Earth and Planetary Science Letters 178 (2000) 315^326
www.elsevier.com/locate/epsl
Helium and argon thermochronometry of the Gold Butte
block, south Virgin Mountains, Nevada
Peter W. Reiners a; *, Robert Brady a , Kenneth A. Farley a , Joan E. Fryxell b ,
Brian Wernicke a , Daniel Lux c
a
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
b
Department of Geological Sciences, California State University, San Bernardino, CA 92407, USA
c
Department of Geological Sciences, University of Maine, Orono, ME 04469, USA
Received 19 November 1999; received in revised form 13 March 2000; accepted 13 March 2000
Abstract
One of the largest exposures of Precambrian crystalline rock in the Basin and Range province of the southwestern
USA is the Gold Butte block of the south Virgin Mountains, about 15 km west of the Colorado Plateau. It has been
interpreted as a largely continuous crustal cross-section about 15^20 km thick that was exhumed by a deeply
penetrating normal fault during Miocene extension. To test this interpretation as well as the use of the newly developed
titanite (U^Th)/He thermochronometer, we examined the low temperature thermal history of the Gold Butte block with
the apatite and titanite (U^Th)/He and muscovite 40 Ar/39 Ar thermochronometers. Apatite He ages average 15.2 þ 1.0
(2c) Ma throughout the block, indicating that the entire section was warmer than 70³C prior to Miocene exhumation.
Titanite He ages increase from 18.6 þ 1.5 Ma near the paleobottom (west) end of the block, to 195 þ 15 Ma near the
paleotop (east) end. A rapid change from mid-Tertiary to increasingly older titanite He ages to the east is observed at
about 9.3 km paleodepth, and is interpreted as a fossil He partial retention zone for titanite. Assuming a preexhumation geotherm of 20³C/km (consistent with earlier apatite fission track work), this depth would have
corresponded to 196³C prior to exhumation, indicating that laboratory-derived He diffusion characteristics for titanite
that yield a closure temperature of about 200³C are applicable and correct. Muscovite 40 Ar/39 Ar ages are 1.0^1.4 Ga
near the paleotop of the block, and 90 Ma near the paleobottom. Together with 207 Pb/206 Pb ages on apatite and titanite,
and an earlier apatite fission track transect across the Gold Butte block, our data indicate that the continental crust at
the western edge of the Colorado Plateau resided at moderate geothermal gradients (and slowly declined in
temperature) from 1.4 Ga to about 100^200 Ma. A 90 Ma cooling event clearly affected the mid-crust (deepest portions
of Gold Butte), which may reflect accelerated cooling or a brief heating and cooling cycle at this time, after which
gradients returned to about 20³C/km prior to rapid exhumation in the Miocene. This work thus supports previous
structural and thermochronologic studies that suggest that the Gold Butte block is the thickest largely continuous crosssection of crust exposed in the southwestern USA. ß 2000 Elsevier Science B.V. All rights reserved.
Keywords: thermochronology; Basin and Range Province; exhumation; helium; Ar/Ar; extension
* Corresponding author. Present address: Department of Geology, Washington State University, Pullman, WA 99164, USA.
Tel.: +1-509-335-3009; Fax: +1-509-335-7816; E-mail: [email protected]
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 0 8 0 - 7
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1. Introduction
The Gold Butte block is a large exposure of
Precambrian crystalline rock in the south Virgin
Mountains of southeastern Nevada, about 15 km
west of the largely undeformed and £at lying sedimentary rocks of the Colorado Plateau (Fig. 1).
The block is bounded on the east by steeply dipping (50^65³) Phanerozoic sedimentary rocks similar to those of the Grand Canyon, on the north
by a large-displacement tear fault, and on the
west and south by unconformably overlying Tertiary sediments [1]. Structural, petrologic, and
thermobarometric lines of evidence have been
used to argue that the block represents a 15^20
km thick section of Proterozoic crust that was
uplifted, tilted to the east, and exhumed, by
deeply penetrating west-dipping normal faulting,
exposing a thick continuous section through the
mid-crust [1^3]. Evidence for the largely continuous nature of the pre-exhumation crustal section
includes systematic east-to-west changes in: (1)
pressure estimates from a variety of thermobarometers, (2) abundance and size of cumulate phenocrysts in the large Gold Butte granite pluton,
and (3) deformation style, from brittle faulting
through brecciation and mylonitization [3]. Some
workers, however, have used sedimentologic and
erosional evidence to argue that most of the Precambrian rock in the Gold Butte block is £at-lying upper crust that was not strongly tilted or
exhumed from deep levels during Miocene extension [4]. Resolving between these two contrasting
interpretations is of critical importance for at least
two reasons. First, the former interpretation implies that the Gold Butte block is one of the largest intact crustal sections in the Cordillera, and
thus provides valuable insights into the petrologic,
thermal, and structural evolution of the crust in
the region of the Colorado Plateau. The latter
suggests that large regions of crystalline mid-crustal rocks were at shallow levels in the Gold Butte
area prior to Miocene extension. Second, the ¢rst
model implies deep exhumation and high angles
Fig. 1. Location and generalized geologic map, after [3], of the Gold Butte block. Numbers beside (U^Th)/He and 40 Ar/39 Ar
sample locations refer to sample numbers in Tables 1^3. Samples dated by apatite FT by Fitzgerald et al. [4] were collected along
the northern and western margin of the block. The locations of these and the Ar-dated samples have been projected onto the
line of the (U^Th)/He samples in Fig. 2. Because of the potential for north^south tilting of the block, the projected paleodepths
of the FT samples may not be exactly the same as the He samples. `MHWF' is the Million Hills Wash fault that may disrupt
the pre-exhumation structural sequence between samples 18 and 20 (see text for details).
