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Low long-term erosion rates and extreme continental stability
documented by ancient (U-Th)/He dates
R.M. Flowers*
S.A. Bowring
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139, USA
P.W. Reiners
Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA
ABSTRACT
Zircon and apatite crystals from the western Canadian shield yield (U-Th)/He dates
that are the oldest yet reported for terrestrial rocks. Zircon dates from 1.73 to 1.58 Ga
are consistent with independent geological and thermochronological constraints, and indicate that the rocks were at temperatures ⱕ180 ⴗC and crustal depths ⱕ7–10 km since
ca. 1.7 Ga. Apatite dates from 0.95 to 0.55 Ga suggest residence of rocks at temperatures
ⱕ40–50 ⴗC and crustal depths ⱕ1.5–2 km for ⬃1.0–0.6 b.y., when interpreted using conventionally accepted apatite He diffusion kinetics and considering the proposed effect of
radiation damage on apatite He retentivity. Our analysis implies long-term integrated
unroofing rates of ⱕ2.5 ␮m/yr since ca. 1.7 Ga. These rates are significantly lower than
the long-term rates suggested by previous thermochronological data sets in continental
interiors, but are within the range of short-term erosional estimates. The results are consistent with the extreme stability of this region since the Proterozoic.
Keywords: (U-Th)/He, craton, continental stability, low erosion rates.
INTRODUCTION
Quantification of long-term erosion rates in
continental interiors is essential to constrain
the full spectrum and longevity of continental
erosional regimes, and to gain a first-order understanding of the potential stability of continents. Although high unroofing rates (⬎5 mm/
yr) have been documented in some
tectonically active regions (Zeitler, 1985;
Crowley et al., 2005), erosion rates within
continental interiors are debated, in part due
to the discrepancy between estimates of longterm (107–108 yr) and short-term (105 yr) denudation rates. Conventional studies of cratonic denudation based on geomorphic
observations, as well as short-term estimates
derived from cosmogenic radionuclide dating,
have been used to advocate extremely low
erosion rates, as long as hundreds of millions
of years, consistent with the tectonic and thermal stability of cratons (Nishiizumi et al.,
1991; Braucher et al., 1998, Bierman and Caffee, 2002). However, recent apatite fissiontrack (AFT) thermochronological data within
continental interiors suggest higher integrated
long-term denudation rates (Harman et al.,
1998; Gleadow et al., 2002; Kohn et al., 2002;
Osadetz et al., 2002; Belton et al., 2004).
In order to address this issue, we used
(U-Th)/He zircon (ZHe) and apatite (AHe)
thermochronometry to determine the lowtemperature exhumation history of rocks in
the east Lake Athabasca region of the western
Canadian shield (Figs. 1A, 1B). Zircon has a
(U-Th)/He closure temperature of ⬃180 ⬚C for
typical grain sizes and a cooling rate of ⬃10
⬚C/ m.y. (Reiners et al., 2004), such that ZHe
dates record exhumation through depths of
⬃7–10 km in cratonic regions. He diffusion
in apatite is sensitive to temperatures ⬃70–30
⬚C (Wolf et al., 1998; Farley, 2000), lower
than the range accessible by apatite fissiontrack analysis. AHe thermochronometry is
widely used to constrain the cooling of rocks
as they are exhumed through the upper 1–3
km of crust, but has only recently been applied in cratonic regions (Reiners and Farley,
2001; Lorencak et al., 2004; Soderlund et al.,
2005). We targeted the east Lake Athabasca
region of the western Canadian shield due to
the broad structural coherency of this area
since ca. 1.7 Ga (Hoffman, 1989) and its exceptionally thick lithospheric mantle root (Godey et al., 2004), suggesting that this continental interior has been effectively shielded
from major perturbation since the Proterozoic.
*E-mail: [email protected]; current address: Division of Geological and Planetary Sciences, California Institute of Technology, MS 100-23,
Pasadena, California 91125, USA.
GEOLOGICAL SETTING
The east Lake Athabasca region of the
western Canadian shield contains extensive
(⬎20,000 km2) tracts of granulite facies rocks
metamorphosed in the deep crust (1.0 to ⬎1.5
GPa) (Fig. 1B). Structural and metamorphic
information, combined with U-Pb and 40Ar/
39Ar thermochronological data, constrains the
detailed multistage exhumation of these rocks
from 1.9 to 1.7 Ga (Mahan et al., 2003; Flowers et al., 2006) (Fig. 1C). 40Ar/39Ar muscovite and biotite dates from 1.78 ⫾ 0.01 to 1.72
⫾ 0.01 Ga record final cooling through 350–
300 ⬚C. Athabasca basin clastic sediments unconformably overlie similar rocks to the
south, and postdate the major exhumation
pulses that unroofed the granulites.
