[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; [email protected]

145

146

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; Danisı´k 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

147

thermochronology of the northern Rocky Mountains, western U.S.A. 112

108

Crazy Mountains Basin BT

104

0

100

200 km

Amoco Beartooth Unit #1

MONTANA

MR

WYOMING Wapiti Mountain traverse

LP

Cloud Peak traverse

Gulf Granite Ridge #1-9-2D

44

BH TR

4 44 4

Bighorn Basin

WR

front

Air Force well

IDAHO

GM

Greater Green River Basin

Laramie Peak traverse

LR

thru st

UTAH

Texaco Gov’t Rocky Mountain #1

Wind River Basin

Gannett Peak traverse Fremont Peak traverse

BL

Powder River Basin

Sevier

MB CB

UM

PR

COLORADO

40

GR

N

40

Denver Basin

Uinta Basin FR

Piceance Basin SR UU

112

108

1 4 104

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

148

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

149

thermochronology of the northern Rocky Mountains, western U.S.A.

A W Bighorn Range

B

Gulf Granite Ridge well

Powder River Basin Pz

1

4

Mz Pz

Mz

-2 -3

0

Air Force well

Wind River Range

Pz

-4 Mz

-6

Pz

-8

C

D SW

4

N

NE

Amoco Beartooth Beartooth Range well

Bighorn Basin

Elevation a.s.l.

T

-2

Mz

-4

Pz

Powder River Basin T

2

2 0

E

Mz T

Pz

-4

Elevation a.s.l.

River Basin

Te Tp

0 -1

W Green

Elevation a.s.l.

Elevation a.s.l.

2

E

Texaco Gov’t S Laramie Rocky Mtn Range well Pz

1 0

Mz

-1 -2

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

152

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 30 eU (ppm)

40

50

BH1652

D

100

80

0

50

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

184

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.

185

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

186

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.

188

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.

190

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

192

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.

References Barker, C., 1996, Thermal modeling of petroleum generation: Theory and applications: 5: New York, Amsterdam, Elsevier, Developments in Petroleum Science, v. 45, 512 p. Beland, P. E., ms, 2002, Apatite fission track and (U-Th)/He thermochronology and vitrinite reflectance of the Casper Arch, Maverick Springs Dome, and the Wind River Basin: implications for Late Cenozoic deformation and cooling in the Wyoming foreland: Laramie, Wyoming, University of Wyoming, M.S. thesis, 97 p. Belton, D. X., Brown, R. W., Kohn, B. P., Fink, D., and Farley, K. A., 2004, Quantitative resolution of the debate over antiquity of the central Australian landscape; implications for the tectonic and geomorphic stability of cratonic interiors: Earth and Planetary Science Letters, v. 219, n. 1–2, p. 21–34, http:// dx.doi.org/10.1016/S0012-821X(03)00705-2 Blackstone, D. L., 1981, Compression as an agent in deformation of the east-central flank of the Bighorn Mountains, Sheridan and Johnson counties, Wyoming: Rocky Mountain Geology, v. 19, n. 2, p. 105– 122. –––––– 1993, Precambrian basement map of Wyoming; outcrop and structural configuration: Wyoming State Geological Survey, Map Series 43, scale 1:1,000,000. Boyd, D. W., 1993, Paleozoic history of Wyoming, in Snoke, A. W., Steidtmann, J. R., and Roberts, S. M., editors, Geology of Wyoming: Geological Survey of Wyoming Memoir 5, p. 165–187. Brown, W. G., 1988, Deformational style of Laramide uplifts in the Wyoming foreland: Geological Society of America Memoir, v. 171, p. 1–25. Bryant, B., and Naeser, C. W., 1980, The significance of fission-track ages of apatite in relation to the tectonic history of the Front and Sawatch Ranges, Colorado: Geological Society of America Bulletin, v. 91, n. 3, p. 156 –164, http://dx.doi.org/10.1130/0016-7606(1980)91具156:TSOFAO典2.0.CO;2 Cerveny, P. F., ms, 1990, Fission-track thermochronology of the Wind River Range and other basement cored uplifts in the Rocky Mountain foreland: Laramie, Wyoming, University of Wyoming, Ph. D. thesis, 180 p. Cerveny, P. F., and Steidtmann, J. R., 1993, Fission track thermochronology of the Wind River Range,

thermochronology of the northern Rocky Mountains, western U.S.A.

