Towards a crustal thermal model for the Archaean Dharwar craton, southern India

Physics and Chemistry of the Earth 28 (2003) 361–373 www.elsevier.com/locate/pce

Towards a crustal thermal model for the Archaean Dharwar craton, southern India Sukanta Roy *, R.U.M. Rao

1

National Geophysical Research Institute, P.O. Bag 724, Uppal Road, Hyderabad 500 007, India Received 30 April 2002; received in revised form 27 November 2002; accepted 4 April 2003

Abstract New heat flow and heat production data from the Archaean Dharwar craton of southern Indian shield refine crustal thermal models and mantle heat flow estimates. The terrain comprises greenstone–granite belts and gneisses, which give way to granulites farther south. Low heat flow, generally spanning a range from 25 to 51 mW m
2 with a mean of 36 mW m
2 characterizes the craton. However the radiogenic heat production of surface rocks varies widely, from 0.2 to 8 lW m
3 . Based on considerations of heat flow, regional geology, crustal sections constrained by deep seismic sounding and gravity data, and extensive heat production data for rock types comprising the crustal sections, four one-dimensional scenarios are envisaged for crustal heat production in the province. These are compatible with the range of the observed surface heat flow and represent the plausible extremes in the crustal heat contribution, on a regional scale. The heat flow at the base of the crust is found to have a small range, 12–19 mW m
2 for the craton. This range is similar to those derived for Archaean provinces in the Canadian and southern African shields. The regional model provides robust bounds for crustal temperature estimates in the craton, with a range of 285–410 °C at the Moho, which points to a generally ÔcoldÕ crust in the region. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Heat flow; Heat production; Mantle heat flow; Crustal temperature estimates; Dharwar craton; South India

1. Introduction Heat flow variations in old, tectonically stable regions essentially reflect variations in heat produced in the crustal column and heat conducted into the crust from the underlying mantle. In contrast, in tectonically active regions the measured heat flow could include effects of thermal transients resulting from recent tectonothermal events such as orogenesis, magmatism and rifting (Armstrong and Chapman, 1998). Estimation of contributions from such events could be quite involved and therefore invariably carry very large amount of uncertainty. Therefore, the Precambrian shields offer the most favourable locales for modeling the crustal thermal structure (Oxburgh, 1981). Mantle heat flow thus de-

*

Corresponding author. Tel.: +91-40-23434700; fax: +91-4027171564/23434651. E-mail addresses: [email protected], [email protected] (S. Roy), [email protected] (R.U.M. Rao). 1 Now at 73, TeachersÕ Colony, Trimulgherry, Secunderabad 500 015, India. Tel.: +91-40-27795454.

duced becomes the datum for inferences regarding thermal conditions in the sub-crustal lithosphere. Interpretation of heat flow data for parts of the Indian shield including the Dharwar craton have been previously carried out within the paradigm of heat flow provinces. That paradigm is based on the concept of the linear relationship between surface heat flow (q0 ) and heat production of surface rocks (A0 ), q0 ¼ qr þ A0 D, proposed from data for plutons by Birch et al. (1968), Roy et al. (1968), and Lachenbruch (1968). The intercept (qr ) and slope (D) have dimensions of heat flow and of length respectively, and are characteristic of a heat flow province. Several models for distribution of heat production with depth, such as step and exponential decrease models, that satisfy the linear relation under one-dimensional, steady state formulations have been adopted for crustal thermal modeling. These early studies stand as classic studies in understanding the thermal structure in the crust from heat flow and steady state formulations in different continental segments. However, the concept of heat flow provinces, which has been extensively followed in geothermal literature, is still widely debated. Unwarranted extension of the

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concept to cover sedimentary and metamorphic terrains has been of particular concern. In the case of the Indian shield, a crustal thermal model was proposed for a large part of the shield based on an observed linear relationship between q0 and A0 involving several metamorphic as well as sedimentary geological provinces (Rao et al., 1976; Rao and Rao, 1983). Gupta et al. (1991) proposed a lithospheric thermal model for the southern Indian shield using a linear relationship through q0 –A0 pairs from three heat flow sites, only two of which are located in the Dharwar greenstone–granite–gneiss province. Also the U, Th and K data used for estimation of heat production for the different rock types were obtained mostly from routine trace element analysis on very small sample quantities. It is now well recognized that such data may not be representative of bulk formations, which are essential for estimating the radioactive heat contribution on a crustal scale. Gupta (1995) added three q0 –A0 pairs and proposed two different linear relationships for the Indian shield: one for ArchaeanEarly Proterozoic cratons and another for Proterozoic mobile belts. A large body of additional heat flow data and heat production data from deep boreholes and exposed crustal sections, and consideration of other important issues have warranted a reconsideration of the interpretation of linear relationships between q0 and A0 over large contiguous continental segments. Discussions covering several of these aspects have been made by Drury (1987, 1989), Morgan et al. (1987), Furlong and Chapman (1987), Fountain et al. (1987), Webb et al. (1987), Vigneresse and Cuney (1991), Ketcham (1996a), Roy (1997), Pribnow and Winter (1997) and Roy and Rao (1999a). Two significant points are discussed here. Heat production of surface rocks could vary widely concomitant with surface geology. However, the heat