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P.W. Reiners et al. / Earth and Planetary Science Letters 178 (2000) 315^326
of tilting of exhumed crustal blocks, suggesting
large amounts of extension in this portion of the
Basin and Range, while the second model implies
only limited or localized exhumation and extension in this region.
Crystalline rocks in the interior of the Gold
Butte block include metamorphic rocks (primarily
orthogneiss and garnet gneiss), and the intrusive
Gold Butte granite, a large pluton of rapakivi
granite [3,5]. Previous Rb/Sr and U/Pb dating indicates that the gneissic lithologies have ages of
about 1.7^1.8 Ga [6], while the 1.45 Ga Gold
Butte granite is representative of the widespread
1.4^1.5 Ga anorogenic magmatism at this time [7].
More recent magmatism in the lower part of the
block has also been documented by 64^66 Ma
monazite U/Pb ages from a two-mica granite in
the western portion of the block [1,8].
Previous ¢ssion track (FT) studies indicate late
Cretaceous through Miocene cooling in the Gold
Butte block. A single biotite FT analysis by Volborth [5] yielded an age of 84.7 þ 0.2, and zircon
and apatite samples from the central portion analyzed by Parolini et al. [9] yielded FT ages of
84.9 and 27.2 Ma (no errors given) respectively.
In the most detailed thermochronologic study of
the block to date, Fitzgerald et al. [10] showed
that apatite FT ages from samples below about
5 km paleodepth (assuming an overlying sedimentary thickness of 2.5 km prior to exhumation)
were essentially invariant at about 14^17 þ 1^3
Ma, whereas ages of samples towards the east
(decreasing paleodepth) increased to about
50 þ 3 Ma near the unconformity. Fitzgerald et
al.'s work supports previous structural and thermobarometric evidence that, at least at paleo-
Table 1
Muscovite
40
317
depths less than 5 km (which show a fossil apatite
partial annealing zone), Gold Butte is a relatively
intact crustal section that was rapidly exhumed at
about 15^16 Ma.
A number of recent studies have shown that
apatite (U^Th)/He thermochronometry provides
reliable cooling ages through temperatures of
about 70³C [11^20]. Most applications of this
technique have focused on age^elevation relationships in vertical transects of mountain ranges to
constrain the timing and rate of exhumation. This
study instead uses (U^Th)/He thermochronometry
to characterize the timing, rate, and structural
style of exhumation of extremely tilted, now
sub-horizontal, crustal sections. This study also
represents the ¢rst application of the newly developed titanite (U^Th)/He thermochronometer
[21,22], which, according to laboratory di¡usion
measurements, provides cooling ages corresponding to closure temperatures (Tc ) of about 200³C
(for a cooling rate of 10³C/Myr). Together with
apatite He ages and muscovite 40 Ar/39 Ar dates
that provide cooling ages corresponding to about
350^425³C [23], titanite He ages in Gold Butte
provide an important test of the closure temperature of this newly developed chronometer.
We collected and analyzed apatite, titanite, and
muscovite samples from a transect running
through a roughly paleo-vertical section of the
Gold Butte block (as suggested by the continuous
crustal section model) for (U^Th)/He, 40 Ar/39 Ar,
and 207 Pb/206 Pb dating. Age^paleodepth relationships for each of these systems provide long-term
thermal histories throughout di¡erent regions of
the block that are consistent with the model of
Gold Butte as a largely continuous section of Pre-
Ar/39 Ar ages of Gold Butte samples
Sample
Paleodepth
(km)
Total gas age
Plateau age
Intercept age
52-1-89
47-5-89
5-10-88
16-1-88
1-10-88
4.0
12.2
16.0
16.5
17.4
1368.5 þ 11.7
999.8 þ 8.8
88.5 þ 1.2
92.6 þ 1.0
92.7 þ 1.2
1374.4 þ 13.0
994.8 þ 11.4
none
89.8 þ 1.1
none
1362.7 þ 12.9
999.3 þ 9.4
89.9 þ 2.0
92.0 þ 2.6
92.8 þ 3.7
Samples were run in nine step-heating extractions, from 730³C to fusion (above 1280³C).
