Heat flow and heat generation in the superior province of the canadian shield

Tectonophysics, 50 (1978) 55-77 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

HEAT FLOW AND HEAT GENERATION OF THE CANADIAN SHIELD *

ALAN M. JESSOP

and TREVOR

IN THE SUPERIOR

55

PROVINCE

LEWIS

Division of Seismology and Geothermal Studies, Earth Physics Bmnch, Department of Energy, Mines and Resources, Ottawa KlA OY3 Ont. (Canada) (Received January 26,1978,

accepted for publication March 3, 1978)

ABSTRACT Jessop, A.M. and Lewis, T., 1978. Heat flow and heat generation in the Superior Province of the Canadian Shield. Tectonophysics, 50: 55-77. Nine new heat flow determinations and several measurements of radioactive heat generation are presented for the Superior Province. The average value of twenty-one heat flows now published for the Superior, corrected for Pleistocene glaciation, is 40 f 8 mW/m2. Heat generation values are low generally less than 3 I.tWlm3. Although individual values of the ratio of thorium to uranium vary considerably, the geometrical average of four is lower than results from other Archean rocks. A linear relation between the heat flow and radioactive heat generation may exist. The reduced heat flow, 21 mW/m2, and the characteristic depth, 14 km, from this relation are quite different from other heat flow provinces. Since large thicknesses of the crust have been eroded away and since the original heat generation was much larger than the values measured now, a linear relation equivalent to those found in younger heat flow provinces is not expected. To account for the large differences in heat flow and heat generation observed in different Archean shields an Archean crustal model is proposed which includes thin (2-4 km) radioactive surface veneers over some areas. The thermal parameters of a young crust may well determine whether or not it will survive. Since Archean times the heat flow of each newly stabilized region has been a constant, and since the time of formation or last orogeny the heat flow in each province has steadily decreased. The geothermal gradients in Archean crust have decreased the most, causing significant underplating, and increasing the strength of the crust. INTRODUCTION

The Superior Province, Archean in age, is the central part of the Canadian Shield. Ever since its formation the amount of heat produced by radioactive decay has been decreasing exponentially. The lithospheric heat flow should reflect the declining heat generation, giving the best possible indication of heat flow from the mantle beneath a stable continental crust. The results from this province present an excellent opportunity to test the concept of * Contribution

from the Earth Physics Branch No. 733.

56

heat flow provinces in a Precambrian metamorphic terrain. The exposed Superior Province extends over 1.6 . lo6 km2, and at least a further 0.5 * lo6 km’ is covered by undisturbed Palaeozoic sediments. The Province contains a wide variety of rocks, including plutonic rocks, greenstone belts containing high proportions of volcanic rocks, and metamorphic rocks of all grades. Stockwell et al. (1970) have identified, on the basis of individual tectonic features, thirteen subprovinces in addition to the large northeastern subprovince. Heat flow measurements are well distributed through these subprovinces; only one major unit in the far northwest does not contain at least one site. Isotopic age determinations throughout the Superior Province show a mean age of 2480 . lo6 years, and the Kenoran orogeny, the most significant metamorphic event, is fixed by these results (Stockwell et al., 19’70). Some of the many recent determinations (e.g., Krogh et al., 1976), are revealing significantly older dates, up to 3550 Ma in some regions (Morey and Sims, 1976). Evidence of pre-Ordovician palaeoplains shows that the shield was stabilized and in its present state by early Palaeozoic time (Ambrose, 1972). Unfortunately age determinations are not available for many of the individual heat flow sites in the Superior Province. In this paper new measurements of heat flow and heat generation are collated with published values, and these measurement are used to study the Archean crust. The presen~tion of the rne~~ernen~ is followed by discussions relating the concept of heat flow province to old crust with its original radioactive heat generation. A linear relation between heat flow and heat production is found in the Superior Province, and its possible significance is discussed. Finally crustal temperatures are calculated for the Superior Province. HEAT FLOW MEASUREMENTS Twenty-one heat flow values in the Superior Province are now available; nine new values are summarized in Table I, and twelve previously published values are shown in Table II. The results are notable for their uniformity. The average of sixteen values corrected for Pleistocene glaciation is 39 mW/m*, with a standard deviation of 8 mW/m2. The average value is 40 4 8 mWfm2 using approximate glacial corrections for uncorrected published results. Omitting the highest two and lowest two values gives the same average and reduces the standard deviation to 3 mW/m*. Half of the sixteen values lie within the narrow range of 40-44 mW/m2, and the median heat flow is 42 mW/m2. The distribution of heat flow data is shown in Fig. 1. All the new results are reliable, being based on good data and sufficient lengths of borehole, as indicated in Table I. Measurement of temperature to an accuracy of O.Ol”C and conductivity by means of the divided bar to an accuracy of 3% is standard throughout. Correction for Pleistocene glaciation has been applied to all new tem~ratures using a standard model (Jessop,

81O18.2’ 81O23.3’ 81°20 3’ 81°21; 74O21 0’ 94Y2;

85O50.9’ 85O48.6’ 85’50.9’ 85’50.9’ 58’50.9’ 85’50 9’ 85’51; 78’26.5’

46O39’ 46O37 7’ 46O39; 46O39’ 49O53 1’ 51°00i

49’10.6’ 49O11.2’ 49Oll.l’ 4g011.0’ 49’10.8’ 49’10 8’ 49Yli 48O38.9’

lOO80-

470 400

150-

305 366

80- 884 300-1279 150- 869 170- 892 200714 lOO- 629

410 428 436 421 430 421

742 311

200-1840 50- 990 150-1680

150-610 200-606 150-605 150-612

Depth Interval (m)

365 340 360

280 340 395 420

Elevation (m)

59

14

54 34 49 49 35 36

23 29

108 64 98

52 58 59

10.7

11.8 13.1 11.7 11.8 11.6 11.9

9.1 11.9

14.5 16.9 16.2

14.2 9.1 14.4 13.5

28

3.66

3.33 3.04 3.28 3.27 3.46 3.15

3.40 4.09

23 29 64 64 56 58 45 45

3.14 3.13 2.85

2.91 2.69 3.00 3.02

53 62 59

55 56 58 60

average (W/mK)

number

number

gradient (mK/m)

Conductivity

Temperature

30 34 30 32 33 27 31 31

41 45 42 43 23 37

35 20 35 34

Heat flow -measured (mW/m2)

36 38 36 38 39 33 37 40

45 51 46 47 30 44

41 25 42 40

corrected (mW/m2)

0.5

0.5 0.3 0.4 0.3 0.9 0.6

5 5 8 5 8 8 5 8

5 5 5 5 5 8

0.2 0.2 0.4 0.4 0.6

5 8 5 5

0.3 0.6 0.3 0.2

Probable error (%) ___ (a) (b)

Limits of probable error are reported in two ways: (a) as the standard deviation of the least squares calculation by the Bullard method, and (b) as a realistic estimate based on the reliability of the raw data and the variation of heat flow within the column.

