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A C T A G E O P H Y S I C A P O L O N Vol. 52, No. 4 I C A 2004 DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE OBTAINED FROM P- AND S-WAVE VELOCITY MODELS ALONG THE EUROBRIDGE’97 SEISMIC PROFILE AND GRAVITY DATA Elena KOZLOVSKAYA1, Tomasz JANIK2, Jukka YLINIEMI1, German KARATAYEV3 and Marek GRAD4 1 Sodankylä Geophysical Observatory/Oulu Unit POB 3000, FIN-90014, University of Oulu, Finland e-mail: [email protected] 2 Institute of Geophysics, Polish Academy of Sciences ul. Księcia Janusza 64, 01-452 Warszawa, Poland 3 Institute of Geological Sciences Zhodinskaya str. 7, Minsk, 220141, Belarus 4 Institute of Geophysics, University of Warsaw ul. Pasteura 7, 02-093, Warszawa, Poland Abstract Traditionally, joint interpretation of seismic refraction and wide-angle reflection data and gravity data is based upon a well-known correlation between seismic P-wave velocity and density proved by numerous laboratory investigations of elastic properties of crustal rocks. One of the problems connected with this approach is that rocks with high content of calcium-reach plagioclase have higher P-wave velocity and do not satisfy the common density-Vp relationship. That is why joint interpretation based upon any conventional relationship between density and P-wave velocity cannot be applied to wide-angle profiles across large anorogenic rapakivi-gabbro-anorthosite massifs composed of rocks with high content of plagioclase. The problem can be solved if both P- and S-wave velocities are used to calculate the density model. The results of laboratory studies of rock properties demonstrate strong correlation between density and S-wave velocity. Moreover, the isotropic S-wave velocity seems to be generally more correlated to density than the P-wave velocity and less affected by E. KOZLOVSKAYA et al. 398 high content of plagioclase. In spite of that, the known relationships connecting density to S-wave velocity or to both P- and S-wave velocities are very seldom used for joint interpretation of seismic and gravity data. The main reason for this is a lower quality of S-wave arrivals in explosion seismology, which makes it difficult to obtain reliable S-wave velocity models. In our paper we present the results of joint interpretation of seismic and gravity data collected along the EUROBRIDGE’97 wide-angle reflection and refraction profile in the Ukrainian Shield, where the absence of thick sediments made it possible to obtain both P- and S-wave velocity models. To calculate the density model along the EUROBRIDGE’97 profile we used a method of gravity data inversion, in which the density model was parameterised by the relationship connecting density to both P- and S-wave velocity models. Such a parameterisation makes it possible to obtain the relationship between density and seismic velocities by inverting the gravity data. As a result, non-linear and scattered relationship between density and seismic velocities was obtained for the EUROBRIDGE’97 profile. Analysis of the relationship demonstrated that the reason for this scattering is difference in density-velocity relationships for largescale geological units crossed by the profile. In order to explain this difference, we compared the relationship between seismic velocities and density in three major geological units crossed by the EUROBRIDGE’97 profile with the petrophysical data from the Ukrainian Shield and other selected Precambrian areas. We demonstrated that the deviations from the averaged density-velocity relationships can be explained by specific mineral composition of rocks resulting from different age and conditions of crust formation. We showed how the analysis of density-velocity diagrams can be used to restrict the composition of the crust and, in particular, the composition and metamorphic grade of the lower crust. Key words: gravity modelling, density-velocity relationship, integrated interpretation, continental crust, Precambrian, Ukrainian Shield. 1. INTRODUCTION The main condition for joint interpretation of refraction and wide-angle reflection data and gravity data is relationship between seismic wave velocity and density established already in the 1960-ties (Birch, 1961) and proved by numerous compilations of laboratory measurements of rock density and seismic velocities throughout the world (Carmichael, 1989; Henkel et al., 1990; Krasovsky, 1981; Christensen and Mooney, 1995). As a rule, these earlier investigations into density-velocity relationships aimed at obtaining linear and non-linear regression curves approximating results of laboratory measurements of density and velocity in different types of lithospheric rocks under various confined pressures and temperatures. These curves are usually used to recalculate seismic velocity models obtained by wide-angle reflection and refraction experiments into density models. DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 399 In spite of numerous investigations into density-velocity relationship, very few of them were devoted to the analysis of variations of this relationship in geological units of different structure and geological age. Such an analysis was made by Krasovsky (1981), who demonstrated differences between regression curve approximating laboratory measurements of density and velocity worldwide and regression curves approximating subsets of global data corresponding to various geological provinces around the world. In addition to laboratory measurements, the density-velocity relationship in lithospheric units of regional scale can be obtained directly from the velocity distribution within the lithosphere provided by large-scale seismic experiments and observed gravity data, as it was proposed by Kozlovskaya and Yliniemi (1999) and Kozlovskaya et al. (2001a, b; 2002). This approach makes it possible to find a density-velocity relationship that gives the best fit of the density model to the observed gravity data, namely, the relationship is obtained as a solution to the inverse gravity problem. The method was used to study variations of density-velocity relationship in largescale geological units with available wide-angle reflection and refraction profiles in the East European Craton (EEC): the SVEKA profile in Finland (Kozlovskaya and Yliniemi, 1999) and the EUROBRIDGE'95-97 profiles (Kozlovskaya et al., 2001a, b; 2002), crossing a number of tectonic units of different age in Finland, Lithuania, Belarus and the Ukraine. The density-velocity relationships obtained for these profiles are generally quasi-linear and demonstrate moderate scattering of P-wave velocity around the corresponding values of density. The scattering is due to variations of the relationship in different tectonic units. The investigations also revealed several cases when the density-velocity relationship deviated from the quasi-linear one. As demonstrated by the EUROBRIDGE'96 profile (Kozlovskaya et al., 2002), the density-VP correlation may be violated due to large-scale seismic anisotropy in geological units composed of tectonically deformed rocks. Deviation from quasi-linear density-VP relationship was also revealed in some large rapakivi-gabbro-anorthosite massifs, containing abundant amounts of rocks with high content of feldspars, i.e., rapakivi granites and anorthosites. Wide-angle reflection and refraction profiles across such massifs reveal very high values of P-wave velocity (up to 6.4 km/s) at a depth of the upper crust. In some cases such areas of high velocity are marked by a negative gravity anomaly, as revealed by BALTIC profile across the Wyborg rapakivi batholith (Luosto et al., 1990) and by the EUROBRIDGE´97 profile across the Korosten rapakivi-gabbro-anorthosite pluton in the Ukrainian Shield (Thybo et al., 2003). However, if any conventional density-VP relationship is used to recalculate these high velocities to densities, it results in positive values of the calculated gravity effect. In our paper we demonstrate how this problem can be treated if the density is calculated using a relationship connecting density to both P- and S-wave velocity models. We use the seismic and gravity data collected along the EUROBRIDGE’97 wide-angle reflection and refraction profile, namely, the Bouguer anomaly and P- and 400 E. KOZLOVSKAYA et al. S-wave velocity models (Thybo et al., 2003). Another purpose of our study was to compare seismic velocities and densities along EUROBRIDGE’97 profile to petrophysical data in order to explain differences between density-velocity relationships for various tectonic units crossed by the profile. 