GEOLOGICA CARPATHICA, 49, 2, BRATISLAVA, APRIL 1998
ANALYSIS OF THE GRAVITY FIELD IN THE WESTERN AND
EASTERN CARPATHIAN JUNCTION AREA: DENSITY MODELLING
Geophysical Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 28 Bratislava, Slovak Republic
(Manuscript received February 24, 1997; accepted in revised form December 11, 1997)
Abstract: An analysis of the gravity field in the Western and Eastern Carpathian junction area is based on local
isostasy and forward density modelling. For the first time the lithosphere/asthenosphere boundary was taken into
account for density modelling of long-wavelength gravity anomalies in this region. The gravity effects of the main
geological structures within the lithosphere were estimated. To give a better view of the present lithosphere structure
in this region, density cross sections were calculated along the profile KP-X. The results demonstrate a slab-like
structure under the mountain range. The slope of the underthrusted lower European Platform is very steep. Modelled
slab dips from about 60
. Density modelling shows that the southern margin of the European basement bends
down to the soutwest into the Carpathian subduction system. The East Slovak Basin is characterized by thinning of the
crust and lithosphere. The extension process is accompained by the existence of lower crustal high-density mass.
Key words: Western and Eastern Carpathian junction zone, gravity field, density modelling, lithosphere, asthenosphere.
The Carpathian arc and the Pannonian back-arc basin are in-
terrelated components of the Mediterranean arc-basin com-
plex (Royden 1988). Royden & Horváth (1988), Csontos et
al. (1992), Horváth (1993) and Ratschbacher et al. (1991a,b)
have developed the extrusion hypothesis, explaining the es-
cape of the Eastern Alps into the Carpathian area during Neo-
gene subduction. The formation of the Central European Al-
pides was influenced by complex processes such as
riftogenesis, crustal thinning, convergence, lateral displace-
ment, rotational movements, collisional suturing, accreation,
transpression-transtension (Soták 1992). The shape of the
Carpathian arc was apparently dictated by the Mesozoic ge-
ometry of an embayment in the European passive margin
(Tomek et al. 1996).
Szafián et al. (1997) proposed that in accord with active
transpressional orogens, a strong strain partitioning occurred
within the Carpathian arc, due to the oblique convergence be-
tween the escaping units of the Western Carpathians (North-
Pannonian units in the sense of Csontos et al. (1992), Tari et
al. (1992) and Horváth (1993)) towards the northeastern and
European margin areas. The process was combined with ex-
tension in the hinterland. The latest stage of development of
the Western Carpathian arc and the Pannonian Basin was
characterized by a lithospheric desintegration which occurred
as a consequence of a transition from a transpressional to an
extensional regime (Kováè et al. 1995; Fodor 1995). Most of
the transpressional deformations were associated with uplift
and erosion, but the final step was connected to the earliest
Miocene sedimentation (Fodor 1995). The Western Car-
pathian (North-Pannonian) units were involved in a Late Oli-
gocene-Early Miocene episode of eastward-directed, large-
scale continental escape, followed by extensional collapse of
the overthickened and gravitationally unstable crustal wedge
(Ratschbacher et al. 1991a,b; Tari et al. 1992; Horváth 1993).
The Intra-Carpathian area was made up of a set of tensional
and transtensional basins. A mechanism of extension varied
with depth-dependent rheology (Horváth 1993). Their initial
subsidence and extension took place mostly during the 17
and 13 Ma interval, synchronously with the deformation of
the external parts of the Carpathians (Royden 1988; Szafián
et al. 1997). It was accompanied by both crustal thinning and,
what is more important, by thinning of the lithosphere which
was associated with an uplift of asthenospheric and partly
molten masses (Ádám 1989, 1990; Praus et al. 1990; Stegena
et al. 1975; Babuka et al. 1987, 1990; Beránek & Zátopek
1981; Pospíil & Vass 1983; Horváth 1993; Kováè et al.
