
5.32
0.57
0.88
1.22
4He
(nmol/g)
(continued)
Table A1
146.7
136.6
179.0
203.3
5.7
6.3
31.4
10.5
27.2
1025.1
30.2
34.0
42.2
26.1
30.7
24.0
23.8
37.2
52.3
66.6
49.1
30.9
39.0
2.4
2.2
15.3
31.4
26.5
16.0
33.0
23.7
8.1
9.1
10.7
raw age
(Ma)
0.64
0.69
0.55
0.65
0.60
0.59
0.58
0.63
0.63
0.68
0.62
0.62
0.74
0.59
0.59
0.69
0.70
0.57
0.64
0.73
0.68
0.65
0.63
0.67
0.67
0.63
0.60
0.76
0.65
0.61
0.66
0.70
0.67
0.62
Ft
227.5
198.9
326.9
313.9
9.4
10.6
54.3
16.7
43.4
1505.8
49.1
54.5
56.9
44.1
52.0
34.8
34.1
65.8
81.7
90.9
72.7
47.6
61.7
3.6
3.3
24.2
52.1
34.7
24.6
54.3
35.8
11.6
13.5
17.3
9.5
7.6
12.6
11.3
1.0
2.5
2.2
0.7
1.8
69.6
2.3
2.8
2.6
2.4
2.8
2.4
2.3
3.4
4.5
5.0
3.8
2.9
3.5
0.9
1.7
3.3
3.8
1.4
2.6
2.8
1.9
1.8
1.6
2.5
corr. age
(Ma)
±2ı (Ma)
3131
3131
3131
1026
1026
1026
526
526
526
526
526
155
155
155
155
-270
-270
-270
-270
-270
-440
-440
-440
-581
-581
-581
-581
-581
-581
-734
-734
-734
-734
-734
Sample
elev. (m)
9
9
9
1296
1296
1296
1796
1796
1796
1796
1796
2167
2167
2167
2167
2592
2592
2592
2592
2592
2762
2762
2762
2903
2903
2903
2903
2903
2903
3056
3056
3056
3056
3056
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Depth subpC unconf.
(m)
# grains
thermochronology of the northern Rocky Mountains, western U.S.A.
203
Rs
(ȝm)
39.7
35.1
54.1
77.1
75.2
42.3
46.4
34.0
36.1
34.5
54.3
51.2
34.4
37.9
37.2
32.5
32.3
half-width
(ȝm)
38.0
36.8
45.0
68.8
71.3
40.0
43.0
31.0
35.0
33.5
48.3
45.0
34.5
32.8
36.0
31.3
28.8
17
18
15
21
18
23
32
29
20
22
25
24
98
16
38
5
17
U
(ppm)
21
16
16
39
23
27
35
30
27
27
32
39
127
15
51
2
22
Th
(ppm)
239
174
NA
NA
NA
507
595
369
468
408
NA
NA
608
246
441
80
NA
Sm
(ppm)
22
22
19
30
23
29
40
37
27
29
33
33
128
20
50
5
22
eU
(ppm)
14.03
14.24
228.71
250.73
298.71
13.35
11.27
29.70
13.85
15.82
511.22
415.63
38.49
4.68
15.90
4.18
108.52
4He
(nmol/g)
116.8
118.8
100.7
68.5
105.2
82.5
50.9
147.2
93.9
100.2
127.7
102.4
55.1
43.1
58.1
142.9
40.7
raw age
(Ma)
0.64
0.60
0.73
0.80
0.80
0.66
0.69
0.59
0.61
0.59
0.73
0.71
0.59
0.63
0.62
0.58
0.57
Ft
182.5
197.9
138.3
85.4
131.7
125.2
74.1
250.0
154.7
169.3
175.4
144.1
93.2
68.8
94.2
247.9
71.7
6.9
7.3
2.2
1.3
2.0
4.6
2.6
9.5
5.4
6.4
2.8
2.3
3.3
3.6
3.6
21.9
2.0
corr. age
(Ma)
±2ı (Ma)
3131
3131
3131
3009
2859
2689
2689
2689
2689
2689
2689
2524
2359
2359
2359
2359
2359
Sample
elev. (m)
9
9
9
131
281
451
451
451
451
451
451
616
781
781
781
781
781
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Depth subpC unconf.