flow measured near the surface is controlled by heat produced in the major litho units comprising the entire crustal thickness, out of which the surficial formations could constitute only a thin layer. The heat flow variation across the Abitibi greenstone belt in the Superior Province of Canadian shield, from 29 to 44 mW m
2 , found over a 800 km long stretch of uniformly low heat producing greenstone volcanics (Pinet et al., 1991) points to the ambiguity in interpretation of the linear relationship for such situations. In India, the point is borne out clearly from the case of the Dharwar province. Heat flow ranges between 25 mW m
2 (at Attibele, near Bangalore) and 50 mW m
2 (in parts of the Pattikonda area) (Table 1). The corresponding range of heat production of surface rocks is 0.4–8 lW m
3 (present study, Table 2). Individual rock samples yield more variable values, from as low as 0.3 lW m
3 to as high as 15 lW m
3 . These observations would imply a very weak correlation and very low value for slope, through q0 –A0 plots. Parameters of a regression line would have large uncertainties when errors in heat flow and heat production are considered. Secondly, continental crustal sections revealed by seismic data and interpreted in terms of regional geology, clearly imply lateral and vertical heterogeneity in the crust that cannot be described by the simple 1-D heat production models proposed for interpreting a linear relation. Results of stochastic modeling studies show that effects of lateral heat conduction can lead to an underestimation of the apparent thickness of the radiogenic layer as obtained from interpretation of a linear relationship, and the depth scale of heat production deduced empirically has little relation with any geophysical dimension (Vasseur and Singh, 1986). Combined analytical and numerical methods to evaluate horizontal averaging effects of heat conduction in re-

Table 1 Heat flow data for the Dharwar craton, Indian shield Location

Number of boreholes

Rock type

Mean heat flowa (mW m
2 )

Reference

Kolar gold fields Gadag Kalyadi Ingaldhal Hyderabad Pattikonda–Adoni area (i) Sub-area Ab (ii) Sub-area Bb Dharmavaram area Tummalapalli Bangalore area Attibelec

Mine 2 3 1 3 16 8 8 8 1 11 2

Hornblende schist Schist Schist Metavolcanics Granite–gneiss Granite–gneiss

40(–) 29(1) 30(1) 30(–) 40(4) 43(9) 50(3) 35(5) 40(2) 51(–) 30(7) 25(1)

Rao (1970) Gupta et al. (1991) Gupta et al. (1991) Roy and Rao (2000) Gupta et al. (1987) Roy and Rao (2000)

a

Granite–gneiss Granite–gneiss Granite–gneiss Granite–gneiss (amphibolite–granulite transition)

Gupta et al. (1991) Roy and Rao (2000) Roy and Rao (2000) Roy and Rao (2000)

Numbers within parentheses after heat flow indicate standard deviation. Sub-area A represents a 25  25 km2 portion within the 85  65 km2 Pattikonda–Adoni area. Sub-area B represents the remaining portion within the study area. c Included in the Bangalore area, but shown separately for correspondence with Section 4 of the text. b

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363

Table 2 Heat production data for major rock types in the Dharwar craton Rock type

Location

Laboratory/ in situ

Tonalite

Kolar area Dharmavaram area

Laboratory Laboratory

Granite–gneiss

Hyderabad area i(i) Grey granites (ii) Pinkgranites Pattikonda–Adoni area i(i) Sub-area Ac (ii) Sub-area Bc

Laboratory

Kolar schist belt Chitradurga schist belt Chitradurga schist belt Attibele area (southeast of Bangalore) SGP

Hornblende schist (metavolcanics) Metavolcanics Metasediments Amphibolite-to-granulite grade transitional gneiss Granulite

Number of samples/ outcropsa

Average heat productionb (lW m
3 )

Reference

29 24

1.5(0.1) 1.7(0.2)

Rao et al. (1976) Present study

152 53

8.2(0.5) 4.2(0.5)

37 40

7.6(0.4) 4.1(0.4)

Laboratory

15

0.25(0.1)

Rao et al. (1976)

Laboratory Laboratory In situ

3 3 25

0.15(–) 0.85(–) 0.4(0.02)

In situ

73

0.2(0.02)

Gupta et al. (1991) Gupta et al. (1991) Unpublished data (R. Srinivasan, pers. comm.) Roy et al. (2003)

Present study

In situ

Present study

a

Samples in case of laboratory measurements, outcrops in case of in situ measurements. Numbers within brackets indicate standard error in heat production. c Sub-area A represents a 25  25 km2 portion within the 85  65 km2 Pattikonda–Adoni area (same as in Table 1). Sub-area B represents the remaining portion within the study area. b