86’58.5’ 89O35.9’ 9OO28.8’ 91O19.3’

48’51.5’ 51O49.5’ 50’42.7’ 49O38.6’

Jackfish Otoskwin River Minchin Lake English River Sudbury Basin Moose Lake - a Onaping Moose Lake - b average value Merrill Island Red Lake Manitouwadge 269 S-158 273 274 309 306 average value Launay

Longitude

Latitude

Name

New heat flow data

TABLE I

80°02’ 78*09’ 79O44) 81°20’ 8Z038’ 8OO56.6’ 82O22.8’ 83O32.1’ 97Oo7.9’ 77O41.0’ 93O43 1’ 7g”o3;

Longitude (OWi

250 230 260 232 3 -

-

330 280

36

42 29 37 31 50 39 27 46 29 25

Heat flow measured (mW/m* )

___..___l”___ Elevation (ml

(43) (35) (441 (36) (60) 43 33 52 38 26 49 42

corrected (mW/m2 )

infor-

Misener et al. (1951) Misener et al. (1951) Misener et al. (1951) Misener et al. (1951) Sass et al. (1968) Cermak and Jessop (1971) Cermak and Jessop (197 1) Cermak and Jessop (1971) Jessop and Judge (1971) Jessop and Judge (1971) AIIis and Garland (1976) Lewis and Beck (1977) -----_ _ -. .

Reference

----_-.^._--.---__I-_____

Figures in brackets represent corrections for Pleistocene climatic disturbance, calculated by the present authors from incomplete mation.

48OlO’ 48OO8’ 48OO6’ 4s030’ 46O26’ 49OO6.2’ 49O25.0’ 49O41.4’ 49O48.7’ 55’23.7’ 49O40 9’ 4B021i

-

Kirkland Lake Malartic Larder Lake Timmins Elliot Lake (7 holes) Cochrane Kapuskasing Hearst Winnipeg Nielsen Island Experimental Lakes (11 sites) Lake Dufault (70 holes)

.____

-

Latitude (ONI

Name

Previous heat flow data

TABLE II

59

Fig. 1. The distribution of heat flow within the Superior Province of the Canadian shield. Open symbols represent old data points for which assumptions were made in order to correct the measured values for glacial disturbances. The exposed part and the boundary of the covered parts of the province are indicated by shading and a dotted line.

1971). Heat flow has been calculated by means of the Bullard method throughout. The ~mperature gradients shown in Table I are the mean of gradients calculated for successive overlapping sections. The conductivity shown is the arithmetic mean of the results within the depth interval quoted. The product of gradient and conductivity as shown in Table I will not always be exactly equal to the calculated heat flow. Individual features of the sites have required special treatment as follows. Jackfish: A topographic correction was calculated for this site, but the result was negligible. Otoskwin River: A correction of 1 mW/m* was added to compensate for the sudden deposition of 60 m of moraine material at the close of the most recent glacial period. Minchin Lake: A correction was calculated for the proximity of several lakes, but the result was negligible. With English River, these three constitute the last four holes to be drilled in Canada as part of the Upper Mantle project. The locations were chosen to give the greatest benefit in the interpretation of heat flow in terms of other geological and geophysical parameters, and they were fully cored to give the best conductivity samples and complete lithological data. They were also sited to ensure the minimum disturbance due to miner~ization, fault zones, water movement, etc. Sudbury Basin: Several holes have been measured in the Sudbury Basin.

60

The three reported here are on the northwest margin near the contact with rocks of the Superior Province. Merrill Island: Corrections for topography and inclination of the hole were required for this site. Manitouwadge: Six deep, fully cored holes were available at this site, and each one gave a good value for heat flow. There was good agreement except for one hole, which gave a result 10% below the mean, Since the product of the average gradient and the average conductivity gave a result much closer to the mean than did the Bullard calculation, the difference was probably due to scatter in the data rather than to any real difference in heat flow. HEAT PRODUCTION

MEASUREMENTS

The heat production of representative cores from most boreholes was measured in the laboratory by y-ray analysis. Measurements were also made on surface samples, especially where geological structures indicated that the core might not be representative of the underlying crust. Samples measured were granitic or highly metamorphosed rocks such as gneisses: results from volcanics and unmetamorphosed sediments were never used. The extent to which values from surface samples measured by laboratory and portable instruments, were used is indicated below in comments on individual sites. The result listed first in Table III for each site is considered to be the most representative of that site. The technique of measurement has been described by Lewis (1974). Since that time a sample size of 330 g has been adopted as standard, and the absorption of background radiation is assumed to be constant. Composite samples were used only once when the individual samples were too small. The results are shown in Table III. The limits shown are the standard deviations, assuming at any one site that the measured concentrations are normally distributed. These standard deviations are much larger than the uncertainty in individual measurements. Measurements of uranium, thorium and potassium concentrations published previously vary as much as the new data. The lower six lines of Table III are previously published data for relevant sites in the Superior Province. Airborne y-ray surveys have crossed parts of the Superior Province, but such surveys are better suited to the detection of strongly anomalous areas rather than to the detailed measurements required for the present purpose (Richardson et al., 1975). Concentrations of radioactive elements in widely spaced surface outcrops were also measured. Results are included in Table IV with previously published average values for the Canadian Shield. These determinations verify the generally low levels found in shield areas, and the large lateral variations. The geometrical average values for surface samples along the surface traverses are higher than similar values for borehole cores, but the average values of Th/U and K/U are very similar. The result for K/U is similar to previous determinations. The average value for Th/U is 4, similar to results from

iB S’ B B 1 1 1 2 3 4

P P S B B S B S S

*

-

41 41 8 17 13 4 11 3 5 6 4 7 20 19

13 15 51 12 17 116

No. of samples

1.2

6.2 7.0 3.9 4.9 0.67 0.79 2.1 0.95 3.8 0.10 1.4 1.28 2.7 2.7 2.5 0.6 3.0 2.9

0.79 1.71 3.4 0.32 1.26 4.9

0.22 0.54 1.6 0.20 0.66 2.4

+ 2.7 t 5.9 + 2.9 t4.2 + 0.13 * 0.10 + 1.3 + 0.14 + 2.7 * 0.03 +0.3 % 0.54 f 3.2 + 2.2

