TERRESTRIAL HEAT-FLOW DETERMINATIONS FROM NORWAY* G. GrØnlie, Institutt for geologi, Universitetet i Oslo, Blindern, Oslo 3, Norway.

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1 TERRESTRIAL HEAT-FLOW DETERMINATIONS FROM NORWAY* GISLE GRØNLIE, KNUT S. HEIER & CHANDLER A. SWANBERG Grønlie, G., Heier, K. S. & Swanberg, C. A. : Terrestrial heat-flow determinations from Norway. Norsk Geologisk Tidsskrift, Vol. 57, pp Oslo Eight new heat-flow determinations from Norway based on data from 15 drill hoies are presented. The new data bring the heat-flow determinations in Norway to a total of 48 values, of which 24 are determined from drill-hole measurements and 24 from lake measurements. Six of the new values are from the Baltic Shield and the Permian Oslo Region. The mean value of these is 0.94 ± 0.29 hfu, which is in good agreement with previously published results from both Norway and the Baltic Shield in general. The new values support the idea of a zone of anomalous low-mantle heat flow to the east of the Caledonian mountains in Norway. Two of the heat-flow determinations are from the Caledonian mountain belt. These are consistent with published results from Norway and other Paleozoic orogenic areas. G. GrØnlie, Institutt for geologi, Universitetet i Oslo, Blindern, Oslo 3, Norway. K. S. Heier, Norges geologiske undersøkelse, P. O. Box 3006, 7001 Trondheim, Norway. C. A. Swanberg, College of Arts and Science, Department of Physics, Box 30, Las Cruces, New Mexico 88003, USA. This determination of terrestrial heat flow in drill hoies was carried out by Institutt for geologi and Mineralogisk-geologisk museum, Universitetet i Oslo, as part of the Norwegian Geotraverse Project (Heier 1969) which is supported financially by the Norwegian Research Council for Science and the Humanities (NAVF). The heat-flow project was started in 1969 and was divided into two parts: To determine heat flow from diamond drill hoies. To determine heat flow from measurements in!akes. The first part of the project was in the beginning performed in cooperation with Professor Simmons of Massachusetts Institute of Technology, but in the past two years we have been able to carry out the conductivity measurements and the data reductions at Institutt for geologi. The second part of the project has been carried out in cooperation with the Niedersachisches Landesamt flir Bodenforschung. We present eight new heat-flow values for Norway based on measurements in diamond drill hoies. The values supplement the previously published fifteen heat-flow determinations from drill hoies published by Swanberg et al. (1974) and the twenty-four heat-flow values from lakes in southern Norway Publication No. 122 in the Norwegian Geotraverse Project.

2 154 G. GRØNLIE, K.S. HEIER & C. A. SWANBERG Bjørnevatn 1,03 o km Fosdalen 1,14 Verdal1 8 s * Fåberg 1;18 Hurdalo,83 Lier 1,25 Hobbøl1,19 "' - Hurum Lassedal 1,06 B Permian ( 1 > Caledonian ( 2 > \\% Precambrian ( 3 > + New values, this paper Swanberg et al Fig. l. Distribution of heat-flow values (hfu) in the Precambrian Baltic Shield (3), the Paleozoic Caledonian orogenic belt (2 ), and the Permian Oslo Region (1). given by Hanel et al. (1974). Our data are from holes all over Norway. We consider the heat-flow determinations from Bjørnevatn and Bidjovagge in north Norway to be especially valuable, not only because heat-flow deter-

3 TERRESTRIAL HEAT-FLOW DETERMINATIONS 155 minations from the Baltic Shield in Scandinavia and the Soviet Union until now have been rather few (Puranen et al. 1968, Lubimova et al. 1972) but also because of the northern latitude of these locations. Geology For the present discussion, it is convenient to consider the geo1ogy of Norway in terms of three geological provinces: the Precambrian Baltic Shield, the Paleozoic Caledonian orogenic belt, and the Permian Oslo Region. The following generalized geology includes only the information pertinent to the present study. For a more complete treatment, see Holtedahl (1960). The Baltic Shield covers a large portion of Fennoscandia, including southem and west central Norway (Fig. 1). It is of Precambrian age, but includes different ages of orogenic and intrusive activity. Rock types include amphibolite to granulite facies metamorphic rocks of varying composition, and numerous ultramafic to felsic intrusions. The segment of the shield in west central Norway was, at least in part, modified during the Caledonian orogeny. The Paleozoic Caledonian orogenic belt extends from south central Norway northeastwards along the axis of the present-day mountain system to north Norway (Fig. l) and includes a large portion of the central Scandinavian Peninsula. Rocks range in age from young Precambrian to Early Devonian, and consist of low-grade (greenschist) to moderate-grade (amphibolite facies) metasediments and metavolcanics. Caledonian intrusives, principally gabbros and trondhjemite, are quite common. The Permian Oslo Region, a sub-volcanic igneous province of alkaline intrusives and extrusives, is confined to the Oslo graben, a tectonic feature generally regarded as representing a Permian rift system. Crustal structure and the nature of the crust-mantle interface in this area are quite complex (Kanestrøm 1971, Ramberg & Smithson 1971, Ramberg 1975), and the thickness of the crust appears to increase from about 32 km in the Oslo area to about 41 km in the interior of central Norway (Kanestrøm & Haugland 1971, Sellevoll & Sundvor 1969). Thermal conductivity The conductivities were measured at Institutt for geologi, Universitetet i Oslo, using a steady-state method similar to that described by Birch (1950), with an apparatus built by Combs & Simmons (1973) at the Massachusetts Institute of Technology. A detailed description of the measuring technique is given by Swanberg et al. (1974). The conductivity of each sample was determined at!east twice to assure repeatability to within + 3 %. Calculation of heat flow The data reduction procedure of Swanberg et al. (1974) is followed in de-