gions with laterally variable vertical scales of heat production show that the depth scale only represents some average of various actual thicknesses of the heat producing layers, and does not provide information on actual vertical distribution at any location (Jaupart, 1986). Numerical modeling study on a 2-D simplified crustal cross-section of the Superior Province (Fountain et al., 1987) brings out the non-uniqueness of heat production distribution with depth as determined from the linear relation. Furlong and Chapman (1987) conclude from a numerical modeling study of a realistic 2-D crustal cross-section that the depth scale determined from the linear relation is significantly affected by horizontal scale of crustal heterogeneity, and results in overestimation of lower crustal heat flow and of temperatures obtained using the linear relation. This overestimation is largest in areas of low mantle heat flow as in the cratonic areas of the shields. Direct approaches for constraining better the heat contribution of the crust have been demonstrated for the Vredefort structure and Witwatersrand basin in the Kaapval craton (Nicolaysen et al., 1981; Jones, 1988), the West African craton (Lesquer and Vasseur, 1992), the Kapuskasing, Abitibi and some other regions of the Canadian shield (Drury, 1991; Pinet et al., 1991; Jaupart et al., 1998; Mareschal et al., 2000); the Ivrea and Strona Ceneri zones in the Southern Alps (Galson, 1983), the Catalina and Harquahala metamorphic core complexes of the Arizona Basin and Range province (Ketcham, 1996a), and parts of the Deccan Volcanic Province (DVP) and Dharwar craton of the Indian shield (Roy and Rao, 1999a,b). It is now felt that better estimates of crustal contribution to the surface heat flow (and hence

robust estimates of mantle heat flow) for a region with thermal equilibrium assumed can be obtained by using: (a) a dense and reliable heat flow coverage for the region under study, (b) a cross-section of the major crustal layers in terms of lithologic types, obtained by high quality seismic data coupled with regional geology and constrained by other geophysical methods, and, (c) a heat production data set obtained by intensive coverage of the major formations, representative of the constituting crustal layers that have been delineated, so that reasonable values could be ascribed for the average heat production of the crustal layers. New heat flow and heat production data emerging out of recent studies in the Dharwar craton (Roy, 1997; Ray et al., 1999; Roy and Rao, 2000), combined with information on crustal structure based on geology and deep seismic sounding investigations, allow us to propose a crustal thermal model for the Dharwar craton based on the above considerations.

2. Geological setting and heat flow data The greenstone–granite–gneiss province of the Dharwar craton is an Archaean terrain (Fig. 1). The greenstone belts are called the Dharwar schist belts, and the gneisses that separate and surround them form a complex of unclassified gneisses known as the Peninsular Gneissic Complex (Pichamuthu and Srinivasan, 1983). The schists and gneisses gradually give way to the granulites (charnockites and khondalites) farther south. The bulk of the Peninsular gneisses has a granodioritic composition with a range from tonalite to granite.

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Fig. 1. Geological map of southern India (modified after Geological Survey of India, 1998). The Dharwar craton is exposed between the orthopyroxene isograd (solid line) in the south and the 65 Ma Deccan flood basalts in the north. The orthopyroxene isograd separates the greenschist and amphibolite facies rocks in the north from the dominantly granulitic facies rocks in the south, which comprise the SGP. The Kavali–Udupi deep seismic sounding profile (grey solid line) cuts across the central part of the Dharwar craton. Heat flow sites (solid circles) along with heat flow data in mW m
2 are also shown. Heat flow shown at each site represents an average of heat flow values obtained from a large number of boreholes distributed in an area around the site (see Table 1). The distribution of boreholes in Hyderabad, Pattikonda, Dharmavaram, Bangalore and Gadag areas are shown in the inset. At Kalyadi, three boreholes are located at the same site. PCL: Palghat-Cauvery lineament.

A long linear belt of potassium-rich granitoid rocks, the Closepet Granite batholith, and other smaller granitoid bodies invade the Dharwar schist belts and the Peninsular Gneissic Complex. Beginning at 3400 Ma, the record of Precambrian events in the province can be traced up to 2100 Ma. The heat flow data for the province are listed in Table 1 and shown in Fig. 1. The data set results from temperature measurements in 45 boreholes and a 2150 m deep mine distributed in nine areas. The majority of the values for individual boreholes range from 25 to 51 mW m
2 ; the area means range from 29 to 51 mW m
2 . The data cover the major geological sub-units constituting the province. The moderately high heat flow value of 51 mW m
2 at Tummalapalli can be attributed to

higher heat production in the uranium-rich gneissic basement rocks relative to the Peninsular gneisses. The greenstone terrains (such as Gadag, Kalyadi and Ingaldhal in the western part) and granulite terrains (such as those close to Bangalore and Attibele in the southern part) are generally characterized by lower heat flow values relative to the gneiss–granite terrains, consistent with the levels of heat production in the corresponding rock types. Heat flow in the Kolar greenstone belt, however, is comparable to that in the Peninsular gneiss terrain due to the presence of only a thin cover of low heat producing hornblende schist at the surface, which is underlain by higher heat producing tonalite gneiss. Overall, the Dharwar craton is characterized by low heat flow with a mean value of 36  8 mW m
2 (Roy and Rao, 2000).

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The province is one of the oldest geological units worldwide and shares the low heat flow characteristic noted for other cratons of similar age such as the Superior Province of Canadian shield (Jessop and Lewis, 1978; Jessop et al., 1984; Drury and Taylor, 1987; Drury, 1991; Mareschal et al., 1989; Pinet et al., 1991; Guillou et al., 1994; Guillou-Frottier et al., 1995, 1996), the Yilgarn block of Western Australian shield (Sass et al., 1976; Sass and Lachenbruch, 1979), the KolaKarelian block of the Baltic shield (Kukkonen, 1989, 1993; Cermak et al., 1992; Balling, 1995; Pasquale et al., 2001), and the Zimbabwe, Tanzania and Kaapval cratons of the southern African shield (Carte and van Rooyen, 1969; Chapman and Pollack, 1977; Jones, 1987, 1988; Ballard et al., 1987; Nyblade et al., 1990).

