Acta Geod. Geoph. Hung., Vol. 39(2-3), pp. 139-159 (2004)

LITHOSPHERE IN THE WESTERN CARPATHIANS AND ITS SURROUNDING TECTONIC UNITS GEOPHYSICAL STUDY M BIELIK1, J SEFARA 2 , M KOVAC 3 , J HOK 4 , J VOZAR 5 , H ZEYEN 6

Geophysical methods are important tools for the investigation of the structure and geodynamic development of the lithosphere. The central and eastern parts of the Western Carpathians are bordered in the north by a thicker and stronger lithosphere of the European platform (100-150 km), which is underthrust (about of 50 km) beneath the margin of the overriding Carpathian orogen. This thickening is interpreted as remnants of subducted slabs. In contrast, the "thin" lithosphere at the western margin of the Western Carpathians can be considered as a result of oblique collision along a deep-seated transform zone between the platform and orogenic lithosphere. Neo-Alpine "soft" collision and retreating subduction of this orogen can also be discovered by means of quantitative interpretation of observed gravity field. The crustal thickness in the Western Carpathians ranges among 27-35 km. The central Western Carpathians are characterized by thicker crust (30-55 km) in comparison with thinner crust (25-30 km) in the Pannonian Basin System. This feature is probably the result of the youngest lithosphere processes from the Middle Miocene. Rheological properties of the Western Carpathian lithosphere show that the mechanical strengths decrease within the whole lithosphere from the area of the European platform via the Western Carpathians to the Pannonian Basin. The most remarkable and important first-order tectonic structures (seismo-tectonic zones) in the Western Carpathians are the zones of the Pieniny Klippen Belt, the Mur-Miirz-Leitha fault zone, the Certovica fault zone and the Hurbanovo line. Map of neo-Alpine fault systems and neotectonic regions (blocks) of Slovakia was defined. Keywords: Carpathian-Pannonian Basin region; crust; flexure; geophysics; integrated modeling; lithosphere; neotectonics; rheology; Western Carpathians

1.

Introduction

The complicated structure of the Western Carpathian lithosphere with specific physical properties is a result of a complex geodynamic development of the orogen. On the surface, the structural pattern is documented by geology, but in the depth can be identified only by means of geophysics. Geophysical methods belong to one of the most important tools for the investigation of the structure and for the 1 Geophysical Institute of the Slovak Academy of Science, Dubravska cesta 9, 845 28 Bratislava 45, Slovak Republic 2Department of Applied and Environmental Geophysics, Comenius University, Mlynska dolina G., 842 15 Bratislava, Slovak Republic 3Department of Geology and Paleontology, Comenius University, Mlynska dolina G., 842 15 Bratislava, Slovak Republic 4Dionyz Stur State Geological Institute, Mlynska dolina 1, 817 04 Bratislava, Slovak Republic 5Geological Institute of the Slovak Academy of Sciences, Dubravska cesta 9, POB 106, 840 05 Bratislava, Slovak Republic 6Department des Sciences de la Terre, Universite de Paris-Sud, Bat. 504, F-91504 Orsay Cedex, France

1217-8977/$ 20.00 ©2004 Akademiai Kiad6, Budapest

140

M BIELIK et al. 16'

24 '

26' 50'

48'

' - -_ _--"ClO1
Fig. 1. Schematic tectonic map of the ALCAPA region. Modified after Lillie et al. (1994) and Kovac (2000)

reconstruction of geodynamic development of the lithosphere in this region (Fig. 1). We do not restrict our research to the Western Carpathians, but we also take into consideration the surrounding units: the Eastern and Southern Carpathians, the Eastern Alps, the European platform (Bohemian Massif) and the Pannonian Basin System. Additionally, the geophysical and geological research can contribute to the study of neotectonics of the Western Carpathians. The article reviews mostly the recent results and knowledge on the geophysical research of deep structure, geodynamics and neotectonics of the lithosphere in the Western Carpathians and partly in their surrounding tectonic units (in the ALCAPA (Alpine-Carpathian-Pannonian region)). For the purpose of that the results of interpretation of deep seismic refraction profiling, deep seismic reflection profiling, gravity, magnetic field, seismicity, geothermal field, geoelectrical conductivity zones, paleomagnetism, crustal thickness, lithosphere thickness, integrated modeling of the thermal lithosphere structure and thickness, flexure of the European platform and rheology are taking into account. In the paper we are interested more in the results obtained by 2D and 3D quantitative interpretation of observed gravity field, flexure of lithosphere, rheology and 2D integrated geophysical modeling.

Acta Geod. Geoph. Hung. 39, 2004

LITHOSPHERE IN THE WESTERN CARPATHIANS

2.

141

Geophysical study of the deep lithosphere structure

The research of the Western Carpathian lithosphere consists mainly of the application of the deep range geophysical methods, such as methods of seismic refraction a reflection profiling, seismology, gravimetry, magnetometry, magnetotelurics, geothermics and paleomagnetism. Seismic methods are of the cardinal importance in the analysis of the geophysical fields. The most important are refraction measurements of the deep seismic sounding (e.g. Beranek 1971, Beranek et al. 1972, 1979, Beranek and Zatopek 1981, Mayerova et al. 1994) and reflection seismic measurements with the prolonged time of registration from 12 to 16 seconds (Tomek et al. 1989, Tomek and Hall 1993, Ibrmajer et al. 1994, Vozar et al. 1996, 1998, Santavy and Vozar 1999). In the Western Carpathians gravimetric map in the scale 1:25 000 with the density of the measured points 3-6 points per km 2 (Sefara et al. 1996, 1998, Szalaiova and Santavy 1996, Santavy and Vozar 1999) has been used up till now. At present reambulation and completing of the total Bouguer anomaly map has already been finished (Grand et al. 2001). Measurements of the magnetotelluric sounding with the deep range (e.g. Praus et al. 1981, Varga and Lada 1988, Nemesi et al. 1996, Ernst et al. 1997) or magnetovariation sounding have also brought interesting results. These measurements found out and defined the Carpathian conductivity anomaly (Praus et al. 1990). Map of the surface heat flow (Cermak and Hurtig 1979, Horvath et al. 1989, Cermak 1994, Kral 1995, Sefara et al. 1996) and deduced temperatures in the depth (Cermak and Bodri 1986, Majcin et al. 1998, Zeyen and Bielik 2000) are very important information, too. The results of the seismology are used very successfully, either as earthquake interpretation (e.g. Schenk et al. 1994, Labak and Broucek 1996) or research of the seismic waves propagation (e.g. Babuska et al. 1984, 1987, 1988, Spakman et al. 1993). Magnetometric map of the aerial (Gnojek and Janak 1986) or the land (e.g. Filo and Kubes 1994, Kubes et al. 2002) measurements play an important role in distinguishing the crust type or other volcanic events. 3.