4 156 G. GRØNLIE, K. S. HEIER & C. A. SWANBERG tail. In all cases, heat flow is taken as a product of the least-squares temperature gradient over the most uniform section of the hole, and the harmonic mean of the conductivity over the same section. The geothermal gradient has been modified where applicable for the following conditions. Topography Because of the rugged and frequently spectacular terrain in Norway, terrain corrections have been calculated for all drill hoies, using the technique developed by Birch (1950). Corrections include the effect of topography out to a radius of 15 km from the drill hole (25 km for the deep hoies) in order to include over 85% of the maximum correction (Birch 1950). The decrease of surface temperature with increasing elevation is taken as Cjkm, a least-squares fit to mean annua! temperature-elevation data from over 80 meteorological stations from the interior of central and southern Norway (Norsk Meteorologisk Årbok ). In most cases, the corrections are small ( < 5 %) and tend to decrease the standard error. Lateral variations in surface temperatures Several of the drill hoies are located in the vicinity of small lakes. Mean annual temperatures at the bottoms of several lakes of comparable size (G. Kjensmo, pers. comm. 1971) are very near the mean annual air temperature (Norsk Meteorologisk Årbok ) for nearby meteorologic stations, the maximum difference being 1.2 C. Maximum corrections for this effect, based on a two-dimensional approximation (Spiegel 1964: 243), range from 0.0 to 0.8 Cjkm, depending upon the distance from the drill hole to the lake and upon the depth range used in deterrnining heat flow. These corrections are subsequently ignored. All drill hoies are sufficiently distant from the ocean so that corrections from this source can be ignored. Re fr action Thermal refraction around bodies of differing conducticity is likely to introduce significant errors in some of the heat-flow determinations, since many of the hoies were drilled in search of highly conductive ore bodies. These errors are difficult, if not impossible, to estimate due to the complexity of the geology and the lack of knowledge of the thermal conductivity contrasts. Further, there is no guarantee that these errors will be manifest in the internal consistency of the data. Swanberg et al. (1974) found maximum variations as high as hfu in different sections within one hole, at Fosdalen and Tverrfjellet mines in central Norway. Anisotropy In metamorphic rocks, especially among quartz-mica schists, conductivity measurements (Diment & Werre 1964, J. M. Østby and K. L. Østby pers. comm.) on saturated samples in directions parallel and perpendicular to the plane of foliation indicate a difference of up to 30 %. Since the samples were

5 TERRESTRIAL HEAT-FLOW DETERMINATIONS 157 too small to measure anisotropy, the error limits must be increased on the hoies with quartz-mica schists. Paleoclimatic temperature variations Heat-flow values corrected for the effects of paleoclimatic temperature variations have not been used in the interpretation, due principally to the large uncertainty in estimating the required parameters and to the observations (Sass et al. 1971) that vertical variations in heat flow attributable to this source were not observed in a 3 km drill hole, even though clear geologic evidence suggests that Pleistocene temperatures were approximately 3.5 C colder than present. Nonetheless, sample calculations based on the technique of Birch (1948) have been made in order to gauge the range of possiple errors attributable to this source. A reasonable model for Pleistocene temperature variations is shown by Swanberg et al. (1974). They found that these corrections were less than 1.8 Cfkm and would increase the heat flow approximately hfu, depending upon the conductivity. These corrections are of comparable magnitude, but are opposite in sign to the effect of post-glacial isostatic uplift (following section), and the omission of both corrections appears justified. Uplift and erosion Swanberg et al. (1974) considered the effect of uplift and erosion (i.e. isostatic rebound) following the retreat of the glaciers as a possible source of error. This effect is largest in the Oslo area and eastern Norway, where shoreline data (Hafsten 1960) indicate an uplift of 250 m in the last 10,000 years. The standard terrain-correction program was modified to include this effect according to the scheme of Birch (1950). Maximum corrections to the geothermal gradient resulting from postglacial uplift are about 2 Cfkm and are opposite in sign to the corrections for paleoclimatic effects. Both corrections have been ignored in the values. Data Because of the large number of data being presented, each drill hole is not described in detail. The following areas, however, bear special mention: Hobbøl. - This hole was drilled especially for heat-flow analysis in the Østfold gneisses, to the east of the Oslo Permian province. Rena (Galå, Slettås). - These hoies were extended ca. 100 m into bedrock (gneiss) for heat-flow analysis after having been drilled through sandstone down to bedrock looking for disseminations. Glamsland. - This hole was drilled into a homogenous pegmatite especially for heat-flow analysis.