+ + f + + f

U (ppm)

4.4

4.6 21.8 18. 21. 9.13 3.46 10.0 39. 17.6 0.26 4.5 5.1 6.0 5.8 6.5 2.5 11.3 10.3

14.0 5.2 4.8 0.60 5.4 9.1

5.4 1.2 1.6 0.25 3.9 5.3

+ 2.2 ? 9.2 + 13. + 14. + 0.82 + 0.88 + 7.5 + 22. + 6.4 + 0.07 + 1.1 f 4.8 + 2.0 f 1.4

Y! + + + * f

Th (ppm)

1.5

0.56 2.90 3.8 3.3 4.35 1.06 1.7 3.85 3.91 0.21 1.4 2.42 1.8 1.72 2.5 1.2 2.8 2.7

2.19 1.95 1.4 0.52 1.46 1.7 t 0.28 f 0.85 + 1.4 + 1.1 + 0.84 f 0.58 + 1.1 f 0.59 + 0.93 Y!Z 0.13 t 0.4 f 1.3 + 1.1 + 0.90

t 0.54 + 0.47 It 0.4 + 0.13 + 0.86 + 1.1

K (%I

3.8

0.7 3.1 4.7 4.2 14. 4.4 4.7 41. 4.7 2.6 3.2 4.0 2.3 2.1 2.6 4.2 3.8 3.6

18. 3.0 1.4 1.9 4.3 1.8

l-h/U

1.3

0.09 0.41 0.96 0.66 6.5 1.34 0.79 4.05 1.03 2.1 1.0 1.89 0.66 0.63 1.0 2.0 0.93 0.93

2.77 1.14 0.41 1.63 1.16 0.35

(x 104)

K/U

2.01 3.7 2.7 3.1 1.25 0.55 1.43 3.4 2.6 0.07 0.83 0.94 1.3 1.31 1.3 0.46 1.8 1.76 0.61 1.3

1.42 1.01 1.41 0.18 0.86 2.1

samples;

+ 0.89 f 2.3 f 1.8 + 2.2 ? 0.17 zt 0.09 + 0.98 + 1.7 + 1.3 * 0.03 f 0.20 + 0.61 f 1.1 + 0.77 f 0.2 + 0.08 to.3

f 0.50 + 0.28 + 0.6 f 0.08 f 0.53 -I 1.3

Heat production (uW/m3 )

* B = borehole samples; S = surface samples; P = portable gamma-ray spectrometer measurements on outcrops; $ = composite 1 = Cermak and Jessop (1971); 2 = Sass et al. (1968); 3 = Ingham and Keevil (1951); 4 = Ailis and Garland (1976). The result appearing first in the table is taken to represent the upper crust, as explained in the text.

Geometrical average

Cochrane Kapuskasing Hearst Elliot Lake Bourlamaque Experimental Lakes

Nielsen Island Lake Dufault Sudbury northwest of Windy Lake west of Wanapitei Lake Merrill Island Red Lake Manitouwadge

English River north of collar south of and near collar

Otoskwin River Minchin Lake

Winnipeg Jackfiih

Site

Measured heat production at sites in the superior province

TABLE III

z

References:

1 = Shaw (1967);

granulite facies or lower facies

average

New Quebec New Quebec Hornblende Amphibolite

Geometrical

Bay

7.0 9.6

6.8

9.3

11.3 5.1 5.5 12.0 21. 7.4 10.2

* 8.7 + 4.3 f 3.5 ? 9.0 +12. f 5.6 ? 9.0

Th (ppm)

of surficial

2 = Fahrig et al. (1967).

0.7 1.3

2.0

2.13

+ 2.8 +0.5 + 1.0 ?I 2.5 + 2.7 It 0.59 + 2.3

3.1 1.1 1.6 3.3 4.4 0.94 2.7

concentrations

North Bay to White R. White R. to Manitouwadge Manitouwadge to Terrace Terrace Bay to Ignace Ignace to Valora Valora to Pickle Crow Ignace to Kenora

and Potassium

U (ppm)

Thorium

IV

Region

Uranium,

TABLE

10.0 7.4

4.4

3.7 4.6 3.4 3.6 4.8 7.9 3.8

2.01 2.4

1.48 1.23 1.42 0.96 0.33 1.16 1.69

3.4

+ f + ? + + -I

Th/U

2.19

2.87

2.80 2.35 2.76 3.38 4.26 1.90 3.19

K(s)

samples

2.9 1.9

1.1

1.35

0.9 1.3

1.2

1.9 0.9 1.1 2.1 3.1 1.0 1.8

__~

(uW/m3)

