Crustal structure at the contact of the Dinarides and Pannonian basin ...

Geophys. J. Int. (2009) 179, 615–633

doi: 10.1111/j.1365-246X.2009.04288.x

Crustal structure at the contact of the Dinarides and Pannonian basin based on 2-D seismic and gravity interpretation of the Alp07 profile in the ALP 2002 experiment 1 ˇ Franjo Sumanovac, Jasna Oreˇsković,1 Marek Grad2 and ALP 2002 Working Group∗ 1 Faculty

of Mining, Geology and Petroleum Eng., University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia. E-mail: [email protected] of Geophysics, University of Warsaw, Pasteura 7, 02-093 Warsaw, Poland

2 Institute

SUMMARY The area of this study includes the contact between the Dinarides and the Pannonian basin, as a relation between the Adriatic microplate and Pannonian segment. Our analysis was carried out on profile Alp07, which is a part of the ALP 2002 experiment. ALP 2002 was large international seismic experiment that focused on the lithospheric structure of the Eastern Alps and surrounding areas. Profile Alp07 is one of several refraction and wide-angle reflection profiles located in the transition from the Adriatic microplate, through the Dinarides, and into the Pannonian basin. This 300-km-long profile extends from Istra, Croatia to the Drava River at Hungarian–Croatian border in a WSW–ENE direction. It is oriented approximately perpendicular to the Dinarides, the main faults in the Adriatic region, and the contact between the Dinarides and Pannonian basin. 2-D seismic modelling was done using tomographic inversion and ray tracing techniques. The Moho depth is the greatest in the area of the Dinarides, reaching about 40 km and is shallowest (30–20 km) in the Pannonian basin area. At the boundary between these provinces, the depth changes rather suddenly. In order to obtain additional constraints on the crustal structure, 2-D gravity modelling was also performed. The layer boundaries were retained from the seismic model as there was no need to change them during modelling, since varying densities in the model produced a good fit to the data. A geological model was constructed based on both geophysical models. Three types of the crust were found along profile: the Dinaridic and the Pannonian crusts that are separated by a relatively narrow transition zone. The Dinaridic upper crust is characterized by low seismic velocities and densities, but its lower crust has high velocities and densities. The Pannonian crust can be seen as unique layer characterized by both low seismic velocities and densities. Large lateral and vertical changes in densities and seismic velocities can be found in the transition zone. Troughs in the seismic model at the level of the Mohoroviˇcić discontinuity are interpreted as major faults in the lithosphere. Three main lithospheric faults were identified: in northeastern part of the Dinarides under the Sava depression and under the Drava depression. The first one may be considered as a result of subduction of the Adriatic microplate under the Pannonian segment. Similar movements are also defined within the transition zone, where the Pannonian segment is gradually rising over on the Adriatic microplate. Key words: Controlled source seismology; Body waves; Continental tectonics: compressional; Crustal structure.

∗ ALP 2002 Working Group: E. Brückl, coordinator (Institute of Geodesy and Geophysics, Vienna University of Technology, Austria), T. Bodoky (Eötvös Lorand Geophysical Institute, Budapest, Hungary), A. Gosar (Environmental Agency of the Republic Slovenia), M. Grad (Institute of Geophysics, University of Warsaw, Poland), A. Guterch (Institute of Geophysics, Polish Academy of Sciences, Poland), Z. Hajnal (Department of Geological Sciences, University of Saskatchewan, Canada), E. Hegedüs (Eötvös Lorand Geophysical Institute, Budapest, Hungary), P. Hrubcová (Geophysical Institute, Academy of Sciences of the Czech Republic), G.R. Keller (School of Geology and Geophysics, University of Oklahoma, Norman, USA), J. Oreˇsković (Faculty of Mining, Geology and ˇ ca´ k (Geophysical Institute, Academy of Sciences of the Czech Republic), F. Sumanovac ˇ Petr. Eng., University of Zagreb, Croatia), A. Spiˇ (Faculty of Mining, Geology and Petr. Eng., University of Zagreb, Croatia), H. Thybo (Geological Institute, University of Copenhagen, Denmark) and F. Weber (Austrian Academy of Sciences, Vienna, Austria). C

2009 The Authors C 2009 RAS Journal compilation

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GJI Tectonics and geodynamics

Accepted 2009 June 8. Received 2009 May 19; in original form 2008 July 26

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ˇ F. Sumanovac et al.

I N T RO D U C T I O N This study is a part of the ALP 2002 experiment that was a large international cooperative experiment whose primary goal was to obtain fundamental data on deep lithosphere structures in the area of the Eastern Alps, the northwest Dinarides, the eastern part of Pannonian basin and western part of the Bohemian massif (e.g. Brückl et al. 2002, 2007). The Alps are the most extensively studied mountain range in Europe, but their eastern extension together with the Carpathians, Pannonian basin and the Dinarides form a very complex tectonic system, whose underlying plate tectonic dynamics remain poorly understood. These regions lie along the southern edge of the relatively stable European lithosphere. Despite many geological and geophysical studies in this area, tectonic and structural relations have not yet been fully explained. Therefore, this study is yet another attempt to offer a contribution towards a better understanding of the lithospheric structure and evolution in this area. The area of this study includes the southern and southeastern portions of the ALP 2002 experiment that primarily cover the contact between the Dinarides and the Pannonian basin. The Pannonian basin is characterized by extensional structures (Posgay et al. 1995), whereas subduction is considered main dynamic process in the Dinarides (Herak 1986; Moretti & Royden 1988). The participants of the ALP 2002 experiment include researchers from European countries (Austria, the Czech Republic, Croatia, Denmark, Finland, Hungary, Poland and Slovenia), as well as from Canada and the USA. ALP 2002 is part of a series of large seismic projects that have covered Central Europe: POLONAISE’97, CELEBRATION 2000 and SUDETES 2003 (Guterch et al. 2003, 2007). At the same time, the project TRANSALP, which focused on a deep seismic reflection profile that crossed the central Alps (TRANSALP Working Group 2002), was undertaken as well. Thus, these deep seismic experiments cover the area from the Baltic Sea, through Central Europe to the Adriatic Sea and encompass the East European craton, Trans-European Suture zone, Bohemian massif, Alpine and Carpathian regions, the Pannonian basin, and a part of the Dinarides. Although the basic concept was receiver deployment along network of 2-D profiles, the ultimate goal of these projects is a 3-D analysis of the each target area (e.g. Behm et al. 2007; Majdański et al. 2007; Malinowski et al. 2008). The study area, the Dinarides and the southwestern part of the Pannonian basin, is crossed by Alp07 profile that is fully located in Croatia, as shown in Fig. 1. Seismic modelling, both inverse and forward, was performed on the data gathered along this profile, for the purpose of determining the structure of the lithosphere and the relationships between the Adriatic microplate and the Pannonian basin of the Eurasian plate. Additionally, 2-D gravity modelling along the profile was performed to provide further constraints for the geophysical interpretation. Finally, the implications of our new integrated geophysical model are discussed in geological and tectonic sense.