Geophysical characteristics of the deep structure and geodynamics of the lihtosphere

Deep seismic refraction profiling

The measurements in the Western Carpathians were conducted in 70's along international profiles and later along regional crustal profiles (e.g. Beranek 1971, Beranek et al. 1972, 1979, Beranek and Zatopek 1981, Mayerova et al. 1994). The results indicate a zone of increased vertical gradient of velocity, which was observable in the upper part down to a depth of around 10 km. Underneath this zone it can be seen the low-velocity channel. In the lower part of the crust there is a zone of increased gradient of velocities, which is probably associated with the transition zone. The Moho does not form a sharp boundary but a transient zone up to Acta Geod. Geoph. Hung. 39, 2004

142

M BIELIK et al.

700 650 600 40

30 20 10 0 · 10 ·20 -30

-40 -SO -00

·70

300

-00

250

·90 ·1 00

200

·,20

· 110

150 100

550

600

650

700

750

800

850

900

Fig. 2. Gravity anomaly map of the ALCAPA region (compiled after Bucha and Blfzkovsky 1994, Szalaiova and Santavy 1996, Vozar and Santavy 2000, Szafian 1999, Szafian et al. 1997, Bouguer-anomaly map of Hungary 1996, Krolikowski and Petecki 1995, Bureau Gravimetrique International 1962, 1964, Ibrmajer 1981). Contour interval is 10 mGal

several kilometers thick. The mentioned data on the Moho discontinuity were also systematically supplemented with data gained by industrial explosions (Bucha and Blizkovsky 1994). Deep seismic reflection profiling In 80's years, the reflection seismic method began to use in research of the deep structure (Tomek et al. 1989, Tomek and Hall 1993, Ibrmajer et al. 1994). Until now, data have been obtained along more than 1000 km of profiles (Vozar et al. 1996, 1998, Santavy and Vozar 1999). The chosen results, briefly presented here, include deep seismic reflection profile 2T, which confirmed, collisional origin of the Western Carpathians. In generally, profile 2T shows for example typical nappe structures of the Western Carpathians and the Veporicum was interpreted as an Upper Cretaceous whole-crust collision suture. Profile also brought a new insight into the understanding of the history of the Tertiary deformation processes in the Western Carpathians. The whole crust flexure of the lower European plate can be explained as a result of subduction movements when passive continental margin of the Krosno Sea was subducted beneath the Carpathian-Pannonian plate (Tomek et al. 1987, 1989).

Acta Geod. Geoph. Hung. 39, 2004

LITHOSPHERE IN THE WESTERN CARPATHIANS

143

IOOlr:m I

Fig. 3. a) The thickness of the sedimentary complexes of the Outer Carpathians. (Compiled by using data published by Poprawa and Nemcok 1989, Kovac 2000, Matenco 1997, KrejCi 1997, Mocanu and Radulescu 1994.) The values are in m. b) The gravitational effect of the 3D density model of the Outer Carpathians. Based on the results of Krolikowski and Petecki (2001) the average densities 2.62 gcm- 3 (2.42 gcm- 3 ) of the foredeep sediments and 2.65 gcm- 3 (2.57 gcm- 3 ) of the Flysch belt were applied for the western (eastern) part of the Outer Carpathians. The values are in mGal

Gravity The Bouguer gravity anomalies in the ALCAPA region decrease from the European platform towards the Outer Carpathians. The axis of maximum values of gravity low minimum correlates approximately with the center of the flysch zone of the Outer Carpathians. The Central Western Carpathians are followed by its increase, when the maximum values of the gravity high can be observed in the Acta Geod. Geoph. Hung. 39, 2004

144

M BIELIK et al.

Pannonian Basin System (Fig. 2). It means that the observed gravity field is characterized by a couple of gravity low and high. The gravity low is called "the Western Carpathian gravity minimum" in sense of Tomek et al. (1979), while the gravity high is called "the Pannonian gravity high" . Tomek et al. (1979) explain the Carpathian gravity low by low-density porous foredeep sediments covered by nappes of the accretionary wedge of the Outer Western Carpathians. A part of the gravity low, which can be identified in the Central Western Carpathians, has always been subject for various interpretations. Bezak et al. (1997) interpreted this anomaly as a segment of the European platform. The lithosphere and mainly the crust in the gravity minimum area of the Central Western Carpathians appears thicker (30-55 km) in comparison with thinner crust (25-30 km) in its surrounding. It is appropriate to assume another lithosphere evolution of the anomalous crust segment because the other geophysical evidence such as flexural bending as seen in seismics (Tomek et al. 1989) or gravimetry (Bielik 1995). Lillie et al. (1994) localized the plate contact in this area. In simultaneity intensive 3D interpretation of gravity field in the Outer Carpathians overshoots (Bielik et al. 2003, Makarenko et al. 2002). The purpose ofthis study is to demonstrate the gravity anomalies due to the density inhomogeneities located beneath the pre-Tertiary basement in the Outer Carpathians region. 3D density model of the sedimentary filling of the Outer Carpathian Molassic Foredeep and Outer Carpathian Flysch zone has already constructed (Fig. 3a). The total gravity effect of this model (Fig. 3b) has been calculated by means of the automatic system developed by Starostenko et al. (1997). The gravity effect of the sedimentary filling of the Outer Carpathians varies from 0 to -85 mGal. The highest value -85 mGal was found in the Western and Eastern Carpathians junction. In the Eastern and Southern Carpathians junction the maximum gravity effect is of about -75 mGal. The smallest gravity response was revealed in the western part of the Western Carpathians. The amplitudes of the Carpathians gravity low in the map of Bouguer gravity anomaly are still a little bit higher than the gravitational effects of the sediments of the Outer Carpathians. From this point of view it is possible to suggest that the Carpathian gravity low can not be explained by gravity effect of the outer Carpathian sediments only. Probably it is a result of superposition of both gravity effects of sediments and deep-seated density inhomogeneities as well. The Pannonian gravity high is the result of significant shallowing of the Moho (to about 25-30 km) beneath the Pannonian Basin system (Bielik 1988, 1998). Relatively low amplitude of gravity low in the Western Carpathians indicates the "soft" or "weak" collision that resulted in only "low mountain topography" (Lillie et al. 1994). The Western Carpathian gravity low with a wavelength of about 100 km indicates only about 50 km of post-collision crustal shortening (Lillie et al. 1994). This means that the process of convergence finished in the "initial" stage of collision or the convergence lasted only for a limited time. The amplitude of this gravity low reaching about -50 . 10- 5 ms- 2 (mGal) indicates that erosion and isostatic rebound amount to about 4 km. Presented values and analysis of the gravity field imply that collision stopped at an early stage in the Western Carpathians, so that transitional or oceanic crust is preserved intact beneath the mountains. Acta Gead. Geaph. Hung. 39, 2004