3. Heat production data Radioelement (U, Th and K) measurements on major rock types constituting the crustal column in the Dharwar craton were carried out in the laboratory using a low-level counting multichannel gamma-ray spectrometer system. The set-up comprises a 5 in.  6 in. NaI(Tl) crystal coupled to a 5 in. diameter photomultiplier tube housed in a 7 in. thick lead shield (Rao, 1974; Roy, 1997). A complete description of the method is provided in Ketcham (1996b). The crushed rock samples sealed in plastic containers are counted after about a weekÕs storage by which time secular disequilibrium due to radon loss is largely repaired. In a recent study Chiozzi et al. (2000) have discussed the possibility of plastic being permeable to radon. However, further experiments are necessary to find alternative container materials. A 256-channel data set covering up to 3 MeV is obtained through a PC-based multichannel analyzing card that provides spectrum stabilization. Each sample weighed about 0.4 kg and the counting time was varied between 10,000 and 25,000 s depending upon the activity level of the sample. Heat production estimate for each sample was obtained from the radioelemental abundances using the relation given by Birch (1954). The laboratory analyses were complemented by in situ gamma-ray spectrometric analysis in several areas. A SCINTREX-make, four-channel, spectrum stabilized, gamma-ray spectrometer with a large (6 in. dia and 4 in. high) NaI(Tl) crystal as the sensor was used. Measurements were made by placing the sensor directly over outcrops, and obtaining counts over a pre-set time in three energy ranges of the gamma-ray spectrum, three channels, 180, 240 and 320 keV wide, and centered around the peaks, 1.46, 1.76, and 2.62 MeV, characteristic of K, U, and Th respectively. Periodic energy calibration was carried out through the course of a dayÕs work using a ThO2 source. Data reduction was carried out in three steps: (i) subtraction of background to ob-

365

tain net count rates, (ii) stripping the net count rate in the U-channel for the effect of Th, and the net count rate in the K-channel for the effects of Th and U, and, (iii) applying pre-determined sensitivity constants to the net, stripped, count rates. The count rates for the background in the three channels were obtained by making measurements over long periods, several kiloseconds, in lakes. The spectrometer with the sensor was placed on a fibre-glass boat and pedaled to the center of a lake, 100–150 m away from the shore, where the water column was >8 m. The stripping factors and the sensitivity constants used are those supplied, by measurements made with this detector over test pads specially built for the purpose by the Geological Survey of Canada, Ottawa. The count time was generally set at 300 s and at times, increased to 600 s when low count rates were encountered. The outcrops over which measurements were carried out are (i) flat over at least 1 m2 area and conforming to 2p-geometry, and (ii) fresh, in the sense that they were either quarries with freshly exposed rock, or, were having only a thin veneer of weathering effects, 1–3 mm, which is negligible compared to the volume sensed, a circle of investigation of 40 cm and a depth up to 12 cm (Lovborg et al., 1971). Reliable estimates of K, U and Th even at low concentrations could be obtained because of accurate determination of background, adequate count times that make the errors due to counting statistics low, and application of well-determined constants. Errors in estimates of K, U and Th concentrations, based on counting statistics (Loevinger and Berman, 1951), were derived, taking both high as well as low heat producing rocks as test cases. In arriving at errors for the net count rates at each site (i.e., measured count rate––background count rate), an error in the background counts that is appropriate to the counting time at the particular site was considered. The stripping constants and the sensitivity factors as determined from test pads were considered to have negligible errors. The estimated errors in K, U and Th concentrations were translated into errors in computed heat production for each of the three elements and an overall error in heat production due to all elements combined. An error of 5% in density values was considered. In rocks having high radioelemental concentrations (heat production > 1 lW m
3 ) as in the case of granites and syenites, it was found that that the error in the case of a 300 s counting time did not exceed 2% for K and 5% for U and Th. The propagated error in the total heat production is invariably less than 5%. However, at low elemental levels as in the case of depleted granulites (heat production < 1 lW m
3 ), even with a 600 s counting time, the error in U and Th exceeded 20% in several cases. Because K accounts for a large fraction of the total heat production, as compared to U and Th in

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such rock types, and the errors in K estimates are always less than 5%, the propagated error in the total heat production arrived at, is less than 20% despite the large errors in measurement of U and Th. Correspondence between the heat production estimates obtained using the laboratory and the in situ methods was established from measurements in relatively homogeneous outcrops. In situ analysis at a very low heat producing granulite (charnockite) quarry located 10 km east of Salem (Fig. 1), yielded estimates of K, U, and Th as 0.72  0.04 (s.e.) %, 0.08  0.02 (s.e.) ppm, and 0.42  0.16 (s.e.) ppm respectively. Sixteen samples from the same quarry analyzed in the laboratory (400 g each) yielded mean values of 0.63  0.01%, 0.14  0.03 ppm, 0.6  0.06 ppm for the respective elements. The estimates of heat production for different rock types comprising the Dharwar crust are listed in Table 2. The Late Archaean charnockites and enderbites, outcropping abundantly to the south of the orthopyroxene isograd up to the Palghat-Cauvery lineament (Fig. 1), are highly depleted in heat producing elements and have the lowest heat production rates with an average of 0.2 lW m
3 . The amphibolite facies rocks have highly variable heat production, ranging from 0.15 to 0.25 lW m
3 for metavolcanic rocks to as high as 8 lW m
3 for granites. The granodioritic and tonalitic gneisses, which constitute a large part of the Dharwar upper crust, have a mean heat production of 1.5 lW m
3 . The amphibolite-to-granulite transitional grade rocks, characteristic of Attibele area close to the orthopyroxene isograd, yield low mean heat production of 0.4 lW m
3 , not much higher than that for granulites.