R E G I O NA L T E C TO N I C S E T T I N G The study area is located in the boundary zone between the African and Eurasian plates that includes the Adriatic microplate, the Alps, the Dinarides and the Pannonian basin (Fig. 2). In the WSW–ENE direction, profile Alp07 stretches from the edge of the Adriatic microplate through the northern part of the External Dinarides and a very narrow belt of the Internal Dinarides. Towards the Pannonian basin, the profile crosses a wide ophiolite zone (Ilic et al. 2005;

Schmid et al. 2008), which is divided into the narrow Dinaridic ophiolite zone and much wider Sava-Vardar zone (Fig. 2). The ophiolites of these two zones differ in their origin, structure and age (Pamić et al. 2002). The profile terminates at eastern part of the Tisia block in the Pannonian basin. The area crossed by Alp07 is characterized by compression and movement of the Adriatic microplate in the NNE direction (Grenerczy et al. 2005) with clockwise rotation (Anderson & Jackson 1987). This motion is nearly perpendicular to the trends of exposed tectonic structures. According to more recent insights based on seismic activity, the Adriatic microplate is not a single coherent feature and consists of at least two major fragments (Favali et al. 1990; Oldow et al. 2002; Ivanˇcić et al. 2006). The boundary runs approximately from Gargano (in southeastern coast of Italy) towards Dubrovnik (Fig. 2). The southern fragment is rotating counter-clockwise, opposite to northern fragment (Favali et al. 1990). As result of the push exerted by the African plate, the Adriatic microplate is overthrusting the European plate, causing the formation of the Alpine–Dinaridic orogenic belt. Tectonic movements during the Palaeogene caused compression in the northeastern direction, whose consequence is the final formation of the Dinarides. In the coastal part of Croatia, the Adriatic microplate is underthrusting the Dinarides (Herak 1986; Moretti & Royden 1988), which is observed in seismicity as a northeastern subduction (Kuk et al. 2000).

Geological setting The Alp07 profile was laid out approximately perpendicular to the trend of the Dinarides and the main faults in the Adriatic region (Fig. 2). Subduction of the Adriatic microplate under the External Dinarides produced NW–SE trending steep reverse faults in a belt stretching from the northeastern coast of Istra towards the north Adriatic islands (Vrabec & Fodor 2006). The profile crosses two significant reverse faults in the northern Dinarides. At the very beginning of the profile, there is a fault at the edge of the External ´ carija fault (CF Dinarides, known as the Trst-Uˇcka-Dugi Otok or Ci´ in Fig. 2). Its location on the mainland is very well defined. In the Adriatic Sea, however, its direction is not accurately defined, and according to some authors, it extends as far as the island Vis (Fig. 2) in the southern Adriatic. The other significant reverse fault parallel to the Dinarides is the Velebit fault (VF in Fig. 2). These faults are part of a zone of reverse and thrust faulting southeast of Trieste that lies along the Adriatic coast and is considered by some authors as the main discontinuity between the External Dinarides and the Adriatic microplate (e.g. Aljinović et al. 1984). However, recent studies show that there is a continuous structural transition between Adriatic microplate and External Dinarides, which consists of a series of folds with smaller southwest-directed thrusts (Poljak & Riˇznar 1996). The Alp07 profile begins on the eastern side of the Istra peninsula, which is part of the former Adriatic carbonate platform, and continues via the islands Cres and Krk (Fig. 1). Thus the southwestern part of the profile crosses predominantly Cretaceous deposits, with a sporadic appearance of Palaeogene deposits. Upper Cretaceous limestones and dolomites with limestone inliers are predominant on the north Adriatic islands and along the coast, whereas on the island Krk and in the coastal area around Crikvenica there are narrow zones of flysch and molasse of Middle and Upper Eocene age. The profile continues through the Dinarides, with interfingering layers of Jurassic and Cretaceous rocks. In the highest regions of the Dinarides, at profile distances of 55–95 km, there are predominantly C

2009 The Authors, GJI, 179, 615–633 C 2009 RAS Journal compilation

Crustal structure on the Alp07 profile

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Figure 1. Locations of the receivers and shot points along Alp07 profile. Sg, Samoborsko Gorje Mountains. Insert shows all profiles from ALP 2002 experiment (main profiles Alp01, Alp02 and Alp04 are shown by thick lines; Brückl et al. 2007; Grad et al. 2009).

carbonate rocks, limestone and dolomites of Upper and Middle Jurassic age, with interfingering layers of limestones and dolomites of Upper and Lower Cretaceous age from 95 to 115 km along the profile (Geologic map of SFR Yugoslavia 1970). A narrow area of the Upper Jurassic carbonate rocks (limestones, dolomites and limestone breccias) reappears further towards the NE. At the transition of the Dinarides and the Pannonian basin, at the distance of about 120 km, there is a narrow area of dolomites, dolomitic limestones and shales of the Upper Triassic age, continued by the Neogene clastic rocks. On the northeastern edge of the Dinarides, the profile crosses the southern marginal fault of the Pannonian basin (FellaSava-Karlovac; SMF in Fig. 2), which is considered to the boundary between the Dinarides and the Neogene deposits of the Pannonian basin (Prelogović et al. 1998). The Pannonian basin and its origin are most frequently considered in relation to the Carpathian range in terms of a ‘backarc’ basin (Stegena et al. 1975; Royden et al. 1983). However, marginal areas of the southern Pannonian basin are influenced by events in the Dinarides (Pamić 1993; Tari & Pamić 1998). The southwestern part of the Pannonian basin is bordered by the Southern Alps in the east and the Dinarides in the south. The largest depressions in this part of the Pannonian basin are the Sava and Drava depressions, whose geological sedimentary fill is well known due to petroleum exploration (Luˇcić et al. 2001; Saftić et al. 2003). Pannonian basin is characterized by significant hydrocarbon potential, and the main hydrocarbon reservoirs in the SW part of the Pannonian basin are in the Sava and Drava depressions. There are 35 oil and 13 gas fields discovered. The Sava depression is located along the very southwestern edge of the Pannonian basin, whereas the Drava depression is further to the north (Fig. 2). Both depressions are of asymmet C