The Athabasca basin currently covers an
area of ⬃100,000 km2 and has a maximum
thickness of ⬃2300 m near its center. Noweroded basinal sediments may have extended
farther north over the east Lake Athabasca region in the basin’s later history (Ramaekers,
1981). The timing of Athabasca basin deposition across the granulites is constrained by
detrital zircon grains as young as 1.661 ⫾
0.034 Ga in the upper part of the basin sequence (Rayner et al., 2003), consistent with
ca. 1.65–1.70 Ga U-Pb dates for basinal authigenic phosphate sediments (Cumming et
al., 1987). Basinal fluid inclusion and clay
mineralogy studies, U-Pb and Pb-Pb analyses
of U-rich minerals, and 40Ar/39Ar and K-Ar
investigation of Fe-rich clay paragenesis indicate an early 150 ⬚C diagenetic event at 1.7–
䉷 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]
Geology; November 2006; v. 34; no. 11; p. 925–928; doi: 10.1130/G22670A.1; 1 figure; Data Repository item 2006207.
925
Figure 1. A: Geological map of western Canadian shield showing major tectonic features.
Rectangle marks location of B. Inset shows location of A within North America. P is pressure. B: Simplified map of east Lake Athabasca region showing domains (Sou—southern,
NW—northwestern, Chip—Chipman domains) and sample locations (1—02–76B, 2—03–52,
3—SZ00–141A, 4—SZ00–196C, a—SZ00–196B, b—02–108B, c—02–123A, d—03–194A).
Range of ZHe and AHe dates for each sample is listed. C: Temperature-time path for exhumation of high-pressure granulites in Chipman domain, reconstructed from U-Pb and
40
Ar/39Ar data (Flowers et al., 2006), and new ZHe data. D: Thermal models that satisfy AHe
dates, assuming conventional apatite He diffusion parameters. See text for details. PRZ is
partial retention zone.
1.6 Ga, ⬎150 ⬚C burial diagenesis between
1.6 and 1.45 Ga, fluid remobilization ca. 0.9
Ga, and Phanerozoic fluid incursion at ⬍50
⬚C (Kotzer et al. 1992; Philippe et al., 1993;
Fayek et al., 2002). Continental reconstructions have proposed that the Paleozoic seas
that deposited sedimentary sequences preserved in much of the North American interior
also inundated most of the western Canadian
shield currently lacking Phanerozoic cover, including our area of study (Ziegler, 1989; Scotese and Golonka, 1992).
(U-Th)/He ZIRCON AND APATITE
DATA
Zircon crystals were selected from four
well-characterized populations that were previously imaged using cathodoluminescence
(CL) techniques and dated by U-Pb isotopedilution thermal ionization–mass spectrometry
(ID-TIMS). Past study has demonstrated that
a high level of accumulated radiation damage
in zircon can lead to He loss and anomalously
young apparent ages (Reiners et al., 2004): for
this reason, zircon grains containing relatively
926
low U and Th concentrations (ⱕ70 ppm) were
targeted for analysis. Apatite grains were picked from one population dated by U-Pb IDTIMS, and from three additional samples. All
analyses are single grain fractions corrected
for ejection of 4He from the crystal assuming
a homogeneous distribution of U and Th (Farley et al., 1996; Hourigan et al., 2005). Analytical methods are described in the GSA Data
Repository1, and Table DR1 in the Repository
lists analytical results and sample lithologies.
Of 10 zircon grains from 3 samples, 8
yielded ZHe dates from 1.73 ⫾ 0.08 Ga to
1.58 ⫾ 0.07 Ga (Fig. 1B). Two grains from
one additional sample yielded distinctly younger dates of 1.39 ⫾ 0.07 Ga to 1.33 ⫾ 0.06
Ga. The CL images of zircon crystals from
this latter sample revealed weakly luminescent
overgrowths suggestive of higher U concen1GSA Data Repository item 2006207, analytical
methods, and Table DR1, (analytical results and
sample lithologies), is available online at www.
geosociety.org/pubs/ft2006.htm, or on request from
[email protected] or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
trations. A higher U rim would cause underestimation of the alpha-ejection correction and
may account for the anomalously young dates.
Dates from 0.95 ⫾ 0.05 Ga to 0.55 ⫾ 0.02
Ga (Fig. 1B) were obtained from 12 apatite
crystals from 3 samples. The range of dates in
individual samples is not fully understood, but
may be due in part to alpha-ejection processes
in heterogeneously zoned apatite grains that
redistribute 4He. This is supported by the
range of Th/U ratios in apatite crystals from
two samples, suggesting the presence of multiple apatite populations with different zoning
characteristics and consistent with the polymetamorphic character of the rocks from
which these grains were extracted. A fourth
sample yielded two younger AHe dates of
0.40 ⫾ 0.03 Ga and 0.29 ⫾ 0.03 Ga. This
latter sample was collected in a recently
burned area, and it is possible that it may have
undergone wildfire-induced He loss (Mitchell
and Reiners, 2003), although it is from a different geological domain than the other dated
samples and thus may reflect true regional variability in low-temperature unroofing histories. Additional work would be needed to fully
evaluate this, and we do not consider it further
in this study.