209

Wyoming: Evidence for timing and magnitude of Laramide exhumation: Tectonics, v. 12, n. 1, p. 77–91, http://dx.doi.org/10.1029/92TC01567 Coney, P. J., and Reynolds, S. J., 1977, Cordilleran Benioff zones: Nature, v. 270, p. 403– 406, http:// dx.doi.org/10.1038/270403a0 Constenius, K. N., 1996, Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt: Geological Society of America Bulletin, v. 108, n. 1, p. 20 –39, http://dx.doi.org/10.1130/00167606(1996)108具0020:LPECOT典2.3.CO;2 Crowley, P. D., Reiners, P. W., Reuter, J. M., and Kaye, G. D., 2002, Laramide exhumation of the Bighorn Mountains, Wyoming: An apatite (U-Th)/He thermochronology study: Geology, v. 30, n. 1, p. 27–30, http://dx.doi.org/10.1130/0091-7613(2002)030具0027:LEOTBM典2.0.CO;2 Danisı´k, M., Sachsenhofer, R. F., Privalov, V. A., Panova, E. A., Frisch, W., and Spiegel, C., 2008, Low-temperature thermal evolution of the Azov Massif (Ukrainian Shield–Ukraine)—Implications for interpreting (U-Th)/He and fission track ages from cratons: Tectonophysics, v. 456, n. 3– 4, p. 171–179, http://dx.doi.org/10.1016/j.tecto.2008.04.022 DeCelles, P. G., 2004, Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S.A.: American Journal of Science, v. 304, n. 2, p. 105–168, http://dx.doi.org/10.2475/ ajs.304.2.105 DeCelles, P. G., Tolson, R. B., Graham, S. A., Smith, G. A., Ingersoll, R. V., White, J., Schmidt, C. J., Rice, R., Moxon, I., Lemke, L., Handschy, J. W., Follo, M. F., Edwards, D. P., Cavazza, W., Caldwell, M., and Bargar, E., 1987, Laramide thrust-generated alluvial-fan sedimentation, Sphinx Conglomerate, southwestern Montana: AAPG Bulletin, v. 71, n. 2, p. 135–155. DeCelles, P. G., Gray, M. B., Ridgway, K. D., Cole, R. B., Pivnik, D. A., Pequera, N., and Srivastava, P., 1991a, Controls on synorogenic alluvial-fan architecture, Beartooth Conglomerate (Palaeocene), Wyoming and Montana: Sedimentology, v. 38, n. 4, p. 567–590, http://dx.doi.org/10.1111/j.13653091.1991.tb01009.x DeCelles, P. G., Gray, M. B., Ridgway, K. D., Cole, R. B., Srivastava, P., Pequera, N., and Pivnik, D. A., 1991b, Kinematic history of a foreland uplift from Paleocene synorogenic conglomerate, Beartooth Range, Wyoming and Montana: Geological Society of America Bulletin, v. 103, n. 11, p. 1458 –1475, http:// dx.doi.org/10.1130/0016-7606(1991)103具1458:KHOAFU典2.3.CO;2 Dickinson, W. R., and Snyder, W. S., 1978, Plate tectonics of the Laramide Orogeny, in Matthews, V., editor, Laramide folding associated with basement block faulting in the western United States: Geological Society of America Memoir, v. 151, p. 355–366. Dickinson, W. R., Klute, M. A., Hayes, M. J., Janecke, S. U., Lundin, E. R., McKittrick, M. A., and Olivares, M. D., 1988, Paleogeographic and paleotectonic setting of Laramide sedimentary basins in the central Rocky Mountain region: Geological Society of America Bulletin, v. 100, n. 7, p. 1023–1039, http:// dx.doi.org/10.1130/0016-7606(1988)100具1023:PAPSOL典2.3.CO;2 Dickinson, W. W., 1986, Analysis of vitrinite maturation and Tertiary burial history, northern Green River Basin, Wyoming: United