Download DENSITY-VELOCITY RELATIONSHIP IN THE UPPER

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. Soc. 87, 195-208.
Belikov, B.P., K.S. Alexandrov and T.W. Rysova, 1970, Elastic Properties of Rock Forming
Minerals and Rocks, Nauka, Moscow (in Russian).
Birch, F., 1961, The velocity of compressional waves in rocks to 10 kilobars (Part II), J. Geophys. Res. 65, 1083-1102.
Bogdanova, S., and EUROBRIDGE colleagues, 1996a, EUROBRIDGE: Palaeoproterozoic
Accretion of Sarmatia and Fennoscandia. In: D.G. Gee and H.J. Zeyen (eds.),
“EUROPROBE 1996 – Lithosphere Dynamics: Origin and Evolution of Continents”,
EUROPROBE Secretariat, Uppsala, 81-86.
Bogdanova, S.V., I.K. Pashkevich, R. Gorbatschev and M.I. Orlyuk, 1996b, Riphean rifting
and major Palaeoproterozoic boundaries in the East European Craton: Geology and
geophysics, Tectonophysics 268, 1-21.
Bogdanova, S.V., I.K. Pashkevich, V.B. Buryanov, I.B. Makarenko, M.I. Orlyuk, V.M.
Skobelev, V.I. Starostenko and O.V. Legostaeva, 2004, The 1.80-1.74 Ga gabbro
-anorthosite-rapakivi Korosten Pluton in the Ukrainian Shield: a 3-D geophysical reconstruction of deep structure, Tectonophysics 381, 5-7.
Carmichael, R.S. (ed.), 1989, Practical Handbook of Physical Properties of Rocks and Minerals, CRC Press, Boca Raton, Florida, 741 pp.
DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE
421
Christensen, N.I., and W.D. Mooney, 1995, Seismic velocity and composition of the continental
crust: a global view, J. Geophys. Res. 100, B7, 9761-9788.
Claesson, S., S.V. Bogdanova, E.V. Bibikova and R. Gorbatschev, 2001, Isotopic evidence of
palaeoproterozoic accretion in the basement of the East European Craton, Tectonophysics 339, 1/2, 1-18.
Červený, V., and I. Pšenčík, 1983, SEIS83 – numerical modelling of seismic wave fields in 2-D
laterally varying layered structure by the ray method. In: E.R. Engdahl (ed.), “Documentation of Earthquake Algorithm, World Data Centre A for Solid Earth Geophysics”, Boulder, Re. SE-35, 36-40.
Dortman, N.B. (ed.), 1992, Petrophysics. A Handbook. Book 1: Rocks and Minerals, Nedra,
Moscow (in Russian).
Egorova, T.P., V.I. Starostenko, V.G. Kozlenko and J. Yliniemi, 2003, Lithosphere of the
Ukrainan Shield and Pripyat Through revealed by the gravity studies, Geophys. J. 25,
4, 26-58.
Elo, S., 1997, Interpretation of the gravity anomaly map of Finland, Geophysica 33, 1, 51-80.
EUROBRIDGE Seismic Working Group, 1999, Seismic velocity structure across the Fennoscandia-Sarmatia suture of the East European Craton beneath the EUROBRIDGE
profile through Lithuania and Belarus, Tectonophysics 314, 193-217.
EUROBRIDGE Seismic Working Group, 2001, EUROBRIDGE’95: deep seismic profiling
within the East European Craton, Tectonophysics 339, 153-175.
Fountain, D.M., T.M. Boundy, H. Austrheim and P. Rey, 1994, Eclogite-facies shear zonesdeep crustal reflectors? Tectonophysics 232, 1-4, 411-424.
Ganchin, Y.V., S.B. Smithson, I.B. Morozov, D.K. Smythe, V.Z. Garipov, N.A. Karaev and
Y. Kristofferson, 1998, Seismic studies around the Kola Superdeep Borehole, Russia,
Tectonophysics 288, 1-16.
Garetsky, R.G., G.I. Karatayev and J.P Khot’ko (eds.), 1991, Deep Structure of the Earth and
Geodynamics of the Belarus Region, Nauka i Technika, Minsk (in Russian).
Gebrande, H., H. Kern and F. Rummel, 1982, Elasticity and inelasticity. In: K.-H. Hellwege
(ed.), “Landolt-Börnstein. Numerical Data and Functional Relationships in Science
and Technology”, New Series; Group V. Geophysics and Space Research. Vol. 1b:
“Physical Properties of Rocks”, Springer-Verlag, Berlin, 1-233.
Goff, J.A., and K. Holliger, 1999, Nature and origin of upper crustal seismic velocity fluctuations and associated scaling properties: combined stochastic analyses of KTB velocity
and lithology logs, J. Geophys. Res. 104, B6, 13,169-13,182.