1995). The mantle xenoliths transported to the surface by the
youngest alkaline basalts support this suggestion (Koneèný et
al. 1995; efara et al. 1996)
Investigation of the geodynamic evolution of the Western
Carpathians was concentrated mainly on a study of deep-
seated crustal and lithospheric structures in its western and
central segments. For an integrated study not only of the
Western Carpathians but also the whole Carpathians it is also
very important to investigate their junction zone. For this pur-
pose it is useful to study the gravity field (besides other geo-
physical fields) in this region. In the present study the gravity
field was analyzed. The analysis was performed by means of
modelling in local isostatic equilibrium using not only gravity
and topographic data but also the thickness of sediments,
crust and lithosphere. This paper also utilizes gravity model-
ling for determining a preliminary density model of the litho-
sphere along the profile KP-X (Fig. 1). Several density mod-
els have been constructed in the Western Carpathians (e.g.
Fusán et al. 1971; Pospíil & Filo 1980; Pospíil & Vass
1983; efara et al. 1987; Bielik et al. 1990, 1991; Vyskoèil et
al. 1992; korvanek & Biela 1993; Lillie et al. 1994, etc.).
New maps of the gravity field in Poland (Królikowski &
Petecki 1995) and in Central Europe (Szafián et al. 1997),
new detailed maps of the crustal and lithosphere thicknesses
(Horváth 1993; efara et al. 1996), coupled with maps of the
thickness of Neogene sediments (Kilényi & efara 1989) and
the Carpathian Flysch Belt and Mollasse Foredeep (Rylko &
Tomas 1995) enable improvement of the density model in the
Western and Eastern Carpathian junction zone. The results
contribute to a more complete picture of the lithospheric
structure, slab evolution, collision and extension in the Car-
pathian mountain arc. This is the first time the lithosphere-as-
thenosphere boundary was taken into account for density mod-
elling of long-wavelength gravity anomalies in this region.
The gravity field
Former studies of the Bouguer gravity anomalies (Fig. 2)
in eastern Slovakia were published, for example, in the pa-
pers of Blíkovský (1961), Ibrmajer (1981), Matouek &
Odstrèil (1975), Pospíil (1977, 1980), utor & Èekan
(1965), korvanek & Biela (1993). The first synthesis of
gravity measurements and their interpretation was done by
Pospíil (1980). These studies provided the first information
about crustal structure.
The gravity field can be devided into two zones. The first
zone is characterized by positive anomalies. It correlates
very well with the area, which includes the whole East Slo-
vak Basin, Zemplínske vrchy Upland, Vihorlatské vrchy
Mts. This positive zone extends far towards the Pannonian
and Transcarpathian basins. The maximum amplitude of the
gravity field in the Zemplínske vrchy Upland reaches the
values of about +35 mGal (+350
). The Humenské
vrchy Upland and the Vihorlatské vrchy Mts. are accompa-
nied by a local gravity high with the maximum amplitude of
about +25 mGal. It is interesting to note that in spite of the
East Slovak Basin representing an expressionless relative
gravity low between the Zemplínske vrchy Upland and Vi-
horlatské vrchy Mts., it is accompanied by positive gravity
values (about +10 mGal). In general, extensional basins
which are filled by low-density sediments should be charac-
terized by negative observed gravity anomalies. On the ba-
sis of the stripped gravity map (Bielik 1988) it is well
known that this is not valid for subbasins of the Pannonian
Basin (e.g. for the Danubian Basin, the Little Hungarian Ba-
sin, the Great Hungarian Basin). This map showed signifi-
cant gravity highs over the deepest subbasins. Pospíil
(1980) and Bodnár & Pospíil (1981) were first to discover
this phenomenon in the East and South Slovak basins, re-
Fig. 1. Tectonic sketch of the eastern part of Slovakia after Biely
et al. (1996). Thick line shows the location of the sector of the pro-
file KP-X. Legend: 1 Gemeric Superunit, 2 Silicic Supernit,
3 Turnaic Superunit, 4 Veporic Cover Superunit, 5 Tatric
Superunit basement, 6 Tatric Cover Superunit, 7 Veporic
basement, 8 Zemplinic Cover Superunit, 9 Fatric Superunit
of the Humenské vrchy Upland, 10 Inner Carpathian Paleogene,
11 Neogene volcanics, 12 Neogene basins, 13 Pieniny
Klippen Belt, 14 Carpathian Flysch Belt [a) Magura Zone, b)
Fig. 2. Bouguer gravity anomaly map of the eastern part of the
Western Carpathians (after Ibrmajer 1981 and efara 1987). Coun-
tour interval 50
ANALYSIS OF THE GRAVITY FIELD 77
The second zone is characterized by negative values of the
Bouguer gravity anomalies. It covers in the northern and
northeastern parts of eastern Slovakia and includes the Low
Beskyds region and the Bukovské vrchy Mts. In the region of
eastern Slovakia the amplitude of the gravity low is only
about 10 mGal. But this negative gravity zone represents
only a part of the third segment large Carpathian gravity min-
imum (Tomek 1988). Most of the third part of the Carpathian
gravity minimum is located in Poland and mainly in Ukraine.