4. Crustal heat production models and mantle heat flow A regional crustal thermal model for the Dharwar craton is envisaged by integrating 1-D heat flow–– crustal heat production models for four areas: Attibele, Kolar, Dharmavaram, and Gadag. The geological setting, locations of heat flow sites, and heat flow data are summarized in Table 1 and shown in Fig. 1. The areas were selected based on two main criteria. First, they cover three possible crustal scenarios spanning a range of heat flow and crustal heat production for the province. (a) Attibele shows the lowest heat flow, 25 mW m
2 . The upper crust in this area comprises granite– gneiss occurring in amphibolite-to-granulite transition zone, and the lower crust is granulitic; (b) Dharmavaram, with a mean heat flow of 40 mW m
2 , has a dominantly tonalitic upper crust and granulitic lower crust; (c) Kolar and Gadag, located in the Kolar schist belt and the Chitradurga schist belt respectively, are characterized by heat flow values of 40 and 29 mW m
2 . In both areas, a metavolcanic and/or metasedimentary

cover overlies a tonalitic layer in the upper crust, the lower crust being granulitic as elsewhere. Most other heat flow sites fall into one of the three scenarios–– Hyderabad and Pattikonda–Adoni areas in scenario (b), Kalyadi and Ingaldhal in scenario (c). Secondly, the areas are typically located away from the edges of the geological units ensuring validity of 1-D heat conduction approximations. Tummalapalli area, where a good heat flow estimate has been obtained, does not satisfy this criterion because it is located at the southwestern edge of the Cuddapah basin (Fig. 1). A borehole drilled in this area reveals a 200 m quartzite layer underlain by a granite–gneiss basement. The basement is exposed in the adjoining Dharwar province to the west, just outside the basin. However, core samples of granites and gneisses from a borehole at Tummalapalli are enriched in heat producing elements relative to those exposed in the adjoining area, possibly a result of uranium mineralisation in the area. This area has therefore not been considered for crustal thermal modeling. We believe that the three scenarios (a–c) provide a broad bounding framework for a plausible crustal thermal structure for the province, and therefore help to estimate the possible crustal temperatures. The principal constraints for deriving a gross structure of the crust have been obtained from interpretation of deep seismic sounding data along a transect cutting across the Dharwar province, from Kavali in the east to Udupi in the west (Fig. 1) (Kaila et al., 1979). The crustal thickness in this region varies from 35 to 40 km. Interpretation of gravity data along a NE–SW profile in the northern part of the Dharwar craton reveals a variation in crustal thickness from 35 to 41 km (Mishra et al., 2002). This result therefore complements the thickness estimates resulting from seismic studies along the Udupi–Kavali profile, which passes through the central part of the craton. Further, it has been shown by Kaila and Sain (1997) that the upper crust is 20 km thick. It is now widely recognized that the lower crust comprises dominantly granulite facies rocks. Several workers have shown that the southern Indian Archaean crust is an inclined cross-section of the earthÕs crust and that the gneisses and granitoids are underlain by granulites in the deeper section (Pichamuthu, 1965; Shackleton, 1976; Raase et al., 1986). From a study of metamorphic pressures in different geological sub-units of the Dharwar craton and the granulite terrain south of it, Percival et al. (1992) have constructed a longitudinal N–S cross-section showing inferred depth relationships beneath the two terrains. The study conclusively establishes a greenschist–amphibolite–granulite sequence from the surface downwards for the region. A general crustal model comprising 20 km granite–gneiss upper layer and a 17 km granulitic lower layer is considered here based on the results of above studies. Departure from this general scenario occurs in the amphibolite-to-

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granulite transitional zone (e.g., near Attibele), where the transitional rocks are exposed. South of this region, continuing along the inclined cross-section of the EarthÕs crust, the granulitic rocks are exposed. Heat production of granulites from various terrains worldwide is found to be generally low and have a small range (cf. Irving, 1974; Dupuy et al., 1979; Nicolaysen et al., 1981; Pinet and Jaupart, 1987; Fountain et al., 1987; Rudnick and Fountain, 1995). For the present study, a value of 0.2 lW m
3 is adopted. This value is the mean heat production recently measured at 73 granulite sites along a 250 km long profile from Krishnagiri to Dindigul in the Southern Granulite Province (SGP), a predominantly granulitic terrain contiguous to and south of the Dharwar craton in the southern Indian shield (Roy et al., 2003). Presence of a transition layer between the amphibolite facies upper crust and the granulite facies lower crust has been very well documented in the Dharwar craton (Pichamuthu, 1961; Janardhan et al., 1982; Hansen et al., 1984). The thickness of this layer, estimated from metamorphic pressure estimates, is 7 km (Janardhan et al., 1979; Gopalakrishna et al., 1986; Sen and Bhattacharya, 1990; Srikantappa and Hansen, 1992; Senthil Kumar, 2001). These transitional rocks, like granulites, are also strongly depleted in radioelements. Heat production measurements on such rocks have been made at 25 sites around Attibele, over a 5  10 km2 area. They yield uniformly low values with a mean of 0.4 lW m
3 . Simple layered models of heat production representative of the major lithologic units beneath these areas, heat flow contributions from individual layers, and mantle heat flow derived by subtracting the crustal contribution from the surface heat flow are shown in Fig. 2 and discussed area-wise.