145

LITHOSPHERE IN THE WESTERN CARPATHIANS

Distribution of the hypocenter depths in Western Carpathians 6 5

r--

o

-

IT

K 3

5

7

11 13 9 Depth, km

15

_n 17

19

Fig. 4. The earthquake hypocenters map in the Western Carpathians (modified after Zsiros et al. 1987 and LaMk 1996)

U sing the calculations of density models in local isostatic equilibrium Lillie et al. (1994), Bielik (1995) and Bielik et al. (1998) found out that it is not completely possible to explain the measured gravity field without taking into account the lithosphere-asthenosphere boundary in the ALCAPA area. The bulge of the asthenosphere under the Pannonian Basin is too large. Its vertical dimensions are even 60 km and horizontal dimensions even 600 km. It influences the observed gravity field in the Carpathians to a high extent. Magnetic field

In general, the geomagnetic field of the Slovak Republic shows variable anomalous patterns (Santavy and Vozax 1999). A slightly changeable field is developed in the Outer Carpathians area where the anomalies mainly indicate the responses to the deep magnetic sources situated within the North European Platform. The anomalous field of the Inner Carpathians is mostly influenced by the effects of numerous relatively shallow and small sources usually causing the anomalies with amplitudes not exceeding 200 nT. On the other hand, the most changeable anomalous field as well as the anomalies locally exceeding 1,000 nT are found in the areas predominantly built by neovolcanics. Seismicity

Earthquake activity in the Western Carpathians is determined by its geological history and tectonics. The most seismically active unit is the belt stretching along the rivers Mur-Murz-Leitha fault zone in the Eastern Carpathians with SE extending the Pieniny Klippen Belt in the Western Carpathians. As a rule, the earthquake foci in the Western Carpathians occur within the upper part of the crust i.e. hypocenter depths vary from 3 to 15 km (Fig. 4). Acta Geod. Geoph. Hung. 39, 2004

146

M BIELIK et al.

Geothermal field The general increase of the heat flow in the direction from the outer Carpathian belt towards the Pannonian Basin system is dominant in the whole Carpathian system. The heat flow in the European platform and the Outer Western Carpathian flysch zone is only 50-60 mWm- 2 , but it attains over 100 mWm- 2 in the Pannonian Basin system. Surface heat flow density in the Pannonian Basin System is one of the highest terrestrial heat flows that have been detected. The high present day heat flow in this region is attributed to the Early-Middle Miocene lithosphere extension (Royden et al. 1983a, b, Lenkey 1999). The areas characterized by a high heat flow are underlain by a thin lithosphere.

Geoelectrical conductivity zones The most known geoelectrical conductivity zone known as Western Carpathians conductivity anomaly interpreted from the MVS (Praus et al. 1981) is located on the surface plane of projection of the deep-seated boundary between the European platform and the Carpathian-Pannonian plate. Based on the results of eerv et al. (1994) and Varga and Lada (1988) it is possible to interpret also other anomalous conductivity horizons. The most interesting is the zone in the bedding of Moho in the depth of about 40 km. It is supposed that they are separated from the original asthenosphere located in the depth of about 100150 km (Sefara et al. 1998). These zones were interpreted on the profile 3T (in the Danube basin) as well as on the profile 2T (in the Luceneckii kotlina fold). The zones of very low resistances were interpreted as the asthenospheric material with the properties of a partially molten material corresponding with the uprising ways of the youngest neovolcanic rocks - basalt (Sefara et al. 1998).

Paleomagnetism Not negligible marker of the dynamics of the lithosphere evolution is the results of the paleomagnetic measurements. Paleomagnetic declinations define the rotational movements of various parts of the lithosphere in the ALCAPA region including their time progression (e.g. Krs et al. 1982, Kovac et al. 1997, Panaiotu 1998, Kovac 2000). In the ALCAPA region it was distinguished a NW rotation located in its western part (in middle Badenian) and a NE rotation affecting only the eastern (north-eastern) part of this area (Sarmatian, up till lower Pannonian?). These rotations are probably related to the youngest evolution of the ALCAPA region. The rotation movements in the older periods are usually less distinctive.

Acta Geod. Geoph. Hung. 39, 2004

147

LITHOSPHERE IN THE WESTERN CARPATHIANS

4.

Crustal thickness

The crust of the Western Carpathians and neighbouring tectonics units has a complex structure and is composed of fragments formed during Alpine and Hercynian orogenic cycles. The flexibility of the Moho discontinuity depends mainly on the temperature relationships in the depths. The existing sections (Tomek and Hall 1993) of the inclined reflectors located on or above this discontinuity as well as the regions where Moho is not accompanied by reflectors indicate that this boundary is probably a demonstration of the vertical material (phase) changes including the transient zones of various thickness. Based on new seismic reflection profiling data as well as stripped gravimetric map Moho was reambulated in the western part of the Western Carpathians. Here it is assumed that the crustal thinning continues to the outer Carpathians. The crustal thinning is probably a result of the youngest (from the middle Miocene - Kovac 2000) extension of the crust in this area, which is at the same time related to the oblique collision and decrease of the transpressing processes. We interpret a similar process also in the Eastern Slovak basin. Essentially, we are talking about a new discontinuity origin by the influence of the thermal conditions changes in the depth (neo-Moho). The Western Carpathians are characterized by crust thickness (Fig. 5) of about

-

retracllon

-

- wide . angle

Fig. 5. Crustal thickness map for the ALCAPA region (compiled after Beranek and Zatopek 1981, Guterch et al. 1984, 1986, Cekunov et al. 1988, Posgay et al. 1995, Tomek et al. 1987, 1989, Tomek and Hall 1993, Horvath 1993, Sefara et al. 1996, Lenkey 1999). Contour interval 5 km

Acta Geod. Geoph. Hung. 39, 2004

148

M BIELIK et al.