6 158 G. GRØNLIE, K. S. HEIER & C. A. SWANBERG Bjørnevatn. - These hoies lie around and are drilled into the iron ore formation at Kirkenes right at the border between Norway and Soviet-Union. The formation lies within the Baltic Shield. Bidjovagge. - Two hoies were measured which penetrate a sulfide formation. The heat-flow values may be low because the measured conductivity may not reflect the true sulfide content, thereby giving a too low conductivity. The Bidjovagge hoies also lie within the Baltic Shield. V er dal. - This hole was drill ed in homogenous limestone and should gi ve a representative value for this part of the Caledonides. Løkken. - The two hoies were situated ca. 400 and 1000 m down in the mine and penetrate mostly greenstone. The values (0.62 hfu) are lower than the average heat flow from the four hoies measured by Swanberg et al. (1.29 hfu). The mean of all six values is 1.07 hfu. Norway east of the Caledonian orogeny Of the eight heat-flow determinations, six are located in the Baltic Shield and the Permian Oslo Region (Table 1). The values range from 0.57 hfu (Bidjovagge) to 1.22 hfu (Hobbøl) with an average of hfu. This is in good agreement with earlier published results from these areas in Norway; Swanberg et al. (1974) have an average of 0.99 hfu (3 values) and Hanel et al. (1974) have an average of hfu (24 values which also include values from the Caledonian mountain chain). Our results seem also to be consistent with the data published from other parts of the Baltic Shield: Lubimova et al. (1972) 0.75 and 0.60 hfu from the Kola Peninsula, Puranen et al. (1968) hfu (5 values) from Finland, and Majorwicz (1973), 1.01 hfu (197 values) from Poland. The assumed anomalous low mantle heat-flow zone east of the Caledonian mountains argued by Swanberg et al. (1974) and apparently confirmed by Hanel et al. (1974) may still be a possibility. The low heat-flow values from the two hoies at Bidjovagge (0.57 hfu) indicate that the low mantle heatflow zone may continue to the east of the Caledonian mountain chain also in north Norway. Caledonian orogenic belt Two values are (Table 2) from the Caledonian mountain. chain, 1.18 hfu (Verdal) and 0.62 hfu (Løkken, 2 hoies). The Verdal values are in good agreement with the average values found by Swanberg et al. (average hfu, 9 values). Our results from Løkken are lower than the mean value that Swanberg et al. presented based on 4 holes. The mean of all six measurements at Løkken give a value of 1.07 hfu.

7 Rock type Average Heat flow heat flo\\ (hfu) (hfu) , (Il..., - > t""' = > ';"l 'li t""' o o..., s:: - z >..., - o z (Il.. Ul \0 Table L Heat-flow data from the Precambrian Baltic Shield and the Caledonian orogenic belt. -- K Depth dt/dz*1 dt/dz 2 10-s cal Location Hole range Error Error ('C/km) ( C/km) (m) cm sec o c Error No. of samples Rena!Galå Rena/Slettås Average Rena Hobbol Bjornevatn Bjornevatn Average Bjornevatn Biddjovagge S40B Biddjovagge S44A Average Biddjovagge Gneiss Gneiss 7 Sandstone 7 Sandstone lron ore lron ore 10 Gneiss Gneiss,.77 8 Gneiss, disseminated ore disseminated ore 4 Amphibolite.20 5 Diabase + 1 Corrected for deviation from the vertical. 2 Corrected for deviation and topography.