367

Fig. 2. Heat flow and crustal heat production models for four representative crustal heat production scenarios in the Dharwar craton. Rock types constituting the crustal sections and their mean heat production values (in lW m
3 ) are given in the legend. The contribution to surface heat flow (in mW m
2 ) from each layer in the crustal column is shown within the layer. Surface heat flow at each site is shown at the top of the corresponding model along with the site name. Mantle heat flow, obtained by subtracting the crustal heat contribution from the surface heat flow for each site, is shown at the bottom of each model.

4.2. Kolar 4.1. Attibele The heat flow in the Attibele area (southeast of Bangalore), derived from two boreholes 207 and 247 m deep and 10 km apart, is 25 mW m
2 (Roy and Rao, 2000). This is the lowest heat flow observed in the Dharwar province. Attibele is located near the northern fringes of the granulite province. The heat production value at Attibele, 0.4 lW m
3 , is representative of granite–gneiss affected by granulite metamorphism. As discussed earlier, the thickness of the transition layer is 7 km. The observed low values of heat production at the surface are therefore likely to continue up to 7 km, and thereafter merge with typical granulitic values (0.2 lW m
3 ) characteristic of the lower crust. Because the whole crust is low in heat production, the uncertainty in such a partitioning would affect the crustal heat-contribution estimate by not more than a few mW m
2 only, which is considered to be insignificant.

The data from Kolar Gold Field (KGF) is representative of a crustal segment with tonalitic gneiss dominating the upper crust. The KGF is located in the central part of the 80 km long, NS-trending, Kolar greenstone belt. The metavolcanic rocks comprising the greenstone belt extend up to at least 4 km depth, and are surrounded and underlain by tonalitic gneisses. Heat flow, measured up to a depth of 2150 m in the Nundydrug mine, which is part of the gold field, is 40 mW m
2 (Rao, 1970). Fifteen samples of metavolcanics yielded a mean heat production of 0.25 lW m
3 , while 29 samples of the tonalitic gneisses (on either side and in the immediate vicinity of the gold field) yielded a mean heat production of 1.5 lW m
3 . The model shown in Fig. 2 comprises a 4 km metavolcanic upper layer, followed successively by a 9 km tonalite layer, a 7 km transition layer, and a 17 km granulite lower crustal layer.

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4.3. Dharmavaram Measurements in eight boreholes, ranging in depth from 150 to 280 m and covering a 10  35 km2 area of the Peninsular Gneissic Complex, define a mean heat flow of 40 mW m
2 (Gupta et al., 1991). Analysis for U, Th and K have been carried out for 24 fresh samples of gneiss collected from rock quarries at sites close to the boreholes. The mean heat production for the rocks in the area is 1.7 lW m
3 , not much different from the value of 1.5 lW m
3 obtained on a larger set of samples typical of tonalitic gneisses in the province. Therefore, for the general, first order model envisaged here, the latter value is adopted. The heat production model comprises 13 km tonalitic gneisses, followed by 7 km of a transition layer and the lower crust of granulites. 4.4. Gadag Heat flow obtained from two boreholes, 520 and 240 m deep, is 29 mW m
2 . The heat flow site is in the center of a 25 km-wide greenstone belt, which comprises a thick sequence of metavolcanic and metasedimentary rocks. The surrounding gneisses are dominantly tonalitic. Measurements on the metavolcanics and the metasediments yield typical heat production averages of 0.15 and 0.85 lW m
3 respectively (Gupta et al., 1991). A 65:35 partitioning for metavolcanics and metasediments based on geological studies (R. Srinivasan, personal communication), yields a mean value of 0.4 lW m
3 . Modeling of gravity data in the northern part of greenstone belt shows that the upper greenstone layer at Gadag likely extends up to a maximum of 8 km (Verma and Subrahmanyam, 1984; Narain and Subrahmanyam, 1986). The heat production model therefore comprises 8 km of greenstone cover, followed by 5 km of tonalitic gneisses, 7 km of a transition layer and then the lower crust of granulites. The tonalite layer immediately underlying the greenstones is thinned to 5 km to account for the low heat flow. In view of the good quality of heat flow data, it appears that heat flow does provide a useful constraint for the crustal heat production structure. The four crustal heat production models yield a range of mantle heat flow estimates, 12–19 mW m
2 , for the Dharwar craton (Fig. 2). The range covers the major crustal heat production scenarios that can be envisaged in the province. The estimates are robust because a large fraction of the crust is very low in heat production and the surface heat flow is also low. Major heat flow variations within the province are attributable to variations in upper crustal heat production. Estimates of mantle heat flow from similar studies in other Archaean terrains such as in parts of the Canadian, Baltic and Indian shields, and southern and western parts of the African shield are in the range 10–20 mW m
2 (Fountain et al.,

1987; Jaupart et al., 1998; Pinet et al., 1991; Mareschal et al., 2000; Pinet and Jaupart, 1987; Kukkonen, 1989; Pasquale et al., 1991, 2001; Balling, 1995; Roy and Rao, 1999a; Nicolaysen et al., 1981; Jones, 1988; Lesquer and Vasseur, 1992).