2. DENSITY-VELOCITY RELATIONSHIP AS A MAIN CONDITION OF JOINT INTERPRETATION OF SEISMIC AND GRAVITY DATA Density-velocity relationship revealed from laboratory studies of elastic properties of rocks The correlation between rock density and compressional wave velocity was originally obtained by Birch (1961) under confining pressures up to 10 kbar and under the assumption that compressional wave velocity Vp in isotropic media depends primarily upon two parameters, i.e., the mean atomic mass mA and material density d: VP = 2.76d − 0.98 + 0.7(21 − mA ) . (1) The mean atomic mass in eq. (1) is an additional parameter that characterises the composition of rock. Anderson (1967) in his theoretical investigation proved the main conclusions made by Birch, i.e., dependence between seismic velocities, density and mean atomic mass. He showed that the density depends upon both compressional and shear wave velocities and derived a relationship between density and seismic velocities in the form: d = a mA Φ n , 4 Φ = VP2 − 3 VS2 , (2) where VS is the isotropic shear wave velocity, Φ is the seismic parameter, a is a coefficient and the exponent n is of the order of 1/4 or 1/3. As it has been noticed already by Birch (1961), rocks with high content of calcium-reach plagioclase have higher P-wave velocity and do not satisfy the common density-VP relationship. Simmons (1964) developed a modification of Birch’s law that makes it possible to take into consideration the effect of the CaO content. Later, Manghnani et al. (1974) developed a similar equation for both compressional and shear wave velocities: VP = 2.58 d − 0.53 + 0.7(21 − mA ) + 4.6 CCaO (3) VS = 1.56 d − 0.63 − 0.21(21 − mA ) + 0.46 CCaO . Equations (3) demonstrate weak influence of the mean atomic mass and CaO content on shear wave velocity. From this it follows that the isotropic S-wave velocity is more correlated to density than the P-wave velocity. Since then, numerous relationships between density and seismic velocities have been compiled from results of velocity and density measurements under laboratory DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 401 conditions and in boreholes for various types of rocks from different geological provinces and at different pressure and temperature. The detailed analysis of these results can be found, for example, in papers by Krasovsky (1981), Barton (1986), and Schön (1998). It was demonstrated that in the most common case the relationship between compressional wave velocity and density for crustal rocks can be approximated by a linear regression curve. One of the most popular density-velocity correlation curves used in joint interpretation of wide-angle reflection and refraction data and gravity data is the Nafe-Drake curve (Ludwig et al., 1970) and the relationship by Christensen and Mooney (1995) connecting compressional wave velocity and density. One of the significant difficulties connected with application of such relationships has always been significant scattering around the mean value revealed for all types of lithospheric rocks. The studies of density-velocity relationship in the KTB superdeep borehole (Kneib, 1995; Goff and Holliger, 1999) demonstrated, however, that the statistical properties of the density-velocity relationship strongly depend on the observation scale. In the short wavelength range (less than 10 m) the correlation between velocity and density logs is very small due to big scattering, but the correlation increases (scattering decreases) with increasing wavelength. This means that scattering of the density-velocity relationship for the geological units with characteristic scale of more than 10 km is significantly less than the scattering in laboratory measurements on rock samples. Therefore, the strong density-velocity correlation is expected for lithospheric units with characteristic scale of dozens and hundreds kilometers, which are studied by wide-angle reflection and refraction experiments. Correlation between density and compressional wave velocity in the continental lithosphere In the continental lithosphere both seismic velocity and density are affected by pressure and temperature, the rock macrostructure and microstructure, cracks and fractures, the presence of fluids and anisotropy of rock-forming minerals. Generally, these factors affect density and velocity in a different way on micro- and macroscale. Therefore, not all these factors must be taken into consideration in lithopsheric studies dealing with large-scale units stretching to a depth of dozens kilometers. The main factor affecting seismic velocities in the upper continental crust is the presence of cracks and fractures that are often filled also with fluids. Detailed study of rock properties in the superdeep boreholes demonstrated that the upper crust is penetrated by fluid-filled fractures and cracks down to a depth of at least 10−12 km (Ganchin et al., 1998; Smithson et al., 2000). As a result, seismic velocities in the upper crust are lower than intrinsic (crack-free) velocities by about 0.2 km/s (Smithson et al., 2000); therefore, the density values calculated from these velocities using any linear density-velocity relationship may be systematically underestimated. In the middle and lower crust, where all the cracks are closed, the seismic velocities and densities depend on pressure and temperature, but the influence of these fac- 402 E. KOZLOVSKAYA et al. tors is weak comparing to the effect of rock composition. In the areas of low heat flow the combined effect of pressure and temperature on seismic velocities and densities cancels out at a depth corresponding to the middle and lower crust (i.e., at pressures more than 2 kbar) (Kern and Richter, 1981; Schön, 1998). Therefore, both velocities and densities in the middle and lower crust depend mainly on elastic properties of rock forming minerals and are controlled by pressure-dependent mineral reactions changing the mineral assemblages from plagioclase-bearing and garnet-free to garnet-bearing and plagioclase-free (Green and Ringwood, 1967). This results in increase of both density and seismic velocities with depth and strong correlation between density and P- and S-wave velocities (Sobolev and Babeyko, 1994). The relationship between density and P-wave velocity in lithospheric units of regional scale obtained for the SVEKA profile in Finland (Kozlovskaya and Yliniemi, 1999) and for the EUROBRIDGE’95−EUROBRIDGE’97 profiles (Kozlovskaya et al., 2001a, b; 2002) generally agrees with the density-VP curve by Sobolev and Babeyko (1994), although variations of the relationship in different tectonic units resulted in scattering of P-wave velocity (0.2−0.5 km/s) around