(m)
# grains
Gray shading is used to highlight different samples. Half-width is the average measured radius of the apatite crystal for single grain aliquots, and mass-weighted
average radius for multi-grain aliquots. Rs is the radius of a sphere with an equivalent surface area to volume ratio as the apatite crystal (mass-weighted radius for
multi-grain aliquots). eU is effective U concentration. Ft is the alpha ejection correction of Farley (2002). Depth sub-pC unconf. is the depth below the Precambrian
unconformity of Blackstone (1993). # grains is the number of apatite grains in each aliquot. NA means not applicable.
sample name
LARAMIE RANGE
Laramie Peak surface samples
02PRLP2aD
1.58
02PRLP2aE
0.96
02PRLP2aB(pr)
5.32
02PRLP3aA(pr)
9.59
02PRLP4aA(pr)
7.28
02PRLP5aA
1.95
02PRLP5aB
1.82
02PRLP5aC
0.76
02PRLP5aD
1.15
02PRLP5aE
1.00
02PRLP5aA(pr)
3.40
02PRLP6aA(pr)
3.00
02PRLP7aA
0.47
02PRLP7aB
1.00
02PRLP7aC
0.85
02PRLP7aD
0.57
02PRLP7aA(pr)
0.71
mass
(ȝg)
Table A1
(continued)
204
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Rho-S
(e5)b
NSc
5.761
1.796
2.193
3.871
6.963
9.85
9.276
8.531
7.18
8.881
11.981
23.129
35.534
34.967
25.526
23.65
53.64
Rho-I
(e5)b
665
228
573
234
712
2637
7095
5901
1859
3726
7170
NIc
62.68
90.29
98.3
25.37
74.84
0
82.6
38.2
51.8
70.8
98.6
P(c))2
(%)d
11.543
11.81
10.086
12.361
12.499
13.05
11.032
11.268
11.346
11.111
11.189
Rho-D
(e5)e
4082
4082
4082
4082
4082
4082
4421
4421
4421
4421
4421
NDf
131.1
45.8
56.7
98.8
133.6
105.7*
54.2
54.0
54.4
54.8
56.6
Pooled
age
*central
(Ma)
8.8
7.4
5.1
12
8.9
6.7
1.6
1.9
2.9
2.0
1.8
±1s
9.8
7.88
8.55
9.16
11.96
22.55
39.63
39.02
28.27
26.78
58.13
U (ppm)
14(39)
13.7(53)
NA
13.8(24)
13.8(54)
Mean
length
(mm) (n)
1.49
1.4
1.46
1.48
2.2
2.4
2.8
2.1
2.3
Dpar
(mm)
0.8/0.1
0.9/0.2
NA/0.4
0.7/0.1
1.0/0.2
SD
(L/Dpar)
AFT analytical data for the Gannett Peak (Wind River Range) and Cloud Peak (Bighorn Range) vertical profiles. Samples analyzed with a Leica DMRM microscope
with drawing tube located above a digitizing tablet and a Kinetek computer-controlled stage driven by the FTStage program (Dumitru, 1993). Analysis is performed with
reflected and transmitted light at 1250⫻ magnification. Samples were irradiated at Oregon State University. Samples were etched in 5.5 M nitric acid at 21 °C for 20 s.