The anomaly runs southeast for more than 500 km along the
Eastern Carpathians. The Carpathian gravity minimum reach-
es about 100 mGal in Ukraine. According to Tomek et al.
(1979) and Pospíil & Filo (1980) the source of this third seg-
ment of the Carpathian gravity minimum can be explained by
the gravity effect of a large accumulation of flysch sediments
and autochthonous molasse.
The stripped gravity map in the East Slovak Basin, which
has been constructed by Pospíil in efara et al. (1987) shows
that eastern Slovakia is covered only by a zone of positive
gravity anomalies. The reason for that is removal of the nega-
tive gravity effect of basin fill and the gravity effect of higher-
density inhomogeneities of the crust (beneath the pre-Tertiary
basement), which extends outwards from the East Slovak Ba-
sin to well within the whole of eastern Slovakia.
The Western and Eastern Carpathian junction zone and its
wider surroundings is characterized by the long-wavelength
positive-negative gravity anomaly couple (Królikowski &
Petecki 1995; Ibrmajer 1981; Szafián et al. 1997). It is known
that this fact is valid for the whole Carpathian belt (Royden
1993; Bielik 1995; Krzywiec & Jochym 1997) and the Alps,
Apennines and Appalachians, too (Karner & Watts 1983;
Royden 1993). This gravity anomaly couple was interpreted
as evidence for flexure of the continental lithosphere by sur-
face and subsurface loading (Royden 1993; Bielik 1995;
Krzywiec & Jochym 1997).
Density modelling in local isostatic equilibrium
Calculation of a simple density model in local isostatic
equilibrium taking into account topography, gravity and
density data together with published maps of thicknesses of
sedimentary fill, crust and lithosphere provides a clue to
analysis of observed gravity field. The aim of density mod-
elling in local isostatic equilibrium is to offer and show the
contributions of the main anomalous zones to the free-air
and Bouguer anomalies.
Topography was available from the topographic map of
Slovakia (SÚGK 1976). In Poland these data were taken from
Fig. 3. Local isostatic model showing gravity contributions from
different levels. The depth of compensation is 250 km. Density
contrasts are in gcm
. The anomaly due to the topography (a), the
Moho (b) and the lithosphere (c). Free-air gravity effect (d) calcu-
lated by summing the three components. For this model a Bouguer
reduction densities of 2.67 gcm
for crust, 2.47 gcm
gene sediments, 2.62 gcm
for outer flysch and molasse sedi-
ments remove the effect of topography, resulting in a Bouguer
anomaly which is similar to the observed Bouguer anomaly in the
the Atlas of average topography in Europe (Geodätischen Di-
enste 1979). The thicknesses of Neogene sedimentary fill of
the East Slovak Basin and Pannonian Basin were modified
after Kilényi & efara (1989). Depths of the Outer Car-
pathian Flysch Belt and Molasse Foredeep basement in Po-
land were available from the results of Rylko & Tomas
(1995). In Slovakia, they were extrapolated on the basis of
Polish data. Input data on crustal thickness were taken from
the map of Moho depths in Slovakia (Beránek & Zátopek
1981; Mayerová et al. 1985; efara et al. 1996), in Hungary
(Horváth 1993) and in Poland (Guterch et al. 1986; Bojdys
& Lemberger 1986). For the thickness of the lithoshere were
adopted the depths published by Horváth (1993) and efara
et al. (1996). These authors constructed a lithospheric thick-
ness map for the Pannonian Basin and surrounding territo-
ries on the basis of seismological (Babuka et al. 1987,
1990; Spakman 1990), geothermal (Èermák et al. 1991) and
magnetotelluric (Praus et al. 1990; Ádám et al. 1989, 1990)
data. Bouguer anomalies were taken and modified from the
gravity maps published by Ibrmajer (1981), Szafián et al.
(1997) and Królikowski & Petecki (1995).