30-35 km, the Eastern Carpathians by 32-42 km and the Vrancea zone by 55 km (new refraction seismic data show a maximum Moho depth of about 40 km in Vrancea - Hauser et al. 2001). The Moho depth increases to 65 km underneath the Tornquist-Teysseyre zone (Sollogub and Cekunov 1988, Gutterch et al. 1984, 1986). But the newest results of the POLONAISE and CELEBRATION 2000 experiments (e.g. CELEBRATION 2000 Working Group 2002) indicate that it is of about 45-50 km only. The Southern Carpathians Moho is at a depth of about 42-50 km. Note that the Carpathians are characterized by relatively thin crust in comparison with other orogens. The thinnest crust (25-30 km) can be observed in the Pannonian Basin system. 5.

Lithosphere thickness

From the map of the lithosphere thickness published by Babuska et al. (1987) and Horvath (1993) it results that the course of the lithosphere-asthenosphere boundary changes extremely in the ALCAPA region. While the mean thickness of the typical continental lithosphere in the central Europe is about 120 km, it is even 210 km in the Eastern Alpine area and, on the contrary, it is only about 80 km in the Pannonian Basin area. This means that the biggest difference in the lithosphere thickness is even 130 km in a relatively small ALCAPA area.

Integrated modeling of the thermal lithosphere structure and thickness A detailed analysis of the results published in the papers of Panza (1985), Babuska et al. (1987, 1988), Praus et al. (1990), Adam et al. (1996), Zhdanov et al. (1986), Horvath (1993), Cermak (1994), Posgay et al. (1995) and Lenkey (1999) showed clearly varieties in determination of the lithosphere thickness by means of different geophysical fields. These problems led Zeyen et al. (2002) to the application of integrated lithosphere modeling. 2D numerical models were based on a combined interpretation of heat flow, gravity data and topographic elevation. They have modeled lithosphere thickening underneath the central and eastern parts of the Western Carpathians (Fig. 6, Profile V). The lithosphere increases in thickness to a maximum of 140-150 km. The apparent lithosphere thickening was interpreted as a remnant of a subducted slab(s) of the European plate. Remnants of deep subduction below the Pannonian Basin have been detected earlier (Spakman 1990, Spakman et al. 1993, Goes et al. 1999). In addition to the slab detachment can be explained by the continuation of convergent movements between the overriding ALCAPA block and the European platform for a short time period after the slab break-off (Zeyen et al. 2002). The western part of the Western Carpathians does not show lithosphere thickening (Zeyen et al. 2002). Taking into account these results (Fig. 6, Profile I) we suggest that the tectonic evolution of continental collision along the Carpathian orogen has changed in time and space. The differences in lithosphere thickness in these both parts of the Western Carpathians could be explained by the different geodynamic evolution during the Oligocene and Miocene. It is assumed that a compressional process during Miocene brought the northern Acta Geod. Geoph. Hung. 39, 2004

149

LITHOSPHERE IN THE WESTERN CARPATHIANS

PROFIL I

PROFIL V

o

100

200

300

400

SOO

SW

NE

SE

NW

0

600

100

200

300

400

500

100

200

300

400

500

120

N' E

~

100

~

80

oS

0

;;; CIl

l:

n; (!)

oS ~

';;:

E

(!)

o

100

200

300

Distance (km)

400

500

600

o

Distance (km)

Fig. 6. Integrated modeling along Profiles I and V (modified after Zeyen et al. 2002). a) Surface heat flow, b) free-air anomalies, c) topography with dots corresponding to measured data with uncertainty bars and solid lines to calculated values, d) profile. Keys for profile I: 1-2 Pannonian Neogene sediments, 3 - Midle Hungarian Mts., 4 - Neogene sediments of the Inner Western Carpathians, 5 - Molasse and flysch of the Outer Western Carpathians and sediments of the Vienna basin, 6 - ALCAPA upper crust, 7 - Male Karpaty Mts., 8 - upper crust of the Bohemian massif, 9 - ALCAPA lower crust, 10 - high density anomalous crust, 11 - lower crust of the Bohemian massif, 12 and 13 - litospheric mantle. Keys for profile V: 1-2 Pannonian Neogene sediments, 3 - Midle Hungarian Mts., 4 - Neogene sediments of the Inner Western Carpathians, 5 - Flysch of the Outer Western Carpathians sediments, 6 - Molasse sediments, 7 - European platform cover, 8 - ALCAPA upper crust, 9 - Upper crust of the European platform, 10 - high density anomalous body, 11 - ALCAPA lower crust, 12 - Lower crust of the European platform, 13 and 14 - litospheric mantle

Acta Geod. Geoph. Hung. 39, 2004

150

M BIELIK et al.

a seizmolOglcol Slolion • mogne10leIuriC SO\X\Oing Site

I Inlegro1ed modeling N

A ,. ~

Fig. 7. Lithosphere thickness map for the ALCAPA region (constructed and modified after Babuska et al. 1988, Horvath 1993, Sefara et al. 1996, Lenkey 1999, Zeyen et al. 2002). Contour interval 20 km