rical shape, with steeper southern limbs that are elongated in the NW–SE direction. There is a smaller depression between them, the Bjelovar depression, and in SE of the Sava depression there is the very narrow, sharp Karlovac depression. Towards the SE in relation to the profile, the Slavonian Mountains of Palaeozoic–Mesozoic age are located between the Sava and the Drava depressions. The Medvednica and Samoborsko Gorje Mountains (Sg in Fig. 1), where outcrops of ophiolites have been discovered (Pamić & Tomljenović 1998), are located at the northern edge of the Sava depression. The main boundary fault of the Drava depression is a right-lateral transcurrent fault with a NW–SE trend (Drava fault, Fig. 2). According to Prelogović et al. (1998), this fault continues to the Periadriatic lineament (PAL; Fig. 2) and is called the Periadriatic-Drava fault (DF). The southwestern part of the Pannonian basin is predominantly filled with Tertiary and Quaternary sediments, with maximum depths of about 7 km in the Drava depression (Velić et al. 2002). However, where the Alp07 profile crosses this depression, it reaches a depth of only about 4 km (Saftić et al. 2003). Neogene–Quaternary deposits are dominant in the Sava and Drava depressions. Above the bedrock formed by Mesozoic and Paleozoic rocks, the lower strata are breccias, conglomerates and limestones that are overlain by mostly sandstones, shales and marls. The shallowest deposits are sands, clays and gravels. Previous geophysical investigations A gravity map of the study area was compiled in the 1950s at a scale of 1:500 000 (Gravity map of SFR Yugoslavia 1972). Bouguer corrections were done with a density of 2.67 g cm−3 . According to

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Figure 2. Tectonic map of the Alp07 investigation area (generalized after Schmid et al. 2008; Tomljenović 2002).

the Explanation for the Gravity map of SFRY–Bouguer Anomalies (Bilibajkić et al. 1979) the density of measurements was greater in the Pannonian part and lower in the Dinarides. In Croatia there are between 6.1 stations per km2 (in some plain regions) and less than 1 station per 10 km2 in southern part of the Adriatic Sea where measurements were done mainly on islands. In the vicinity of the profile Alp07 the density of measurements is in the range 6.1–0.94 stations per km2 , with average density of 2.7 stations per km2 . Errors for the Bouguer corrections were calculated separately for three regions. The first region is Pannonian basin with rms error of Bouguer anomalies ±0.08 mGal. The second region is transition zone between Pannonian basin and Dinarides with RMS error ±0.23 mGal. The third region includes central parts of Dinarides and other high mountains where Bouguer anomalies have less accuracy with RMS error ±0.51 mGal, but the Alp07 profile doesn’t stretch across this area. It can be concluded that this precision and density of measurements satisfy the requirements for regional scale gravity research. Regional gravity anomalies (between –70 and +30 mGal) mostly express the depths of the Mohoroviˇcić discontinuity (Mohoroviˇcić 1910) as the main density contrast in the lithosphere. Local anomalies originate from shallower structures, showing geological relations in the near surface parts of the crust, and are thus most frequently targeted in petroleum exploration, thermal and mineral water exploration and other geological research. The largest gravity minimum in the territory of Croatia is located in the Dinarides (–40 mGal, Fig. 3). Thus, maximum depths of the Mohoroviˇcić discontinuity are expected there. Higher values are located in the Pannonian basin area, and in the Adriatic region

as well (around the Istra and island Cres, Fig. 1), and therefore, a thinner crust can be expected in these areas. In the Pannonian basin area, there is also a series of local anomalies, which originate from depressions or other shallow geological structures. In the vicinity of Karlovac (Fig. 3), there is a very sharp local minimum (down to –25 mGal), which at first sight looks like an anomaly caused by an error in measurement or data processing. Thicknesses of sedimentary rocks could not fully explain this anomaly either (Saftić et al. 2003). However, through subsequent checks and inclusions of the newest data (Saftić 2003), it was determined that the Karlovac depression is significantly deeper and sharper than previously thought. Gravity modelling with new data showed that the Karlovac depression, filled with clastic rocks, might be the cause of this anomaly. The main minima in the Pannonian basin are caused by the Sava and Drava depressions, located at the end of the profile. Between these basins, a local maximum is located, caused by the shallower Bjelovar depression (Fig. 3). Previous deep lithosphere research on the territory of the Republic of Croatia was performed as part of a research program that covered Slovenia, Croatia and Bosnia-Herzegovina. This research program included both deep electrical sounding and deep seismic sounding (DSS method) using refraction surveying. Deep electrical soundings were performed in the 1960s, and the first results were published by Kovaˇcević (1966). Deep refraction surveying was performed on three profiles, with general southeast–northwest trends: Palagruˇza–Valpovo profile (Andrić & Zeljko 1971), Dugi Otok– Virovitica profile (Joksović & Andrić 1982), and Pula-Maribor profile (Joksović & Andrić 1983). The processed and interpreted data from these profiles were presented in numerous publications (Zeljko C



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Figure 3. Location of the Alp07 profile on the background of generalized Bouguer gravity map of Croatia, Slovenia and Bosnia and Herzegovina (after Gravity map of SFR Yugoslavia 1972).

1972; Aljinović et al. 1981; Dragaˇsević & Andrić 1982; Aljinović & Blaˇsković 1984; Skoko et al. 1987). Two basic discontinuities were mapped, the Mohoroviˇcić discontinuity and the basement beneath the sedimentary cover. EXPERIMENT DESIGN In the controlled-source portion of the ALP 2002 experiment, single-channel receivers (RefTek 125, ‘Texan’) were used. These instruments were described in detail by Guterch et al. (2003). The basic Texan programming parameters were as follows: (1) (2) (3) (4) (5)

243 time windows; recording time in each window: 4 min 59 s; sampling frequency: 100 Hz; first window: 2002 July 2, 17:59:00–18:03:59 UTC; last window: 2002 July 5, 03:59:00–04:03:59 UTC.