States Geological Survey Bulletin 1886, p. F1–F13. Donelick, R. A., O’Sullivan, P. B., and Ketcham, R. A., 2005, Apatite Fission-Track Analysis: Reviews in Mineralogy and Geochemistry, v. 58, n. 1, p. 49 –94, http://dx.doi.org/10.2138/rmg.2005.58.3 Dumitru, T. A., 1993, A new computer-automated microscope stage system for fission track analysis: Nuclear Tracks and Radiation Measurements, v. 21, n. 4, p. 575–580, http://dx.doi.org/10.1016/13590189(93)90198-I Dumitru, T. A., Gans, P. B., Foster, D. A., and Miller, E. L., 1991, Refrigeration of the western Cordilleran lithosphere during Laramide shallow-angle subduction: Geology, v. 19, n. 11, p. 1145–1148, http:// dx.doi.org/10.1130/0091-7613(1991)019具1145:ROTWCL典2.3.CO;2 Dunkl, I., 2002, Trackkey: windows program for calculation and graphical presentation of EDM fission track data, version 4.2: http://www.sediment.uni-goettingen.de/staff/dunkl/software/trackkey.html Ehlers, T. A., 2005, Crustal Thermal Processes and the Interpretation of Thermochronometer Data: Reviews in Mineralogy and Geochemistry, v. 58, n. 1, p. 315–350, http://dx.doi.org/10.2138/rmg.2005.58.12 Erslev, E. A., 1993, Thrusts, back-thrusts and detachment of Rocky Mountain foreland arches, in Schmidt, C. J., Chase, R. B., and Erslev, E. A., editors, Laramide basement deformation in the Rocky Mountain foreland of the western United States: Geological Society of America Special Paper, v. 280, p. 339 –358. Fan, M., and Dettman, D. L., 2009, Late Paleocene high Laramide ranges in northeast Wyoming: Oxygen isotope study of ancient river water: Earth and Planetary Science Letters, v. 286, n. 1–2, p. 110 –121, http://dx.doi.org/10.1016/j.epsl.2009.06.024 Farley, K. A., 2000, Helium diffusion from apatite; general behavior as illustrated by Durango fluorapatite: Journal of Geophysical Research, v. 105, n. B2, p. 2903–2914, http://dx.doi.org/10.1029/1999JB900348 –––––– 2002, (U-Th)/He dating: Techniques, Calibrations, and Applications, in Porcelli, D., Ballentine, C. J., and Wieler, R., editors, Noble gases in geochemistry and cosmochemistry: Reviews in Mineralogy and Cosmochemistry, v. 47, p. 819 – 843, http://dx.doi.org/10.2138/rmg.2002.47.18 Farley, K. A., Wolf, R. A., and Silver, L. T., 1996, The effects of long alpha-stopping distances on (U-Th)/He ages: Geochimica et Cosmochimica Acta, v. 60, n. 21, p. 4223– 4229, http://dx.doi.org/10.1016/S00167037(96)00193-7 Fitzgerald, P. G., Baldwin, S. L., Webb, L. E., and O’Sullivan, P. B., 2006, Interpretation of (U-Th)/He single grain ages from slowly cooled crustal terranes: A case study from the Transantarctic Mountains of southern Victoria Land: Chemical Geology, v. 225, n. 1–2, p. 91–120, http://dx.doi.org/10.1016/ j.chemgeo.2005.09.001 Flowers, R. M., Shuster, D. L., Wernicke, B. P., and Farley, K. A., 2007, Radiation damage control on apatite (U-Th)/He dates from the Grand Canyon region, Colorado Plateau: Geology, v. 35, n. 5, p. 447– 450, http://dx.doi.org/10.1130/G23471A.1 Flowers, R. M., Wernicke, B. P., and Farley, K. A., 2008, Unroofing, incision, and uplift history of the