Following irradiation, the mica external detectors were etched at 21 °C in 40% hydrofluoric acid for 45 min. The pooled age is reported for all samples that pass the ␹2
test, suggesting that they represent a single population. Sample BH090406-1 failed the ␹2 test and is reported as a central age. Error is 1␴, calculated using the zeta
calibration method (Hurford and Green, 1983) with zeta of 359.25 ⫾ 4.46 for apatite [unpublished data, 2006, B. Carrapa]. Length data are reported as not corrected
for c axis to allow comparison with previous studies.
a
No. XIs is the number of individual crystals dated.
b
Rho-S and Rho-I are the spontaneous and induced track density measured, respectively (tracks/cm2).
c
NS and NI are the number of spontaneous and induced tracks counted, respectively.
d 2
␹ (%) is the chi-square probability (Green, 1981; Galbraith and Green, 1990). Values greater than 5% are considered to pass this test and represent a single
population of ages.
e
Rho-D is the induced track density in external detector adjacent to CN5 dosimetry glass (tracks/cm2).
f
ND is the number of tracks counted in determining Rho-D.
13
6
14
9
21
26
413
48
175
102
416
1123
No.
Xlsa
BIGHORN RANGE – Cloud Peak samples
BH090606-1
gneiss
2941
BH090406-6
gneiss
3097
BH090406-5
gneiss
3272
BH090406-4
gneiss
3452
BH090406-3
gneiss
3620
BH090406-1
gneiss
4011
Elevation
(m)
2001
1581
498
1009
2029
Lithology
WIND RIVER RANGE – Gannett Peak samples
GP5
granite
3146
25
10.022
GP2
granite
3298
25
9.368
GP1
granite
3573
13
6.838
GP4
granite
3978
25
6.404
GP3
granite
4208
25
15.18
Sample
number
Table A2
thermochronology of the northern Rocky Mountains, western U.S.A.
205
206
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Age (Ma)
m)
(
eR
Siz
ain
Rs (
Gr
ize
m)
in S
(pp
Gra
m)
Age (Ma)
eU
)
pm
(p
eU
Fig. A1. Variation of AHe age with both eU and grain size for apatite held at 30 °C, 50 °C, 70 °C and
90 °C for 1000 Ma. Ages calculated using the RDAAM of Flowers and others (2009). Both displays show the
same data but viewed from different angles. Color gradient shows AHe age and is meant as a visual aid.
Whereas apatite held at 70 °C would be below the PRZ (have zero ages) for the Durango He diffusion model,
it is within the PRZ for the RDAAM and hence ages vary with eU and grain size.
from sample WY5089; however, for all other samples solutions were found only if three age-eU pairs were
Fig. A2. Nodal points from good (pink) and acceptable (green) time-temperature paths from the
inversion of AHe age-eU pairs from sample BH761. Black line is best-fit solution.
thermochronology of the northern Rocky Mountains, western U.S.A.
207
Fig. A3. Nodal points from good (pink) and acceptable (green) time-temperature paths from the
inversion of AHe age-eU pairs from sample WY5089. Black line is best-fit solution.
Fig. A4. Nodal points from good (pink) and acceptable (green) time-temperature paths from the
inversion of AHe age-eU pairs from sample BT⫹2122. Black line is best-fit solution.
208
S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature
Fig. A5. Nodal points from good (pink) and acceptable (green) time-temperature paths from the
inversion of AHe age-eU pairs from sample NLR2761. Black line is best-fit solution.
input into the inversion. No solutions were found if an AFT age was included as one of the input data points
for a sample.
Input data for inversion were entered into HeFTy as raw AHe age (that is, uncorrected for alpha
ejection), U, Th and Sm concentration, and radius of a grain with an equivalent surface area to volume ratio
as our hexagonal prism apatite grain (Meesters and Dunai, 2002b). HeFTy calculates a corrected age by
applying a spherical alpha-ejection correction to the raw age (Farley and others, 1996; Farley, 2002). To
compare inversion results directly with real data, figures that show predicted age-eU distributions from
inversion results (for example, figs. 13B, 13C and 13D) always show real ages with a spherical alpha-ejection
correction applied. However, figures of age versus elevation (for example, fig. 14) always show AHe ages that
have a hexagonal-prism alpha-ejection correction.
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