|Heat flow distribution and geothermal provinces in the Baltic basin|
|Belarus-Baltic Granulite Province|
|East Lithuanian province|
|West Lithuanian Granulite Province|
Heat flow data of the Baltic sedimentary basin was discussed in different studies (Zui et al., 1996; Sliaupa, Rasteniene, 2000; Urban, 1993; Kukkonnen, Joeleht, 1996). Also, extensive geothermal information was collected in adjacent Scandinavia (Balling, 1995; Pasquale et al., 1991), Belarus (Zui, 1993), and Poland (Sokolowski, 1995). Concerning the heat production of the sedimentary and basement rocks there are only limited measurements (Zhuk, 1993; Sliaupa, Rasteniene, 2000; Urban, 1989; Kukkonen, Joeleht, 1997), while the thermal conductivity data are even scarcer (Rasteniene et al., 1998; Kukknonen, Joeleht, 1997; Urban, 1993).
Heat flow measurements are rather unevenly distributes throughout the basin, also data quality are quite different. In most cases temperatures were measured by using electric temperature logging in oil exploration and mapping wells depth of which range from a few hundred meters to a few kilometers. According to conventional methodology, the wells were kept in quiescence for more than 10 days to ensure temperature equilibrium. The central part of the basin is best studied in western Lithuania and western Latvia relating to extensive oil exploration (Sliaupa, Rasteniene, 2000). Thermal logging was performed in more than 100 wells, depth of which ranges in order of 1-2 km. It is important that considerable part of the sections in this part of the basin is represented by Silurian shales showing only miserable variations in lithology (accordingly, thermal conductivity) and they are not affected by any active hydrodynamic processes that might considerable disturb geothermal field in overlying and underlaing aquifers. Therefore, Silurian represents excellent geologic body to derive the heat flow values. The basin flanks in the north-east, east and south are less representative in terms of studied wells and quality of data. The thermal measurements were performed mainly in scarce mapping wells depth of which is in a range of several hundred meters. Moreover, they are dominated by high-permeability rocks that are affected by intense water flow disturbing geothermal field. As a result, geothermal measurements in the basin flanks are less representative. The Baltic sea, confined to the axial part of the basin, remains the least studied area. Temperature data were only reported from several offshore oil exploration wells drilled by Petrobaltic company in eighties (Laskova, 1994). The quality of measurements is lower compared to onshore studied due to shorter equilibrium period before thermal logging. Still, these wells provide invaluable information on heat flow in vast offshore part of the basin.
Heat flow distribution and geothermal provinces in the Baltic basin
Heat flow measured at the surface reflects at large the loss of heat generated by radioactive production with the crust, mantle heat loss that includes radioactive production, thermal convection and gravity energy and heat loss from the core. Still, the other factors also contributed to heat flow variations in the basin.
The crustal heat production is one of most important parameters controlling the heat flow pattern as it was recognised from good correlation of the heat production of the crystalline basement rocks and heat flow density in different parts of the basin (Zhuk, 1993; Sliaupa, Rasteniene, 2000; Urban, 1993; Zui et al., 1996; Baling, 1993). Some convective heat transfer in the sedimentary cover and crystalline basement seems to disturb heat flow along some faulted path ways (Zui et al., 1998; Sliaupa, Rasteniene, 2000). Some hydrodynamic cooling is suggested to affect the shallow basin flanks (Zui, 1986; Sliaupa, Rasteniene, 2000). In hydrogeological terms the basin periphery represents the recharge area (high topography, domination of high-permeability lithologies deposited in shallow water environments throughout the Phanerozoic thus resulting in intense downflushing of the meteoric waters) (Sliaupa, Puronas, 2001). Palaeoclimate changes also imprint temperature profiles, which was used for palaeoclimate reconstructions in Belarus (Zui, 2000) and Lithuania (Zui et al., 2000).