Texan instruments with 4.5 Hz geophones were distributed along a network of profiles, covering a very wide area (see insert in Fig. 1). Locations were carefully selected to reduce natural seismic noise. During ALP 2002, a total of 926 instruments were deployed along C


14 profiles, with a total length of 4313 km, and 32 shots were carried out (Brückl et al. 2003). The profile Alp07 extends from Istra over the island Krk, passes near Karlovac, east of Zagreb and ends on the Drava River (Fig. 1). On the profile, 73 receivers were deployed at spacings of 3–4 km. Four shot points were employed: near Koromaˇcno in Istra (31160), near Tribalj in Crikvenica (37020), near Ivanić Grad (32070) and in Hungary, near the border with Croatia (38070). The list of shot points along Alp07 profile with coordinates in the WGS84-system are listed in Table 1. All shots were fired by the Polish team, which used a GPS-controlled blasting device.

S E I S M I C D ATA A N D WAV E F I E L D It can be generally stated that higher quality data were obtained from the shot points located in the Pannonian basin (38070 and 32070) than from the points located in the Dinarides (37020 and 31160). This variation was expected based on the results of the earlier deep refraction exploration projects. These studies showed that shot points located in carbonates, that is, generally mountain massifs, provide lower quality data than those located in clastic deposits,

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ˇ F. Sumanovac et al. Table 1. List of shot points on the Alp07 profile. SP 31160 37020 32070 38070

Location Istra, HR Crikvenica, HR Ivanić-Grad, HR Kaposvár, H

x (km)

λ (◦ )

φ (◦ )

0.0 51.3 203.0 295.8

14.1324 14.6982 16.4649 17.5408

44.9711 45.1994 45.7668 46.1325

Time (UTC)

Charge (kg)

2002:186:04:40:00.00 2002:185:02:50:00.00 2002:185:02:40:00.00 2002:183:22:10:00.00

400 300 300 640

Note: SP, shot point number; x, distance along profile and λ, φ , longitude and latitude in WGS84 system.

Figure 4. Seismic sections from the profile Alp07 for the shot points located in carbonate terrain – SP 31160, SP 37020. Marked are the main seismic phases Pg and Sg (refracted P and S waves from the crust), P crustal (overcritical crustal phase), PmP (refracted and reflected waves from the Moho); reduction velocity is 8 km s−1 .

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Crustal structure on the Alp07 profile that is, sedimentary basins (Figs 4 and 5; see also Grad et al. 2006; ´ Sroda et al. 2006). There are numerous causes for this phenomenon, but the most important is probably the near surface heterogeneity in the areas of the mountain massifs. Strongly developed discontinuities, fault and fissure zones act as boundaries off which seismic P waves refract, reflect and convert into S waves, causing a strong dispersion of seismic energy. Thus, the signal-to-noise ratio can be low and the scattering of the seismic signals can prevent the desired

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amount of energy from reaching the deepest parts of the Earth’s crust. Data of very high quality, in which the first, and even later arrivals are very clear, can be seen on the record sections for the shot points located in the Pannonian basin (Fig. 5), both at Ivanić Grad (32070) and near Kaposvár in Hungary (38070). For the shot point Ivanić Grad, the Pg first arrivals are clear to offsets of greater than 150 km (profile coordinate 50 km). At offsets of about 160 km, weak

Figure 5. Seismic sections from the profile Alp07 for the shot points located in Pannonian basin – SP 32070, SP 38070. Marked are the main seismic phases Pg (refracted waves from the crust), Pcrustal (overcritical crustal phase), Pn, PmP (refracted and reflected waves from the Moho), PI (upper-mantle reflection); reduction velocity is 8 km s−1 . For SP 32070 note surface waves in sediments of the Pannonian basin (highlighted in grey)—for similar waves see also profile ´ CEL05 (Grad et al. 2006) and profiles CEL01 and CEL04 (Sroda et al. 2006).

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refracted Pn waves from the Mohoroviˇcić discontinuity appear. In this area, the later arrivals are stronger, originating as reflected waves (PmP) from the Moho. Shot point 38070 was located off the NE end of the receiver spread and provided the most complete data on the deep structures and the Moho (Fig. 5). Weak first arrivals are seen out to offsets of 90 km. In the offset range of 90–150 km, the Pg first arrivals are hardly identifiable. In the offset range of 150–250 km, the first arrivals are Pn waves. The dominant events on the profile are the later arrivals, including the PmP reflection from the Moho, the overcritical phase Pcrustal, and the reflection from within the mantle lithosphere. Data quality obtained from the shot points at Istra (31160) and Crikvenica (37020) vary in a typical manner for carbonate areas, Fig. 4. However, Pg first arrivals and Moho reflection (PmP) from the shot point 31160 were identifiable to an offset of 150 km. At the far end of profile, no clear arrivals could be reliably identified. Shot point 37020 produced good Pg arrivals to more than 100 km and Pn arrivals are seen to the end of the record section. We were also able to identify some Sg arrivals. Although the locations of these two shot points were chosen in flysch deposits that were dominantly marly (not carbonates), relatively strong attenuation of seismic energy in the area of the Dinarides is evident. The thickness of flysch deposits is relatively small, and the structural complexity of the Dinarides perpendicular to the profile (Fig. 2) also probably plays a role in this attenuation. There are no reliable data about a thickness of flysch deposits, but according to the surface geological data, it can be estimated in the range of 100–300 m. Similar problem also occurred in the Alpine region along the profile Alp02 in Slovenia and Austria (Brückl et al. 2007).