8 Table 1. Contin... 0\ o Location Hole Depth K Average dt/dz*1 dt/dz*2 Er ror Error 1Q--3 cal No. of range Er ror Rock type Heat flow heat flow 0 (0C/km) CC/km) samples (m) cm sec C (hfu) o :;ti '0. Glamsland Pegmatite 1.11 z Lassedal Gneiss.80 t: Lassedal Gneiss Lassedal Gneiss 1.01 Lassedal Gneiss 1.12 Lassedal Gneiss 1.33 ::r: Average 1.06 Lassedal :;ti Verdal Limestone 1.18 Ro Lokken Greenstone.63 0 Lokken Greenstone.60 ;> A verage Lokken.62 Vl *1 Corrected for deviation from the vertical. > *2 Corrected for deviation and topography..34 z t:l:l :;ti o

9 TERRESTRIAL HEAT-FLOW DETERMINATIONS 161 The values presented from the Caledonian orogenic belt are similar to model values from other stable continental areas, including areas of Paleozoic orogenic activity (Lee & Uyeda 1965). Acknowledgements. - The data could not have been compiled without considerable assistance from Norges Geologiske Undersøkelse and the following Norwegian mining companies: NS Bleikvassli Gruber, Firma H. Bjorum, NS Norfloat, Norsk Hydro NS, Orkla Gruber NS, and NS Sydvaranger. We hereby express our appreciation for all hel p. NAVF supported the project financially through the Norwegian Geotraverse project (D ). We would also like to thank Bjorums Legat for help in financing this project. The following persons are thanked for their assistance: K. L. and J. M. Østby for helping with the data collection and for doing the conductivity measurements, and O. Moller for technical assistance. March 1976 REFERENCES Birch, F. 1949: The effects of Pleistocene climatic variations upon geothermal gradients. Am. l. Sei. 246, Birch, F. 1950: Flow of heat in the Front Range, Colorado. Bull. Geol. Soc. Am. 61, Combs, J. B. & Simmons, G. 1973: Terrestrial heat-flow determinations in the north central United States. J. Geophys. Res. 78, Diment, W. H. & Werre, R. W. 1964: Terrestrial heat flow near Washington, D.C. J. Geophys. Res. 69, Hafsten, U. 1960: Pollen-analytic investigations in South Norway. In: O. Holtedahl (Editor), Geology of Norway, Nor. Geo[. Unders. 208, Heier, K. S. 1969: Introduction of a Norwegian geotraverse project. Nor. Geol. Tidsskr. 49, Holtedahl, O. 1960: Geology of Norway. Nor. Geo[. Unders. 208, 540 p. Hiinel, R., GrØnlie, G. & Heier, K. S. 1974: Terrestrial heat flow determinations from lakes in southem Norway. Nor. Geol. Tidsskr. 54, Kanestrom, R. 1971: A crust-mantle model for the NORSAR area. In: Crustal structure in southeastem Norway from seismic refraction measurements. Seismol. Obs. Univ. Bergen, Sei. Rep. 5 (2), Kanestrom, R. & Haugland, K. 1971: A refraction profile through southeastern Norway. In: Crustal structure in southeastem Norway from seismic refraction measurements. Seismol. Obs. Univ. Bergen, Sei. Rep. 5 (1), Lee, W. H. K. & Uyeda, S. 1965: Review of heat-flow data. In: W. H. K. Lee (Editor). Terrestrial Heat Flow, Geophys. Monogr. Ser, 8 AGU, Washington, D.C Logn, Ø. & Evensen, E. 1973: Thermal conductivities of some ores and rocks in Norway. Nor. Geo[. Unders. 300, Lubimova, E.A., Karus, E. V., Firsov, F. V., Starikova, G. N., Vlasov, V. K., Lyusova, L. N. & Koperbakh, E. B. 1972: Terrestrial heat flow on Precambrian shields in the U.S.S.R. Geothermics, l, Majorwicz, J. A. 1973: Heat flow data from Poland. Nat. Phys. Sei. 241, Norsk Meteorologisk Årbok, 1970, Prep. Det Norske Meteorologiske Institutt. Aschehoug, Oslo. Puranen, M., Jiirvimiiki, P., Miimiiliiinen, U. & Lektinen, S. 1968: Terrestrial heat flow in Finland. Geoexploration 6, Ramberg, I. B. 1976: Gravity interpretation of the Oslo Graben and associated igneous rocks. Nor. Geol. Unders. 325, 194 p. Ramberg, I. B. & Smithson, S. V. 1971: Gravity interpretation of the southem Oslo graben and adjacent Precambrian rocks, Norway. Tectonophysics 11, Sass, J. H., Lachenbruch, A. H. & Jessop, A. M. 1971: Uniform heat flow in a deep

10 162 G. GRØNLIE, K. S. HEIER & C. A. SWANBERG hole in the Canadian shield and its paleoclimatic implications. J. Geophys. Res. 76, Sellevoll, M. & Sundvor, E. 1969: APn timeterm survey in southeastern Norway. Seismol. Obs. Univ. Bergen, Sei. Rep. 2, 13 p. Spiegel, M. R. 1964: Theory and Problems of Complex Variables. Schaum, New York, 313 p. Swanberg, C. A., Chessman, M. D., Simrnons, G., Smithson, S. B., GrØnlie, G. & Heier, K. S.: Heat flow generations studies in Norway. Tectonophysics 23,