DISCUSSION
Previous Low-Temperature
Thermochronometry in Continental
Interiors
Almost all other investigations aimed at
constraining low temperature histories in cratonic regions have employed AFT thermochronometry. This method relies on analysis
of preserved spontaneous 238U fission tracks,
which, for grains with typical compositions
and cooling rates, are annealed over geological time scales at temperatures ⱖ110 ⬚C and
largely preserved at temperatures ⱕ60 ⬚C.
Early AFT studies in the mid-continent of
North America yielded dates from ca. 0.93–
0.5 Ga (Crowley et al., 1986; Crowley and
Kuhlman, 1988), but recent AFT studies in the
southern Canadian shield and in the Kaapvaal,
Yilgarn, Pilbara, and Brazilian cratons produced dates from ca. 0.4–0.05 Ga that have
been interpreted to reflect increased Phanerozoic unroofing rates (Harman et al., 1998;
Gleadow et al., 2002; Kohn et al., 2002; Osadetz et al., 2002; Belton et al., 2004).
The AHe dates reflect the combined effects
of He loss due to diffusion and He ingrowth
due to radiogenic decay of U and Th, integrated over the cooling history in the He partial retention zone (PRZ), the temperature
range (⬃70–30 ⬚C) over which He is only
partly retained in the crystal (Wolf et al.,
1998; Farley, 2000). The oldest consistent
AHe dates previously reported are ⱕ0.36 Ga
GEOLOGY, November 2006
from the Wyoming craton and southern Canadian and Baltic shields (Reiners and Farley,
2001; Lorencak et al., 2004; Soderlund et al.,
2005).
Recent studies have recognized an inconsistency between AFT and AHe dates in slowly cooled rocks relative to expectations based
on annealing and diffusion characteristics
(Green and Duddy, 2006; Hansen and Reiners,
2006) that may be due to aspects of both systems that are not yet fully understood. Hendriks and Redfield (2005) proposed that the
discrepancy may be due in part to greater rates
of fission-track annealing at low temperatures
than commonly thought, leading to erroneously young AFT dates. However, recent investigations of He diffusion kinetics in apatite,
and previous study of Durango apatite, suggest that He retentivity increases in apatite due
to accumulation of radiation damage, leading
to AHe dates that are anomalously old in some
situations (Farley, 2000; Shuster et al., 2006).
We first use conventional apatite He diffusivity characteristics to interpret our apatite data,
and then consider the potential effects of radiation damage on this interpretation. Future
acquisition of AFT data in the east Lake Athabasca region may provide further insight into
its low-temperature evolution.
Low-Temperature Evolution of the East
Lake Athabasca Region
Our ZHe dates are the oldest yet reported.
The 1.73 ⫾ 0.08 Ga to 1.58 ⫾ 0.07 Ga dates
are interpreted to record the timing of cooling
to ⬍180 ⬚C during exhumation. These results
are consistent with the record of cooling to
⬍300 ⬚C constrained by 1.78 ⫾ 0.01 Ga to
1.72 ⫾ 0.01 Ga 40Ar/39Ar mica dates, and
subsequent deposition of ca. 1.70 to ca. 1.65
Ga Athabasca basin sediments across the region (Fig. 1C). The new data require that
rocks in the east Lake Athabasca region were
at depths ⬍7–10 km for the last ⬃1.7 b.y.
Our AHe dates from 0.95 ⫾ 0.05 Ga to 0.55
⫾ 0.02 Ga are the oldest reported for terrestrial rocks. Some of the variability in dates
between samples may have geological significance that reflects subtle lateral variability in
thermal regimes and/or vertical displacements
by late brittle faulting. Poor reproducibility of
AHe dates is also anticipated in slowly cooled
terranes, where small variations in temperature, apatite He retentivity, and grain size may
induce large differences in the integrated AHe
dates (e.g., Reiners and Farley, 2001). Regardless, our results are consistently older
than the ca. 0.36 Ga and younger dates previously obtained in cratonic regions.
We consider geological constraints, assume
the conventionally accepted diffusion parameters of Durango apatite (Farley, 2000), use an
GEOLOGY, November 2006
apatite half-width of 50 ␮m, and apply a standard He production-diffusion model using the
HeFTy program (Ketcham, 2005) to constrain
feasible thermal histories. The temperaturetime (T-t) paths can be used to estimate nearsurface burial and exhumation histories using
a typical cratonic geothermal gradient of 20
⬚C/km and mean surface temperature of 10 ⬚C.