For analysis of relatively long-wavelength gravity anoma-
lies it is useful to simplify density models into anomalous
zones characterizing major density contrasts (Lillie et al.
1994). In the paper density contrasts are relative to typical
crustal materials. The study prefers density contrasts to abso-
lute densities. This approach has the effect of approximating
normal density increases with depth. Density contrast for sed-
iments is relative to upper crustal materials, while the lower
lithosphere and asthenosphere are relative to the lower crust.
The mean densities for the upper (2.75 gcm
= 2750 kgm
and the lower crust (2.95 gcm
), the lower lithosphere
) and asthenosphere (3.22 gcm
) are estimated
by using formulae published by (Rybach & Bunterbarth
1984; Lachenbruch & Morgan 1990). The mean densities of
the anomalous bodies in the upper crust were defined after
Eliá & Uhmann (1968), Pospíil (1980), efara et al. (1987),
Bielik et al. (1990, 1991), Vyskoèil et al. (1992). In spite of
knowledge that the density of Neogene sediments increases
with depth (Planèár in Biela 1978; efara et al. 1987; Bielik
1988; Meskó 1988; Kovácsvölgyi 1994) for our purpose it is
sufficient to suggest mean densities for both Neogene sedi-
ments of the East Slovak Basin and the Pannonian Basin and
the Outer Carpathian Flysch Belt and Molasse Foredeep.
In this study the following anomalous zones and their den-
sity contrasts are considered:
(1) Neogene sediments filling the East Slovak Basin and
the Pannonian Basin (0.20 gcm
Fig. 4 . Two-dimensional density models of the lithosphere along profile KP-X. Density contrasts are in gcm
. Density models: variant I
(a), II (b) and III (c). Legend: 1 Neogene sediments, 2 Carpathian Flysch Belt and Molasse Foredeep sediments, 3 Pieniny Klip-
pen Belt, 4 Mesozoic of the Humenské vrchy Upland, 5 Neogene volcanics, 6 high-density anomalous body.
ANALYSIS OF THE GRAVITY FIELD 79
(2) Flysch and molasse sediments of the Outer Car-
pathians (0.05 gcm
(3) Mantle part of the lithosphere (+0.30 gcm
(4) Asthenosphere (+0.27 gcm
, i.e. 0.03 gcm
contrast with the lower lithosphere).
The elements assigned zero density contrasts include crust-
al rocks. In local isostatic modelling density contrasts are for
topographic relief relative to air (2.67 gcm
for crust; 2.47
for Neogene sediments; 2.62 gcm
for outer flysch
and molasse sediments).
The three average topographic data are:
a) 0.2 km for the region of the East Slovak Basin and the
b) 0.6 km for the collision region;
c) 0.4 km for the region of the Outer Carpathian Flysch
Belt and molasse Foredeep.
The density contrast between crust and upper mantle
) and lower lithosphere and asthenosphere
) results in isostatic equilibrium (Fig. 3d) for an
approximately 10 km deeping of Moho discontinuity
(Fig. 3b) and about 70 km deeping of the lithosphere/astheno-
sphere boundary from the Pannonian Basin to the European
Platform (Fig. 3c). The maximum gravity contribution of
Moho to the free-air gravity effect is about 105 mGal
(Fig. 3c). This effect is not fully compensated by the gravity
effect of topography (Fig. 3a). The compensation in the Outer
Carpathians is about half (+50 mGal). In the collision region
it is a little bit more (about +70 mGal). For the second part of
compensation is also necessary to consider the gravity effect
of the lithosphere/asthenosphere boundary. Its gravity effect
on the free-air gravity effect is about +50 mGal (Fig. 3c). The
total of all anomalous zones gives free-air gravity effects
(Fig. 3d), which are positive over the Pannonian Basin and
the East Slovak Basin and negative over the Outer Car-
pathians. In spite of rough approximation of crustal and litho-
spheric geometry the calculated Bouguer anomaly correlates
relatively well with the observed gravity effect.
Lithosphere density cross section
To obtain a better view of the present lithosphere structure
in the Western and Eastern Carpathian junction zone a densi-
ty cross section was calculated along the profile KP-X
(Fig. 4). The line of profile KP-X (Fig. 1) starts in the Pan-
nonian Basin 150 km southwest of the Slovak-Hungarian
border. In a northeastern direction the profile runs across the
Zemplínske vrchy Upland and through the East Slovak Basin.
