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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.
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
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
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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
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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. REFERENCES CITED 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 in-
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