The heat flow intensity reflects the tectonic setting and igneous history of the crustal lithotectonic units (Mareschal et al, 1999). This is well reflected in the Baltic basin that shows quite regular pattern in the heat flow, which in turn, shows good correlation with defined Early Precambrian domains.
Fig.1. Heat flow map of the Baltic region
Fig.2. Heat production of the crystalline basement rocks
Belarus-Baltic Granulite Province (BBGP)
The BBGP is confined to the large-scale lithotectonic terrain underlying the eastern shallow periphery of the basin. The heat flow ranges in order of 30-40 mW/m2 which is less than the average heat flow 43 mW/m2 of the East European Platform (Fig.1). The heat flow values decrease to the east and are only 12-20 mW/m2 in the adjacent Central Belarus terrain (Zui et al., 1998). Some hydrodynamic cooling and palaeoclimate effect might have distorted the heat flow in this area. Still, the crustal lithology and low mantle heat flow are the main parameters that led to the low heat flow in the basin periphery.
In Belarussian part the heat production of dominating lithologies ranges from 0.07 mW/m3 to 1.5 mW/m3 (Fig.2). Zhuk (1993) noted good correlation of the heat production of the basement rocks with heat flow along the Grodno-Starobin DSS profiles crossing the Belarus High including the BBGP from the west to the east. This correlation is described by linear equitation
That sort of correlation provides important characteristics of geothermal provinces. At large, the surface heat flow measured in wells is a sum of heat flow generated in the earthxs crust and mantle. The discrimination between crustal heat production and the mantle component of the heat flow is essential to characterize the thermal structure of the lithosphere, which is yet a complex problem. The empirical relationship Q=Qy+D*A might be helpful (Birch et al., 1968; Roy et al., 1968). Qy is the intercept of a best-fitting line to data points and actually quantifies mantle and lower-middle crustal heat flow (mW/m2), coefficient D has dimensions of length and is interpreted as the depth scale of the surfacial heat producing layer (km). Accordingly, the density of the deep sourced heat flow in southern part of BBGP is 20 mW/m2 and the thickness of the upper thermal crust is 8.7 km that is close to the standard values of 10 km in the continents (Sclater et al., 1980). These values are rather typical for the Precambrian platforms. Estimates from different regions provide the range of the mantle flow in the cratonic areas from 5 mW/m2 to 28 mW/m2, typically it is 10-20 mW/m2. Compared to other uplifted cratonic areas, the mantle heat flow in Finland is suggested to be as high as 12x5 mW/m2 (Kukkonen, 1998; Kukkonen, Joeleht, 1996), in Canadian shield it is 13-15 mW/m2 (Jaupert et al., 1998). The low mantle heat flow in the Belarus High might be accounted to thermal insultation of the thick lithosphere (e.g. Ballard, Pollack, 1987; Nyblade, Pollack, 1993), thickness of which is suggested as much as >250 km (Zui, 1993). Assuming mantle and lower crustal heat flow as high as 20 mW/m2, it leaves only 10-20mW/m2 heat flow produced by thermal upper crust that is consistent with average low heat production of the early Precambrian rocks of the BBGB. The lowest values 0.07-0.09 µW/m3 were estimated for mafic intrusions, whereas mafic granulites of the Shuchin Complex have higher heat production 0.20 µW/m3. Granulitic metapelites show 1.4-2.5 µW/m3 heat production (Sliaupa, Rasteniene, 2000).
The northern granulite lithologies of the BBGP (Estonia, eastern Latvia) show similar low average heat production, most typical values range between 0.4 µW/m3 and 0.9 µW/m3 (Kukkonen, Joeleht, 1997; Urban, 1989; Zui et al., 1996), only local bodies show higher heat production 1.8-3.1 µW/m3. The correlation of the heat production of the crystalline basement rocks and surface heat flow is described by empirical equation Q=30+5.7A that implies higher heat flow from the mantle and lower-middle crust compared to that estimated in the southern half of the granulite belt, whereas thickness of the upper thermal layer is less. The former phenomenon might be related to the thinning of the mantle in the north. As it was mentioned before, more than 250 km thick lithosphere is suggested in the Belarus High (Zui, 1993), while twice as thin lithosphere is proposed in Estonian part of the belt by Kukkonen & Joelehte (1997).