210

S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles—Low-temperature

southwestern Colorado Plateau from apatite (U-Th)/He thermochronometry: Geological Society of America Bulletin, v. 120, n. 5– 6, p. 571–587, http://dx.doi.org/10.1130/B26231.1 Flowers, R. M., Ketcham, R. A., Shuster, D. L., and Farley, K. A., 2009, Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and annealing model: Geochimica et Cosmochimica Acta, v. 73, n. 8, p. 2347–2365, http://dx.doi.org/10.1016/j.gca.2009.01.015 Galbraith, R. F., and Green, P. F., 1990, Estimating the component ages in a finite mixture; Nuclear Tracks and Radiation Measurements, v. 17, n. 3, p. 197–206, http://dx.doi.org/10.1016/1359-0189(90)90035-V Gautheron, C., Tassan-Got, L., Barbarand, J., and Pagel, M., 2009, Effect of alpha-damage annealing on apatite (U-Th)/He thermochronology: Chemical Geology, v. 266, n. 3– 4, p. 157–170, http://dx.doi.org/ 10.1016/j.chemgeo.2009.06.001 Gehrels, G. E., Valencia, V. A., and Ruiz, J., 2008, Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry: Geochemistry Geophysics Geosystems, v. 9, Q03017, http://dx.doi.org/10.1029/2007GC001805 Green, P. F., 1981, A new look at statistics in fission track dating: Nuclear Tracks and Radiation Measurements, v. 5, n. 1–2, p. 77– 86, http://dx.doi.org/10.1016/0191-278X(81)90029-9 Green, P. F., Crowhurst, P. V., Duddy, I. R., Japsen, P., and Holford, S. P., 2006, Conflicting (U-Th)/He and fission track ages in apatite: Enhanced He retention, not anomalous annealing behaviour: Earth and Planetary Science Letters, v. 250, n. 3– 4, p. 407– 427, http://dx.doi.org/10.1016/j.epsl.2006.08.022 Gregory, K. M., and Chase, C. G., 1994, Tectonic and climatic significance of a late Eocene low-relief, high-level geomorphic surface, Colorado: Journal of Geophysical Research, v. 99, n. B10, p. 20,141– 20,160, http://dx.doi.org/10.1029/94JB00132 Gries, R., 1983a, Oil and Gas Prospecting Beneath Precambrian of Foreland Thrust Plates in Rocky Mountains: American Association of Petroleum Geologists Bulletin, v. 67, n. 1, p. 1–28. –––––– 1983b, North-south compression of the Rocky Mountain foreland structures, in Lowell, J. D., and Gries, R. R., editors, Rocky Mountain foreland basins and uplifts: Denver, Colorado, Rocky Mountain Association of Geologists, p. 9 –32. Grose, L. T., 1972, Tectonics of the Rocky Mountain Province, in Mallory, W. W., Mudge, M. R., Swanson, V. E., Stone, D. S., and Lumb, W. E., editors, Geologic atlas of the Rocky Mountain region: Denver, Rocky Mountain Association of Geologists, p. 35– 44. Heasler, H. P., and Hinckley, B. S., 1985, Geothermal resources of the Bighorn Basin, Wyoming: Geological Survey of Wyoming Report of Investigations, n. 29, 28 p. Hendriks, B. W. H., and Redfield, T. F., 2005, Apatite fission track and (U-Th)/He data from Fennoscandia: An example of underestimation of fission track annealing in apatite: Earth and Planetary Science Letters, v. 236, n. 1–2, p. 443– 458, http://dx.doi.org/10.1016/j.epsl.2005.05.027 Hoffman, P. F., 1989, Precambrian geology and tectonic history of North America, in Bally, A. W., and Palmer, A. R., editors, Volume