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P.W. Reiners et al. / Earth and Planetary Science Letters 178 (2000) 315^326
Table 2
(U^Th)/He ages of apatite and titanite from Gold Butte
Sample name
Paleodeptha
(km)
Apatite
He age
þ 2c
Average apatite
He age
Titanite
He age
þ 2c
Average titanite
He age
18.9 þ 1.5
22.5 þ 1.8
16.6 þ 1.3
16.4 þ 1.3
18.6 þ 1.5
26.6 þ 2.1
24.8 þ 2.0
72.5 þ 5.8
74.0 þ 5.9
25.7 þ 2.1
107 þ 8.5
112 þ 8.9
94.8 þ 7.6
139 þ 11
137 þ 11
190 þ 15
194 þ 16
150 þ 12
104 þ 8.3
98PRGB4
14.4
16.6 þ 1.0
18.1 þ 1.1
17.4 þ 1.0
98PRGB6
12.6
14.4 þ 0.9
98PRGB9
10.9
98PRGB12
9.0
98PRGB14
7.6
98PRGB16
6.7
14.3 þ 0.9
14.5 þ 0.9
12.8 þ 0.8
14.5 þ 0.9
14.7 þ 0.9
14.6 þ 0.9
15.4 þ 0.9
17.9 þ 1.1
17.9 þ 1.1
98PRGB15
6.1
12.0 þ 0.7
98PRGB20
4.8
11.1 þ 0.7
13.0 þ 0.8
98PRGB18
3.4
17.2 þ 1.0
16.7 þ 1.0
17.0 þ 1.0
13.6 þ 0.9
14.6 þ 0.9
17.1 þ 1.0
73.2 þ 5.9
138 þ 11
192 þ 15
150 þ 12
a
Pre-exhumation paleodepth is depth beneath basal Tapeats sandstone (distance from crystalline rock Tapeats unconformity corrected for 65³ east dip of the overlying sedimentary rocks) plus an assumed pre-exhumation sedimentary thickness of 2.5 km.
cambrian basement. These data also support the
experimentally derived 200³C (U^Th)/He Tc for
titanite.
2. Samples and methods
To characterize (U^Th)/He age^depth relationships in a steeply dipping pre-exhumation pro¢le,
we collected samples of the Gold Butte granite
from a roughly east^west transect, perpendicular
to the strike of the overlying sedimentary rocks
on the east side of the block (Fig. 1). Many samples from the western and central portions of the
block did not contain apatite or titanite. Samples
collected for muscovite 40 Ar/39 Ar dating were collected from pegmatite dikes and gneissic lithologies in the eastern and western portions of the
block. We show these results along with locations
of samples analyzed for muscovite K/Ar dating
(Fig. 1) by Wasserburg and Lanphere [6].
Muscovite 40 Ar/39 Ar dates were obtained by
step-heating experiments at the University of
Maine (Table 1). Apatite and titanite (U^Th)/He
dates (Table 2) were obtained by standard procedures at Caltech (e.g., [18] for apatite ; [21,22] for
titanite), involving heating of inclusion-free grains
in a resistance furnace and measurement of released 4 He by isotope dilution on a Balzers quadrupole mass spectrometer. 4 He contents of some
titanite grains were also measured by peak height
comparison to standards on a sector mass spectrometer (MAP 215-50). Grains were then recovered, spiked with 230 Th and 235 U, dissolved in
HNO3 , HCl, and/or HF. Th and U isotope ratios
were then measured on a single-collector doublefocussing ICP^MS (Finnigan Element).
Raw apatite ages were corrected for the e¡ects
of alpha-ejection ([24]; FT parameters were 0.68^
0.79), and estimated uncertainties on apatite He
ages are 6% (2c). Titanite ages were not corrected
for alpha-ejection because of uncertainties in orig-
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P.W. Reiners et al. / Earth and Planetary Science Letters 178 (2000) 315^326
inal grain sizes and shapes. We do not expect
these corrections to exceed about 8% however,
based on typical titanite crystal sizes and replicate
analyses of other samples with known ages (primarily Fish Canyon Tu¡ and Mt. Dromedary
specimens [22]). An additional complication with
titanite He ages is the possibility of di¡usive
rounding of He pro¢les within individual crystals
[22]. Crystals with histories involving slow cooling
or prolonged residence at temperatures near the
closure temperature will result in strongly zoned
He concentrations from core to rim. Because our
sample preparation procedure produced irregular
fragments (about 200U500 Wm) of larger titanite
crystals (about 300U800 Wm), it is possible that
fragments with a large fraction of material from
either the original core or rim of a whole crystal
record ages that are di¡erent from those of bulk
grains [22]. This is especially true for samples
from the eastern part of the block, where low
pre-exhumation temperatures and partial He retention would be expected. Nonetheless, based
on the relative sizes of observed whole crystals
and the fragments we selected for analyses, as
well as the reproducibility of ages from individual
samples, we do not expect these e¡ects to exceed
about 8%. Our estimated analytical uncertainty
on titanite He ages of 8% is based on the reproducibility of these and other samples, and is greater than the propagated uncertainty on U, Th, and
He content determinations (V2^3%).