G E O P H Y S I C A L I N T E R P R E TAT I O N S For purposes of 2-D seismic interpretation, two techniques were used. Seismic tomographic inversion was performed on the first arrivals (Hole 1992). Forward modelling using the ray tracing method, ˇ was performed using the SEIS83 package (Cerven´ y & Pˇsenˇc´ık 1983), supported by the graphical interfaces ZPLOT (Zelt 1994; ´ Sroda 1999) and MODEL (Komminaho 1997). Following seismic modelling, 2-D gravity modelling was carried out to determine the densities in the lithosphere. Gravity modelling was performed by using the software GM-SYS, based on the Talwani method (Talwani et al. 1959).

grid with equidistant nodes. The distance between nodes was 1 km. The computation is performed in several steps. In the first step picks up to 50 km were used. In the next steps the distance was increased to 100, 200 and 300 km, in order to gradually enlarge the maximum depth of the ray penetration. The rms error between observed and calculated data for the final model is 0.12 s. In the uppermost few kilometres of the model obtained by tomographic inversion, at distance 130 km till the NE end of the profile, very high vertical velocity gradients occur, which somewhat change laterally (Fig. 6a). The highest gradient occurs at distance from 150 to 220 km, while the velocity down to the depth of 6 km reaches up to 6 km s−1 . These high vertical gradients occur in the area of the Pannonian basin, and are due to the existence of local depressions. The highest mentioned gradients of seismic velocities occur in the area of the Sava depression, encompassing in the northeast end of profile also the Drava depression, with the Bjelovar depression located as a threshold in between. At depths from 5 to about 15 km, mild lateral changes in velocities are evident. Increased seismic velocities are evident at the southwest end of the profile at smaller depths, and at the distances 150–230 km and 230–290 km at greater depths. The velocities are generally relatively low and change in the range from 5.9 to 6.2 km s−1 , after which their increase with increasing depth is much stronger, and at depths in the interval of 30–45 km, they reach the value of 8 km s−1 . At the southwest end of the profile, the increase in velocities starts at greater depths than at the end of profile. The highest density of isolines is between 7 and 8 km s−1 , therefore the main discontinuity, that is, the Mohoroviˇcić discontinuity, is located in this interval. It can be generally said, based on previous explorations by forward seismic modelling, that the velocities in the lower crust are about 7.1 km s−1 , and in the upper part of the mantle about 8.1 km s−1 , so it can be assumed that the isoline 7.6 km s−1 best defines the Moho (e.g. Janik et al. 2005; Malinowski et al. 2005b; Grad et al. 2006). Accordingly, in the area of the Dinarides, at the southwest end of the profile, the velocity isoline 7.6 km s−1 is located at the depth of 42 km, whereas in the area of the Pannonian basin at the northeast end of profile, it is located at the smallest depth, about 28 km. The shapes of isolines indicate irregular rather than continuous changes in depth of the boundary, and the greatest depth change is located at the distance of 120– 150 km.

Seismic tomographic inversion

Seismic modelling using ray tracing

In the first step of seismic modelling we applied tomographic inversion based on the first arrivals (Hole 1992; Fig. 6a). The advantage of this method is that it relatively quickly provides a generalized model of seismic velocity distribution based on traveltimes of first arrivals. Each automatic inversion, for purposes of achieving inversion stability, necessarily requires smoothing of measured data, with the resulting loss of higher frequencies, which has further impacts on the reduction of the resolution, therefore some details are lost. Furthermore, a limiting factor is the use of only first arrivals, which in some cases can be even more difficult to read than the later arrivals. For tomographic modelling, 252 picks of the first arrivals were used. The program of Hole (1992) uses iterative back projection method. It uses linearization of the relation between traveltime and the slowness. The velocity model is defined in a 2-D rectangular

The distribution of the velocities in the tomographic model was used in construction of an initial four-layer model for 2-D forward ray tracing. There were only 4 shot points on this profile so there was concern that complex shallow structures would hinder modelling the deeper structures that were the main targets of this study. Thus, we sought out drilling and seismic reflection data from petroleum exploration in the data to define the structure in the sedimentary rocks above the crystalline basement. The thickness of the sedimentary section was obtained using a relatively dense network of seismic reflection profiles, and data from numerous exploration boreholes (Saftić et al. 2003). Velocity changes with increased depth were determined by applying characteristic relationships obtained from velocity measurements in deep exploration boreholes using data provided by INA-Naftaplin. These data provided a reliable and relatively detailed definition of the shallowest seismic structure in the model (Fig. 6b), which remained unchanged in the modelling C



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Figure 6. 2-D velocity models along Alp07 profile obtained using ‘smooth’ tomographic inversion based on P wave first arrival traveltimes; white parts of the model have no ray coverage (a) and using forward ray tracing modelling (b). Above topography and main tectonic units are shown. Shot point locations are marked by red triangles; numbers are P-wave velocities in km s−1 ; SMF, Southern Marginal Fault of Pannonian basin.

process. This meant that we could derive a more reliable determination of the deeper velocity structure and discontinuities. The second layer with seismic velocities of about 6 km s−1 constitues the crystalline upper crust and extends to depths of ∼15 km C


at the southwest end of the profile and all the way to the Moho (∼25 km) along the northeast end of the profile (Fig. 6b). A lower crustal layer with velocities as high as 7.1 km s−1 extends across the model from 0 to 220 km. In the initial model velocity below

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the Moho was assumed to be 8 km s−1 . However significant lateral changes were required to fit the Pn arrivals and an uppermost mantle reflector was also modelled. Velocities of about 6 km s−1 were generally found in the upper crust, but there were significant lateral velocity changes. A very shallow anomaly with slightly higher seismic velocities (>6.10 km s−1 ) at the southwest end of the profile in the area of Istra and the Adriatic coast stands out and is bounded to the northeast by a feature in which velocities are as low as 5.7 km s−1 . However, along the end of profile, the velocities are low and there is no anomaly evident such as the one that appears on the tomographic profile (Fig. 6a). The boundary between the lower and upper crust is located at depths of 12–17 km. Its depth gradually increases from the southwest to the middle of the profile, and it disappears as a clear boundary beginning at a distance about 220 km (Fig. 6b). In the lower crust of the Dinarides, there are relatively high seismic velocities (6.6–7.1 km s−1 ), whereas the velocities of the Pannonian basin are very low, only about 6.0–6.2 km s−1 . The velocity distribution on the profile shows that there is no need to divide the Pannonian crust into distinct layers. Similar low crustal velocities in the Pannonian crust were also observed beneath profiles CEL07 and CEL08 in western Hungary (Malinowski et al. 2005a). On the Alp02 profile, whose southeastern part stretches through the Pannonian part of Croatia, a multi-layer crust was however interpreted (Brückl et al. 2007). Crustal velocities at the crossing point between the Alp02 and Alp07 profiles (Fig. 1) beneath SP32070 are similar, being in the range of 5.7 km s−1 near the surface and about 6.2 km s−1 in the deepest crust at about 27 km depth (compare with fig. 14 in Brückl et al. 2007). However, the Alp02 profile extends along the edges of the ophiolite zone, thus the profile Alp07, which stretches almost perpendicularly to the contact zone of the Dinarides and the Pannonian basin, offers data that are relatively free of 3-D effects. Beneath the Alp07 profile, the Moho is deepest in the area of the Dinarides, reaching about 40 km (Fig. 6b). The depth in the area of the Pannonian basin ranges from 30 to 20 km and is the smallest at the northeast end of the profile. The boundary depth changes suddenly, thus making evident the uneven relief. The uplift on the profile in the distance of about 140 km is documented by clear Pn-arrivals and PmP Moho reflections (Figs 7–10). The synthetic seismograms and ray tracing diagrams explain very clearly why Pn-arrivals cannot be observed from SP 38070 in the distance from 130 to 200 km (Fig. 10). Namely, due to the relief of the Moho, there are no refracted rays from the larger part of this boundary. A strong later arrival at the southwest end of the profile, which is very clearly seen in the seismic section from SP 38070 (Fig. 10), can be explained by reflection from within the lithospheric mantle, and the modelled traveltimes and synthetic seismograms correspond well with the observed data.