(X 104) 0.9 2.1 1.7 1.0 1.0 2.0 1.2

Heat production

K/U

2 2

1

Reference

63

Australia (Bunker et al., 1975), but it is quite different from values of S-15 reported by Lambert (1976) for other analyzed Archean rocks. Although our samples have not been studied ~tro~aphic~ly, the areal distribution and the large number of individual samples ensure that they represent a substantial part of the Archean crust of the Superior. Eight of the borehole sites were chosen to be typical of the region in which they were drilled, but boreholes at the other sites were drilled for mineral exploration purposes. The variation in the value of Th/U at these eight sites is probably related to the degree of metamorphism. Average concentrations of the radioactive elements in Archean rocks of the Ukrainian Shield are similar to those reported here. Belevtsev and Komarov (1975) found that the uranium content decreases with increasing metamorphic grade, and that the ratio Th/U increases from 4 to about 13. The English River site presented a problem in determin~g the value of heat generation most representative of the crust beneath it. The heat generation of the core is 3.1 ,uW/m3, the highest value measured. Surficial samples from near the collar and to the south show similar high heat generation, with an average potassium content of 4.26 Lf:0.33%. G.F. West (personal communication) has also found high values in this area. A sheetlike intrusive with a depth of 2 km lies to the south of this hole, as determined by interpretation of gravity surveys (Szewczyk and West, 1976). A more representative result comes from north of the site where the value is 2.01 pW/m3. At Sudbury samples from the cores were chosen from sections of Superior Province rocks from below the irruptive, giving a heat generation of 1.38 ~W/rn3, Surface samples from northwest and northeast of the basin produce 3.4 and 2.6 ~W/m3, respectively. Since Charbonneau (1972) has discovered a granitic body having a high radioactive element content cIose to the northeast flank of the basin, the deep borehole samples are regarded as more likely to represent the general upper crust. The average heat generation from a different, small suite of surface samples from north of the basin was 1.5 @W/m3, similar to that obtained from the borehole samples. However, individu~ samples vary by a factor of 10, and the suite is not necessarily representative of the crust. HEAT FLOW PROVINCES

The concept of heat flow provinces has been developed from a study of the relation between heat flow and heat generation in Phanerozoic intrusive rocks (Birch et al., 1968; Roy et al., 1968). The linear relationship between heat flow Q and heat generation A Q=Qo+Ab (1) yields a reduced heat flow Q0 and a characteristic depth b, which are uniform within certain areas known as “heat flow provinces”. The constant b, with the dimensions of length, is less than normal crustal thickness. The uni-

64 TABLE V Parameters of heat flow provinces Province

$W,m2)

.~ Eastern U.S.A. Basin and Range Sierra Nevada Au&r. Shield Yilgarn Block Central Austr. Indian Shield Superior Prov. Combined data Superior Prov. Superior Prov. *

..~

15 12 6 4 9 10 6 3 4 11 11

-_ __~._...

Reference

n

33i- 1 59 +_4 17% 1 26 30 27 39 21 34 21 + 1 28f 1

--

pkm) 7.5 9.4 10.1 4.5 3.0 11.1 14.8 13.9 7.1 14.4 13.6 -___

-* 0.2 +_1.3 * 0.1

c 0.5 t 0.6

-~.

Roy et al. (1968) Roy et al. (1968) Roy et al. (1968) Jaeger (1970) Sass et al. (1976) Sass et al. (1976) Rao et al. (1976) Cermak and Jessop (1971) Rao and Jessop (1975) ~-

* This result is devised from data that are corrected for Pleistocene climatic changes. All other results in this table do not include this correction.

form heat flow coming from below an upper layer of varied heat generation throughout the heat flow province is related to QO. Various models such as a step model, an exponential model and a linear model can satisfy this relation (Lachenbruch, 1968). Three large heat flow provinces from the United States are listed at the top of Table V, where the figures are taken from Roy et al. (1968) with conversion into SI. The original measurements that define the heat flow provinces were made on large plutons, but measurements in the Superior Province are not associated with large homogeneous bodies, and at many sites it is the heat production of metamorphic rocks that best represents the exposed crust. In some regions the amount of erosion that has taken place since Kenoran time approaches the value of b of 7.5 km that is found for the eastern U.S.A. Under such conditions it is necessary to consider the crust as a whole, and it is not clear that a linear relationship between heat flow and heat generation is to be expected. Even if the relationship is conclusively demonstrated by the data, the underlying cause may not be the same as in Phanerozoic areas. Cermak and Jessop (1971) found that three good heat flow holes in the metamorphic rocks of the Superior Province defined a straight line that was different from the United States lines, but the small number of data points meant that the result was vulnerable to local anomalies and was perhaps of low statistical significance. Rao and Jessop (1975), in a review of all geothermal data in shields, found that the addition of one point from the Churchill Province (Sass et al., 1971) and two points from rocks of Grenville age in the northern Appalachians (Roy et al., 1968) produced an ambiguity between two possible lines, depending on which points were regarded as

65

Fig. 2. Heat flow, Q, and heat generation, A, of the Superior Province of the Canadian Shield. Heat flow is not corrected for Pleistocene glaciation. Open circles indicate data that were omitted when fitting the straight line to the data by the least squares method. The dashed line is the observed relation for the eastern United States heat flow’province. The ellipses indicate limits of probable error (see Tables I and III).

anomalous. One line was close to the line of Cermak and Jessop (1971) and the other was similar to the line for the eastern United States. The second was chosen as the more probable, and the low heat flow in the Kapuskasing area was interpreted as an anomaly, related to the gravity anomaly of the same name. Details are shown in Table V. Results from crustal rocks having such different ages should not be combined. The results of the present study indicate that a line can be drawn through the plot of heat flow against heat production with standard errors of slope and intercept that are better than those for the Basin and Range Province and comparable with the other established provinces. These statistical parameters are calculated from the accepted data and do not take into account the limits of error of the individual points, which are indicated by ellipses in Fig. 2. The line for the Superior Province defined by eleven data points is very close to the original line of Cermak and Jessop (1971). The plot of heat flow, uncorrected for Pleistocene glaciation, against heat production is shown in Fig. 2. Uncorrected values were used for comparison with previous work since all the earlier data were used in the uncorrected form. It is probable that the corrections required for the data from the unglaciated United States are smaller than those needed for the Canadian data, and so this comparison is not entirely valid. The fully corrected Canadian data are repeated in a similar format in Fig. 3 and both sets of derived parameters are shown in Table V.

Fig. 3. Heat flow, Q, corrected for Pleistocene glaciation, and heat generation, A, in the Superior Province of the Canadian Shield. Open circles indicate data that were omitted from the calculation fitting the best line, and the triangle indicates data of Allis and Garland (1976). The dashed line is from the eastern United States heat flow province.