the corresponding values of density. It has been also found out that the density-VP relationship deviates from this general rule in several tectonic units. For example, in the Central Belarus belt crossed by the EUROBRIDGE’96 profile the linearity of density-VP relationship is violated due to seismic anisotropy in tectonically deformed rocks (Kozlovskaya et al., 2002). The other unit was Korosten rapakivi-gabbro-anorthosite anorogenic massif (Korosten pluton), where the EUROBRIDGE’97 profile revealed high values of P-wave velocity in the upper crust (Thybo et al., 2003). The area of high P-wave velocity is marked by a negative gravity anomaly. The gravity modelling of this profile (Kozlovskaya et al., 2001b; Egorova et al., 2003) demonstrated, however, that this negative gravity cannot be explained, if any quasi-linear density-VP relationship is used. The Korosten pluton is composed of two main rock types, that is, rapakivi granites and gabbro-anorthosites. Therefore, the high P-wave velocity in the uppermost crust that is accompanied by a negative gravity anomaly can be attributed to anorthosites. Due to a high amount of plagioclase and CaO, these rocks have high P-wave velocity comparable with that of gabbroid rocks, while their density is significantly less (Henkel et al., 1990; Kern et al., 1993). However, the average density of anorthosites is of about 2.7 g/cm3, which is too high to explain the negative gravity. Another explanation of a high P-wave velocity and negative gravity may be the influence of pressure on P-wave velocity of rapakivi granites having low density comparing to other types of granitoids. Extensive laboratory studies of elastic properties of granitoids of the Ukrainian Shield under confining pressures and temperatures corresponding to the present-day geotherm were made by Lebedev (1989); Lebedev and Korchin (1982); Lebedev et al. (1972; 1983; 1990). Based on these results, Schön (1998) demonstrated strong dependence of P-wave velocity in granites on rock texture. The highest pressure effect was found in coarse-grained rapakivi granites, in which VP increases rapidly from about 6.0 km/s at the surface up to 6.5 km/s at a depth DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 403 of about 5 km. In fine- and medium-grained granites the effect of pressure on VP is much less. Due to this non-linear effect of pressure on P-wave velocity in rapakivi granites, they cannot be distinguished from the more dense anorthosites, if only P-wave velocity is used. However, laboratory studies demonstrated (Lebedev et al., 1982) that the S-wave velocity in rapakivi granites and anorthosites is generally less affected by the pressure in the same depth interval. As can be concluded from eqs. (3), the S-wave velocity is also less affected by high content of plagioclases. This suggests that the interpretation based upon both P and S waves may help to obtain more realistic density models and better fit to the observed gravity data. Relationship between shear wave velocity and density Like the more conventional density-VP relationship, the correlation between density and shear wave velocity was studied in a number of laboratory experiments and several relationships approximating the laboratory measurements have been obtained (see, for example, Gebrande et al., 1982; Dortman, 1992; Schön, 1998). Gebrande et al. (1982) and Manghnani et al. (1974) demonstrated that isotropic shear wave velocities depend mainly on rock density and weakly depend on mA. These studies also demonstrated that isotropic VS is also less affected by the CaO content than VP ; see eq. (3). The linear regression approximating the relationship between VP, VS and density in lithospheric rocks was obtained by Rosental (Dortman, 1992): d = 0.763 + 0.402 VP − 0.138Vs . (4) A non-linear regression connecting density to both compressional and shear wave velocities was obtained also by Khalevin et al. (1986): 2 2 d = 2.66 − 0.107 VP − 0.0535VS + 0.026 VP VS + 0.0463(VP − 1.3333VS ) . (5) The relationships connecting density to S-wave velocity or to both P- and S-wave velocities are very seldom used for the purpose of integration of seismic and gravity data, because the velocity models obtained by wide-angle reflection and refraction experiments are usually based upon interpretation of P waves. The reason is the lower quality of S-wave arrivals that are masked by coda of P waves, which makes it difficult to obtain reliable S-wave velocity models. However, in shield areas the quality of S waves in wide-angle refraction and reflection data is comparable with that of P waves due to absence of thick sediments. In such areas, independent S-wave velocity models can be obtained; thus, both P- and S-wave velocities may be used for the purpose of gravity modelling. A method of gravity modelling based on relationship between density and P- and S-wave velocity models was proposed by Kozlovskaya and Yliniemi (1999) for the SVEKA profile in Finland. In the present study we apply the same technique to obtain the density model along the EUROBRIDGE’97 profile and to 404 E. KOZLOVSKAYA et al. analyse the relationship between density, VP and VS in geological units crossed by the profile. 3. SEISMIC P- AND S-VELOCITY MODELS ALONG THE EUROBRIDGE’97 WIDE-ANGLE REFLECTION AND REFRACTION PROFILE The EUROBRIDGE’97 wide-angle reflection and refraction experiment The seismic experiment was part of the EUROBRIDGE project aiming at establishing the deep lithospheric structure of the EEC between the exposed Proterozoic and Archaean complexes of the Baltic and Ukrainian Shields (Bogdanova et al., 1996a). The deep seismic sounding measurements along 1200 km on shore transect were made at three parts: 1995 (EUROBRIDGE Seismic Working Group, 2001), 1996 (EUROBRIDGE Seismic Working Group, 1999) and 1997 (Thybo et al., 2003). The EUROBRIDGE’97 experiment was carried out as an international co-operation between Belarussian, Ukrainian, British, Danish, Finnish, German, Polish and Swedish institutes. Seismic data acquisition was undertaken in August and September 1997 along a 530 km N-S transect within the Sarmatian segment of the EEC (Thybo et al., 2003). The EURO-BRIDGE’97 crossed several terranes of Sarmatia, and particular attention was paid to achieving higher spatial resolution in the vicinity of the Korosten Pluton (Fig. 1). The detailed description of the EUROBRIDGE’97 seismic experiment was presented by Thybo et al. ( 2003). The northern part of EUROBRIDGE’97 profile is located within the OsnitskMikashevichi Igneous Belt (OMIB) near to the north-western margin of the Sarmatian crustal segment. The OMIB is about 100−150 km wide and formed of Proterozoic igneous rocks of different composition, essentially free of regional metamorphism. Complexes include metagabbro-diabase, the dominant diorite-granodiorite-granite and quartz-syenite-granite (2.1−2.0 Ga). The OMIB is partly covered by the sediments of the Pripyat Trough and Volyn-Orsha aulacogen (Bogdanova et al., 1996b; Claesson et al., 2001). The Pripyat Through (PT) is part of the Phanerozoic Pripyat-Dnieper-Donetsk Palaeorift (Stephenson et al., 1996). The PT is about 280 km long and 150 km wide and is filled by sediments down to a depth of 6 km in some places. The oldest formations are terrigenous and carbonate strata of about 200−300 m of the Middle Devonian age. The most widespread formations range from the Upper Devonian to Middle Triassic and include terrigenous, carbonate, saliferous and volcanogenic lithologies rocks. They form strata of several kilometres thickness, whereas the youngest, Upper Triassic to Quaternary strata have a thickness of