A N A LY S I S O F A C C U R A C Y, R E S O L U T I O N A N D U N C E RTA I N T I E S OF SEISMIC MODEL Uncertainties of velocity and depth in the model obtained using the ray tracing technique result first of all from the uncertainties of subjectively picked traveltimes (Fig. 11a). Using modern techniques, the shot times and locations for shot points and receivers were measured very precisely, on the order 1 ms and few metres, respectively, with GPS systems, and these errors are insignificant in a crustal scale experiment. On the other hand, uncertainties due to erroneous interpretation of arrivals cannot be estimated, but the

probability of their accuracy increases with increasing quality and amount of data (number and effectiveness of shot points, signal-tonoise ratio, spacing between seismic stations, reciprocity of traveltime branches, ray coverage in the model). In most cases, the ray tracing technique used to model the picked traveltimes produced theoretical traveltimes that fitted the observed (experimental) traveltimes for both refracted and reflected waves with an accuracy of ±0.1–0.2 s (see e.g. Grad et al. 2008). In addition, synthetic seismograms show good qualitative agreement with relative amplitudes of observed refracted and reflected waves. The following conclusions about the resolution and uncertainties of models derived from refraction and wide-angle reflection data are based in part on the experience we obtained from the POLONAISE 97, CELEBRATION 2000 and SUDETES 2003 experiments, which were characterized by similar methodology, source and receiver density, and comparable data quality (e.g. Janik et al. 2002; Grad et al. 2003, 2006, 2008). For the Alp07 profile, we conducted sensitivity tests of the SEIS83 ray tracing results calculated for Pg, Pc and PmP arrival times for SP31160 (Fig. 11). Thick lines are traveltimes calculated for the final model shown in Fig. 6(b). The arrival times of the Pg phase with the velocity perturbed by +0.2 and –0.2 km s–1 shown by lighter lines are significantly too early and too late in relation to the recorded first breaks. This illustrates that the velocities in the upper crust can be determined from Pg waves with an accuracy of ±0.1 km s–1 . However, it should be remembered that these velocities are averages for regions of the crust with dimensions on the order of the recording station interval (3–4 km). In similar fashion, the arrival times of the midcrustal reflection Pc (∼15 km depth) and PmP phase reflected from the Moho in the final model (30–40 km depth) are shown together with arrival times with the depth perturbed by ±2 km, marked by lighter lines (Figs 11b and c). The misfits for the distance range of 20–70 km indicate that it is reasonable to claim that the depth of the midcrustal reflector and the Moho are resolved to at least ±2 km. In considering these estimates of uncertainty, the limitations of ray theory must be kept in mind. Also, our modelling was 2-D in nature, and thus does not allow for the presence of out-of-plane refracted and reflected arrivals, which must be present to some extent in such a structurally complex region.

Gravity modelling The data for 2-D gravity modelling along the Alp07 profile were extracted from the gravity map of the SFR Yugoslavia (Fig. 3; Gravity map of SFR Yugoslavia 1972). The main purpose of gravity modelling was to impose additional constraints on lithospheric structure and reduce the ambiguity of our integrated geophysical interpretation. It was expected that gravity modelling would assist in the definition of the density characteristics of individual layers and anomalous zones determined by seismic modelling. However, some features of the seismic model attained a new meaning during the gravity modelling so the exercise was particularly valuable to our integrated geophysical and geological interpretation. Considering that the seismic data had higher spatial resolution, the boundaries of individual layers and lateral boundaries of the initial gravity model were taken from the ray tracing seismic model (Fig. 6b). The density model extends to +30,000 km and −30,000 km along the profile to eliminate edge effects. The first 50 km beyond the edges the structure is extended based on the gravity data (Gravity map of SFR Yugoslavia 1972), and further with horizons parallel to the surface. The densities in individual layers C



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Figure 7. Forward ray tracing modelling of the seismic record section for SP 31160: (a) synthetic section; (b) seismic section with calculated travel-times for the final model and (c) ray diagram. Reduction velocity is 8 km s−1 .

were determined on the basis of seismic velocities, mostly by using Gardner’s formula (Gardner et al. 1974), and formulas proposed by Christensen & Mooney (1995), as well as the classic Nafe-Drake curve (Ludwig et al. 1970). Table 2 shows the velocities from the seismic model, densities calculated from seismic velocities by using different empirical formulas, and the final densities obtained for Alp07 profile in gravity modelling. Gardner’s formula gives mean density values of sedimentary rocks (with the exception of evaporites), thus the values obtained from this formula were used C


in the shallowest part of the initial density model. For crystalline crustal rocks, the linear density–velocity relationship of Christensen & Mooney (1995) was used. Crustal densities were also calculated by a formula derived by Brocher (2005) from the Nafe-Drake curve by the application of polynomial regression, which is, in fact, the Nafe-Drake empirical curve in the form of an equation. For sedimentary rocks, densities calculated by the Gardner’s formula corresponded very well with the final densities in the gravity model (Fig. 12). In order to achieve an acceptable fit between