A, The geology of North America: An overview: Boulder, Geological Society of America, p. 447–512. Hourigan, J. K., Reiners, P. W., and Brandon, M. T., 2005, U-Th zonation-dependent alpha-ejection in (U-Th)/He chronometry: Geochimica et Cosmochimica Acta, v. 69, n. 13, p. 3349 –3365, http:// dx.doi.org/10.1016/j.gca.2005.01.024 House, M. A., Wernicke, B. P., Farley, K. A., and Dumitru, T. A., 1997, Cenozoic thermal evolution of the central Sierra Nevada, California, from (U-Th)/He thermochronometry: Earth and Planetary Science Letters, v. 151, n. 3– 4, p. 167–179, http://dx.doi.org/10.1016/S0012-821X(97)81846-8 House, M. A., Farley, K. A., and Stockli, D., 2000, Helium chronometry of apatite and titanite using Nd-YAG laser heating: Earth and Planetary Science Letters, v. 183, n. 3– 4, p. 365–368, http://dx.doi.org/10.1016/ S0012-821X(00)00286-7 Hoy, R. G., and Ridgway, K. D., 1997, Structural and sedimentological development of footwall growth synclines along an intraforeland uplift, east-central Bighorn Mountains, Wyoming: Geological Society of America Bulletin, v. 109, n. 8, p. 915–935, http://dx.doi.org/10.1130/0016-7606(1997)109具0915: SASDOF典2.3.CO;2 –––––– 1998, Deformation of Eocene synorogenic conglomerate in the footwall of the Clear Creek thrust fault, Bighorn Mountains, Wyoming: The Mountain Geologist, v. 35, p. 81–90. Hurford, A. J., and Green, P. F., 1983, The zeta age calibration of fission-track dating: Chemical Geology, v. 41, n. 4, p. 285–317, http://dx.doi.org/10.1016/S0009-2541(83)80026-6 Kelley, S. A., 2002, Unroofing of the southern Front Range, Colorado: A view from the Denver Basin: Rocky Mountain Geology, v. 37, p. 189 –200, http://dx.doi.org/10.2113/gsrocky.37.2.189 –––––– 2005, Low-temperature cooling histories of the Cheyenne Belt and Laramie Peak shear zone, Wyoming, and the Soda Creek–Fish Creek shear zone, Colorado, in Karlstrom, K. E., and Keller, G. R., editors, The Rocky Mountain Region: An evolving lithosphere: American Geophysical Union, Geophysical Monograph Series, v. 154, p. 55–70, http://dx.doi.org/10.1029/154GM05 Kelley, S. A., and Blackwell, D. D., 1990, Thermal history of the multi-well experiment (MWX) site, Piceance Creek Basin, northwestern Colorado, derived from fission-track analysis: Nuclear Tracks and Radiation Measurements, v. 17, n. 3, p. 331–337, http://dx.doi.org/10.1016/1359-0189(90)90055-3 Kelley, S. A., and Chapin, C. E., 1997, Internal structure of the southern Front Range, Colorado, from an apatite fission-track thermochronology perspective, in Bolyard, D. W., and Sonnenberg, S. A., editors, Geologic history of the Colorado Front Range field trip guidebook: Denver, Rocky Mountain Association of Geologists, p. 43– 48. Ketcham, R. A., 2005, Forward and Inverse Modeling of Low-Temperature Thermochronometry Data: Reviews in Mineralogy and Geochemistry, v. 58, p. 275–314, http://dx.doi.org/10.2138/rmg.2005.58.11 Ketcham, R. A., Carter, A., Donelick, R. A., Barbarand, J., and Hurford, A. J., 2007, Improved modeling of fission-track annealing in apatite: American Mineralogist, v. 92, n. 5– 6, p. 799 – 810, http://dx.doi.org/ 10.2138/am.2007.2281