5. Crustal temperatures The computation of crustal temperatures is primarily governed by the surface heat flow and distribution of heat production and thermal conductivity with depth. Assuming 1-D heat conduction to be the dominant mechanism of heat transfer in stable shield areas, temperatures at different depths in the crust have been computed following the method described by Chapman (1986). Four 1-D models for A(z) were considered covering all the likely crustal heat production scenarios in the craton. The models can therefore provide useful upper and lower bounds on the crustal thermal structure of the craton. For the thermal conductivity distribution with depth, the temperature and pressure dependent conductivity distribution proposed by Chapman (1986) was adopted. Conductivity was considered to be 3.0 W m
1 K
1 for the upper crust and 2.5 W m
1 K
1 for the granulitic lower crust, based on several hundred measurements on different rock types from southern India (cf. Roy and Rao, 2000; Roy et al., 2003). Temperatures were computed by dividing the crustal column into thin layers of 0.1 km thickness so that heat production and thermal conductivity as functions of depth are adequately approximated. The temperature-depth profiles computed for the four plausible crustal scenarios in the Dharwar craton are shown in Fig. 3. The temperature estimates at the base of the crust range from 285 °C at Gadag to 410 °C at Kolar. Temperatures at Attibele and Dharmavaram are intermediate to those at Gadag and Kolar. Higher crustal temperatures result from a combination of higher heat flow and lower crustal radioactive heat production, and vice versa. These temperature estimates are generally consistent with the steady state geotherms for stable continental crust proposed by Chapman (1986). The study therefore presents a strong case for a relatively cold crust beneath the craton. Uncertainties in thermal conductivity, heat production and thicknesses of the different rock layers constituting the crust cause overall uncertainties in temperature estimates at different depths. The individual uncertainties in these parameters are allowed to propagate through the computation procedure to obtain an overall uncertainty estimate for temperature. In the present study, the individual parameters were allowed to vary within 10% for thermal conductivity, 30% for crustal heat production, and 10% for layer thickness, in keeping with the general nature of variation observed

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369

in a small range, 12–19 mW m
2 . The mantle heat flow at Killari (18°030 N, 76°330 E), located in the southeastern part of the DVP, estimated from similar considerations, is 12 mW m
2 (Roy and Rao, 1999a). The flood basalt sequence in the southern part of the DVP vary in thickness, generally from several tens of metres in the east to a maximum of 3 km in the west. The uniformity in mantle heat flow in the two regions suggests that the crustal thermal structure in the Dharwar craton extends to the north beneath the relatively thin, 65 My old, Deccan flood basalt cover, at least in the southern part of the DVP. This result is corroborated by the similarity in upper mantle seismic velocity structures (1–3% higher than normal) obtained from tomographic experiments carried out in the two regions (Ramesh et al., 1993). The mantle heat flow estimates from the two regions in the shield compare favourably with the mantle heat flow estimates reported earlier for similar age tectonic provinces such as the Canadian shield, the southern African shield and the Baltic shield. Consideration of four realistic, one dimensional, crustal heat production models representative of the dominant crustal scenarios in the craton allowed us to estimate upper and lower bounds for crustal temperatures in the craton. The four scenarios cover the extremes in heat flow and crustal heat production in the province. The crustal columns are representative of three situations: (a) a dominantly tonalitic upper crust, (b) upper crust comprising amphibolite-to-granulite transitional rocks, and (c) upper crust comprising greenschist and amphibolite facies rocks, the lower crust comprising granulite facies rocks in all the cases. The temperature estimates at the base of the crust have a range from 285 to 410 °C, pointing to a generally ÔcoldÕ crust in the region. For sub-crustal depths, the estimates are progressively degraded because of their dependence both on the model of heat transfer adopted and on thermal conductivity and heat production estimated for the lower crust and uppermost mantle, all of which carry very large uncertainties. Therefore, calculations have not been attempted for greater depths. There exist areas in the Dharwar craton where heat flow values have a range characteristic of the province but the heat production of the gneissic rocks at the surface are abnormally high. Two areas, Hyderabad and Pattikonda (Fig. 1) are discussed here. Heat flow

Fig. 3. Crustal temperature-depth profiles for four sites representing possible heat flow and crustal heat production scenarios in the Dharwar craton. A temperature and pressure dependent conductivity distribution KðT ; zÞ (given as inset) has been used. K0 is the conductivity at 0 °C and 1 atmospheric pressure, b is the temperature coefficient, c is the pressure coefficient, and T is the temperature in °C. Uncertainty in temperature estimates has been discussed in the text; uncertainties in temperatures at 10, 20, 30 and 37 km depths are given in Table 3.

in the large data sets generated from several provinces in India. The uncertainty thus introduced in the model is intended to provide realistic bounds to the temperature estimates obtained from such calculations. Computations were carried out for 5000 sets of input parameters using a Monte Carlo algorithm. The mean and standard deviation of the temperature estimates at depths of 10, 20, 30, and 37 km (i.e., Moho in the present case), for the heat flow and crustal heat production models considered in this paper, are given in Table 3.