Then it enters the Vihorlatské vrchy Mts. and passes the Out-
er Carpathian Flysch Belt and Molasse Foredeep and termi-
nates in the European Platform (200 km from the Slovak-Pol-
ish border). The Fig. 4 shows a sector of the interpretation
profile with a length of 450 km. The GMSYS software pack-
age, which was used for calculation of density models, en-
ables avoidance of the marginal effects and to use the lithos-
phere/asthenosphere boundary for density modelling by
prolonging both ends of the profile by several hundred kilo-
The method of density modelling, like every geophysical
method of interpretation, is ambiguous from the mathemati-
cal point of view. Even the best correlation between observed
and calculated gravity anomalies does not guaranty that the
density model reflects real geological structure. It is only one
possibility from many solutions. On the other hand additional
geophysical data bind the modelling due to ambiguity of the
gravity field. Taking into account these facts three most opti-
mal solutions corresponding most with current geophysical
and geological knowledge are presented.
Density models are based on consideration of the four
anomalous zones mentioned above. Moreover, the models in-
clude and interpret other crustal anomalous bodies, of which
the sizes and density contrasts effect observed gravity anoma-
lies more than about ±5 mGal (e.g. Mesozoic of the Humen-
ské vrchy Upland together with the Pieniny Klippen Belt). A
significant anomalous body of high-density mass beneath the
East Slovak Basin basement is also a result of density model-
ling. To give a better image of the influence of anomalous
zones and bodies (density inhomogeneities) upon the total
gravity field some chosen gravity effects are shown in Fig. 5.
The picture illustrates that the largest contribution to the Bou-
guer anomalies comes from the Moho discontinuity. The dif-
ferences between gravity effects of the Moho and lithosphere/
asthenosphere boundary from the Pannonian Basin to the Eu-
ropean Platform are about 200 mGal and +130 mGals, re-
spectively. The Carpathian Flysch Belt and Molasse Fore-
deep give a maximum gravity effect of about 30 mgal. The
gravity effects of the Neogene sediments in the Pannonian
Basin and East Slovak Basin vary from about 0 to 42 mGal.
The results of density modelling (Fig. 4) demonstrate that,
a slab-like structure appears to be required under the moun-
tain range with dipping to the southwest to obtain a good fit
between the calculated and observed gravity anomalies. The
slope of the underthrusted lower European Platform is very
steep. The modelled slab dips from about 60
(Fig. 4a,b) to
(Fig. 4c). Density modelling shows that the southern
margin of the European basement bends down to the south-
west into the Carpathian subduction system. The bending is
in accord with the seismic results obtained in the Western
Carpathians (Tomek et al. 1989, 1996) and with density
modelling results obtained by Szafián et al. (1997) and Bie-
lik & Mocanu (1998) in the Eastern and Southern Car-
pathians. For the Carpathian system Royden (1993) sug-
gests that the crustal slab dips about 60
Under the East Slovak Basin region it was necessary to in-
terpret a striking high-density anomalous body (density con-
trast +0.30 gcm
). It is located within the lower crust (gravity
effect is about +37 mGal) and it compensates for the isostati-
cally low-density basin fill (42 mGal). The geometry of the
anomalous body was interpreted by two approaches. In the
first case the body has an almost symmetrical shape (Fig. 4a),
while in the second case it has a elonqated shape with a slope
toward southwestern (Fig. 4b,c). The upper boundary of the
anomalous body is at a depth of 1214 km. This study con-
firmed the results obtained by Pospíil (1980).
Starting cross sections showed varying degrees of agree-
ment between observed and calculated gravity anomalies.
For the Western Carpathian and the Pannonian Basin region
(Lillie et al. 1994) it was found that most of the disagree-
ment could be corrected simply by adjusting Moho depths.
Existing maps of crustal thickness (e.g. Beránek & Zátopek
1981; Mayerová et al. 1985; Guterch 1986; Èekunov et al.