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measured and calculated Bouguer anomalies, densities within the layers in the model were varied within the ranges of the predicted by the velocity–density relationships with the exception of two regions in the upper crust. One of these exceptions was the need for higher density body in the distance range of 150–200 km that can be explained by the presence of ophiolites that outcrop near the profile, in the area of Medvednica and Samoborsko Gorje (Fig. 1). The results of modelling with lower density (2.73 g cm−3 ), which is in the range of surrounding zones gives a sharp minimum with difference of

28 mGal at the position of 185 km (Fig. 12, green line). The other area was the region of low seismic velocities in the upper crust between 80 and 130 km (Fig. 6) where relatively high density of 2.78 g cm−3 was required. The rms error between observed and calculated gravity values (in Fig. 12 dots and red line, respectively) for the final model is 2.16 mGal. Using a constant mantle density of 3.30 g cm−3 resulted in maximum differences of 35 mGal between calculated and observed gravity values (Fig. 12, blue line). When this uniform density was C



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assumed, a fit to the observed gravity values could only be obtained by employing a very low density in the Pannonian crust, 2.59 g cm−3 . If a uniform density was set at 3.20 g cm−3 , an extremely low density (2.47 g cm−3 ) was required for the Pannonian crust. Since the seismic model showed changes of seismic velocities in the upper mantle, matching lateral variations in density were applied. Gravity data are definitely affected by the changes in density of the deeper mantle and also variation in lithospheric thickness but this is not discussed in this model as the interfaces are taken from C


the seismic model. All these effects are compensated by applied changes in the upper mantle density, which in this case belongs to the regional trend (Fig. 12, blue line). In summary, the overall density of the Pannonian and Transition zone crust is lower than in the Dinarides (Fig. 12). The Pannonian crust is characterized by the lowest density (2.69 g cm−3 ). The highest crustal density values occur in the Dinaridic lower crust (2.89 g cm−3 ) and the ophiolite zone under the Sava depression (2.83 g cm−3 ). The increase in the depth of the Moho at a distance

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about 245 km was also confirmed by gravity modelling because attempts to even out this interface lead to lack of agreement between measured and calculated values. The seismic model showed the existence of a Transition zone between the Dinaridic and Pannonian crust, but in the density model, it is more evident. In the seismic model, this 70 km wide zone appears somewhat narrower and extends from 150 to 220 km (Fig. 6). It is very difficult to define the density values of the zones in the crust and upper mantle through gravity modelling, but we emphasize that density relations between

the vertical and lateral zones in the crust and upper mantle are rather precisely determined.

T E C T O N I C I N T E R P R E TAT I O N Based on seismic and gravity 2-D modelling along the Alp07 profile, three types of crust can be defined: Dinaridic, transitional zone and Pannonian (Fig. 13). The Dinaridic crust is comprised of C



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Figure 11. Test of resolution of calculated traveltimes using the SEIS83 ray tracing technique for SP31160. (a) Seismic section with picked Pg, Pc and PmP arrival times (black, blue and red dots, respectively). (b) The same section with traveltimes; thick lines marked by Pg, Pc and PmP are traveltimes calculated for the final model of the structure shown in Fig. 6(b): the arrival times of the Pg phase with the velocity perturbed by +0.2 km s−1 and –0.2 km s−1 shown by lighter lines are significantly too early and too late in relation to recorded first breaks indicating that the velocity of the upper crust could be determined from the Pg wave with accuracy ±0.1 km s–1 ; the arrival time of the Pc and PmP phases reflected from the midcrustal discontinuity and the Moho in the final model (at ∼15 and 30–40 km depth, respectively) are shown together with arrival times with the depth perturbed by ±2 km marked by lighter lines. (c) Ray diagram with corresponding rays of Pg, Pc and PmP phases. Traveltimes and rays are in black, blue and red for Pg, Pc and PmP phases, respectively. See text for more discussion. Reduction velocity is 8 km s−1 .

C


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ˇ F. Sumanovac et al. Table 2. P-wave seismic velocities and corresponding densities. Structural unit

Sediments Dinaridic upper crust Dinaridic lower crust Pannonian crust Transition zone Mantle

P-wave velocity (km s−1 )

2.5–4.0 5.8–6.1 6.4–7.1 5.7–6.1 5.6–6.4 7.75–8.3

Density (g cm−3 ) Nafe-Drake Christensen and Mooney

Gardner

Used (final)

2.09–2.39 2.68–2.74 2.81–3.00 2.66–2.74 2.64–2.81 3.20–3.40

2.18–2.46 2.70–2.73 – 2.68–2.73 – –

2.26–2.58 2.70–2.78 2.89 2.69 2.73–2.83 3.25–3.32

– 2.63–2.74 2.85–3.10 2.59–2.74 2.56–2.85 –

Figure 12. Results of gravity modelling along the Alp07 profile. HDB is high-density body, and numbers are densities in g cm−3 ; V.E. = 1.6.

distinct upper and lower layers, whereas the Pannonian crust is virtually one layer. The Dinaridic upper crust is characterized by low seismic velocities (about 6 km s−1 ) but somewhat higher density, while the lower crust has high seismic velocities (6.5–7.1 km s−1 ) and high density (2.89 g cm−3 ), Fig. 12. The Pannonian crust is characterized by similar seismic velocities as the upper Dinaridic crust, and low density (2.69 g cm−3 ). Between these units there is a relatively wide Transition zone. On the gravity model, it is somewhat wider than on the seismic model, and due to the clearer picture of the lateral boundaries (130–255 km), the gravity model was used as the basis for determination of the zone’s width, which is 125 km. The southeastern boundary of this zone corresponds with the southern marginal fault of the Pannonian basin (130 km), and the northwestern boundary corresponds with the Drava fault (255 km; Fig. 13). The main faults in the Sava depression are related to the appearance of rocks of the highest densities in the Transition zone. The troughs in the Moho were obtained by seismic forward modelling, and were also confirmed by gravity modelling. They may in-

dicate deep faults or shear zones in the crust and uppermost mantle. The correlations of these features with the depressions in the Pannonian basin indicate that they might relate to major upper crustal features such as the Sava and the Karlovac depressions. Three main steps in the Moho were constructed in the geological model (Fig. 13), and the first one is located at 100 km in the model. Based on geometric relations and the fact that the Adriatic microplate is being squeezed against the Eurasian plate as confirmed by the data from geodetic and geodynamic measurements (Grenerczy et al. 2005), we suggest that this feature is an indication that the Adriatic microplate at the level of the Mohoroviˇcić discontinuity is at least locally being thrust beneath the Pannonian region. Similar conclusions were also drawn from the interpretation of the Alp01 and Alp02 profiles (Brückl et al. 2007). The other two steps in the Moho are located under the Sava (180 km) and Drava (260 km) depressions. The step beneath the Drava depression corresponds with the Drava fault on the profile, but this connection cannot be established laterally (Fig. 2). The first step (100 km) cannot be continuously followed C