only 150−200 m (Aizberg et al., 1987). The Volyn Block (VB) is composed entirely of Proterozoic rocks. The Palaeoproterozoic gneiss complexes occur in the SE and SW parts of the VB and are metamorphosed in amphibolite to epidote-amphibolite facies and their age of formation DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 405 1 III EB’97 200 km SARMATIA 6 T Berlin h 7 11 II file Pro 12 Dn iep r KP 13 TeterevTs Be Belt erk lay ov aBe lt 15 16 17 do Dn iep r SARMATIA - Do ne ts BLACK SEA 100 km Pa lae or ift KNM 18 Blo ck VI lian ile of Pr Po Kiev Kiev I A NS Block Warszawa Z TH 9 10 S Vilnius Minsk CARPA Chernigov 8 E EB ’9 5 96 Tro ug BALTIC SEA ’ EB 5 Pripyat Riga FENNOSCANDIA 4 EB’97 EB Prip yat 43 ’9 6 O M IB Volyn EB ’9 VIII 2 Donetsk Middle Dnieper NM Block Kirovograd Block Azov Block Dn ie s tr Kishinev Fig. 1. Location map of the EUROBRIDGE’97 DSS profile and relevant regional units of the Ukrainian Shield. Inset map shows the location of the entire EUROBRIDGE DSS profiles, the thick dashed line indicates the near surface location of the suture zone separating the Fennoscandia and Sarmatia segments of the East European Craton. (KNM – Korsun-Novomyrgorod Massif, KP – Korosten Pluton, NM – Novoukrainka Massif, OMIB – Osnitsk-Mikashevichi Igneous Belt). ranges from 2.2 to 2.1 Ga (see Stepanyuk et al., 1999 and Bogdanova et al., 2004). Some rocks are represented by anatectic granites (2.06−2.02 Ga) in the southern and western parts of the block (Shcherbak et al., 1989) and intrusive rock complexes of various compositions formed along deep faults (2.02−1.98 Ga) (Shcherbak et al., 1989; Skobelev et al., 1991). Peridotite-pyroxenite-gabbronorite, peridotite-troctoliteanorthosite, alkali ultramafic, gabbronorite-monzonite, gabbro-syenite-granite and granodiorite-granite rock assemblages have been distinguished among them (Skobelev et al., 1991). 406 E. KOZLOVSKAYA et al. The Korosten Pluton (KP) is a distinctive, regionally significant layered rapakivi-gabbro-anorthosite anorogenic plutonic complex (1.8−1.74 Ga) of some 6 km thickness, below which the crust is believed to be extensively intruded by mafic melts. The KP is composed mainly of rapakivi granites, gabbro-anorthosites and gabbronorites (Lichak, 1983; Verkhogliad, 1995). The Podolian Block (PB) occupies the southern part of the western Ukrainian Shield. Its oldest geological units (> 3.4 Ga) are composed of mafic granulites and enderbite-gneisses that are intruded by enderbites (Shcherbak et al., 1989; Lesnaya et al., 1995). Intense tectonic movements, metamorphism and anatexis occurred during the Palaeoproterozoic, forming various anatectic granitoids of 2.08−2.02 Ga (Shcherbak et al., 1989; Skobelev et al., 1991; Lesnaya et al., 1995). These rocks are widespread in the north, close to the VB. Seismic velocity models along the EUROBRIDGE’97 profile A starting 2-D P-wave velocity model was obtained by tomographic inversion of the first arrivals of P waves (Thybo et al., 2003). Final 2-D model of the lithosphere down to 80 km depth was developed using the ray tracing package SEIS83 (Červený and Pšenčík, 1983), supported by the programs MODEL and XRAYS (Komminaho, 1998). In the ray tracing process the travel times were calculated and compared to the observed travel times, then the corresponding corrections to the model were made until a reasonable agreement between the observed and model-derived travel times of the order of 0.2 s was achieved. In addition, synthetic seismograms were calculated to control the velocity gradients within the layers and the velocity contrast at the seismic boundaries. Figure 2a is the final, detailed P-wave velocity model along the EUROBRIDGE’97 profile (Thybo et al., 2003). The P-wave velocity model demonstrates pronounced lateral variations of P-wave velocity in the crust that can be spatially correlated with the OMIB, VB, PB and KP, respectively. The thickness of the crust is about 45 km, increasing slightly up to about 50 km in the south. The most complicated is the structure of the upper crust down to a depth of 10−15 km. In the northern and southern parts of the profile the velocity values in the upper crystalline crust are of about 6.1−6.2 km/s. Low velocity (2.4−4.2 km/s) down to a depth of ~4 km in the northern part of the profile corresponds to thick sediments of the PT. The KP is characterised by extremely high velocities of 6.4−6.7 km/s at a shallow depth (down to 10 km). In the south, a weak low velocity zone at a depth of 10−12 km with velocity of ~6.1 km/s was found. The middle crust is composed of two layers with average velocities of ~6.4 and ~6.7 km/s. The lowermost crust is characterised by velocities of 7.0−7.4 km/s. At the base of the crust, and particularly in the central part of the profile within the VB, a complicated structure with velocities exceeding 7.4 km/s (the high-velocity lower crust) has been revealed. The velocity beneath the EUROBRIDGE’97 profile DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 407 Osnitsk - Mikashevichi Igneous Belt Volyn Block Podolian Block N Pripyat Trough Korosten Pluton S (a) P-wave velocity model Vp [ km/s ] (b) 3.00 3.10 3.20 3.30 3.40 3.50 3.60 3.70 3.80 3.90 4.00 4.10 4.20 4.30 4.40 4.50 3.57 3.63 3.55 3.60 3.73 3.65 3.46 3.60 3.70 3.70 3.80 3.80 3.80 3.85 3.90 3.95 4.37 4.38 4.55 4.50 4.55 S-wave velocity model Vs [km/s] ( c) Vp/Vs Vp/Vs ratio distribution Distance [ km ] Fig. 2. Two-dimensional P-wave (a) and S-wave (b) velocity models and VP/VS ratio (c) along the EUROBRIDGE’97 profile (by Thybo et al., 2003). Thick black lines represent major velocity discontinuities (inter faces) that have been constrained by reflected or/and refracted arrivals of P or/and S waves; thin lines represent velocity isolines; colours represent the distribution of velocity and VP/VS ratio. 408 E. KOZLOVSKAYA et al. Moho is 8.3−8.4 km/s in the southern part of the profile and ~8.1 km/s in the central part. In addition, a dipping reflector has been revealed beneath the Moho in the central part of the profile. Refracted and reflected S-waves observed on vertical component record sections were used for correlation of S-wave arrivals along EUROBRIDGE’97 (Thybo et al., 2003). Generally, the observed S waves are stronger and more distinct in the southern part of the profile, not covered by thick sediments. In the north, up to distance of 200 km, arrivals of S waves are usually very weak against the background of P-wave coda. The limited data set of picked travel times of S waves did not allow modelling of an independent S-wave velocity model. Therefore, the VP /VS velocity ratio for the PT was adapted from detailed laboratory study of sedimentary rocks by Molotova and Vasiliev (1960), and the best branches of correlated S-wave travel times were used to estimate the VP /VS velocity ratio for principal layers of the crystalline crust. The distribution of the VP /VS ratio within the lithosphere beneath the EUROBRIDGE’97 is shown in Fig. 2c. It was used to calculate the initial S-wave velocity model from the P-wave velocity model shown in Fig. 2a. The final S-wave velocity model (Fig. 2b) was obtained interactively by a trial and error method, in order to achieve a good fit to the observed travel times of refracted and reflected S waves. The VP /VS ratio (Fig. 2c) in the upper and middle crystalline crust varies from 1.68 to 1.78. The two main layers of the lower crust and the upper mantle in the central part of the model have P-wave velocities of 7.1, 7.4−7.6 and 8.35 km/s, and Svelocities of about 3.95, 4.25−4.45 and 4.55 km/s respectively (Fig. 2b). These values indicate VP /VS ratio of 1.78 and 1.71−1.74 for the two layers of the lower crust and 1.84 for the upper mantle. The KP has generally higher values of VP /VS ratio for the upper and middle crust than the neighbouring blocks (1.77−1.84), while the PB has the lowest values of VP /VS ratio in the model (1.67−1.69). 