RADIOACTIVE

LEVELS

OF EARLIER

TIMES

Once the upper crystalline crust has cooled sufficiently to retain an apparent age, it may be considered to be a closed system as far as radioactive trace elements are concerned. At greater depths migration of heavy elements may continue for a longer period. Although the heat flow from the mantle may be unknown, the heat produced by the upper crust can be calculated for any time after the last cooling. The Superior Province has been deeply eroded and the heat generation has had time to decrease significantly by the process of natural decay. As a result the measured heat flow and heat generation are low and the line defining the parameters of the present heat flow province is not well defined. Table VI shows the factors by which heat generation has decreased as a function of metamorphic age, calculated from the isotopic abundances and decay constants given by Wetherill(l966). The heat generation due to potassium has decreased by a factor of almost four since Kenoran time. These factors clearly show that a distinction should be made between younger and older shields. If we assume that the concentration of each heat producing element

61 TABLE VI Relative changes in original concentrations of age 238~

Age

235~

U*

and radioactive heat generation as a function

Th

Heat generation

K

Pa) h(s-1): 4.88 . lo-‘8 0.5 1.0 1.5 2.0 2.5 3.0 3.5

1.08 1.17 1.26 1.36 1.47 1.59 1.71

3.09 . 10-l’ 1.62 2.64 4.30 6.99 11.36 18.5 29.9

1.56 . 10-l’ 1.10 1.23 1.39 1.59 1.88 2.29 2.88

1.03 1.05 1.08 1.10 1.13 1.16 1.19

1.31 1.70 2.34 2.91 3.79 4.90 6.42

a **

b ** _-

1.13 1.28 1.48 1.74 2.08 2.52 3.13

1.17 1.37 1.64 1.98 2.43 3.01 3.81

* This assumes a present isotopic composition of Uranium of 99.2886% 0.7114% 2w. ** a. Based on Th/U = 4, K/U = 20000. b. Based on ‘kh/U = 4, K/U = 40000.

decreases exponentially with depth we can express any time t, and at any depth z in the crust: A =

2

A,. exp(-z/b,)

exp(--X, 0

238U and

the heat generation

at

(2)

r=l

where A, is the initial heat generation due to any isotope at the present surface, b, is the exponential depth factor and A, is the exponential time factor for that isotope. The parameter b determined from Fig. 3 may be defined by: A = A0 exp(-z/b)

(3)

Equation 3 is an approximation to Eq. 2 where n = 4, and the observed values of A,, and b are functions of the initial concentrations of the isotopes, the four decay constants and the four exponential depth constants. This complexity can produce a value of b for the exponential model that varies significantly over 2.6 Ga if the values of b, differ significantly. For the purpose of calculating crustal heat generation in the past, it is assumed throughout this paper that the relative concentrations of the radioactive isotopes are constant as a function of depth and equal to those of the surface rocks. The trend of low heat flow from areas of long stability has been commented on by several authors (Polyak and Smirnov, 1968; Hamza and Verma, 1969; &later and Francheteau, 1970). Changes in the quantity of residual heat escaping to the surface after the first 10’ years are small, and the major part of the decrease in heat flow after this initial period must be due to the fall in radioactivity levels. It is possible that the geothermal

68

-80

Q I

j mW/mz

L40

i-..

*w _ ..

1 . _ _.I

GY

2 I

Fig. 4. Observed heat flow, Q, against age according to Hamza and Verma (1969) (line A), and adjustments for radioactive decay (line B) and a maximum amount of erosion (line 0

parameters determining the maximum crustal temperature in a province early in its history also determine whether the province survives. The curve that Hamza and Verma (1969) published relating the observed heat flow to the age of the rocks indicates 34 and 60 mW/m2 for the heat flow crust of Superior and Grenville ages, respectively, whereas the average of fourteen plotted heat flow values from the Superior Province is 33 mW/m2 (uncorrected for glaciation). Table VI gives the heat generation as a function of age, based on two alternative assumptions of the relative concentrations of potassium and the heavy isotopes. The two results are similar and the present potassium/uranium ratio of 2 - lo4 is preferred; in the Archean the potassium concentration was the most critical parameter. These factors have been applied to the heat flow curve of Hamza and Verma (1969), as shown in Fig. 4. It has been assumed that the greater heat generation was reflected in the heat flow at the same time, and that the entire heat flow is subject to the same adjustment. The observed curve is marked A and the original heat flow at the present surface is shown by the curve B.A further increment has been added to account for the maximum loss of heat production by erosion, resulting in curve C, based on a constant erosion rate of 3 -10T6m/y. A present surface heat generation of 0.8’7 ~W/m3 was used in these calculations. The result is an approximation of the average heat flow in newly stabilized crust, as a function of time of origin. Such a calculation can only indicate general trends, but it is apparent that, for crustal provinces that have survived, the heat flow due to radioactive heat generation and conductive transport in young continents crust has been fairly constant since Archean time. In crust of age less than about lo9 y escaping residual heat is still observed, and this probably increased the heat flow in the young Archean crust also. It is probably valid to infer that, since it represents the heat flow from a newly

69

established crust, 80 mW/m2 is the maximum average heat flow due to radioactivity that may be expected from a stable continental area. Escape of residual heat from the event that reformed the crust and reset its age, from mass movement, and from enhanced transport by hydrothermal activity cause irregular high heat flow in areas of young crust and account for the upward trend of curve B and C for times less than lo9 y. Observations that the heat flow and heat generation in the Basin and Range province are not linearly related agree with this concept. OBSERVED

PARAMETERS

FOR SUPERIOR

PROVINCE

The accepted lines in Figs. 2 and 3 are drawn after rejecting three data points. Two of these points are less reliable than the majority and the third is geographically separated from the others and fails to conform to the same line. The rejected points are shown by open circles, whereas the others are shown as solid circles. All of the data that are rejected lie below the line defined by the others, and consequently they could not invalidate the conclusion that the line for the Superior Province lies generally below the line for the eastern United States. The reasons for rejection are as follows. English River: As described above, there is considerable ambiguity in the heat generation data, and this requires its omission from subsequent calculations. Nielsen Island: This site lies on the edge of the Northern Superior Subprovince, where rocks are generally of a high grade of metamorphism and erosion is believed to have progressed much further than in other parts of the Superior Province. The heat generation associated with the site is moderate, but the heat flow is low. The concentration of potassium is very high, much higher than values for this Northern Subprovince published by Fahrig et al. (1967), and so the site appears anomalous. It is worth noting that the heat flow is as low as the heat flow intercept in Fig. 3. Winnipeg: There is some ambiguity in the heat flow result from this borehole because of a non-random trend of decreasing heat flow with depth (Jessop and Judge, 1971). Strictly, the site is outside the boundary of the exposed Superior Province, but only 190 m of Palaeozoic sediments cover the Precambrian rocks. Unfortunately these sediments prevent the collection of extra samples for gamma ray analysis from the surrounding area. The cores from the borehole contain small amounts of the mineral allanite, which is rich in thorium and produces a high Th/U ratio of 18. If we assume that the thorium content is anomalous and we replace it by a figure based on the uranium content and a Th/U ratio of 4, the heat production is reduced to 0.65 ~W/m3. This figure brings the point much closer to the line, but it is preferable to omit the point from the calculations, The line of Fig. 2 is shown with other established lines in Fig. 5, and the main conclusion to be drawn is that the Superior Province is different from the eastern United States. The reduced heat flow is only about two thirds of