Apatite and titanite U/Pb and Pb/Pb dates (Table 3) were obtained by dissolving and spiking
approximately 0.1 mg of inclusion-free apatite
and titanite grains with 235 U and 205 Pb, and measuring peak intensities on a single-collector, double-focussing ICP^MS. Initial Pb isotopic compositions were assumed to be those of potassium
319
Fig. 2. Ages (left: linear scale; right: log scale) and paleodepths (assuming a 2.5 km thick sedimentary cover dipping
65³, i.e., paleodepth is the distance of the sample from the
unconformity along a perpendicular transect multiplied by
sin[65³], plus 2.5 km) of samples from this study and those
of Fitzgerald et al. [10] and Wasserburg and Lanphere [6].
Dashed lines are isotherms corresponding to the Tc s of apatite and titanite (U^Th)/He and the lower limit of muscovite
40
Ar/39 Ar, assuming a geothermal gradient of 20³C/km and
ambient surface temperature of 10³C. The westernmost, 77
Ma, apatite FT sample of Fitzgerald et al. is not shown
here, due to uncertainties in its structural position. The total
gas ages of the muscovite 40 Ar/39 Ar analyses (Table 2) and
average 207 Pb/206 Pb ages of replicate samples (Table 3) are
shown here. In the linear plot (left side) 2c error bars are
smaller than symbols, except for titanite He ages, which are
shown. Lines through points of the same phase and system
are not formal regressions and are meant as guides for the
eye only.
Table 3
Pb/Pb and U/Pb ages of apatite and titanite from Gold Butte
Sample name
Apatite
98PRGB4
1130
98PRGB18
1389
207
Pb/206 Pb age
Titanite
207
Pb/206 Pb age
1384
1417
1391
1392
Titanite
1457
1439
1435
1467
206
Pb/238 U age
Titanite
207
Pb/235 U age
1450
1452
1439
1459
Estimated analytical errors, based on reproducibility of ages on these and other samples on the element, are 2% and 4% for titanite and apatite Pb/Pb ages, respectively, and about 4% for titanite U/Pb ages.
EPSL 5439 10-5-00
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P.W. Reiners et al. / Earth and Planetary Science Letters 178 (2000) 315^326
feldspar we measured from a sample of the Gold
Butte granite (98PRGB4; 206 Pb/204 Pb = 16.25 and
207
Pb/204 Pb = 15.45) While not as precise as conventional TIMS Pb dates, based on our experience with these and other Precambrian age samples, after correction for mass fractionation,
isobaric interferences, and blanks, we estimate
that the method provides suitably precise 207 Pb/
206
Pb ages (V2% for titanite and V4% for apatite), for samples greater than several hundred
million years old. Titanite U/Pb ages were slightly
more variable (V4%) than titanite Pb/Pb dates
(above). Apatite U/Pb ages were considerably
more variable (in some cases greater than 20%);
this method was not fully evaluated and developed, and these results are not discussed here.
3. Results
Fig. 2 shows all ages as a function of location
in the block, expressed as pre-exhumation paleodepth, with sample locations projected onto our
transect parallel to the strike of the overlying sedimentary rocks. We have assumed a pre-exhumation sedimentary cover of 2.5 km thickness, based
on thicknesses of the stratigraphic section on the
east side of the block (Cambrian through Permian
units [3]), which is similar to sections in adjacent
areas, including the Grand Canyon. Also shown
in Fig. 2 are the apatite FT ages of Fitzgerald et
al. [10] and K/Ar ages of Wasserburg and Lanphere ([6]; ages recalculated using constants from
[25]). The (77 Ma) southwesternmost sample of
Fitzgerald et al. [10] is not shown in Fig. 2 because it was inferred to have been excised from a
higher structural position and transported by motion along the overlying fault that exhumed the
block.
Apatite (U^Th)/He ages from the Gold Butte
block show no consistent relationship with location, averaging 15.2 þ 1.0 Ma (Table 1, Fig. 2). In
detail, the youngest ages (as young as 12.0 þ 0.7
Ma) are found in the central part of the block,
and older ages are found on the western and
eastern edges (17.0 þ 1.0 and 17.4 þ 1.0, respectively). Titanite was not found in samples to the
west of 98PRGB4, nor between 98PRGB4 and
98PRGB12. Replicate analyses of the westernmost titanite bearing sample yielded an average
age of 18.6 þ 1.5 Ma, slightly older than the apatite He ages at the same depth. Titanite in
98PRGB12, in the central portion, yielded an
average age of 25.7 þ 2.1 Ma. Titanites east of
this point become progressively older to the
east, from 73.2 þ 5.9 to 192 þ 16 Ma. An exception
to this eastward-increasing age rule is the easternmost sample, 98PRGB18, with an age of 150 þ 12
Ma, which is younger than the adjacent sample to
the west (Fig. 2). As a whole these apatite and
titanite (U^Th)/He ages are consistent with apatite FT ages (with the exception of the 77 Ma
westernmost sample of Fitzgerald et al.; not
shown here) which average 16.1 þ 1.5 Ma from
estimated paleodepths of 16.5^5 km, then increase
to 50 þ 3.0 Ma from about 5 to 3.5 km (Fig. 2).