(northeastern) segment of the Western Carpathians into frontal collision with the European platform, whereas the western segment suffered transpressional deformation due to oblique collision with the Bohemian massif (Csontos et al. 1992, Kovac et al. 1993, Csontos 1995, Decker and Peresson 1996, and others). Based on results of Zeyen et al. (2002) we constructed a new map of the lithosphere thickness in the ALCAPA region (Fig. 7). Modification of the lithosphere depth was done only in the region, in which the profiles I, II, III, IV and V are located. Moreover, in this region we concentrated especially on the Western Carpathian area. Note that the investigation of the Carpathians lithosphere by means of integrated geophysical modeling is going on in the Eastern Carpathians now. The first results have already indicated that the thickness of the lithosphere will be modified in this area, too. The hinterland of the Western Carpathians is characterized by a thin lithosphere (back arc basin). Based on seismic and magnetotelluric measurements, the thinnest lithosphere (40 km) is located beneath the Bekes Basin in Hungary (Posgay et al. 1995, Adam et al. 1996). The lithosphere under the other subbasins of the Pannonian Basin system reaches thickness of about 60 km (Horvath 1993, Lenkey 1999). The integrated modeling of Zeyen et al. (2002) indicates a little bit larger lithosphere thickness (about 80 km) in this region. The extreme lithoActa Geod. Geoph. Hung. 39, 2004

LITHOSPHERE IN THE WESTERN CARPATHIANS

151

sphere thinning was probably caused by stretching of the overriding plate and associated asthenospheric mantle up doming (e.g. Bielik 1988, 1998, Lillie et al. 1994, Kovac 2000).

Flexure of the European plate beneath the front of the Western Carpathian orogen Deformation of the lithosphere caused by loading can be calculated according to the elastic plate theory (Caldwell et al. 1976). This model very well approximates the foredeep basin geometry situated in front of the colliding orogen (Watts et al. 1975). Based on the results of Karner and Watts (1983), Royden (1993) and Bielik and Strizenec (1994) it was possible to relate the lithosphere flexure of the southern margin of the European platform under the Western Carpathians with loading (Fig. 8), which is caused by surface loading (corresponds to the thrust sheets and nappes of the Western Carpathians) and subsurface loading. The latter corresponds to the buried obducted blocks/flakes in sense of Karner and Watts (1983) and/or to a mid-crustal mass anomaly or to the subducted slab at depth (Royden 1993).

6.

Rheology

The ALCAPA region is one of the key areas for the study of the influences of various mechanical parameters on the lithosphere rheology because it is possible to observe several thermo-tectonic units in a relatively small area. Lankreijer et al. (1999) and Bielik et al. (2000) presented in their pioneer studies the results concerning the prediction of the lithosphere rheological behaviour in studied region. They calculated a decrease of the lithosphere mechanical strength from the European platform via the Western Carpathians to the Pannonian Basin system. The effective elastic thickness (EET) changes approximately in the interval from 5 to 23 km (EET values of 5-11 km are predicted for the Pannonian Basin, 8-23 km for the Western Carpathians, and 13-21 km is for the foreland of the Western Carpathians). Compared with the older European platform the mantle lithosphere rheological strength beneath the Western Carpathians significantly decreases and completely disappears in the Pannonian Basin region (Lankreijer et al. 1999 and Bielik et al. 2000, Fig. 9). A striking feature in predictions of the lithosphere strength is the extremely strong Bohemian core, rooting deep into the mantle lithosphere (up to 100 km depth). The highest calculated EET values for this region were 20-40 km. A single, relatively thin rigid layer located in the uppermost 10 km of the crust, characterizes of the Pannonian Basin system (Fig. 9). The rheological strength is absolutely missing in the lower crust and mantle lithosphere in this region. Extreme low rheological strength of the Pannonian lower crust and the mantle lithosphere is the result of high surface heat flow and very shallow and warm asthenosphere. The predicted EET is only 0-10 km. Acta Geod. Geoph. Hung. 39, 2004

152

M BIELIK et al.

oj ~RAL

DefiecHon

BUUlE

b)~ X-----==--------+----~,

c)

.1----1_ _.1.---" .

o

100 Km

Fig. 8. Flexure of the lithosphere. a) Schematic cross section across a generalized thrust belt formed during subduction process. b) Simple scheme of a deflection theoretical curve. c) Locations of the profiles. d) Comparison of the theoretical deflection curves (the dashed lines) and the topographic profiles (the solid lines) for the profiles A, Band C

Acta Geod. Geoph. Hung. 39, 2004

153

LITHOSPHERE IN THE WESTERN CARPATHIANS

0)

b)

Heatflow (mWm" )

100 i ................................... .. oo ~----

__----__------__----________

+ _ ·_ + __ +_+_·~~~_+_+_ _ · _ +_ +_ _ · _ +_ +_ _ + _+~

c)

Predicted Temperature field (deg C)

°1i_IflI;~R

-50

ooooo,o LAB

-1 00

o o

-150

o

o

o

~----~----~----~----~------~----+---~~----~-----+

-1 00

d)

o00

50

150

100

300

200

300

. 180 Compressional Strength prediction (MPa)

40

0 0 0 oULCB 0 0 0 0

o -50

o

-50

0

0 MOHO 0 0 0

02~~ 0 0 0 "2 0

0

0:> 0 0

~o 0 0 0

0

0

0

00 0

o~ 00 ",~~~~0~0~-=="'"iiJi '80~~~14~0~~ 4

• • • • • • • • LAB • • • • • • • • •

0

• o

-100

o Strainrate 10e-15

••

••

- 100 ~----~----~----~----~------~----~----~----~-----+

-100

-50

o

50

100

100

200

250

300

350

Fig. 9. Rheological cross-section C (modified. after Bielik et al. 2000). a) Location of the profile. b) Observed surface heat flow after Sefara et al. (1996). c) Lithosphere temperature field. d) Yield-strength contour plot for compressional deformation, at a strain-rate of 15 S-I. Legend: ULCB-upper/lower crust boundary, MOHO-Mohorovicic discontinuity, E = 10LAB-lithosphere / asthenosphere boundary

The determined rheology with its strong rigid layer in the upper part of the upper crust of the Carpathian-Pannonian Basin system and with its thickness of 0 to 15 km is in a good agreement with the earthquake hypocenters that are limited to the upper 15-17 km of the crust (e.g. Zsiros et al. 1987, Labak 1996).

Acta Geod. Geoph. Hung. 39, 2004

154

M BIELIK et al.

7.