70

Fig. 5. Linear relationship between heat generation, A, and heat flow, Q, for the following heat flow provinces: BR = Basin and Range, EU = eastern U.S.A., IS = Indian Shield, SP = Superior Province, SN = Sierra Nevada, YB = Yilgarn Block, and CA = postulated Superior line during Archean time.

the level for the eastern United States, but is still greater than the same parameter for the Sierra Nevada. Fig. 5 includes a highly speculative line to represent the sate of the Superior Province in Archean time. This line is generated by multiplying currently observed heat production and heat generation by the appropriate figure for 2.5 Ga in Table VI. This line falls very close to the line for the Basin and Range Province, which is now an area of young crust. The observed heat flow intercept is controlled by the small values of heat flow, including the result from Otoskwin River which falls below the calculated straight line. This heat flow measurement is reliable, and is not likely to be sufficiently in error that the point could lie on the line. The heat production value is less reliable since the area is covered by glacial material and it was impossible to gather surface samples to confirm the low radioactivity levels found in the drill core. There is no reason to doubt the available data, since any higher value of heat production could only move the point further from the line. The Otoskwin River site is in the gravity feature known as the “Central Patricia Low”, a belt of low Bouguer gravity anomaly extending eastwards

71

from the southern part of the Lake Winnipeg and curving northeastwards towards Cape Henrietta Maria. This gravity feature has been interpreted as being due to the surviving root of a major mountain chain (Innes, 1960).The levels of radioactivity in the lower crust are normally low, compared with surface levels. At Otoskwin River where the surface heat generation is only 0.18 pW/m3, the generation in the lower crust is probably less than 0.1 pW/m3. It would take a thickness of at least 10 km of this lower crust to generate a heat flow of 1 mW/m*, which is the limit of reliability of a good heat flow value. The heat flow is thus unable to confirm the hypothesis of deep mountain roots, but the granite gneisses of amphibolite facies show a low level of radioactivity that is to be expected in material that has been deeply buried under a mountain chain. The characteristic depth b, given by the slope of the line, is considerably greater than values in most other areas and is almost twice as great as in the eastern United States. This means that heat flow in the Superior Province is lower than in neighboring Phanerozoic areas having the same low level of crustal radioactivity. The characteristic depth, like previous results, cannot be related to any other feature, and it does not necessarily have the same meaning as earlier results. The depth of the Moho is generally 30-35 km, and probably as deep as 40 km in the Northeastern Subprovince (Berry, 1973). The exponential model,_ as suggested by Jessop (1968) in connection with Nielsen Island site and proposed by Lachenb~ch (1968) to account for the Sierra Nevada heat flow, is probably applicable to Precambrian regions where metamorphism and associated redistribution of heat producing elements have taken place on a wide scale. The depth of the Moho is between two and three times as great as the depth parameter, which means that expected heat generation just above the Moho is less than that at surface level by a factor of between 7 and 20. The high value of the characteristic depth b suggests a less advanced state of differentiation of the radioactive elements at the end of the Kenoran metamorphic event than is observed in the plutonic rocks of Phanerozoic age. Alternatively these elements may have been differentiated to different degrees, the potassium being most differentiated. Over the period of 2.5 Ga the difference between eqs. 2 and 3, including the effect of erosion, can cause the observed b to change by a few kilometres. The low characteristic depth of the Australian shield implies the greatest degree of differentiation that is observed. The different degrees of differentiation in Archean shields leaves an uncertainty in the choice of the typical Archean crust. Kutas (1972) found no correlation between heat flow and heat generation in the Ukranian shield. Each Archean crater may have survived because of its greater strength and thickness compared to the surrounding crust. This could be accounted for by a relatively low heat production due to a low potassium concentration. Alternatively, rapid erosion of a thin highly differentiated surface layer may have provided the necessary reduction in heat generation and crustal temperature.

72

The diversity in this linear relation suggests a crustal model for the Archean that includes a thin (2-4 km) surface layer having a relatively high heat production; this layer probably was not present everywhere. Erosion removed it from most areas of the present Archean shields leaving the second layer exposed. An upper layer was suggested by Hyndman et al. (1968), but it was a continuous layer with a thickness comparable to the characteristic depth of younger heat flow provinces. An example of a remnant of the upper layer suggested here is the Indian Lake batholith (Szewczyk and West, 1976) at the English River heat flow site. Compared to other nearby batholiths it is shallow (2-3 km) and potassium rich with a much larger heat production. The second layer or main member of this suggested crust has a relativeiy low heat generation and a large characteristic depth, indicating a much smaller degree of differentiation. The measurements of heat production in the Ukrainian and Baltic shields and the Superior Province come mostly from this part of the crust. If this model is tenable, the Yilgarn Block represents an area where the rich upper layer, depleted in heat production by age, has not been eroded away. It thus shows a high level of differentiation. The Superior Province, on the same basis, represents a crust where the upper layer has been mostly removed by erosion, Any remnants of the upper layer would show as points in Fig. 2 to the right of the straight line, and the results at English River and Nielsen Island represent such remnants. CRUSTAL TEMPERATURE