Green, D.H., and A.E. Ringwood, 1967, An experimental investigation of the gabbro to eclogite transformation and its petrological applications, Geochimica et Cosmochimica
Acta 31, 767-833.
Henkel, H., M.K. Lee and C.-E. Lund, 1990, An integrated geophysical interpretation of the
2000 km FENNOLORA section of the Baltic Shield. In: R. Freeman, P. Giese and
St. Mueller (eds.), “The European Geotraverse: Integrative studies”, ESF, Strasbourg,
1-48.
422
E. KOZLOVSKAYA et al.
Hurich, C.A., S.J. Deemer and A. Indares, 2001, Composition and metamorphic controls on
velocity and reflectivity in the continental crust: An example from the Greenville
Province of eastern Quebec, J. Geoph. Res. 106, B1, 665-682.
Karatayev, G.I., 1966, Correlation Technique of Geological Interpretation of Geophysical
Anomalies, Nauka, Novosibirsk (in Russian).
Karatayev, G.I., R.E. Girin and I.V Dankevich (eds.), 1993, Geophysical Models of the Earth’s
Crust of the Belarus-Baltic Region, Nauka i Technika, Minsk (in Russian).
Kern, H., and A. Richter, 1981, Temperature derivates of compressional and shear wave velocities in crustal and mantle rocks at 6 kbar confining pressure, J. Geophys. 49,
47-57.
Kern, H., Ch. Walther, E.R Flüh and M. Marker, 1993, Seismic properties of rocks exposed in
the POLAR profile region. Constraints on the interpretation of the refraction data,
Precambrian Research 64, 169-187.
Khalevin, N.N., A.L. Aleynikov, E.N. Kolupaeva, A.M. Tyunova and F.F. Yunusov, 1986, On
the joint use of longitudinal and transverse waves in deep seismic sounding, Geologia
i Geofizika 10 (in Russian), 94-98.
Kneib, G., 1995, The statistical nature of the upper continental crystalline crust derived from
in situ seismic measurements, Geophys. J. Int. 122, 594-616.
Komminaho, K., 1998, Software manual for programs MODEL and XRAYS – a graphical interface for SEIS83 program package, University of Oulu, Dept. of Geophys., Report
20.
Kozlovskaya, E., and J. Yliniemi, 1999, Deep structure of the Earth's crust along the SVEKA
profile and its extention to the north-east, Geophysica 1-2, 111-123.
Kozlovskaya, E., G. Karatayev and J. Yliniemi, 2001a, Lithosphere structure along the northern part of EUROBRIDGE in Lithuania; results from integrated interpretation of DSS
and gravity data, Tectonophysics 339, 177-191.
Kozlovskaya, E., J. Yliniemi, G Karatayev and EUROBRIDGE Seismic Working Group,
2001b, Integrated density-velocity modeling along EUROBRIDGE’95-97 wide-angle
reflection and refraction profiles: Main results, Journal of EUGXI, Confer. Abstracts
6, p. 365.
Kozlovskaya, E., L. Taran, J. Yliniemi and G. Karatayev, 2002, Deep structure of the crust
along the Fennoscandia-Sarmatia Junction zone (central Belarus): Results of a geophysical-geological integration, Tectonophysics 358, 97-120.
Krasovsky, S.S., 1981, Reflection of Continental-type Crustal Dynamics in the Gravity Field,
Naukova Dumka, Kiev (in Russian).
Kretz, R., 1983, Symbols for rock-forming minerals, Am. Mineral. 68, 277-279.
Lebedev, T.S., 1989, Study of the physical properties of mineral substances in the lithosphere
at high pressure and temperature, Geophys. J. 7, 6, 796-822.
Lebedev, T.S., and V.A. Korchin, 1982, The relationship between the elastic characteristics of
rocks and their mineral composition at high pressure, Geophys. J. 4, 3, 396-410.
DENSITY-VELOCITY RELATIONSHIP IN THE UPPER LITHOSPHERE
423
Lebedev, T.S., V.I. Shapoval and V.A. Korchin, 1972, New data on longitudinal wave velocities in rocks at high thermodynamic parameter, Geofiz. Sbornik 49, 9-27 (in Russian).
Lebedev, T.S., V.A. Korchin and P.A. Burtny, 1983, Elastic wave velocity in ultrabasic and
magmatic rocks under high pressure and temperature and their variation with depth,
Geofiz. Zhur. 5, 5, 36-45 (in Russian).
Lebedev, T.S., G.Ya. Novik, V.A. Korchin, M.G. Zil’bershmit and T.K. Zavorykina, 1990,
Effect of structural transformations in rocks on changes in their elastic properties under different thermobaric conditions, Geophys. J. 8, 4, 449-467.