In the north-east the granulites give way to amphibolite facies terrain (NE Estonia) that associates with increasing heat flow to 45-56 mW/m2. Accordingly, the average heat production increases considerable, exceeding 2 µW/m3 in most studied samples. The maximum heat flow reaching 62 mW/m2 was reported form the area of the Bay of Finland, which partly relates to high heat production of basement lithologies ranging from 2.1 to 5 µW/m3 (Kukkonen & Joeleht, 1997) and high mantle heat flow.
The heat flow systematically decreases north of the Baltic basin and attains minimum intensity 24 mW/m2 in the Karelian terrain that is accounted to the low heat production of crustal rocks and greater thermal resistance of the thick lithosphere. Actually, it follows rather classical spatial pattern between heat flow and the proximity of the Archean cratons (Nyblade, 1999).
The intensity of the heat flow averages 38-42 mW/m2, which is a little higher than that in adjacent BBGB. Some wells indicate anomalous high heat flow to 52-59 mW/m2, which is likely related to convective heat transfer along some faults (Rasteniene et al., 1998). The increase in heat flow intensity compared to the BBGB is evidently related to higher radiogenic activity of crustal lithologies of ELP. The heat production of dominating gneisses is around 1.4-1.5 µW/m3, supracrustal amphibolites also show rather high production 1.1-1.3 µW/m3 (Sliaupa, Rasteniene, 2000). These values are close to average heat production 1.0-1.2 µW/m3 (Jaupert et al., 1998) of cratonic rocks metamorphosed in amphibolite facies. This is consistent to 38-42 mW/m2 heat flow of the ELP that is close to average heat flow 40 mW/m2 of the Precambrian amphibolite facies terranes. The highest values were measured for the granitic rocks. Radioactive heat production of the migmatites reaches 3.1 µWm/m3, the maximum production was identified for anorogenic granites which is as high as 4.6-9.1 µWm/m3.
In the north the ELP passes into the Inchukalns terrane also metamorphosed in amphibolite facies. It is characterized by higher heat flow ranging from 49 to 69 mW/m2. The heat production varies from 0.5-1.4 µWm/m3 in the south dominated by gravity and magnetic high pointing to rather mafic crustal lithologies, while heat production in average is higher in the north reaching 3.9 µWm/m3 (Urban, 1989).
The number of available data is not sufficient to define consistent correlation between the heat production of the basement rocks and heat flow in the ELB.
West Lithuanian Granulite Province (WLGP)
The West Lithuanian granulite domain is characterised by highest heat flow density in the Baltic basin. Moreover, the geothermal anomaly is recognised in the western Lithuania, which is the most intensive anomaly mapped in the East European Craton. The West Lithuanian geothermal anomaly has the rather distinct boundaries. In the east it is bounded by deep-seated Taurage-Ogre shear zone, in the north the sharp drop of abour 40-50 mW/m2 over short distance is related to mafic periphery of northerly situated Riga pluton, in the south the anomaly is bounded by Mazury-Suwalki tectonic zone of Early Precambrian onset. The extent of anomaly to the west is not clear due to scarce wells available in the Baltic sea. The heat flow ranges from 45-60 mW/m2 in the periphery of the WLGP to more than 90 mW/m2 in the central part of domain (Sliaupa, Rasteniene, 2000; Zui et al., 1998). The heat flow in the WLGP is rather differentiated that is related to several factors, including crustal heat production, fault tectonics, mantle heat variations.
