Tectonic Overview of Central Europe

The portion of the region targeted by CELEBRATION 2000 north of the Carpathian Mountains has experienced a long and complex tectonic history. This history began with the assembly of the Archean and Proterozoic terranes that are exposed in the Baltic and Ukrainian shields (e.g., EUROBRIDGE Seismic Working Group, 1999). The resulting ancient continental mass was rifted at the end of the Precambrian (e.g., Poprawa et al., 1999) to form the paleocontinent Baltica. During the Paleozoic, a series of terranes were accreted to the rifted southwestern margin of Baltica (the Trans-European Suture zone region) to begin the formation of the European continent. However, the origin, nature, and extent of these terranes and the bounding sutures are major subjects of debate in European tectonics. There are many puzzling structural blocks in the region such has the Malopolska massif (USB-HCM region). This feature is probably a fragment of Baltica that experienced lateral transport such as that experienced by the Salinian block that has moved northwestward along the west coast of North America. The largest truly exotic terrane is the Avalonian microcontinent that is recognized on both sides of the Atlantic (e.g., Pararoh 1999; Keller and Hatcher, 1999).

When viewed from a larger perspective, the Paleozoic tectonic evolution of Europe involved a series of orogenic pulses resulting from the collision and suturing of Baltica, Laurentia (the North American paleocontinent), Gondwana (Africa/South America), intervening terranes like Avalonia and the Bohemian massif, and transported crustal blocks to form the supercontinent Pangea (e.g., Pararoh, 1999; Keller and Hatcher, 1999). As the Paleozoic era ended, the formation of this supercontinent was complete, and the northern part of the TESZ region was actively subsiding to form the southern Permian basin. This basin consists of several sub-basins (e.g., Northeast German basin and Polish trough) extending from central Poland, across northern Germany and Denmark, into the North Sea region. The origin of these sub-basins is in doubt, but they indicate the first phase of the break-up of Pangea (e.g., Bayer et al., 1999). North of the Carpathian Mountains, the area targeted by CELEBRATION 2000 experienced relatively minor extension during the Mesozoic as evidenced by further subsidence in the Polish trough. This extension was followed by tectonic inversion along the Tornquist-Teisseyre zone due to the Alpine orogeny.

CELEBRATION 2000 also targeted a younger region, the Carpathian Mountains - Pannonian basin, whose tectonic history is dominated by plate interactions to the south. The breakup of Pangea in the early Mesozoic created the Tethys Ocean and an irregular continental margin across what was then southern Europe. This rifting also produced a collage of microplates between the major paleo-Eurasian and paleo-Afro-Arabian plates. The tectonic development of the region generally reflects the relative movements between the large plates, and the complications posed by the intervening microplates produced the puzzling geology we see in the Mediterranean region today. The tectonic evolution of the Carpathian Mountains - Pannonian basin is complicated even by Mediterranean standards (e.g., Csontos and Nagymarosy, 1998; Linzer et al., 1998). By Jurassic time, deformation of this region had begun, and continues to today as evidenced by active seismicity to depths of ~200 km in the Vrancea region north of Bucharest (e.g., Linzer et al., 1998).

The relative plate movements between the paleo Eurasian and Afro-Arabian plates were at first (Jurassic - early Cretaceous) mostly transverse in nature and then more compressional. During the Cenozoic, the Carpathian arc evolved to assume its strongly arcuate shape that is probably related to the irregularity of the paleo-Eurasian continental margin and the interaction of crustal blocks (e.g., Csontos and Nagymarosy, 1998). These blocks experienced both rotations and translations (e.g., Linzer et al., 1998). The subduction of oceanic areas between these blocks and paleo-Europe involved convoluted slab geometries and movements and produced considerable Neogene volcanism. The resulting arc-related terranes were accreted to Paleozoic terranes (and parts of Baltica?) to the north and east resulting in the formation of the Carpathian fold and thrust belts that have covered the older terranes to varying but often unknown degrees (e. g., Jarosinski, 1998). Back arc extension played the major role in the formation of the Pannonian basin that contains up to 8 km of Neogene strata in its sub-basins.