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Figure 13. 2-D geophysical model of the crust and upper mantle along the Alp07 profile. SMF, Southern Marginal Fault of Pannonian basin. Numbers are P-wave velocities (in km s−1 ); numbers in brackets are densities (in g cm−3 ). The dashed line indicates the subduction zone in the mantle.

through the crust, but there is a clear appearance of two separate faults in the crust shifted towards NE. Relative movements of individual blocks (marked by yellow arrows in Fig. 13) seem to show that also in the area of the Transition zone, the Pannonian segment is gradually rising over the Adriatic microplate, which is also considered in the previous references (Pamić 1998). Some authors also emphasize the subduction of the Adriatic microplate under the Dinarides (Herak 1986; Moretti & Royden 1988; Kuk et al. 2000). The location of subduction, according to our seismic and gravity data, could be located near the northeastern part of the Dinarides (Fig. 13). The strongest lateral changes in seismic velocities and densities were found in the ophiolite zones, that is, on the margin of the Pannonian basin and in the area of the Sava depression and the Bjelovar sag. Based on both seismic and gravity models, it can be concluded that the lower crust under the Dinarides is relatively compact. Tectonic phenomena in the area of the Dinarides cannot be correlated to features at the Moho level, unlike the Pannonian basin, where there is a good correlation between the surface fault zones and the interpreted faults in the Moho. Therefore, the main Velebit fault (Fig. 13) has no influence even at the lower crustal level. More pronounced lateral changes in seismic velocities, which could possibly be related to these faults, are located only in the uppermost crust (Fig. 6b), with no significant changes evident in the densities (Fig. 12). On the other hand, the Sava fault and the fault under the Drava depression can be interpreted to extend through the entire crust, even at the Moho. Whereas, the southern marginal fault of the Pannonian basin impacts both the upper and the lower crust as a separate discontinuity. The results of our modelling lead to the conclusion that there are different velocities of movement as a result of different viscosities of individual parts of the lithosphere, the upper and lower crust and the upper mantle. The question arises, however, of the explanation of the Dinaridic massif origin in this model. Our velocity/density model is a snapshot in time that can not alone answer this question, C


but dynamic modelling, considerations of stress and deformations over time, could produce a satisfactory answer. It must be said, however, that the existing simple conclusions in literature about the appearance of the Dinarides as caused by the subduction of the Adriatic microplate have not been confirmed by dynamic modelling (e.g. Herak 1986; Moretti & Royden 1988).

C O N C LU S I O N S Both gravity and seismic models clearly show that three types of crust can be defined, the Dinaridic and the Pannonian crusts, with a relatively wide Transition zone between them. The Dinaridic crust is markedly two-layered: the upper crust, which is characterized by low seismic velocities, and the lower crust, characterized by very high velocities and high densities. The Pannonian crust can be seen as one-layered, characterized by both very low seismic velocities and very low densities. In the Transition zone, there are medium densities and lower velocities, but there are also clearly evident strong lateral and vertical changes in densities and velocities. There is a marked anomaly with relatively high densities under the Sava depression (2.83 g cm−3 ). This anomaly is probably caused by ophiolite rocks, which are located in the surrounding areas on the surface as outcrops. The boundaries of the Transition zone can be more clearly and accurately defined on the model of densities rather than on the model of velocities, and its width equals 125 km. In the seismic model, the Moho depth is the greatest in the area of the Dinarides, about 40 km, while in the area of the Pannonian basin it ranges from 30 to 20 km, being the shallowest at the end of the profile. The modelling clearly showed that there is significant relief along the Moho with troughs that can be interpreted as a consequence of major faults in the lithosphere. Three faults were interpreted. The first one is located in northeastern part of the Dinarides, the second under the Sava depression, and the third under the Drava depression. Based on geometric relations and the fact

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that the Adriatic microplate is thrusting into the Eurasian plate, it may be concluded that the Adriatic microplate, at the level of the Mohoroviˇcić discontinuity, is subducting under the Pannonian segment. Similar movements were also defined within the Transition zone, where the Pannonian segment is gradually rising over on the Adriatic microplate. In the literature, the subduction of the Adriatic microplate under the Dinarides is considered as the cause of their origin (Moretti & Royden 1988; Herak 1991). The point of subduction according to our data and modelling, both seismic and gravity, should be at the northeastern part of the Dinarides. It can be also concluded that the lower crust under the Dinarides is relatively compact while the surface tectonic phenomena cannot correlate to the phenomena at the level of the Moho, unlike the Pannonian basin. The question of origin of the Dinarides cannot be solved by using a static model, only dynamic modelling may shed additional light on the problem. In any case, the main issue is the relationship between the Adriatic microplate and the Pannonian segment of the European plate, the answer to which shall be further attempted to find by the analysis of the passive seismic data obtained in the framework of the ALPASSDIPS project (Hegedüs et al. 2006).

AC K N OW L E D G M E N T S The implementations of the part of the ALP 2002 project in Croatia has been made possible through the support by the Ministry of Science, Education and Sports of the Republic of Croatia, Vienna University of Technology and INA-Oil industry, Inc. We extend our special thanks to the ALP 2002 project leader, Prof. Ewald Brückl from the Vienna University of Technology, whose involvement helped us obtain support from Austria as well as to Prof. Hans Thybo from the University of Copenhagen, who made available to us their instruments for field measurements. Transport, preparation and programming of instruments were performed by an experienced team of the Geological Institute, lead by Dr Peer Jørgensen. We are very grateful to G. Randy Keller for editing the manuscript and for helpful comments. Our sincerest thanks to the numerous employees of the Faculty of Mining, Geology and Petroleum Engineering and our external collaborators who participated in field measurements. The final data processing and interpretation was performed within the project ‘Geophysical explorations of water-bearing systems, environment and power sources’, approved by the Ministry of Science, Education and Sports.

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