Neotectonics

Seismic activity in the Western Carpathians is closely related to the crustal rheology and combined structural pattern within the brittle upper crust. The structural pattern is a combination of three structural levels. The deepest level is represented by the Paleo-Alpine suture zones dissected by neo-Alpine fault structures, above all strike-slip fault zones (second level). The contemporary stress field and vertical movements control the recent tectonic regimes (third level). Following the analyses of structures and various geophysical data, the principal seismo-tectonic zones of the Western Carpathians are defined (Kovac et al. 2002). The most remarkable and important first-order tectonic structure in the Western Carpathians (Fig. lOa) is the zone of the Pieniny Klippen Belt, which represents the contact of the Western Carpathian internides and the stable European Platform. The Mur-Miirz-Leitha fault zone in the area of the Vienna Basin represents the contact of the Eastern Alps with the block of Western Carpathian internides. Both these tectonic lines represent subvertical boundaries with Tertiary tectonic activity. The Certovica fault zone is a surface projection of the thrust plane of the Veporic thick-skinned sheet over the Tatric unit. Based on geological and geophysical data it is assumed that the Certovica zone is recently active due to its extensional reactivation. Earthquake events are released here mostly along the Hron fault system of the ENE-WSW direction. The next important tectonic structure is the Hurbanovo line, which is most probably the continuation of the Raba line into the Slovak territory. Based on reflection seismic interpretations, there are several low-angle fault surfaces dipping to the SE. These thrust planes were reactivated during the Miocene as low-angle extensional faults. The seismic events (e.g. Komarno area) are most probably generated on these low-angle surfaces. Map of neo-Alpine fault systems and neotectonic regions (blocks) of Slovakia (H6k et al. 1997, H6k and Kovac 1999, H6k 2000) compared with epicenters of macroseismically observed earthquakes for the territory of Slovakia for the period 1034-1990 (Labak and Broucek 1996) is illustrated in the Fig. lOb. 8.

Conclusions

A significance of the study of the structure, geodynamics and rheology of the lithosphere results from a fact that neotectonics, seismic hazard and natural risks are connected very closely to the deep processes, which take place within the solid Earth. It is well-known that, mostly, all sources of the surface changes must be looked for in deep-seated processes.

Acta Geod. Geoph. Hung. 39, 2004

155

LITHOSPHERE IN THE WESTERN CARPATHIANS

a)

N

I ?-I r.hs(r.,a!Id(wll,J)'McmI(u.) ~

thfKJlppe8fkt1

Komirom

o 0 III. IV.

00000 v.

VI. VII.

VIII.

IX.. MSK - 64

....

bl

• fA ••

N

A

.

~,

.1

_

2. the Klippen Selt

_

3. leitha fault zone

_

5. Central Siova kjon fault zone

",.

4. We, tern Slovakian block

0000000 10 : III. IV.

V. VI. VII. VIII.

_

6. Central Slovo kJon block

_

7. Eo,toSlovoklon block

IX. MSK - 64

Fig. 10. a) Map of suture and major shortening zones of the Western Carpathian (Pla.§ienka et al. 1997) in comparison with the epicenters of macroseimically observed earthquakes on the territory of Slovakia for the period 1034-1990 (Labak and Broucek 1996). b) Map of Neo-Alpine fault systems and neotectonic regions (blocks) of Slovakia (H6k et al. 1997,2000, H6k and Kovac 1999) compared with epicenters of macroseismically observed earthquakes for the territory of Slovakia for the period 1034-1990 (Labak and Broucek 1996)

Acta Geod. Geoph. Hung. 39, 2004

156

M BIELIK et al.

References

Adam A, Nagy Z, Nemesi L, Varga G 1990: Acta Geod. Geoph. Mont. Hung., 25, 291-307. Adam A, Szarka L, Pracser E, Varga G 1996: Geophysical Transactions, 40, 45-78. Babuska V, Plomerova J, SHeny J 1984: Geophys. J. R. Astr. Soc., 79, 363-383. Babuska V, Plomerova J, SHeny J 1987: In: Composition, Structure and Dynamics of the Lithosphere-Asthenosphere System, K Fuchs, C Froidevaux eds, Amer. Geoph. Un., Geodynamics, 16, 239-251. Babuska V, Plomerova J, PajduMk P 1988: In: Proceedings of the 4th EGT Workshop. The Upper Mantle. Commission of the European Communities, European Science Foundation, 37-48. Beranek B 1971: Studia geoph. et geod., 15, 316-330. Beranek B, Zatopek A 1981: In: Geophysical syntheses in Czechoslovakia. A Zatopek ed., VEDA, 243-270. Beranek B, Dudek A, Fejfar M, Hrdlicka A, Suk M, Zounkova M, Weiss J 1972: Geop. Trans., spec. ed. Budapest, 87-98. Beranek B, Lesko B, Mayerova M 1979: In: Geodynamics investigations in Czechoslovakia. Veda, Bratislava, 201-226. Bezak V, Jacko S, Janak M, Ledru P, Petrfik I, Vozarova A 1997: In: Geological evolution of the Western Carpathians. P Miner, P Grecula, D Hovorka, M Putis eds, Slovaca Corporation-Geocomplex, a.s., Geofyzika Bratislava and Geological Survey of Slovak Republic, Bratislava, Bielik M 1995: Gontr. Geophys. Inst. Slov. Acad. Sci., 25, 81-93. Bielik M 1988: Phys. Earth Planet. Int., 51, 185-189. Bielik M 1998: Geologica Garpathica, 49, 75-83. Bielik M, Strlzenec P 1994: Gontr. Geophys. Instit. Slov. Acad. Sci., 24, 87-104. Bielik M, Sefara J, Sotak J, Bezak V, Kubes P 1998: In: Geodynamic development of the Western Carpathians. M Rakus ed., Geological Survey of Slovak Republic, Bratislava, Monograph IBSN 80-85314-94-0, 259-272. Bielik M, Lankreijer A, Zoetemeijer R 2000: Osterreichische Beitriige zur Meteorologie und Geophysik, 26, 47-60. Bielik M, Makarenko I, Legostaeva 0, Starostenko V, Dererova J, Sefara J 2003: Osterreichische Beitriige zu Meteorologie und Geophysik (in press) Bouguer-anomaly map of Hungary 1996: Manuscript, ELGI, Budapest Bucha V, BHzkovsky M eds 1994: Crustal structure of the Bohemian Massif and the West Carpathians. Springer Verlag and Academia, Berlin, Heidelberger, New-York and Praha, 177-188. Bureau Gravimetrique International (en collaboration avec les Services N ationaux Interesses), Cartes mondiales de anomalies de Bouguer, scale 1:1,000, 000 (Vienna Sheet), Paris, France, 1962 Caldwell J G, Haxby W F, Karig D E, Turcotte D L 1976: Earth and Planetary Science Letters, 31, 239-246. Cekunov A V, Adam A, B!lzkovsky M, Bormann P, Guterch A, Dacev Ch, Kornea I, Kutas R J, Magnickij V A, Sollogub V B, Chain V E, Sollogub N V, Starostenko V J 1988: (in Russian). Naukova dumka, Kijev, 165. CELEBRATION 2000 Working Group 2002: Geologica Garpathica, 53, special issue, CDROM