The primary parameter controlling variations in the heat flow density is the high heat production of the crust (Urban, 1989; Sliaupa, Rasteniene, 2000; Motuza et al., 2004; Sliaupa et al., 2005). Two geothermal groups of granulites are defined (i) granulites that show low heat production in the range of 0.27-0.89 µWm/m3, the migmatitized varieties are more active heat producers averaging 1.47 µWm/m3; (ii) The granulites that indicate higher heat production 1.6-2.5 µWm/m3, up to 3.1 µWm/m3. Both groups of granulites show higher radioactive heat production than the average cratonic granulites 0.2-0.4 µWm/m3 (Jaupert et al., 1998). Furthermore, the granulites were intruded by numerous granitoid bodies that commonly show high heat production. The highest heat production is recognised for the anorogenic granitoids that were established in Middle Proterozoic time (4-18 µWm/m3).
The small wavelength heat flow variations are believed to be only sensitive to shallow heat production contrasts (Jaupert, 1983). The most intense Zemaiciu Naumiestis heat flow anomaly 75-95 mW/m2 is confined to cratonic granitoids dominated. Similarly, the smaller Salantai anomaly in the north is confined to granitoid massif that has high heat production 2-5 µWm/m3. These anomalies are separated by xcoldx Telsiai zone (50-55 mW/m2) dominated by low-production 1.1-1.8 µWm/m3 charnockites.
The high-intensity Zemaiciu Naumieastis anomaly passes into the NE-SW anomaly that extends to the central Latvia. It is notable that this geothermal appendix is confined to large scale Taurage-Ogre shear zone. Following magnetic modelling by using Werner deconvolution it has thinn-skined tectonic fabric and is traced down to as deep as the lower crust (Sliaupa, Popov, 1998). There are two alternative explanations of this association.
The lithological variations well explain short-wavelength variations in the heat flow in WLGP that is advocated by positive correlation +0.40 between these two parameters. Still, they can hardly explain the origin of the vast West Lithuanian geothermal anomaly rather pointing to additional intense heat source. The correlation between the heat production of the crystalline basement rocks and the surface heat flow of WLGP is fitted to linear relationship Q=45+11.7A.
The surface heat flow drastically decreases in north-western Lithuania and western Latvia. The lowest heat flow values 19-38 mW/m2 in the Baltic basin are related to the large Riga Pluton. The lithological factor satisfactorily explains low heat flow values (30-38 mW/m2) in the periphery of the pluton, which is dominated by maffic phases having very low heat production 0.2-0.48 ?Wm/m3 (Urban, 1989). The central port part of the massif the rapakivy granites crop out, they have high radiogenic heat production 1.6-3.78 µWm/m3 that comes in conflict to minimum values of the heat flow 19-22 mW/m2. This caused that no correlation between the heat production and heat flow was reported from the Riga pluton (Zui et al., 1998). Assuming mantle heat flow as high as 10-12 mW/m2 that is typical for cratonic areas this leaves only 10 mW/m2 for the crustal source. This would imply only 1-2 km thickness of the rapakivi granites, while the rest of the crust should be dominated by mafic rocks and depleted metapelites with average heat production 0.2 µWm/m3. The latter does not seem impossible provided the rapakivi granites were produced by melting of the granulite metapelites that lost U, K and Th in this processes the heat producing elements being concentrated in the felsic melt (Rudnick, Fountain, 1995). The modelling of the potential fields support this mechanism (Sliaupa, Korabliova, 2003).
The heat flow of the NE Poland in the Mazury High, encountering the Baltic basin in the south, is lower than the average of the East European Craton. The heat flow ranges from 30 to 40 mW/m2, in the Dobrzyn massif it reaches 45 mW/m2. The low heat flow values are in concert to low heat production of the basement rocks (Ryka, Kubicki, 1979). Enderbites have only 0.3 µWm/m3, granioids have µWm/m3 heat production.
Balling (1995) provided correlation of the basement rocks heat production and surface heat flow of southern Fennoscandian Shield Q=32.8+7.6A that is compatible to that of Estonia. It indicates rather high mantle and lower-middle crustal heat flow, which is much higher than that in the Finland geothermal province. The maximum heat production ranging from 1.5 µWm/m3 to 3.5 µWm/m3 is related to Trans-Scandinavian igneous belt.