Muscovite 40 Ar/39 Ar and K/Ar ages (total gas)
from samples at paleodepths of 18^16 km are
tightly constrained to 91.3 þ 1.3 Ma (Table 2,
Fig. 2). Samples at shallower paleodepths are consistently older, increasing from 1000 þ 8.8,
through 1196 þ 35, and 1385 þ 39 Ma to the
east; the easternmost sample is 1368 Ma (the
1196 and 1385 Ma ages are recalculations of Wasserburg and Lanphere's [6] dates).
Apatite and titanite 207 Pb/206 Pb ages from the
easternmost sample (98PRGB18) are 1389 þ 56
Ma, and 1392 þ 28 Ma, respectively (Table 3),
and are concordant with 40 Ar/39 Ar ages from
samples at comparable paleodepths (Table 2). Titanite U/Pb ages are slightly older, between
1435 þ 57 and 1467 þ 57 Ma. The westernmost
sample (98PRGB4) also yielded titanite Pb/Pb
and U/Pb ages between 1384 þ 55 and 1457 þ 58
Ma, similar to the ages of the easternmost sample.
The apatite Pb/Pb age of the westernmost sample,
however, is 1130 þ 45 Ma, distinctly younger than
any of the other Pb ages.
4. Discussion
Age^distance relationships among the di¡erent
chronometers in this study provide a range of
constraints on the structure of Gold Butte, the
timing and rate of exhumation, and the pre-exhu-
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P.W. Reiners et al. / Earth and Planetary Science Letters 178 (2000) 315^326
mation thermal structure of the block. The ¢rst
important result is that with the possible exception of unexpectedly young titanite He ages in the
easternmost portion of the block, which may represent duplication of section by faulting (as discussed below), all of these data are consistent with
the interpretation of Gold Butte as a single, intact, steeply dipping section of crust that was rapidly exhumed and cooled at 15^16 Ma. The preexhumation vertical thickness of the block is constrained by the correlation between paleodepth
and titanite He age through paleodepths shallower than about 14 km. These age^paleodepth
relationships suggest a continuous depth^temperature increase, which we interpret as a continuous
crustal section, to at least 14 km. Thus the internally consistent age results of each chronometer in
this study and those of Fitzgerald et al. [10]
strongly support the interpretation made by previous workers that the Gold Butte block is a
largely intact, continuous crustal section with paleodepth increasing towards the west [1^3]. This
recognition also provides an important basis for
more detailed thermochronologic interpretations
of the ages.
Generally uniform V15^16 Ma apatite (U^
Th)/He ages throughout the block indicate that
at this time, within less than about 2 Myr (the
standard deviation of apatite He ages in the
block) all the samples in this transect were rapidly
cooled below 70³C. This is consistent with Fitzgerald et al.'s [10] evidence for rapid exhumation
at 16 Ma from the age of the base of the apatite
FT partial annealing zone (PAZ) [10]. Assuming
2.5 km of overlying sediments, a 10³C ambient
surface temperature, and a 20³C/km geothermal
gradient (consistent with the 5 km paleodepth of
Fitzgerald et al.'s apatite FT PAZ), the depth of
the 85³C isotherm that is typically assumed to be
the base of the apatite He partial retention zone
(PRZ [14,18]) would be 3.8 km, slightly deeper
than the inferred paleodepth of the easternmost
sample (3.4 km; Fig. 2). Thus the average
17.4 þ 1.0 Ma age determined for this sample indicates that this pre-exhumation geotherm was at
least 20³C/km. The approximately 2 Myr spread
in apatite ages also provides a crude constraint on
the maximum duration for the cooling that ac-
321
companied exhumation at this time. Assuming
that the approximately 13 km section of paleovertical relief represented by the eastern- and
westernmost apatite samples was exhumed in
2 Myr yields an exhumation rate of 7 mm/yr. This
is only a minimum estimate of the exhumation
rate however, because it relies on the estimated
range of apatite He ages as the temporal constraint (i.e., cooling of the block was too rapid
to produce a non-in¢nite age^paleodepth trend
that could be used to constrain exhumation
rate). Thermochronologic constraints on other
large-o¡set extensional faults in the southwestern
USA have determined slip rates that are similar or
slightly higher than this (3^9 mm/yr; generally 7^
8 mm/yr) [26^31]. Due to the inferred steep dip of
the exhuming normal fault [1^3], in the case of
Gold Butte the exhumation rate was probably
only slightly less than the slip rate.
In detail, apatite He ages do show a variation
between 12.0 þ 0.7 Ma and 17.4 þ 1.0 Ma, with the
youngest ages in the central part of the block.
This may re£ect the topography and sur¢cial
drainage patterns of the block following exhumation and tilting to an approximately 50^65³ eastward dip.
In contrast to apatite, titanite shows a wide
range of (U^Th)/He ages through the section.