Lesnaya, I.M., T.E. Plotkina, L.M. Stepanyuk and Ye.N. Bartnitsky, 1995, Age stages of formation of the mafic-enderbite assemblage of the near Bug region. In: “Geochemistry
and Ore Formation”, Naukova Dumka, Kiev, 21, 56-69 (in Russian).
Lichak, I.L., 1983, Petrology of the Korosten Pluton, Naukova Dumka, Kiev (in Russian).
Ludwig, J.F., J.E. Nafe and C.L. Drake, 1970, Seismic refraction. In: A.E. Maxwell (ed.), “The
Sea”, vol. 4, 53-84, Wiley Inc., New York.
Luosto, U., T. Tiira, H. Korhonen, I. Azbel, V. Burmin, A. Buyanov, I. Kosminskaya, V. Ionkis and N. Sharov, 1990, Crust and upper mantle structure along the DSS BALTIC
profile in SE Finland, Geophys. J. Int. 101, 89-110.
Manghnani, M.H., and C.S.P. Ramananotoandro, 1974, Compressional and shear wave velocities in granulite facies rocks and eclogites to 10 kbar, J. Geophys. Res. 79, 54275446.
Markwick, A.J.W., and H. Downes, 2000, Lower crustal granulite xenoliths from the Archangelsk kimberlite pipes: petrophysical, geochemical and geophysical results, Lithos 51,
135-151.
Markwick, A.J.W., H. Downes and N. Veretennikov, 2001, The lower crust of SE Belarus:
petrological, geophysical and geochemical constraints from xenoliths, Tectonophysics
339, 215-237.
Molotova, L.W., and J.I. Vasilev, 1960, On the ratio value of P and S waves in the rocks, Izv.
AN SSSR., ser. Geophys. 8, 1097-1116 (in Russian).
Schön, J.H., 1998, Physical Properties of Rocks: Fundamentals and Principles of Petrophysics, Pergamon, the Netherlands, 583 pp.
Shcherbak, N.P., G.V. Artemenko and Ye.N. Bartnitsky et al., 1989, Geochronological
scale of the Precambrian of the Ukrainian Shield, Naukova Dumka, Kiev (in Russian), 144 pp.
Simmons, G., 1964, Velocity of compressional waves in various minerals at pressure to 10
kbars, J. Geophys. Res. 69, 6, 1117-1121.
Skobelev, V.M., B.G. Yakovlev, S.A. Galiyet et al., 1991, Petrogenesis of Nickel-Bearing
Gabbroic Intrusions of the Volynsky Megablock of the Ukrainian, Naukova Dumka,
Kiev (in Russian), 140 pp.
Smithson, S.B., F. Wenzel, Y.V. Ganchin and I.B. Morozov, 2000, Seismic results at Kola and
KTB deep scientific boreholes: velocities, reflections, fluids, and crustal composition,
Tectonophysics 329, 301-317.
424
E. KOZLOVSKAYA et al.
Sobolev, S.V., and A.Y Babeyko, 1994, Modelling of mineralogical composition, density and
elastic wave velocities in anhydrous magmatic rocks, Surveys in Geophysics 15, 515544.
Stepanyuk, L.M., E.B. Bibikova, S. Claesson, H.J. Stein, S.V. Bogdanova and V.M.Skobelev,
1999, Geochronological and petrological evidence for far-field effects of 2.1–2.0 Ga
convergent tectonics in the western Ukrainian Shield, Abstracts of the 7th EUROBRIDGE Workshop, 80-83.
Stephenson, R.A., S. Bogdanova and C. Juhlin, 1996, Palaeozoic rifting in a 2.0 Ga Andeantype magmatic belt in the East European Craton: structural and rheological implications, 7th Intern. Symp. on “Deep Seismic Profiling of the Continents”, 15-20 September 1996, Asilomar, California, Abstracts, p. 46.
Thybo, H., T. Janik, V.D. Omelchenko, M. Grad, R.G. Garetsky, A.A. Belinsky, G.I.
Karatayev, G. Zlotski, M.E. Knudsen, R. Sand, J. Yliniemi, T. Tiira, U. Luosto,
K. Komminaho, R. Giese, A. Guterch, C.-E. Lund, O.M. Kharitonov, T. Ilchenko,
D.V. Lysynchuk, V.M. Skobelev and J.J. Doody, 2003, Upper lithospheric seismic velocity structure across the Pripyat Trough and the Ukrainian Shield along the
EUROBRIDGE’97 profile, Tectonophysics 371, 41-79.
Verkhogliad, V.M., 1995, Age Stages of Magmatism of the Korosten Pluton. In: “Geochemistry and Ore Formation”, Naukova Dumka, Kiev, 21, 70-84 (in Russian).
Voigt, W., 1910, Lerhbuch der Kristallphysic, Teubner-Verlag, Leipzig.
Received 26 July 2004
Accepted 29 July 2004