4. DENSITY MODEL ALONG THE EUROBRIDGE’97 PROFLIE Gravity data The gravity data used in our study are the Bouguer anomaly along the EUROBRIDGE’97 profile and its long-period regional component (Fig. 3a). The data were taken from the corresponding digital gravity maps compiled at the Institute of Geological Sciences of the National Academy of Sciences of Belarus from the results of gravity surveys in Belarus and the Ukraine (Garetsky et al., 1991; Egorova et al., 2003). The regional components of the gravity field were calculated using the technique of non-linear filtration by Karatayev (1966). As it can be seen in Fig 3a, the gravity field along EUROBRIDGE’97 has the pronounced regional gravity low that is spatially coincident with the PT. However, this DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 409 trend can be explained by the effect of thick sedimentary cover only in the northern part of the profile, where the northern flank of the PT is marked by an area of high horizontal gradient in the observed Bouguer anomaly (Fig. 3a). In the southern part of the PT, the gravity field increases rather gradually to the south and has a clear regional maximum between approximately 350 and 450 km of the profile. This regional trend is superimposed by several positive and negative anomalies of smaller amplitude caused by density variations in the upper crust. The gravity field over the KP does not possess the “classic” round-shape negative anomaly that is sometimes considered as a common feature of rapakivi-anorthosite batholiths and that is observed, for example, over the Wyborg rapakivi massif in Finland (Elo, 1997). The KP is a complex, multi-stage intrusion complex consisting of several large units composed of rocks with contrasting density (i.e., granites and gabbro-anorthosites). Therefore, its gravity effect is an alternation of positive and negative anomalies of various intensities (Fig. 3a). The Bouguer anomaly across the KP increases from nearly −60 mGal in the Pripyat Trough to +20 mGal south of the KP. In the northern part of the KP (190−275 km) it is mainly negative, partly due to the effect of the thick sediments of the Pripyat Trough. The pronounced minimum of the Bouguer anomaly is observed south from the Pripyat Trough at a distance of ~218 km. This local anomaly spatially corresponds to the Ovruch trough filled with sediments. Further south along the profile, in the south-western part of the KP, the gravity field is dominated by the positive gravity effect of the Volodarsk-Volynsky anorthosite massif. The results of forward 2-D gravity modelling along the EUROBRIDGE’97 was presented by Egorova et al., (2003), who noticed that the gravity minimum in the northern part of the KP cannot be fit, if the standard linear density-VP relationship is used to recalculate VP to density. Inversion of gravity data along the EUROBRIDGE’97 profile In our study, the 2-D density model along the EUROBRIDGE’97 profile (Fig. 3b) was calculated from both P- and S-wave velocity models (Fig. 2) using the technique of gravity data inversion described by Kozlovskaya and Yliniemi (1999). The technique is based on the non-linear relationship between VP, VS and density similar to that obtained by Khalevin et al. (1986) and described by eq. (5). Basing on this equation, it can be assumed that the relationship between density and seismic velocities in the 2-D section under study can be approximated by the following function: d ( x, z ) = N ∑ A U ( x, z ) k =0 k k (6) where: U0(x, z) = 1, U1(x, z) = VP(x, z), U2(x, z) = VS(x, z), U3(x, z) = VS(x, z)VP(x, z), U4(x, z) = VP2(x, z), U5(x, z) = VS2(x, z), U6(x, z) = 1/VS(x, z), U7(x, z) = VP(x, z)/VS(x, z). Functions Uj(x, z), j = 8…N, represent some additional geophysical parameters measured along the profile that can provide information about differences between large E. KOZLOVSKAYA et al. 410 EUROBRIDGE’97 profile Osnitsk - Mikashevichi Igneous Belt Pripyat N a) Volyn Block Trough Podolian Block Korosten Pluton S Gravity field [ mGal ] 40 b) 20 0 -20 regional gravity -40 Bouguer anomaly calculated effect -60 2.30 2.40 2.50 2.60 2.70 2.80 2.90 3.00 3.10 3.20 3.30 Density model 3.40 3.50 Distance [ km ] d) 8.5 4.6 8.0 4.4 7.5 4.2 Vs [km/s] Vp [km/s] c) 7.0 6.5 4.0 3.8 3.6 6.0 3.4 3.2 2.5 2.7 2.9 3.1 3.3 3.5 2.5 2.7 2.9 3.1 3.3 3.5 DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 411 lithospheric blocks in the area under investigation. Inclusion of such functions into eq. (6) makes it possible to model variations of density-velocity relationship in large-scale units of the lithosphere. These functions can be, for example, regional magnetic field or variations of the heat flow along the profile. The information about differences between large-scale lithospheric blocks can be obtained also directly from seismic models. As was demonstrated by Karatayev et al. (1993) and Christensen and Mooney (1995), the geological units of different geological age and evolution have also different structure of the crust, namely, depth to the major boundaries (the Moho, the basement surface, the upper crust/middle crust boundary and the middle crust/lower crust boundary) and the average crustal velocity. That is why variations of these parameters along the profile can be used to model variations of density-velocity relationship in large-scale geological units. In our study, function U8(x, z) = VPmean(x) is P-wave velocity from model in Fig. 2a averaged over z-axis and functions U9(x, z) = H1(x), U10(x, z) = H2(x), U11(x, z) = H3(x) are three main seismic boundaries from the seismic model, that is, the basement surface, the boundary between upper and middle crust and the Moho. Functions U12(x, z) = H4(x) and U13(x, z) = H5(x) are two additional seismic boundaries from the seismic model, i.e. the boundary between middle and lower crust and the boundary between lower crust and the high velocity lower crust. In addition, the information about variations of density-velocity relationship was represented also by functions U14(x, z) = VPmean(x)VP(x, z) and U15(x, z) = dVPmean(x, z)/dx. The coefficients Ak in eq. (6) are unknown and can be obtained as a solution to inverse gravity problem, as described in details by Kozlovskaya and Yliniemi (1999), Kozlovskaya et al. (2001a). Then eq. (6) can be used to calculate the density distribution from known P- and S-wave velocity models using the conventional integral operator for gravity effect of an arbitrary 2-D density distribution and numerical integration. For this purpose, velocity and density models are parameterised by regular rectangular grids. In our study we used the square grid with size of 2×2 km to represent adequately velocity and density steps on major discontinuities. Figures 3c and 3d demonstrate the density-velocity relationships for the EUROBRIDGE’97 profile obtained as a result of the gravity data inversion. The plots show the values of density and corresponding values of velocities extracted from the 2-D Fig. 3. The 2-D density model along the EUROBRIDGE’97 profile calculated from the P- and S-wave velocity models of Fig. 2: (a) Comparison of the observed gravity field (black line), regional gravity (blue dashed line) and calculated gravity effect of the 2-D model (pink line); (b) 2-D density model (dashed line shows the range of seismic model); (c) and (d) densityvelocity diagrams compiled from the