Acta Geod. Geoph. Hung. 39, 2004

LITHOSPHERE IN THE WESTERN CARPATHIANS

157

Cermak V 1994: In: Crustal structure of the Bohemian Massif and the West Carpathians. V Bucha, M Blizkovsky eds, Springer, Berlin, Heidelberg, New York and Praha, 85121. Cermak V, Bodri L 1986: J. Geodyn., 5, 133-163. Cermak V, Hurtig E 1979: Heat flow map of Europe 1:5 000 000. In: Terrestrial heat Flow in Europe. V Cermak, L Rybach eds, Springer, Berlin Heidelberg New York Cerv V, Pek J, Praus 0 1984: J. Geophys., 55, 161-168. Cerv V, Pek J, Picha B, Praus 0, Tobyasova M 1994: In: Crustal Structure of the Bohemian Massif and the West Carpathians. V Bucha, M Blizkovsky eds, Springer Verlag and Academia, Berlin, Heidelberger, New-York and Praha, 147-156. Csontos L 1995: Acta Vulcanologica, 7, 1-13. Csontos L, Nagymarosy A, Horvath F, Kovac M 1992: Tectonophysics, 208, 221-241. Decker K, Peresson H 1996: In: Oil and gas in Alpine Basins thrustbelts and Basins of central and eastern Europe. G Wessely, W Liebl eds, AEGE Spec. pub!. N. 5, 69-77. Ernst T, Jankowski J, Semenov V Yu, Adam A, Hvozdara M, J6zwiak W, Lefeld J, Pawliszyn J, Szarka L, V Wesztergom 1997: Acta Geophysica Polonica, 45, 33-43. Filo M, Kubes P 1994: In: Crustal Structure of the Bohemian Massif and the West Carpathians. V Bucha, M Blizkovsky eds, Springer - Verlag and Academia. Berlin, Heidelberger, New-York and Praha, 200-207. Fodor L 1995: Tectonophysics, 242, 151-182. Gnojek I, Janak F 1986: 1:50 000. MS. Geofond, Bratislava Goes S, Spakman W, Bijwaard H 1999: Science, 286, 1928-1931. Grand T, Sefara J, Pasteka R 2001: In: The 4th Slovak geophysical Conference. Abstracts. Contributions to Geophysics and Geodesy, 84. Guterch A, Grad M, Materzok R, Pajchel J, Percuc E, Toporkiewicz S 1984: Acta Geophys. Pol., 32, 25-41. Guterch A, Grad M, Materzok R, Perchuc E, Toporkiewicz S 1986: In: Publications of the Institute of Geophysics, Polish Academy of Sciences A-17, Paristwowe wydawnictwo naukowe. Warszawa-L6dz (in Polish with English summary), 102, 84. Hauser F, Raileanu V, Fielitz W, Bala A, Prodehl C, Polonic G, Schulze A 2001: Tectonophysics, 340, 233-256. H6k J, Bielik M, Yanko J, Kovac P, Sujan M 2000: Mineralia Slovaca, 32, 459-470. H6k J, Kovac P, Madaras J, Maglay J, Kovac M, Barath I, Sabol I, Slavik M, Lukaj M 1997: Manuskript, archiv GSSR, Bratislava. 1-77. Horvath F 1993: Tectonophysics, 226, 333-357. Horvath F, Szalay A, Dovenyi P, Rumpler J 1989: In: Thermal modeling in sedimentary basins, M Mahel' ed., Technip. Paris, 339-358. Ibrmajer I, Tomek C, Korab T, Dvorakova L 1994: In: Crustal Structure of the Bohemian Massif and the West Carpathians. V Bucha, M Blizkovsky eds, Springer, Berlin, Heidelberg, New York and Praha, 21-44. Karner G D, Watts A B 1983: J. Geophys. Res., 88, 10449-10477. Kovac M 2000: Veda Bratislava, 202. (in Slovak) Kovac M, Nagymarosy A, Sotak J, Sutovska K 1993: Tectonophysics, 226, 401-416. Kovac M, Bielik M, Lexa J, Pereszlenyi M, Sefara J, Tunyi I, Vass D 1997: In: Geological evolution of the Western Carpathians. P Grecula, D Hovorka, M Putis eds, Miner. Slovaca Corporation-Geocomplex, a.s., Geofyzika Bratislava and Geological Survey of Slovak Republic, Bratislava, 43-64. Kral M 1995: In: Atlas of Geothermal Energy of Slovakia, 0 Frako, A RemSik, M Fendek eds, GUDS, Bratislava

Acta Geod. Geoph. Hung. 39, 2004

158

M BIELIK et al.