The average age of 18.6 þ 1.5 Ma for the westernmost sample is slightly older than that of apatite
throughout the block, but if the single sample
with the 22.5 Ma age is removed, the average
age is 17.3 þ 1.0 Ma, which is indistinguishable
from the apatite He ages. Assuming that the
22.5 Ma age is spurious (possibly because of inclusions of undissolved U- and Th-bearing minerals within the grains), these data indicate that the
temperature at this pre-exhumation depth was
higher than 200³C, the closure temperature of titanite [21,22], consistent with crustal depths greater than 10 km for a geothermal gradient of 20³C/
km.
The 25^27 Ma titanite He ages at 9.3 km paleodepth are consistent with cooling below 200³C at
this depth signi¢cantly before 15^16 Ma. However, rocks at this depth were probably not signi¢cantly cooler than 190³C just prior to exhumation, because : (1) apatite He ages show that at
EPSL 5439 10-5-00
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P.W. Reiners et al. / Earth and Planetary Science Letters 178 (2000) 315^326
least up to 3.4 km paleodepth, rocks were still
hotter than about 85³C, and (2) apatite FT ages
show that at least up to 5 km paleodepth, rocks
were still hotter than about 110³C. Both of these
results are consistent with a 20³C/km geotherm,
which predicts a temperature of 196³C at 9.3 km.
Isothermal holding of titanite at this temperature
prior to 15^16 Ma exhumation would result in
su¤cient He retention to produce an age greater
than 15^16 Ma. For example, Wolf et al. [14]
have shown that all crystals, including those
open to He di¡usion due to residence at temperatures near Tc , will achieve steady state He ages,
teq V(1/15)U(D/a2 )31 . Assuming an activation energy and frequency factor for titanite of 44.6 kcal/
mol and 59 cm2 /s [22], an average grain radius of
200 Wm, and an isothermal holding temperature
of 196³C, the titanite teq is 8.7 Ma. Additional
isothermal holding for 16 Ma at ambient surface
temperatures following exhumation yields an age
of 25 Ma, in good agreement with the observed
titanite ages of 25^27 Ma at 9.3 km paleodepth.
The rapidly increasing titanite He ages above
9 km are consistent with a fossil titanite He PRZ
established some time prior to about 192 Ma. The
upper 9 km of the Gold Butte block has been
cooler than 200³C since about this time, although
it is possible that it was signi¢cantly cooler than
200³C well before then, and was subsequently
heated and cooled again before 192 Ma. The unexpectedly young 150 Ma age of the easternmost
sample is problematic in the interpretation of a
continuous fossil He PRZ. One possibility is
that this sample represents a slightly greater structural depth than the one immediately to the west,
due to motion along an extension of the Million
Hills Wash fault, a fairly large o¡set ( s 1 km),
shallowly dipping, top-to-the-west normal fault
with a southern projection between these two
samples [1,3]. This would mean that sample
98PRGB20 is the structurally shallowest sample
in the He transect. This interpretation is supported by a 15.6 þ 2.3 Ma apatite FT age for
98PRGB18 [32]. Projection of the apatite FT
ages of Fitzgerald et al. to our transect indicate
that samples in a continuous vertical sequence at
the paleodepth of sample 98PRGB18 should have
ages of about 50 Ma. Thus the 15.6 Ma apatite
FT age on our sample also suggests that this easternmost sample in the transect may be out of
structural sequence.
With closure temperatures in the 350^425³C
range [23] muscovite 40 Ar/39 Ar and K/Ar ages
provide higher temperature thermochronologic
constraints than (U^Th)/He or apatite FT ages.
The 1.37^1.39 Ga ages at the top of the section
are consistent with slow regional cooling of the
crust following intrusion of the 1.45 Gold Butte
granite, while the decreasing 1.0 and 1.2 Ga ages
in the next two samples to the west probably re£ect partial Ar retention over long time periods.
Together with the tight cluster of 89^93 Ma ages
towards the western end of the block, this age^
paleodepth relation resembles a fossil PRZ [33]
with a break-in-slope somewhere between 12 and
16 km paleodepth. A 20³C/km geotherm predicts
a 350³C isotherm at about 17 km depth, so 15^16
Ma Ar ages re£ecting zero Ar retention prior to
exhumation would only be expected at greater
crustal depths than observed here. Most importantly, the cluster of 90 Ma ages at the 18^16
km paleodepths clearly represents a cooling event
at this time not recorded by the (U^Th)/He chronometers. It is unlikely that heating from a young
(64^66 Ma) two-mica granite [8] in the west-central part of the block signi¢cantly a¡ected the
muscovite Ar ages, because all three samples are
concordant at 90 Ma, and Parolini et al. [9] also
reported a single zircon FT age from the Gold
Butte townsite (north-central portion of the
block) with a similar age of 85 Ma. Dumitru et
al. [34] also observed late Cretaceous apatite FT
ages from V65^90 Ma in samples from the
Grand Canyon region. Possible causes of this
late Cretaceous cooling include cessation of Sevier
thrusting [35], the end of intense plutonism in the
Sierra Nevada and a regional decrease in geothermal gradients at this time [16,35,36], or erosion
caused by Laramide deformation [34]. In any
case, Ar age^depth relationships also suggest a
structurally continuous block, and indicate a
mid-crustal depth for the western end of the block
from the late Cretaceous through Miocene.