Four model T-t paths are depicted in Figure
1D that can generate the observed AHe dates,
although a restricted set of similar histories is
possible for each path depending on the timing and magnitude of each exhumation or
burial pulse. The simulations are: (1) singlephase monotonic cooling and unroofing at
moderate rates through the AHe PRZ at 1.0–
0.6 Ga followed by extended residence at
near-surface conditions (path 1); (2) long-term
residence (1.7–1.6 b.y.) at temperatures of
⬃30 ⬚C and depths of ⬃1.0 km followed by
Phanerozoic unroofing (path 2); (3) episodic
unroofing through the AHe PRZ synchronous
with basinal fluid circulation events (path 3);
and (4) early unroofing to near-surface conditions, with subsequent burial beneath as
much as 1.5–2 km of Phanerozoic sediments
at maximum temperatures of ⬃40–45 ⬚C
(path 4).
The key result of this analysis is that all
simulations require that the apatite grains resided at temperatures ⱕ40–45 ⬚C and crustal
depths ⱕ1.5–2 km for the last 1.0–0.6 b.y.
Higher temperatures during this time interval
would yield AHe dates younger than those observed. Additional constraints on early or recent exhumation and burial events would allow better discrimination between the models
by narrowing the suite of feasible T-t paths for
portions of the history for which no such further information is available. For example, in
T-t path 4, the preservation of ancient AHe
dates despite a hypothetical period of younger
sedimentary accumulation requires an earlier
history characterized by much lower temperatures than those in T-t path 3, with residence
at crustal depths ⬍0.5 km for prolonged periods of the Proterozoic (Fig. 1D).
Consideration of the potential effect of radiation damage on apatite He retentivity does
not change the primary conclusions we derive
from our data set regarding long-term residence at crustal depths ⱕ2 km. We evaluate
the T-t paths using the recently proposed
radiation-damage trapping model, a production-diffusion model that incorporates He diffusion kinetics that evolve with He concentration due to increasing radiation damage
(Shuster et al., 2006). This analysis predicts
that the oldest apatite grains remained at temperatures ⱕ50 ⬚C and crustal depths ⱕ2 km
for at least 0.6 b.y. since 1.7 Ga, although a
short interval (0.05–0.1 b.y.) of burial by as
much as ⬃3 km of Phanerozoic sediments is
permissible.
Implications for Long-Term Erosion Rates
and Continental Stability
The antiquity of our AHe dates suggests
that rocks in the east Lake Athabasca region
remained at temperatures ⱕ40–50 ⬚C, equivalent to the upper 1.5–2 km of the crust, for
at least 1.0–0.6 b.y. These constraints imply
low long-term (107–108 yr) integrated unroofing rates of ⱕ2–2.5 ␮m/yr since 1.7 Ga. Previous AFT and AHe studies typically suggest
minimum long-term rates ⱖ5 ␮m/yr; most
data imply rates ⱖ10 ␮m/yr. In contrast, cosmogenic studies in continental interiors that
rely on the in situ buildup of cosmogenic nuclides such as 10Be and 26Al on bedrock surfaces yield short-term (105 yr) erosional estimates of 0.3–5.7 ␮m/yr (Nishiizumi et al.,
1991; Braucher et al., 1998; Bierman and Caffee, 2002; Belton et al., 2004). Thus, our new
data imply rates significantly lower than those
indicated by previous thermochronological
data sets in cratonic regions, but consistent
with short-term erosional estimates.
The preservation of ancient continental
crust through billions of years of Earth history
has long served as qualitative testimony to the
extended stability of portions of the continents. These regions owe their survival to a
cratonic architecture characterized by cold,
thick (⬎250 km), chemically depleted lithospheric mantle roots that have shielded the
crust from subsequent plate margin processes
and underlying asthenospheric mantle activity
(Jordan, 1978). Deciphering low-temperature
histories may constrain episodes of lowamplitude burial and exhumation for which
there is little geological evidence, and thereby
identify potentially subtle tectonic or mantle
perturbations to cratonic stability. Our ancient
(U-Th)/He data from the western Canadian
shield suggest that low denudation rates are
possible in quiescent continental interiors for
hundreds of millions of years. The results are
consistent with the first-order tectonic and
thermal stability of the cratonic cores of some
continents.
ACKNOWLEDGMENTS
This work was supported by a National Science
Foundation (NSF) graduate fellowship to Flowers
and NSF grant EAR-0310215 to Bowring. We thank
Kevin Burke for helpful discussions, Stefan Nicolescu for analytical assistance, and Barry Kohn and
Andrew Gleadow for thorough reviews that improved the manuscript.
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Manuscript received 8 February 2006
Revised manuscript received 12 June 2006
Manuscript accepted 16 June 2006
Printed in USA
GEOLOGY, November 2006