thermochronology of the northern Rocky Mountains, western U.S.A.

211

Kluth, C. F., and Coney, P. J., 1981, Plate Tectonics of the Ancestral Rocky Mountains: Geology, v. 9, n. 1, p. 10 –15, http://dx.doi.org/10.1130/0091-7613(1981)9具10:PTOTAR典2.0.CO;2 Kohn, B., Gleadow, A., Frei, S., and Kohlmann, F., 2008a, Low-temperature thermochronology in ancient terranes: a progress report, in Garver, J. I., and Montario, M. J., editors, Proceedings from the 11th International Conference on thermochronometry, Anchorage, Alaska: Schenectady, New York, Union College, p. 142–143. Kohn, B., Spiegel, C., Phillips, D., and Gleadow, A., 2008b, Rubbing out apatite helium age-spread in fast-cooled rocks, in Garver, J. I., and Montario, M. J., editors, Proceedings from the 11th International Conference on thermochronometry, Anchorage, Alaska: Schenectady, New York, Union College, p. 144. Lopez, D. A., 2001, Preliminary geologic map of the Red Lodge 30⬘ ⫻ 60⬘ quadrangle south-central Montana: Montana Bureau of Mines and Geology Open File Report 423, scale 1:100,000. Love, J. D., 1960, Cenozoic sedimentation and crustal movement in Wyoming: American Journal of Science, v. 258-A, p. 204 –214. Mackin, J. H., 1947, Altitude and local relief of the Bighorn area during the Cenozoic: Wyoming Geological Association Field Conference Guidebook, v. 2, p. 103–120. Mallory, W. W., Mudge, M. R., Swanson, V. E., Stone, D. S., and Lumb, W. E., 1972, Geologic atlas of the Rocky Mountain region: Denver, Rocky Mountain Association of Geologists, 331 p. Maughan, E. K., 1993, The ancestral Rocky Mountains in Wyoming, in Snoke, A. W., Steidtmann, J. R., and Roberts, S. M., editors, Geology of Wyoming: Geological Survey of Wyoming Memoir 5, p. 188 –207. McKenna, M. C., and Love, J. D., 1972, High-level Strata Containing Early Miocene Mammals on the Bighorn Mountains, Wyoming: American Museum of Natural History Novitates, n. 2490, p. 1–31. McMillan, M. E., Heller, P. L., and Wing, S. L., 2006, History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau: Geological Society of America Bulletin, v. 118, n. 3– 4, p. 393– 405, http:// dx.doi.org/10.1130/B25712.1 Meesters, A. G. C. A., and Dunai, T. J., 2002a, Solving the production-diffusion equation for finite diffusion domains of various shapes: Part II. Application to cases with ␣-ejection and nonhomogeneous distribution of the source: Chemical Geology, v. 186, n. 1–2, p. 57–73, http://dx.doi.org/10.1016/S00092541(01)00423-5 –––––– 2002b, Solving the production-diffusion equation for finite diffusion domains of various shapes: Part I. Implications for low-temperature (U-Th)/He thermochronology: Chemical Geology, v. 186, n. 3– 4, p. 333–344, http://dx.doi.org/10.1016/S0009-2541(01)00422-3 Miller, E. L., Miller, M. M., Wright, J. E., and Madrid, R., 1992, Late Paleozoic paleogeographic and tectonic evolution of the western U.S. Cordillera, in Burchfiel, B. C., Lipman, P. W., and Zoback, M. L., editors, The Cordilleran Orogen: Conterminous U.S.: Geological Society of America, The Geology of North America, v. G-3, Decade of North American Geology, p. 57–106. Mitchell, S. G., and Reiners, P. W., 