Kr6likowski C, Petecki Z 1995: Gravimetric atlas of Poland. Panstwowy instytut geologiczny, Warszawa, 12. Krs M, Muska P, Pagac P 1982: Geol. Prace, Zpravy, 78, 39-58. Kubes P, Fil'o M, Kucharic L, Bezak V, Konecny V 2002: In: Krystalinikum - Contributions to geology and petrology of Crystalline Complexes, M Suk ed., 28, 109-127. Labak P 1996: In: Data Analysis in Seismology and Engineering Geophysics. Z Kalab ed., Institute of Geonics, Academy of Sciences of the Czech Republic, Ostrava-Poruba (in Slovak), 83-93. Labak P, Broucek I 1996: Catalogue of macroseismically observed earthquakes on the territory of Slovakia (Version 1996). Manuscript, Geophys. Instit. Slov. Acad. Sci. Bratislava, 15. Lankreijer A C 1998: Rheology and basement control on extensional basin evolution in central and eastern Europe. Variscan and Alpine-Carpathian-Pannonian tectonics. NSG publication No. 980101, Amsterdam, 158. Lankreijer A, Bielik M, Cloetingh S, Majcin D 1999: Tectonics, 18, 1139-1153. Lenkey I 1999: Geothermics of the Pannonian Basin and its bearing on the tectonics of basin evolution. NSG publication No. 990112. Vrije Universiteit Amsterdam, 215. Lillie J R, Bielik M, Babuska V, Plomerova J 1994: Tectonophysics, 231, 215-235. Linzer H G 1996: Geology, 24, 167-170. Majcin D, Dudasova V, Tsvyashchenko V A 1998: Gontr. Geophys. Instit. Slov. Acad. Sci., 28/2, 107-114. Makarenko I, Legostaeva 0, Bielik M, Starostenko V, Dererova J, Sefara J 2002: Geologica Garpathica, 53, special issue, CD-ROM. Mayerova M, Novotny M, Fejfar M 1994: In: Crustal structure of the Bohemian Massif and the West Carpathians, V Bucha, M Blizkovsky eds, Academia Press, Springer Verlag, 13-20. Nemesi L, Sefara J, Varga G, Kovacsvolgyi S 1996: Geophysical Transactions, 41, 133-159. Panaiotu C 1998: Report of Geodesy, 37, 205-211. Panza G F 1985: In: Second EGT Workshop. The southern segment. D A Galson, St Mueller eds, ESF, Strasbourg, France, 47-51. Peresson H, Decker K 1997: Tectonics, 16, 38-56. Plasienka D, Putis M, Kovac M, Sefara J, Hrusecky I 1997: In: Geological evolution of the Western Carpathians. P Grecula, D Hovorka, M Putis eds, Mineralia Slovaca Monograph, Bratislava, 35-42. Posgay K, Bodoky T, Hegedus E, Kovacsvolgyi S, Lenkey L, Szafian P, Takacs E, Timar Z, Varga G 1995: Tectonophysics, 252, 467-484. Praus 0, Pecova J, Petr V, Hvozdara M, eerv V, Pek J, Lastovickova M 1981: In: Geophysical Syntheses in Czechoslovakia, A Zatopek ed., Bratislava, Veda, 297-316. Praus 0, Pecova J, Petr V, Babuska V, Plomerova J 1990: Phys. Earth Planet. Inter., 60, 212-228. Ratschbacher L, Frisch W, Linzer H G, Merle 1991a: Tectonics, 10. 257-271. Ratschbacher L, Merle 0, Davy P, Cobbold P 1991b: Tectonics, 10, 245-256. Royden L H 1988: In: The Pannonian Basin, a study in basin evolution, L H Royden, F Horvath eds, Am. Assoc. Pet. Geol. Mem., 45, 195-233. Royden L H 1993: Tectonics, 12, 303-325. Royden L H, Horvath F, Rumpler J 1983a: Tectonics, 2, 63-90. Royden L H, Horvath F, Nagymarosy A, Stegena L 1983b: Tectonics, 2,91-137.

°

Acta Geod. Geoph. Hung. 39, 2004

LITHOSPHERE IN THE WESTERN CARPATHIANS

159

Santavy J, VOZ;lX J eds 1999: Electronical atlas of deep reflection seismic profiles of the Western Carpathians. Ministry of environment SR, Geological Syrvey SR and Geocomplex, Inc. Bratislava, CD-ROM Schenk V, Prochazkova D, Schenkova Z 1994: In: Crustal structure of the Bohemian Massif and the West Carpathians. V Bucha, M Blizkovsky eds, Academia Press, Springer Verlag, 13-20. Sefara J, Bielik M, Konecny P, Bezak V, Hurai V 1996: Geol. Carp., 47, 339-347. Sefara J, Bielik M, Bezak V 1998: In: Geodynamic development of the Western Carpathians. M Rakus ed., GS SR Bratislava, 273-280. Sollogub V B, Cekunov A V 1988: (in Russian). Naukova Dumka, Kiev Spakman W 1990: Terra Nova, 2, 542-553. Spakman W, van der Lee S, van der Hilst R 1993: Phys. Earth Plan. Int., 79, 3-74. Szafian P 1999: A case study in the Pannonian Basin and the surrounding mountain belt. NSG publication No. 990102. Vrije Universiteit Amsterdam Szafian P, Horvath F, Cloetingh S 1997: Tectonophysics, 272, 233-247. Szalaiova V, Santavy J 1996: Slovak Geol. Mag., 3-4, 369-377. Tomek C, Hall J 1993: Geology, 21, 535-538. Tomek C, Svancara J, Budik L 1979: Earth Planet. Sci. Lett., 44, 39-42. Tomek C, Dvorakova L, Ibrmajer I, Jitlcek R, Korab T 1987: Geophys. J. R. Astr. Soc., 89, 383-388. Tomek C, Ibrmajer I, Korab T, Biely A, Dvorakova L, Lexa J, Zbofil A 1989: Miner. Slovaca (in Slovak with English summary), 21, 3-26. Varga G, Lada F 1988: Magnetotelluric measurement on the profile 2T (in Czech). Manuscript ELGI Budapest, Geofyzika Brno Vozar J, Santavy J eds 2000: MINIST. IV.PROSTREDIA SR. ISBN 80-88974-06-2, 76. Vozar J, Tomek C, Vozarova A, Mello J, Ivanicka J 1996: Geol. Prrice, Spr., 101, 32-34. Vozar J, Szalaiova V, Santavy J 1998: In: Geodynamic development of the Western Carpathians, Monogr. M Rakus ed., Dionyz Stur Pub!., Bratislava, 241-257. Watts A B, Cochran J R, Selzer G 1975: J. Geophys. Res., 80, 1391-1398. Wortel M J R, Spakman W 2000: Science, 290, 1910-1917. Zeyen H, Bielik M 2000: Geoph. J., 5, 70-82. Zeyen H, Dererova J, Bielik M 2002: Phys. Earth Planet. Int., 134, 89-104. Zhdanov M S, Golubev N G, Varentsov I M, Abramova L M, Shneer V S, Berdichevsky M N, Zhdanova 0 N, Gordienko V V, Bilinsky A I 1986: Ann. Geophys., Ser. B, 335-341. Zsiros T, M6nus P, T6th L 1987: Gerlands Beitr. Geophys., 98, 146-154.

Acta Geod. Geoph. Hung. 39, 2004