P- and S-wave velocity models in Fig. 2 and the density model in Fig. 4b. 412 E. KOZLOVSKAYA et al. velocity and density grids. It is seen that the method allowed us to model the nonlinear and scattered density-velocity relationship similar to that revealed by the laboratory measurements in real rocks. Figure 3a demonstrates that the gravity field computed from this density model fits rather well to the observed Bouguer anomaly along EUROBRIDGE’97, with the exception of the ends of the profile. In the northern end of the profile the gravity field is dominated by the gravity effect of thick sediments of the PT; therefore, the gravity modelling requires detailed knowledge of the structure of the sedimentary cover. Although the velocity structure of the PT was constrained by results of previous seismic investigations in the area (Thybo et al., 2003), the boundaries within the sedimentary cover were smoothed to adopt them for the SEIS83 ray tracing program. Due to smoothing, some details of the sediment structure were lost, which resulted in a poor fit to the observed Bouguer anomaly within the PT. The values of density in the crust beneath the Pripyat Trough are in agreement with those obtained by the gravity modelling for the EUROBRIDGE’96 profile (Kozlovskaya et al., 2002). The values of density in the upper crystalline crust down to a depth of 5 km agree well with the a priori data for rock density in the main geological units along the EUROBRIDGE’97 profile (Egorova et al., 2003). The highest values of density in the upper crust were found in the southern part of the profile corresponding to the PB. In the central part of the profile, a complicated density distribution in the lower crust and upper mantle beneath the KP has been revealed. The density in the High Velocity Lower Crust (HVLC) is of about 3.1 g/cm3, while the density of the lower crust in the adjoining OMIB and PB is about 2.95−3.0 g/cm3. The modelling also indicates a layer of relatively low density in the upper mantle beneath the KP limited by the southward dipping mantle boundary identified from the seismic data. The density of this layer is of about 3.25−3.30 g/cm3, compared to 3.35−3.40 g/cm3 immediately below the Moho in the adjoining areas. This result can be considered as additional evidence of the contrast of elastic properties on this mantle interface or zone. The density contrast of the layer with the neighbouring structures in the upper mantle is sufficient to create a negative gravity effect responsible for the regional trend of the gravity field in the southern part of the PT and KP region. 5. DENSITY-VELOCITY RELATIONSHIP IN VARIOUS GEOLOGICAL UNITS CROSSED BY THE EUROBRIDGE’97 PROFILE Figures 3b and 3c demonstrate the density-VP and density-VS relationships, respectively, obtained as a result of gravity data inversion. It is seen that the relationship between density and VP for the whole EUROBRIDGE’97 profile consists of several branches and cannot be approximated by any simple linear relationship. The densityVS relationship demonstrates the same feature, although its scattering in the density range of 2.5−3.0 g/cm3 is less than that of the density-VP relationship. For the upper DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE a) 413 9.0 8.5 Osnitsk-Mikashevichi Igneous Belt 8.0 7.5 Vs [ km/s ] 7.0 6.5 6.0 5.5 2.5 b) 2.7 2.9 3.1 3.3 Density [ g/cm ] 4.5 4.0 3.5 2.5 3.5 2.7 2.9 3.1 3.3 Density [ g/cm ] 3.5 9.0 Podolian Block 8.0 7.5 7.0 Vs [ km/s ] Vp[ (km/s ] 8.5 6.5 6.0 4.0 3.5 5.5 2.5 c) 4.5 2.7 2.9 3.1 3.3 Density [ g/cm ] 2.5 3.5 2.9 3.1 3.3 Density [ g/cm ] 3.5 9.0 8.5 Volyn Block and Korosten Pluton 8.0 7.5 7.0 Vs [ km/s ] Vp [ km/s ] 2.7 6.5 6.0 5.5 2.5 2.7 2.9 3.1 3.3 Density [ g/cm ] 3.5 4.5 4.0 3.5 2.5 2.7 2.9 3.1 3.3 Density [ g/cm ] 3.5 Fig. 4. Relationships between density, VP and VS for major geological units: OMIB (a), PB (b), VB and KP (c), crossed by the EUROBRIDGE’97 profile. The left panels show density-VP relationship, the right panels show density-VS relationship (for VS, axis is scaled by factor 1.73). The reference density-velocity relationships are shown by stars. 414 E. KOZLOVSKAYA et al. mantle rocks with density of more than 3.2 g/cm3 both density-VP relationship and density-VS relationship consist of two branches corresponding to different values of velocity in the upper mantle revealed by the EUROBRIDGE’97 seismic profile (Fig. 2). To understand the reason for scattering of the density-velocity relationships, we plotted the relationship between density and seismic velocities separately for three major tectonic units crossed by the profile. The values of density and corresponding values of velocities were extracted from the parts of 2-D velocity and density grids corresponding to OMIB, VB together with the KP and PB, respectively. The sedimentary rocks of the PT were not included into our analysis. The result is shown in Fig. 4, where the left panel shows the relationship between density and VP and the right one demonstrates the relationship between density and VS in three major tectonic units mentioned above. It can be seen that the density-VP and density-VS relationships for these units are different. As the profile is located in the area of low heat flow, the pressure and temperature have minor effect upon densities and velocities comparing to the influence of rock composition. Therefore, the difference between density-velocity relationships for the OMIB, VB/KP and PB can be explained in terms of compositional variations by comparison with the corresponding relationships for rocks with known chemical and mineral composition. However, the density-velocity relationships obtained from laboratory measurements on rock samples are affected by other factors like fractures and porosity, which can mask the effect of composition. To exclude the influence of these non-lithological factors, the density and seismic velocities can be calculated from known modal mineralogy, assuming that elastic stiffness moduli and densities for minerals of specific composition are known. Monte-Carlo simulation of the relationship between seismic velocities and density in the rocks with known composition For calculation we selected several types of igneous and metamorphic rocks representing the main crustal lithologies and containing different proportions of main rockforming minerals. The modal mineralogy of selected rocks was taken from various petrological studies in the Ukrainian Shield and other Precambrian areas (Kozlovskaya et al., 2002; Hurich et al., 2001; Lichak, 1983; Markwick and Downes, 2000; Markwick et al., 2001) and used to estimate the range of modal proportions of main minerals in each rock type. The mineralogical data are summarised in Table 1. Then seismic velocities and density were estimated by Monte-Carlo simulation, in which 1000 random combinations of modal proportions of minerals were generated for each rock type. The technique of Belikov et al. (1970), which is a modification of Voigt’s homogeneous strain approach (Voigt, 1910), was used to calculate elastic parameters from modal mineralogy at room pressure and temperature. The detailed description of the technique can be found also in Schön (1998). The elastic stiffness moduli and densities for the main rock-forming minerals were taken from Dortman et al. (1992). The relationships between density and seismic velocities obtained by Monte-Carlo simulation are shown in Fig. 5. DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 415 T ab le 1 Modal mineralogy for selected rock types used for modelling (in percents) An in Pl is shown in brackets Qtz Kfs Pl(An20) Granite 30-40 30-40 