Given the surface heat flow and the distribution of conductivity and heat generation in the crust, it is possible to calculate a temperature profile. Fig. 6 shows temperature profiles calculated for the three heat flow provinces of the U.S.A., the Yilgarn Shield and the Superior Province. In each model the thermal conductivity has been set at 3.25 W/mK for crustal rocks and 4.00 W/mK for mantle rocks, and heat generation and corresponding heat flow have been set at about the mid-point of the observed straight line. The temperatures under the Superior Province during the Archean are similar to the temperatures at present under the Basin and Range. The addition of a thin, highly radioactive layer will increase the temperatures by a maximum of 100 K. Moho depth has been set at 30 km for the Basin and Range Province, 40 km for the Sierra Nevada and 35 km for the others. These sample models show temperatures at a depth of 50 km that are lowest in the Sierra Nevada (314”C), slightly higher in the Canadian and Australian Shields (368°C and 409”C), slightly higher in the eastern U.S.A. (558°C) and much higher in the Basin and Range Province (91O’C) and the young crust of the Superior Province in Archean time. The arcs at the bottom of Fig. 6 indicate the possible spread of temperature, corresponding on one side to the lowest heat flow and zero heat pro-

73

Temperoture

OC

40

Fig. 6. Crustal temperature profiles for: BR = Basin and Range, EU= eastern United States, SN = Sierra Nevada, YB = the Yilgarn block of the Australian shield, and SP = the Superior Province of the Canadian shield.

duction,

and on the other side to the highest heat flow and heat production in the province. These are one-dimensional, conductive models, and they take no account of the lateral heat transfer that must occur as a result of horizontal temperature gradients, The three cool provinces show overlapping temperature ranges, but the eastern U.S.A. and the Basin and Range Provinces have ranges of possible temperature that are quite distinct. Hydrothermal transfer probably reduces the real temperature range during the initial 10’ y to less than the values calculated here. Figure 7 shows the effect of different models on the calculated temperature profiles. All models refer to the eastern U.S.A., and curve A is the same as is shown in Fig. 6. Models A and B have an exponential distribution of heat generation with depth, but in model B the thermal conductivity is assumed to correspond to basalt (1.9 W/mK) between 25 km and 35 km. Models C and L) have a uniform heat generation that is equal to A0 down to 7.5 km. Model C has conductivity and heat generation typical of crustal rocks (3.25 W/mK and 0.5 pW/m3) from 7.5 km to 25 km and typical of basalt (1.90 W/mK and 0.1 pW/m3) from 25 km to 35 km. Model D has the values of basalt throughout the entire section from 7.5 km to 35 km. Model D shows the highest temperature because of the large thickness of low conductivity material, and it is probably the least realistic. Models B and C show temperature at 50 km that lie just within the range indicated by the observed

Fig. 7. Crustal temperature in the eastern U.S.A., calculated from four different models.

arc in Fig. 6. Thus the calculated temperature in the lithosphere can be adjusted by changing the model by about the same amount as by using the extreme high or low observed heat flow, Since there are three distinct temperature ranges shown in Fig. 6, it is unlikely that the differences could be resolved by adopting different reasonable models for each province. Compared with young crust, Precambrian crust now has a generally low temperature. Temperature has been decreasing since the time of last metamorphism causing possible sub-crustal accumulation and development of greater strength. As, mentioned above, crustal temperature in the Superior Province in Archean time was at least as high as present temperatures in the Basin and Range Province, and probably higher if there was a thin highly radioactive upper layer. It may be that the formation of this thin upper layer, and its rapid removal by erosion was a necessary condition for the survival of the young Archean crust. Consequently this crust was cooler and stronger than the surrounding crust, and was not typical of the Archean crust. CONCLUSIONS

The heat flow and heat generation of the Superior Province of the Canadian Shield are very low. Average crustal heat generation was not only much higher in the Precambrian than it is now, but it decreased considerably between the times of formation of provinces such as Superior and Grenville. Therefore the data from shields of different ages should not be considered to form one heat flow province. It is possible that the Superior was not one

75

heat flow province originally since the differences in age of its various parts comprise a period during which a large change in radioactive heat generation occurred ~approximately 20%). The values of heat flow and heat generation in the Superior Province are approximately linearly related and can define a heat flow province similar to those observed in the U.S.A. The statistical significance of this relation is small since the uncertainties in measuring the low values are relatively large. The slope, 14 km, and the intercept, 21 mW~m*, are different from those of any other heat flow province. The small intercept or low heat flows set a maximum limit on heat flow from the mantle beneath the Superior. The slope is much larger than slopes of other heat flow provinces. In the western Australian Archean shield the higher heat flow and heat generation and the different relationship between them require some explanation. A crustal model for the original Archean crust that includes thin, highly radioactive surface veneers in some places can account for the observed differences. In comparison to younger heat flow provinces, such a model suggests that the crust that has survived from Archean time was more differentiated. The surface that we now see in the Superior Province may be near the transition to less radioactive crust. Over a period of 3 Ga the initial value of heat flow has been a constant in newly formed stable cratons. This does not include the initial heat lost immediately following formation of the craton. Higher heat flows probably existed in crust that has not survived due to its relative weakness. Calculation of temperature in the crust and upper mantle shows that the shields and the Sierra Nevada are now signific~tly cooler than the eastern U.S.A., an area of stable platform and early Palaeozoic orogeny, which in turn is much cooler than the Basin and Range Province. However, the temperatures under the original Superior Province were probably higher than the temperatures under the Basin and Range at present. REFERENCES Allis, R.G. and Garland, G.D., 1976. Geothermal measurements in five small lakes of Northwest Ontario. Can. J. Earth Sci., 13: 987-992. Ambrose, J.W., 1972. Tectonic implications of exhumed paleoplains (The Superior Province). In: R.A. Price and R.J.W. Douglas (Editors), Variations in Tectonic Styles in Canada. Geol. Assoc. Can., Spec. Pap,, 11: 603-606. Belevtsev, Ya.N. and Komarov, A.N., 1975. Radioactive elements in the metamorphic process (in Russian). In: Radioactive Elements in Rocks. Nauka, Novosibirsk. Trans. Inst. Geol. Geophys., 286: 133-140. Acad. Sci. U.S.S.R., Siberian Branch. Berry, M.J., 1973. Structure of the crust and upper mantle in Canada. Tectonophysics, 20: 183-201. Birch, F., Roy, R.F. and Reeker, E.R., 1968. Heat flow and thermal history in New England and New York. In: W.S.E.-an Zen and J.B. Hadley (Editors), Studies of Appalachian Geology: Northern and Maritime. Interscience, New York, N.Y., pp. 437451.