1988) were mainly constructed by seismic refraction experi-
ments. While these maps are useful in showing general
changes in crustal thickness (e.g. crustal thinning from the
European Platform to the Pannonian Basin), they do not ad-
equately portray shorter-wavelength changes in Moho
depth. These shorter-wavelength changes are important in
presenting geometries that can be interpreted in terms of
tectonic evolution (Lillie et al. 1994). Therefore the dis-
agreement was partly corrected by adjusting the Moho con-
figuration. This approach is significant supported by the fact
that in the eastern part of Slovakia the published Moho
depths were only extrapolated. Seismic refraction and re-
flection data are not available. Moreover, discrete points at
which the Moho depths were determined from industrial ex-
plosions do not exist either (Mayerová et al. 1985). Density
modelling shows that the adjusted Moho correlates, in gen-
eral, with published Moho maps (Beránek & Zátopek 1981;
Èekunov et al. 1988; Horváth 1993; efara et al. 1996). It
means that the thickness of the crust increases gradually in
the direction from the Pannonian Basin (26 km) to the Euro-
pean Platform (3550 km). A sharp thickness contrast is in-
terpreted between the colliding plates. The results of the
modelling also demonstrate that the relief of the Moho under
the outer flysch and molasse zone has a rolling character.
Investigation of the lithospheric structure in the junction
of the Western and Eastern Carpathians by means of density
modelling is only the first step in complex geophysical and
geological interpretation. Unfortunately, the presented inter-
pretation cannot be supported by available seismic refrac-
tion and reflection profiling observations or seismic tomog-
raphy, because they are missing here.
Discussion and conclusions
The view of the current structure of the lithosphere ob-
tained by density modelling in the junction of the Western
and Eastern Carpathians appears to be compatible with
lithosphere geometry in the thrust belts formed at retreating
subduction boundaries which were defined by Royden
(1993). At retreating plate boundaries the rate of overall
plate convergence is slower than the rate of subduction. Lin-
zer (1996) has estimated between the stable East European
and Moesian plates and the Carpathian nappe systems that
the hanging-wall plate is convergence rate of 2 cm/yr is less
than the subduction rate of the foot-wall plate. These oro-
genic systems (e.g. the Apeninnes, the Carpathians and the
Fig. 5. Illustrating contributions to gravity from different density
inhomogeneities for density model (variant I) along profile KP-X.
Legend: 1 observed Bouguer anomaly, 2 calculated Bouguer
anomaly, 3 gravity effect of the lithosphere/asthenosphere
boundary, 4 crustal gravity contribution, 5 gravity effect of
the Moho, 6 gravity anomaly due to high-density anomalous
body, 7 stripped gravity anomaly (the Bouguer gravity anoma-
ly corrected by gravity effect of sedimentary fill), 8 gravity
contributions of Neogene sedimentary fill of the East Slovak Ba-
sin and the Pannonian Basin and Carpathian Flysch Belt and Mo-
ANALYSIS OF THE GRAVITY FIELD 81
Hellenides Royden 1993) are evidenced by significant
back arc extension contemporaneous with subduction. Pre-
sented lithospheric cross sections show clearly the presence
of regional extension both beneath the East Slovak Basin
and the Pannonian Basin within the overriding plate.
Interpretation of the gravity field also suggests the exist-
ence of a slab-like structure in an area of colliding plates
with dipping under the overthrusted plate. The existence of
the subducted slab is also assumed, for example, by Giese
& Morelli (1977), Royden (1993) and proved by Linzer
(1996). Similar results were also obtained by Száfian et al.
(1997) and Bielik & Mocanu (1996) in the Eastern and
Southern Carpathians. In this study the crustal slab was
modelled as the anomalous body which is submerged at a
depth of about 40 km. This depth was required to obtain
good agreement between the observed and calculated Bou-
guer anomalies. It is very probably that during initial stage
of subduction the slab submerged into the deeper parts of
the lower lithosphere and asthenosphere. During subduction
the slab in oceanic form resulted in melting of andesite
magma at a depth of about 100 km (Tomek personally
communication). It is speculated that modelled crustal slab
is only a remnant after breaking and submerging of the sub-
ducted plate. Using implications for the crustal structure
variation along the Carpathian arc (Szafián et al. 1997) it
could be assumed that the subducted slab has detached and
sunk into the deeper asthenosphere or has been heated up
and largely assimilated to the surrounding asthenosphere. In
the Vrancea region the dipping of rigid subducted plate goes
on, probably, into large depths (170) km and it is still active
(Oncescu 1984, 1987). It is documented and accompanied
by recent strong intermediate earthquakes. The latter are li-
mitted to the Vrancea area and are distributed along a verti-
cal plane. These intermediate earthquakes are not observed
in the Western and Eastern Carpathian junction zone
(Kárnik 1968). Underthrusting of the Vrancea plate beneath
the Eastern Carpathians is almost vertical (Oncescu 1984,
1987; Tomek et al. 1996).