20-40 Rapakivi granite 17-27 46-53 14-24 Pl(An60) Cpx Opx Ol Amph Grt 0 0 0 0 0 0-3 0 0-1 0 0-1.5 2-6 0 Igneous rocks Anorthosite Gabbro-norite Gabbro 0 0-2 0 90-95 0-2 0-2 0-2 0-2 0 0-2 0-10 0 43-65 10-24 0-20 0-23 0-5 0 0 2-3 0 50-60 7-10 6-14 10-30 3-6 0-3 Metamorphic rocks Amphibolite 0-4 0 30-40 5-10 0 0 0 55-60 0 Mafic granulite 0-4 0 0 40-50 4-20 8-12 0 20-30 0-9 Mafic garnet granulite 0 0 0 17-41 5-10 4-10 0 19-30 20-43 Eclogite 0 0 0 0 5-41 6-16 30-15 13-57 20-40 Mineral abbreviations after Kretz (1983): Qtz – quartz, Kfs – K-feldspar, Pl – plagioclase, Cpx – clinopyroxene, Opx – orthopyroxene, Ol – olivine, Amph – amphibolite, Grt – garnet, An − anorthite. The density-VP relationship shown in Fig. 5a is in good agreement with the relationship obtained by Hurich et al. (2001), who analysed the influence of various minerals on density and P-wave velocity in magmatic and meta-morphic rocks from the Grenville Province in Canada. The analysis of S-wave velocity was not presented there. Figure 5 demonstrates that different modal mineralogy results in different density-velocity relationships, although all of them have a similar trend. To understand the influence of different minerals on density-velocity relationships it was necessary to compare the calculated relations with some reference curves, i.e., the density-velocity relationships for rocks with some averaged chemical composition that did not undergo metamorphic reactions. Sobolev and Babeyko (1994) obtained such relationships for anhydrous magmatic rocks representing major crustal lithologies and having different average chemical composition, i.e., granites, granodiorites and gabbro. The reference relationships are shown by stars in the corresponding density-velocity plots in Fig. 5. It is seen that generally the influence of different minerals on the density-VP relationship is stronger than on the density-VS relationship. Comparison of the calculated density-velocity relationship to the reference curves (Fig. 5a, b) demonstrates that rocks with the high content of anorthitie plagioclase and low content of amphibole (i.e., anorthosites, gabbro-norites and gabbro) E. KOZLOVSKAYA et al. 416 IGNEOUS ROCKS a) Granite METAMORPHIC ROCKS Amphibolite Rapakivi granite 8.5 8.0 Anorthosite Mafic granulite Gabbro-norite Mafic garnet granulite Gabbro Eclogite 7.5 b) Vs [ km/s ] 7.0 6.5 6.0 4.5 4.0 3.5 5.5 2.6 2.8 3.0 3.2 Density [ g/cm 3 ] 3.4 2.6 2.8 3.0 3.2 Density [ g/cm 3 ] 3.4 Fig. 5. Relationships between density and seismic velocities obtained by Monte-Carlo simulation for selected types of igneous and metamorphic rocks from the Ukrainian Shield and other Precambrian areas: (a) density-VP relationship; (b) density-VS relationship (for VS, axis is scaled by factor 1.73). The reference density-velocity relationships are shown by stars. have the density-VP relationship shifted up with respect to the reference curve. However, the high content of anorthite does not affect significantly the density-VS relationship for anorthosites, which is rather close to the reference curve. The change in average plagioclase composition from labradorite to oligoclase and the increase of feldspar content in granites and rapakivi granites shifts the density-VP relationships for these rocks down from the reference curve, while the corresponding density-VS relationships are rather close to it. Both density-VP and density-VS relationships for amphibolite are shifted down from the reference curve, although the shift is more pronounced in the density-VP plot. This shift is explained by the combined effect of the change in plagioclase composition from labradorite to oligoclase decrease in the total amount of plagioclases and increase in amphibole content. Granulites and mafic garnet granulites, in which relative proportions of amphibole and Ca-rich plagioclase are nearly equal, have the density-velocity relationships close to the reference curve. However, in mafic garnet granulites and eclogites the increase in garnet and decrease in plagioclase content results in significant increase of both velocities and density comparing with the protoliths. The density-velocity relationships for mafic garnet granulites have the similar trend as the reference curves, while the density-velocity relationships for eclogites are shifted up with respect to the reference curve, which may be explained by absence of plagioclase and high content of pyroxenes. DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 417 From the above it can be concluded that an analysis of density-velocity diagrams can be used to restrict the composition of the crust, and, in particular, the composition and metamorphic grade of the lower crust. Therefore, if the density-velocity relationships are close to the reference curve in the range of velocities corresponding to the lower crust, it may indicate that the lower crust is composed of rocks of mafic composition and granulite metamorphic grade. The shift of the density-velocity relationships up from the reference curve indicates the high plagioclase content and absence of amphibole, which is typical for igneous rocks. On the contrary, the shift down from the reference curve may indicate the high content of amphibolite facies rocks in the lower crust. The density values less than 3.0 g/cm3 for the lower crust indicate that the lower crust contains minor amounts of high- and ultrahigh-pressure garnet-bearing metamorphic rocks, i.e., mafic garnet granulites and eclogites, while the density values in excess of 3.0 g/cm3 indicate that the lower crust was metamorphosed under high pressure conditions and contains mafic garnet granulites and/or eclogites. Composition of the crust along the EUROBRIDGE’97 profile as deduced from the analysis of density-velocity relationships The proposed analysis was applied to density-velocity diagrams for major tectonic units crossed by the EUROBRIDGE profile (Fig. 4a, b), where the reference densityvelocity relationships are shown by stars. The density-velocity relationships for the OMIB (Fig. 4a) are close to the reference curves for the upper and middle crust. However, they are shifted up from the reference curves for the lower crust. That implies that the crust there is most probably composed of igneous rocks like granite, granodiorite and gabbro for the upper, middle and lower crust, respectively. The density-velocity relationships for the PB are close to the reference curves for the upper and middle crust and shifted up slightly for the lower crust. This indicates some average granite-granodiorite composition and low metamorphic grade for the upper and middle crust and mafic composition and granulite metamorphic grade for the lower crust within the PB. Increased crustal thickness in the PB implies that the pressure-temperature conditions within the lower crust correspond to eclogite facies. In spite of that, the density values of the lower crust are lower than 3.0 g/cm3, indicating minor content of garnet and increased content of plagioclase, which implies that the lower crust did not undergo eclogitisation. This may be in the case when the thick crust of the PB was formed under anhydrous conditions (Austrheim et al., 1997). The most complicated density-velocity relationships were obtained for the VB and KP (Fig. 4c). It is seen that the density-VP relationship is strongly scattered around the reference curve and the density-velocity pairs are shifted both up and down from it. This indicates that the crust within the VB/KP is composed of rocks of different composition and origin. However, the density-VS relationship for the KP is almost linear in the density and velocity