2003, Influence of wildfires on apatite and zircon (U-Th)/He ages: Geology, v. 31, n. 12, p. 1025–1028, http://dx.doi.org/10.1130/G19758.1 Naeser, C. W., Bryant, B., Kunk, M. J., Kellogg, K. S., Donelick, R. A., and Perry, W. J., Jr., 2002, Tertiary cooling and tectonic history of the White River uplift, Gore Range, and western Front Range, central Colorado: Evidence from fission-track and 39Ar/40Ar ages, in Kirkham, R. M., Scott, R. B., and Judkins, T. W., editors, Late Cenozoic evaporite tectonism and volcanism in west-central Colorado: Geological Society of America Special Paper, v. 366, p. 31–54, http://dx.doi.org/10.1130/0-8137-2366-3.31 Naeser, N. D., 1986, Neogene thermal history of the northern Green River Basin, Wyoming— evidence from fission-track dating, in Gautier, D. L., editor, Roles of organic matter in sediment diagenesis: Society of Economic Paleontologists and Mineralogists Special Publication, v. 38, p. 65–72, http://dx.doi.org/ 10.2110/pec.86.38.0065 –––––– 1989, Thermal history and provenance of rocks in the Wagon Wheel No. 1 well, Pinedale anticline, northern Green River Basin— evidence from fission-track dating, in Law, B. E., and Spencer, C. W., editors, Geology of tight gas reservoirs in the Pinedale anticline area, Wyoming, and at the multiwell experiment side, Colorado: United States Geological Survey Bulletin 1886, p. E1–E13. –––––– 1992, Miocene cooling in the southwestern Powder River Basin, Wyoming; preliminary evidence from apatite fission-track analysis: United States Geological Survey Bulletin 1917-O, 17 p. Nuccio, V. F., 1994, Vitrinite reflectance data for the Paleocene Fort Union and Eocene Wind River Formations, and burial history of a drill hole located in central Wind River Basin, Wyoming: United States Geological Survey Open-File Report OF 94-0220, p. 1– 43. Omar, G. I., Lutz, T. M., and Giegengack, R., 1994, Apatite fission-track evidence for Laramide and post-Laramide uplift and anomalous thermal regime at the Beartooth overthrust, Montana-Wyoming: Geological Society of America Bulletin, v. 106, n. 1, p. 74 – 85, http://dx.doi.org/10.1130/00167606(1994)106具0074:AFTEFL典2.3.CO;2 Perry, W. J., Jr., Haley, J. C., Nichols, D. J., Hammons, P. M., and Ponton, J. D., 1988, Interactions of Rocky Mountain foreland and Cordilleran thrust belt in Lima region, southwest Montana, in Schmidt, C. J., and Perry, W. J., Jr., editors, Interactions of the Rocky Mountain foreland and the Cordilleran thrust belt: Geological Society of America Memoir, v. 171, p. 267–290. Reiners, P. W., and Farley, K. A., 2001, Influence of crystal size on apatite (U-Th)/He thermochronology: an example from the Bighorn Mountains, Wyoming: Earth and Planetary Science Letters, v. 188, n. 3– 4, p. 413– 420, http://dx.doi.org/10.1016/S0012-821X(01)00341-7 Reiners, P. W., Spell, T. L., Nicolescu, S., and Zanetti, K. A., 2004, Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with 40Ar/39Ar dating: Geochimica et Cosmochimica Acta, v. 68, n. 8, p. 1857–1887, http://dx.doi.org/10.1016/j.gca.2003.10.021 Reiners, P. W., Thompson, S. N., Tipple, B. J., Peyton, S. L., Rahl, J. M., and Mulch, A., 2008, Secondary