Similar 1389^1392 Ma 207 Pb/206 Pb ages of both
titanite and apatite for the easternmost sample are
also consistent with regional cooling of the crust
EPSL 5439 10-5-00
P.W. Reiners et al. / Earth and Planetary Science Letters 178 (2000) 315^326
following 1.45 Ga plutonism. The approximately
20 Myr age di¡erence between the Pb and Ar ages
from the easternmost samples probably re£ect the
higher closure temperature of the U/Pb system
(approximately 700³C for titanite [37] and 450³C
for apatite [38]). The markedly younger 1130 Ma
apatite 207 Pb/206 Pb age for the westernmost sample probably re£ects signi¢cantly slower regional
cooling at 15 km crustal depths.
Combining U/Pb, Pb/Pb, Ar/Ar, FT, and (U^
Th)/He dates on samples from the eastern and
western ends of the Gold Butte block provides a
detailed time^temperature history for the di¡erent
crustal levels of the block (Fig. 3). Zircon U/Pb [7]
and titanite Pb/Pb dates indicate rapid cooling of
the pluton from magmatic temperatures at 1.45
Ga to temperatures less than that of the titanite
Pb Tc (V700³C [37]) by about 1.4 Ga. Apatite
Pb/Pb ages indicate that rapid cooling of the eastern portion through about 450³C [38] continued
quickly (1.39 Ga), while the western portion did
not cool below the apatite Pb Tc until about 1.13
Ga. The largest di¡erence in eastern and western
ages for the same system is muscovite 40 Ar/39 Ar;
the eastern part of the block cooled through this
blocking temperature (assumed to be the median,
388³C, of the 350^425³C range suggested by
Hodges [23]) by 1.37 Ga, while the western portion did not close with respect to Ar di¡usion
until about 90 Ma. Assuming a pre-Miocene paleodepth of 17 km for the muscovite samples at
the west end of the block, geotherms of 20^25³C/
km predict temperatures from 350 to 435³C at
this depth. The 90 Ma muscovite ages in this portion of the block could therefore represent either a
decrease in the geotherm below 25³C/km at this
time, or a transient heating and cooling cycle
from a 20³C/km geotherm (Fig. 3). Subsequent
to this late Cretaceous cooling event, the only
other cooling event recorded in rocks of the western portion of the block is the 15^16 Ma Miocene
exhumation re£ected in all of the titanite (U^Th)/
He, apatite FT, and apatite (U^Th)/He systems.
The timing of cooling through 200³C is more
di¤cult to ascertain for the eastern portion of the
block, as the location of the top of the titanite He
PRZ is not clear, possibly due to structural complications in the easternmost portion of the block.
323
Fig. 3. Long-term thermal history of Gold Butte, based on
data from all available geo- and thermochronometers. Zircon
U/Pb is 1.45 Ga age of the Gold Butte granite [7]; an arbitrary high Tc of 900³C is assumed for this age. Other Tc s
are assumed to be 700³C for Pb in titanite, 450³C for Pb in
apatite, 350³C for Ar in muscovite, 200³C for He in titanite,
110³C for apatite FTs, and 70³C for He in apatite. The trend
labeled `west end' includes He and Pb ages from sample
98PRGB4, and the average Ar age of the three westernmost
muscovite samples. The trend labeled `east end' includes the
easternmost (50 Ma) apatite FT sample of Fitzgerald et al.
[10], sample 52-1-89 for muscovite Ar age, 98PRGB18 for
Pb ages and the apatite He age, and 98PRGB20 for the titanite He age (due to the anomalously young age in the easternmost titanite). The bottom of the block clearly has a
slower cooling history until about 16 Ma, re£ecting its greater depth. The possible increase in temperature at 90 Ma
could also be modeled as simply a gradual cooling through
the muscovite Ar Tc , but the fact that all three of the westernmost samples yield 90 Ma ages suggests that a signi¢cant
section of the mid-crust closed to Ar di¡usion at this time,
and this may be more consistent with cooling from a higher
geotherm (25³C/km), or regional exhumation, at this time.
The top of the block clearly cooled more quickly than the
bottom, as indicated by the di¡erence in muscovite Ar ages
between the east and west ends. The 192 Ma titanite He age
probably re£ects prolonged residence at a temperature near
160³C (gray line) for some interval between the late Proterozoic and early Tertiary. This could have been caused by either a slightly greater thickness of overlying sediments than
the pre-Miocene 4.8 km, or a higher geothermal gradient of
V30³C/km, rather than the 20³C/km inferred for the more
recent history by the other data.