6. Discussion and conclusions Based on a good coverage of heat flow for the Dharwar craton, heat production data for the major rock types constituting the crustal column, and structural constraints inferred from available deep seismic sounding, gravity and geological investigations, robust estimates of mantle heat flow for the Dharwar craton have been obtained. Mantle heat flow is estimated to be

Table 3 Mean and standard deviation (SD) of temperature estimates at different crustal depths Location Attibele Kolar Dharmavaram Gadag

T (10 km), °C

T (20 km), °C

T (30 km), °C

TMoho (37 km), °C

Mean

SD

Mean

SD

Mean

SD

Mean

SD

111 152 140 119

8 15 14 9

196 242 214 183

16 30 25 15

272 331 284 242

26 49 41 25

321 388 329 279

32 63 53 33

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measurements at Hyderabad, carried out at three localities covering an area of 20  40 km2 , have yielded a mean heat flow of 40 mW m
2 (Gupta et al., 1987) (Table 1). Extensive heat production measurements carried out on major rock types present in and around Hyderabad yield consistently high albeit variable values. The dominant rock types are grey and pink granites. The mean heat production, resulting from measurements at 205 sites, is 6.2 lW m
3 (Table 2). Although Hyderabad is located more than 200 km north of the Kavali–Udupi deep seismic sounding profile, recent crustal thickness estimates for the area using teleseismic receiver function analysis show that the crust is only marginally thinner (33  2 km) than the average value of 37 km for the region as considered in the present study (Saul et al., 2000). A decrease in the thickness of the upper crust could affect the crustal heat-contribution estimate, whereas a thinning of the lower crust would have little effect. However, no information on resolution of the upper and lower crustal thicknesses are available. Heat flow measurements at nine localities distributed over a 25  25 km2 sub-area ÔAÕ in the Adoni–Pattikonda area have yielded a mean heat flow of 50 mW m
2 (Roy and Rao, 2000) (Table 1). Heat production measurements carried out over outcrops at 40 sites in the area have yielded consistently high values with a mean of 7.6 lW m
3 (Table 2). Measurements conducted at another set of 40 sites in the surrounding region (subarea ÔBÕ) yielded a mean heat flow of 35 mW m
2 and heat production of 4.1 lW m
3 . The heat production values observed at the surface in both the areas, Hyderabad and Pattikonda, if continued deeper up to 7 km would account for the entire observed heat flow. This would be unrealistic. The study therefore shows that robust heat flow estimates for an area, obtained from a number of well-distributed boreholes, place strong constraints for the high heat producing rocks at the surface to be confined to a thin veneer only, which would be followed deeper, by the regionally extensive tonalitic and granodioritic gneisses. Therefore the crustal heat production models for the two areas could be considered to be broadly similar to that of Kolar or Dharmavaram, with the uppermost 0.5–1 km of the tonalitic layer replaced by the high heat producing surface granitoid layers. Such situations have been previously observed in other shields. The Doodlakine site in Western Australia, the Lac du Bonnet batholith and the English River sites in the Superior province of Canadian shield, and the Ilimaussaq intrusion in southern Greenland shield stand out as examples of such sites (Sass and Lachenbruch, 1979; Jessop and Lewis, 1978; Drury and Taylor, 1987; Sass et al., 1972). Heat flow values in these areas are only marginally higher than the regional heat flow range, but the heat production are several fold that of the regional heat production.

A complete characterization of the thermal structure of the southern Indian Precambrian crust requires a detailed study of the SGP, which comprises dominantly high grade metamorphic rocks exposed over a large region to the south of the Dharwar craton (Fig. 1). From a very limited data set for the SGP, Gupta et al. (1991) suggested that the mantle heat flow in the terrain was likely to be low, similar to that in the Dharwar craton. Because the granulite facies rocks are generally depleted in heat producing elements, the crustal contribution to heat flow would be smaller than for the Dharwar crust. Such a scenario could lead to potentially significant differences in mantle heat flow and crustal temperature estimates between the two regions. Data are available for only one site in the SGP. Detailed heat flow and heat production measurements coupled with good structural control for the crustal column in the SGP are essential for a better understanding of any possible differences between the crustal thermal structures of the two adjoining regions. The study re-emphasizes the significance of acquiring detailed structural information using high resolution seismic and other geophysical methods as well as better constraining the heat flow and crustal heat production in order to characterize the thermal structure for large continental regions.

Acknowledgements Extensive discussions on regional geology with R. Srinivasan resulted in constraining better the crustal structure and composition. The paper has benefited from a constructive, pre-submission review by David S. Chapman, and from valuable suggestions for improvement by two anonymous reviewers. We are thankful to G. Ramacharyulu and C. Shankar for technical assistance. We acknowledge the help received from A. Yadagiri and G. Ramulu during field and laboratory measurements. The work was carried out during the tenure of a research fellowship provided to SR by the Council of Scientific and Industrial Research, India. We are grateful to Director, NGRI for support to the heat flow studies programme of the Institute.

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