212

S. L. Peyton, P. W. Reiners, B. Carrapa, and P. G. DeCelles

weathering phases and apatite (U-Th)/He ages: Geochimica et Cosmochimica Acta, v. 72, n. 12, Supplement, p. A784. Riihimaki, C. A., Anderson, R. S., and Safran, E. B., 2007, Impact of rock uplift on rates of late Cenozoic Rocky Mountain river incision: Journal of Geophysical Research, v. 112, F03S02, http://dx.doi.org/ 10.1029/2006JF000557 Roberts, L. N. R., and Kirschbaum, M. A., 1995, Paleogeography of the Late Cretaceous of the western interior of middle North America— coal distribution and sediment accumulation: United States Geological Survey Professional Paper 1561, 115 p. Roberts, L. N. R., Finn, T. M., Lewan, M. D., and Kirschbaum, M. A., 2008, Burial history, thermal maturity, and oil and gas generation history of source rocks in the Bighorn Basin, Wyoming and Montana: United States Geological Survey Scientific Investigations Report 2008-5037, p. 1–27. Roberts, S. V., and Burbank, D. W., 1993, Uplift and thermal history of the Teton Range (northwestern Wyoming) defined by apatite fission-track dating: Earth and Planetary Science Letters, v. 118, n. 1– 4, p. 295–309, http://dx.doi.org/10.1016/0012-821X(93)90174-8 Saleeby, J., 2003, Segmentation of the Laramide slab— evidence from the southern Sierra Nevada region: Geological Society of America Bulletin, v. 115, n. 6, p. 655– 668, http://dx.doi.org/10.1130/00167606(2003)115具0655:SOTLSF典2.0.CO;2 Shuster, D. L., and Farley, K. A., 2009, The influence of artificial radiation damage and thermal annealing on helium diffusion kinetics in apatite: Geochimica et Cosmochimica Acta, v. 73, n. 1, p. 183–196, http://dx.doi.org/10.1016/j.gca.2008.10.013 Shuster, D. L., Flowers, R. M., and Farley, K. A., 2006, The influence of natural radiation damage on helium diffusion kinetics in apatite: Earth and Planetary Science Letters, v. 249, n. 3– 4, p. 148 –161, http:// dx.doi.org/10.1016/j.epsl.2006.07.028 Shuster, M. A., ms, 1986, The origin and sedimentary evolution of the northern Green River basin, western Wyoming: Laramie, Wyoming, University of Wyoming, Laramie, Ph. D. thesis, 323 p. Smithson, S. B., Brewer, J., Kaufman, S., Oliver, J., and Hurich, C., 1978, Nature of the Wind River thrust, Wyoming, from COCORP deep-reflection data and from gravity data: Geology, v. 6, n. 11, p. 648 – 652, http://dx.doi.org/10.1130/0091-7613(1978)6具648:NOTWRT典2.0.CO;2 Snoke, A. W., 1993, Geologic history of Wyoming within the tectonic framework of the North American Cordillera, in Snoke, A. W., Steidtmann, J. R., and Roberts, S. M., editors, Geology of Wyoming: Geological Survey of Wyoming Memoir, v. 5, p. 2–56. Spencer, S., Kohn, B., Gleadow, A., Norman, M., Belton, D., and Carter, T., 2004, The importance of residing in a good neighbourhood: rechecking the rules of the game for apatite (U-Th)/He thermochronometry: Abstract Volume of the Proceedings from the 10th International Conference on Fission Track Dating and Thermochronology, Amsterdam, p. 20. Spiegel, C., Kohn, B., Belton, D., Berner, Z., and Gleadow, A., 2009, Apatite (U-Th-Sm)/He thermochronology of rapidly cooled samples: The effect of He implantation: Earth and Planetary Science Letters, v. 285, n. 1–2, p. 105–114, http://dx.doi.org/10.1016/j.epsl.2009.05.045 Steidtmann, J. R., and Middleton, L. T., 1991, Fault chronology and uplift history of the southern Wind River Range, Wyoming: Implications for Laramide and post-Laramide deformation in the Rocky Mountain foreland: Geological Society of America Bulletin, v. 103, n. 4, p. 472– 485, http://dx.doi.org/10.1130/ 0016-7606(1991)103具0472:FCAUHO典2.3.CO;2 Steidtmann, J. R., Middleton, L. T., and Schuster, M. W., 1989, Post-Laramide (Oligocene) uplift in the Wind River Range, Wyoming: Geology, v. 17, n. 1, p. 38 – 41, http://dx.doi.org/10.1130/00917613(1989)017具0038:PLOUIT典2.3.CO;2 Strecker, U., ms, 1996, Studies of Cenozoic continental tectonics: Part I, Apatite fission-track thermochronology of the Black Hills uplift, South Dakota; Part II. Seismic sequence stratigraphy of the Goshute Valley halfgraben, Nevada: Laramie, Wyoming, University of Wyoming, Ph.D. thesis, 344 p. Vermeesch, P., 2008, Three new ways to calculate average (U-Th)/He ages: Chemical Geology, v. 249, n. 3– 4, p. 339 –347, http://dx.doi.org/10.1016/j.chemgeo.2008.01.027 Vermeesch, P., Seward, D., Latkoczy, C., Wipf, M., Gu¨nther, D., and Baur, H., 2007, ␣-emitting mineral inclusions in apatite, their effect on (U-Th)/He ages, and how to reduce it: Geochimica et Cosmochimica Acta, v. 71, n. 7, p. 1737–1746, http://dx.doi.org/10.1016/j.gca.2006.09.020 Waples, D. W., Pacheco, J., and Vera, A., 2004, A method for correcting log-derived temperatures in deep wells, calibrated in the Gulf of Mexico: Petroleum Geoscience, v. 10, n. 3, p. 239 –245, http://dx.doi.org/ 10.1144/1354-079302-542 Wise, D. U., 1997, Pseudo tear faults at Red Lodge, MT; clues to late-state basin understuffing and tectonics of the greater Beartooth Block, in Campen, E. B., editor, Bighorn Basin Symposium Guidebook: Yellowstone Bighorn Research Association, Wyoming Geological Associate, and Montana Geological Association, p. 71–99. Wolf, R. A., Farley, K. A., and Silver, L. T., 1996, Helium diffusion and low-temperature thermochronometry of apatite: Geochimica et Cosmochimica Acta, v. 60, n. 21, p. 4231– 4240, http://dx.doi.org/10.1016/ S0016-7037(96)00192-5 Ye, H., Royden, L., Burchfiel, C., and Schuepbach, M., 1996, Late Paleozoic deformation of interior North America: The greater ancestral Rocky Mountains: American Association of Petroleum Geologists Bulletin, v. 80, n. 9, p. 1397–1432.