It is also assumed that the current shape of the crustal slab
and its slope results from considerable rollback effect (Balla
1981; Tomek et al. 1996) which is connected with Krosno-
Menilite subduction (Tomek et al. 1989). It means that the dip
of the slab could be flatter at the beginning of subduction.
The existence of the rollback process along the Carpathians
was clearly shown by Linser (1996). The andesic volcanism
started in the Western Carpathians and the Apuseni Mts. dur-
ing the Middle Miocene (16 Ma) and migrated continuosly to
the east, ending in the Eastern Carpathians at 0.2 Ma (Vass et
al. 1988; Kalièiak & Pospíil 1990; Szakács & Seghedi
1995). Therefore, the rollback process of the ratreating slab
occurred between 16 Ma and the Holocene over a distance of
600 km, indicating an average displacement velocity of 3.75
cm/yr and the width of the rollback area about 50 km (Linzer
It seems that the collision process in the Western and East-
ern Carpathian junction zone has finished or is coming to its
end (Vass et al. 1988), even though in this region and its sur-
roundings shallow crustal earthquakes can be observed
(Zsiros et al. 1988; Labák & Brouèek 1996). There is evi-
dence that the Upper Miocene and Pliocene collision (11.0
1.8 Ma) of the European Platform and the Carpathian-Pan-
nonian plate in the region investigated is older than in the
Vrancea area of the Eastern Carpathians. The established K-
Ar dates of the Carpathian volcanic rocks show decreasing
age toward the southeast (Szakács & Seghedi 1995). The oro-
genic activity of the Alpine-Carpathian chain is also charac-
terized by a continuous eastward progression of deformation
along the leading thrust systems (Linzer 1996). The present
east-south and east-directed convergence in the easternmost
part of the Carpathians, the Vrancea area (Schmitt et al.
1990), probably marks the final stage of retreating subduc-
tion. This convergence is accompanied by recent earth-
quakes of crustal and intermediate depth (Linzer 1996).
Tomek et al. (1996) also speculates that the Vrancea seis-
mic zone is related to a more-or-less detached lithospheric
slab, perhaps the final expression of Carpathian subduc-
tion along the European margin.
Two-dimensional density models of the lithosphere struc-
ture presented in the paper indicate that the Western and
Eastern Carpathian junction zone is a very complicated area
in which interaction of compression, strike-slip and exten-
sion can be observed. This interplay led to the formation of
the East Slovak Basin (Kalièiak & Pospíil 1990; Soták
1992; Soták et al. 1995; Kováè et al. 1995). The basin cross
section is characterized by a larger thickness of sediments,
both crustal and lithospheric thinning. The extensional pro-
cess is accompanied by the existence of high-density (upper
mantle?, eclogical?) mass within the lower crust. Pospíil
(1980) suggested that the high-density anomalous body can
be explained by a suture associated with basic and ultraba-
sic rocks and/or a diapiric intrusion of upper mantle material
into the thin crust along the axis of the East Slovak Basin. It
is also possible (Soták personal communication) that the
anomalous body could also represent a detached part of an
older and shallower dipping of the subducted plate, when its
higher crustal position is a result of the tectonic exhumation
and extensional unroofing, which culminated in the formation
of the East Slovak Basin. Similar tectonics are suggested by
Tomek et al. (1997) for the Danube Basin. All extensional
movements have been placed on older Alpine-Carpathian
thrust faults and have made the tectonic exhumanation of the
Kolárovo enigmatic lower crustal body possible.
Acknowledgements: Author is thank Prof. Ján efara, Dr.
Èestmír Tomek, Dr. Vladimír Bezák, Dr. Adrián Panáèek
and anonymous reviewer for their critical comments, valu-
able suggestions and fruitful discussion which helped to im-
prove the quality of the manuscript. This research (Grant
No. 95/5305/418) was supported by the Scientific Grant
Agency (VEGA) of the Ministry of Education of the Slovak
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