Assuming the simplest possible cooling path,
judging by the rapid Proterozoic cooling indicated
by Pb and Ar ages and assuming the pre-Miocene
paleodepth of about 5.1 km and inferred geother-
EPSL 5439 10-5-00
324
P.W. Reiners et al. / Earth and Planetary Science Letters 178 (2000) 315^326
mal gradient of 20³C/km, the east end of the
block near sample 98PRGB20 should have been
about 112³C by the late Proterozoic, well below
the titanite He Tc . However, sample 98PRGB20
yields a 192 Ma titanite He age. A modestly increased geotherm (30^33³C/km) or slightly greater
paleodepth (V6 km, rather than 5.1 km) any time
between the late Proterozoic and about 90 Ma
could have caused considerable He loss and decreased the titanite He ages in the eastern portion
of the block. For example, assuming the same
titanite He di¡usion parameters as above, an isothermal holding temperature of 163³C (achievable
by either 5.1 km paleodepth at 30³C/km, or 6.1
km paleodepth at 25³C/km) would generate a teq
of 184 Ma. With an additional 16 Ma of postMiocene holding at 10³C, this would produce a
titanite He age of 200 Ma, not far from the observed age of 98PRGB20. Thus the mid-Jurassic
titanite He ages for the eastern portion of the
Gold Butte block probably re£ect low but partial
He retention at V150^170³C temperatures over
long time periods (any time between V500 Myr
and 1 Gyr) prior to Miocene exhumation, rather
than any speci¢c geologic event.
The 50 Ma apatite FT age for the eastern portion of the block [10] may also be related to isothermal holding at temperatures at which FTs
anneal at slow rates, rather than any particular
cooling event. Alternatively, it could represent
the latest cooling associated with Laramide-age
tectonism, as suggested by other studies of the
region [34]. Laramide uplift near Gold Butte has
been inferred from evidence for a basement high
to the south (e.g., the Kingman uplift of [4,39],
Paleozoic and Mesozoic strata erosionally bevelled southward towards Gold Butte, Eocene
gravel deposits ¢lling northeastward-draining paleocanyons near the edge of the Hualapai plateau
[40], and drainage patterns to the northeast in
southeastern Nevada and northwestern Arizona
[4]). Because of the 15^16 Ma apatite He ages in
the eastern portion of the block however, one ¢rm
constraint we can place on the eastern (shallow
paleodepth) end of the block is that the temperature was above 70³C until the Miocene exhumation event.
Taken as a whole these data suggest that rocks
at middle crustal depths at the western edge of the
Colorado Plateau resided at moderate geothermal
gradients (and slowly declined in temperature)
from 1.4 Ga to about 90 Ma. The 90 Ma event
may re£ect accelerated cooling, a brief heating
and cooling cycle (after which gradients returned
to a 20³C/km prior to rapid exhumation in the
Miocene), or a regional exhumation event as suggested by Dumitru et al. [34]. Similar pre-extensional geothermal gradients of 20^25³C/km have
also been interpreted from thermochronologic
data in other areas of the Basin and Range
[41,42]. The shallower crustal history is more dif¢cult to interpret between 1.4 Ga and 192 Ma,
but these data are consistent with rapid cooling
through the muscovite Ar Tc (350^425³C) following intrusion of the Gold Butte granite by V1380
Ma, followed by residence at temperatures typical
of the upper 5^6 km of crust (V110^160³C)
probably until the Tertiary. A Laramide-age cooling event may have a¡ected this shallow part of
the crust, but the most clearly expressed cooling
event is the Miocene exhumation that rapidly decreased the temperature of this part of the crust to
below 70³C at 15^16 Ma.
5. Conclusions
Systematic spatial variations in U/Pb, Ar/Ar,
FT, and (U^Th)/He ages of samples from Gold
Butte are consistent with the interpretation of the
block as a single, largely intact, steeply dipping
cross-section of crust s 15 km thick. Apatite
and titanite He ages throughout the block indicate that prior to rapid (at least 7 mm/yr) exhumation at about 15^16 Ma, crust just west of the
present-day Colorado Plateau resided at geothermal gradients of about 20^25³C. A cooling event
at 90 Ma a¡ected the mid-crust in this area, represented by samples from s 15 km depth, bringing rock to a temperature between the Tc of Ar in
muscovite and He in titanite (200 to V388³C).
Prior to this event the time^temperature path of
the block is di¤cult to interpret, but taken as a
whole, the He, Ar, and Pb ages from the upper
and lower portions of the block suggest di¡erent
cooling patterns from near magmatic tempera-
EPSL 5439 10-5-00
P.W. Reiners et al. / Earth and Planetary Science Letters 178 (2000) 315^326
tures at 1.45 Ga that re£ect di¡erent pre-exhumation depths. The lower portion of the block did
not cool through 450³C until about 1.13 Ga, and
probably remained at temperatures near 350³C
until about 90 Ma, consistent with a geotherm
of about 20^25³C/km for this pre-exhumation
depth. In contrast, the upper portion of the block
cooled rapidly from near magmatic temperatures
through 350^425³C by 1.37 Ga, consistent with a
much shallower depth. Finally, the agreement between the apparent depth of the fossil He PRZ for
titanite and the inferred depth of the pre-exhumation 200³C isotherm supports the laboratory-derived di¡usion measurements indicating a 200³C
Tc for this chronometer.[RV]
[11]
[12]
[13]
[14]
[15]
[16]
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