76 Bunker, C.M., Bush, C.A., Munroe, R.J. and Sass, J.H., 1975. Abundances of uranium, thorium and potassium for some Australian crystalline rocks. U.S. Geol. Surv., Open File Rep., 75-393. Cermak, V. and Jessop, A.M., 1971. Heat flow, heat generation, and crustal temperature in the Kapuskssing area of the Canadian shield. Tectonophysics, 11: 287-303. Charbonneau, B.W., 1972. Gamma Ray Support. In: Report of Activities. Geol. Surv. Can., Pap., 72-l: 45-46. Fahrig, W.F., Eade, K.E. and Adams, J.A.S., 1967. Abundance of radioactive elements in crystalline shield rocks. Nature, 214: 1002-1003. Hamza, V.M. and Verma, R.K., 1969. The relationship of heat flow with age of basement rocks. Bull. Volcanol., 33: 123-152. Hyndman, R., Lambert, I.B., Heier, K.S., Jaeger, J.C. and Ringwood, A.E., 1968. Heat flow and surface radioactivity measurements in the Precambrian shield of Western Australia. Phys. Earth Planet. Inter., 1: 129-135. Ingham, W.N. and Keevil, N.B., 1951. Radioactivity of the Bourlamaque, Elzevir and Cheddar batholiths, Canada. Bull. Geol. Sot. Am., 62: 131-143. Innes, M.J.S., 1960. Gravity and isostasy in northern Ontario and Manitoba. Publ. Dom. Obs., Ottawa, 21: 260-338. Jaeger, J.C., 1970. Heat flow and radioactivity in Australia. Earth Planet. Sci. Lett., 8: 285-292. Jessop, A.M., 1968. Three measurements of heat flow in eastern Canada. Can. J. Earth Sci., 5: 61-68. Jessop, A.M., 1971. The distribution of glacial perturbation of heat flow in Canada. Can. J. Earth Sci., 8: 162-166. Jessop, A.M. and Judge, AS., 1971. Five measurements of heat flow in southern Canada. Can. J. Earth Sci., 8: 711-716. Krogh, T.E., Davis, G.L., Ermanovics, I. and Harris, N.B.W., 1976. U-Pb isotopic ages of zirons from the Berens block and English River Gneiss belt. Proc. 1976 Geotraverse Conf., University of Toronto. Kutas, R.I., 1972. Investigation of heat flow anomalies in some regions of the Ukraine. Geothermic, 1: 35-39. Lachenbruch, A.H., 1968. Preliminary Geothermal model of the Sierra Nevada. J. Geophys. Res., 73: 6977-6989. Lambert, R.St.J., 1976. Archean thermal regimes, crustal and upper mantle temperatures and a progressive evolutionary model of the earth. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, New York, N.Y. pp. 363-373. Lewis, T.J., 1974. Heat production measurement in rocks using a gamma ray SPeCtiOmeter with a solid state detector. Can. J. Earth Sci., 11: 526-532. Lewis, T.J. and Beck, A.E., 1977. Analysis of heat-flow data - detailed observations in many holes in a small area. Tectonophysics, 41: 41-59. Misener, A.D., Thompson, L.G.D. and Uffen, R.J., 1951. Terrestrial heat flow in Ontario and Quebec. Trans. Am. Geophys. Union, 32: 729-738. Morey, G.B. and Sims, P.K., 1976. Boundary between two Precambrian W terranes in Minnesota and its geologic significance. Geol. Sot. Am. Bull., 87: 141-152. Polyak, B.G. and Smirnov, Y.A., 1968. Relationship between terrestrial heat flow and the tectonics of continents. Geotectonics (Eng. Trans.), 4: 205-213. Rao, R.U.M. and Jessop, A.M., 1975. A comparison of the thermal characters of shields. Can. J. Earth Sci., 12: 347-360. ho, R.U.M., Rao, G.V. and Narain, Hari, 1976. Radioactive heat generation and heat flow in the Indian Shield. Earth Planet. Sci. Lett., 30: 57-64. Richardson, K.A., Killeen, P.G. and Charbonneau, B.W., 1975. Results of reCOnnaiSSanCC type airborne gamma-ray spectrometer survey of the Blind River - Elliot Lak , area, Ontario. In: Report of Activities, Geol. Surv. Can. Pap., 75-l: 133-135. Roy, R.F., Blackwell, D.D. and Birch, F., 1968. Heat generation of plutonic rocks and continental heat flow provinces. Earth Planet Sci. Lett., 5: l-12.

Sass, J.H., Killeen, P.G. and Mustonen, E.D., 1968. Heat flow and surface radioactivity in the Quirke Lake Syncline near Elliot Lake, Ontario, Canada. Can. J. Earth Sci., 5: 1417-1428. Sass, J.H., Lachenbruch, A.H. and Jessop, A.M., 1971. Uniform heat flow in a deep hole in the Canadian shield and its paleoclimatic implications. J. Geophys. Res., 76: 85868596. Sass, J.H., Jaeger, J.C. and Munroe, R.J., 1976. Heat flow and near-surface radioactivity in the Australian continental crust. U.S. Geol. Surv., Open File Rep., pp. 76-250. Sclater, J.G. and Francheteau, J., 1970. The implications of terrestrial heat flow observations on current tectonic and geochemical models of the crust and upper mantle of the earth. Geophys. J.R. Astron. Sot., 20: 509-542. Shaw, D.M., 1967. U, Th and K in the Canadian Precambrian shield and possible mantle compositions. Geochim. Cosmochim. Acta, 31: 1111-1113. Stockwell, C.H., McGlynn, J.C., Emslie, R.F., Sanford, B.V., Norris, A.W., Donaldson, W.F., Fahrig, W.F. and Currie, K.L., 1970. Geology of the Canadian Shield. In: R.J.W. Douglas (Editor), Geology and Economic Minerals of Canada. Geol. Surv. Can., pp. 43-150. Szewczyk, Z.J. and West, G.F., 1976. Gravity study of an Archean granitic area northwest of Ignace, Ontario. Can. J. Earth Sci., 13: 1119-1130. Wetherill, G.W., 1966. Radioactive decay constants and energies. In: S.P. Clarke (Editor), Handbook of Physical Constants of Rocks. Geol. Sot. Am. Mem., 97: 513-519.