range corresponding to the crust. That is why a good fit 418 E. KOZLOVSKAYA et al. to the observed gravity data was obtained, in spite of high P-wave velocities in the upper crust. The points shifted up from the reference curve in the density-VP plot for the densities less than 3.0 g/cm3 correspond to igneous rocks of the KP, i.e., rapakivi granites, gabbro-norite-anorthosites and gabbro-norites. The content of plagioclase in anorthosites, gabbro-norite-anorthosites and gabbro-norites of the KP reaches 90−95%, 75−85% and almost 50%, respectively (Lichak, 1983). The points that are shifted down in the density-VP plot for the velocity and density ranges corresponding to the upper and middle crust indicate the presence of metamorphic rocks. These rocks are associated with a high velocity body in the northern part of the KP, where seismic modelling revealed high values of both VP and VS (6.40−6.46 km/s and 3.66−3.72 km/s, respectively) at depths of 5−12 km. The values of VS within the body are too high to be attributed to either rapakivi granites or anorthosites. As can be judged from the density model (Fig. 3a), the densities within the body are 2.75−2.80 g/cm3. This combination of VP, VS and density may be attributed to metamorphic rock with high amphibole content (Lebedev et al., 1983). The values of VP, VS and density determined in the upper part of the KP (0−5 km depth) suggest that it is composed of both rapakivi granites and anorthosites, although the resolution of the DSS data is not enough to resolve the detailed structure. It is evident that KP is indeed a complex and heterogeneous body. It should be noted that the pairs of density and P-wave velocity corresponding to the uppermost crust of the KP are shifted up with respect to the reference curve, while the values calculated from modal mineralogy (Fig. 5a) are shifted down. This is surprising, as the content of Kfs in rapakivi-granites of the KP is very high, i.e., about 45−50% (Lichak, 1983). Therefore, these values of density and seismic velocities cannot be explained by the effect of composition only. As it has already been mentioned in Section 2 (page 7), the VP in coarse-grained rapakivi and rapakivi-like granites is strongly affected by pressure. The VP in these rocks increases rapidly from about 6.0 km/s at surface to 6.5 km/s at a depth of about 5 km, remains almost constant at depths of 5−15 km, and then decreases to approximately 6.3 km/s at a depth of about 20 km. This explains why the density-velocity pairs corresponding to rapakivi granites are shifted up in the density-VP plot in Fig. 4. The HVLC revealed beneath the KP has the values of density of more than 3.0 g/cm3, which implies that it may be composed of mafic garnet granulites. However, the density-VP relationship is shifted up with respect to the reference curve in the range of densities and velocities corresponding to the HVLC. This may indicate the increased content of plagioclase comparing with the high-pressure granulites from Table 1. Lower crustal rocks with similar values of density and P-wave velocity were observed within a Grenvillian garnet granulite-facies gabbro-anorthosite terrain in the Bergen Arcs of Norway and classified as gabbroic granulite facies anorthosites by Fountain et al. (1994). DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE 419 6. CONCLUSIONS 1. The present study demonstrated that joint interpretation of wide-angle reflection and refraction data that is based upon relationship between density and both P- and S-wave velocities makes it possible to obtain more realistic density models and better fit to observed gravity data than conventional joint interpretation based upon P-waves alone. The calculated effect of the density model of the EUROBRIDGE’97 profile shows good agreement with the observed Bouguer anomaly and explains the main features of it. 2. The relationships between density and seismic velocities (VP and VS) obtained for large-scale tectonic units crossed by the EUROBRIDGE’97 profile are close to linear. However, they are scattered, differ from each other and deviate from the corresponding relationships for anhydrous magmatic rocks with averaged chemical composition selected as reference curves. 3. The scattering of density-VP relationship is generally higher than the scattering of density-VS relationship. This may be explained by weak dependence of VS on mean atomic mass and on the content of calcium-reach plagioclase. 4. Generally, the difference between the density-velocity relationships for particular tectonic units crossed by the profile and deviation of these relationships from reference curves can be explained by different composition and metamorphic grade of the crust within these units, resulting in different modal proportions of main rockforming minerals. 5. The igneous rocks of the OMIB with increased plagioclase content and small amphibole content have the density-VP relationship shifted up from the reference curve. It can be concluded that the processes of regional metamorphism did not change significantly the structure of the crust within the OMIB. 6. The density-VP relationship for the PB is rather close to the reference curve, indicating some average granite-granodiorite composition and low metamorphic grade for the upper and middle crust and mafic composition and granulite metamorphic grade for the lower crust. In spite of the increased crustal thickness, the lower crust of the PB did not undergo eclogitisation, indicating that the thick crust of this unit was formed under anhydrous conditions. 7. The most scattered and complicated density-VP relationship was obtained for the VB together with the KP, which indicates the presence of both igneous and metamorphic rocks within this unit. However, generally the density-VP relationship for the VB/KP is shifted up with respect to the reference curve, which may indicate high content of plagioclase. In coarse-grained rapakivi and rapakivi-like granites of the KP, the deviation from the reference curve is explained also by non-linear influence of pressure in the upper crust upon P-wave velocity. A c k n o w l e d g e m e n t s. This work was carried out as part of the EUROBRIDGE project within the framework of EUROPROBE (ILP/ESF) and IGCP Project 371 420 E. KOZLOVSKAYA et al. COPENA (“Structure and Correlation of the Precambrian in NE Europe and the North Atlantic Realm”). The authors offer their sincere thanks to the many people and organisations from Belarus, Denmark, Finland, Germany, Poland, Sweden, Ukraine and the United Kingdom who have contributed to the EUROBRIDGE’97 project. Financial support for EUROBRIDGE’97 field experiment was provided by the Danish Natural Science Research Council, Academy of Finland, Deutsche Forschungsgemeinschaft and GFZ Potsdam (Germany), the Institute of Geophysics of the Polish Academy of Sciences (Poland), Memorial University (Canada) and other organisations. References Aizberg, R.E., R.G. Garetsky, S.V. Klushin and E.A. Levkov, 1987, Deep structure and geodynamic of the Pripyat palaeorift and its frame. In: A.L. Yanshin (ed.), “Actual Problems of Tectonic of Oceans and Continents”, Nauka, Moscow, 200-211 (in Russian). Anderson, D.L., 1967, A seismic equation of state, Geophys. J. 13, 9-30. Austrheim, H., M. Erambert and A.K. Engvik, 1997, Processing of crust in the root of the Caledonian continental collision zone: the role of eclogitization, Tectonophysics 273, 129-153. Barton, P.J., 1986, The relationship between seismic velocity and density in the continental crust − a useful constraint?, Geophys. J. Roy. astron. 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