Experiment Design

The layout of the CELEBRATION 2000 experiment was a network of interlocking recording profiles whose total length was about 8900 km and the station spacing along the profiles was 2.8 or 5.6 km. Shots were fired along most of the recording profiles so that in addition to forming an array, about 5400 km of traditional profile data were obtained. Covering this network of profiles required three deployments over a period of 1 month (June, 2000).

Realizing that the lithospheric structure in the target area is very complex, the need for a 3-D approach was recognized early in our planning process. Recent advances in seismic instrumentation have made many more instruments available so that at 3-D approach could be implemented. In fact by pooling European and U. S. resources, >800 new matched seismic instruments that were jointly developed by the University of Texas at El Paso and Refraction Technology were available for this experiment. Thanks to Canadian, European, and IRIS/PASSCAL resources, the total number of instruments deployed was 1230.

A large number of seismic sources was also required, and again the pooling of resources proved to be effective. Ultimately, scientific organizations in Poland, Hungary, the Czech Republic, the Slovak Republic and Austria funded 142 sources. An additional 5 shots were provided by Russia, Belarus and Germany. These sources ranged in size from 15 metric tons to 90 kg with the average being ~500 kg. Since some of these sources were small, we estimate that about 100,000 useable vertical component seismograms were obtained. In addition, about 15% of the stations were occupied by 3-component recorders and included two horizontal seismometers producing even more seismograms.

Initial Results

The seismic instrumentation used in CELEBRATION 2000 worked very well, with over 95% of the recorders deployed returning useful data. In general, the efficiency of the seismic sources was also very good. It will take many months to fully interpret and model the massive data set produced by this experiment. However, the data have been processed, compiled into shot records, and plotted and some initial observations can be made at this time. Data from profiles on the East European craton produce a wave field that is very regular. The sedimentary cover is thin, and the diving wave traveling in the upper and middle crust (Pg) produces clear first arrivals with apparent velocities increasing smoothly from about 6 to 7 km/s. A strong reflected wave from the base of the crust (PmP) is observed starting at offsets of about 110 km and reduced arrival time about 9 s. The arrival times of this phase and the refracted phase from the uppermost mantle (Pn) indicate a crustal thickness of about 45 km. Moving to the south into the TESZ region, we observe a complicated wave field in the area of the Holy Cross Mountains and Upper Silesia block. First arrivals with low apparent velocities extend to offsets of up to ~100 km indicating the presence of a thick pile of sedimentary rocks. There are strong ringing reflections from the lower crust and the base of the crust (Moho discontinuity). To the northeast, the PmP phase arrivals are late indicating a crustal thickness of >40 km. The prominent P phase (P?) seen to the southwest is probably from a dipping interface under the Carpathians.

Although seismic waves are attenuated by the sedimentary strata that comprise the Carpathian Mountains, we were able to obtain some good data in the Carpathian region. The PmP reflection and the refraction from the uppermost mantle (Pn) indicate that the crust is ~ 40 km under the Carpathian Mountains in the region of eastern Slovakia, which is much thinner than in the Alps. Seismic wave propagation across the Bohemia massif was efficient and Pg, PmP and Pn phases are clear and the Pn phase becomes a first arrival at ~170 km indicating a crustal thickness of less than 40km. In the Pannonian basin area, the Pn phase becomes a first arrival at a distance of only ~100 km, which indicates that the crust is < 30 km thick. In addition, a phase originating in the lithospheric mantle (P1) appears to be present on this record section. These observations show that the crustal structure of this region varies in a complex fashion that would be expected given the region's tectonic history. The 3-D model that will ultimately be produced from these data should produce an intriguing image of the crustal scale structure.

CELEBRATION 2000 also targeted the lithospheric mantle, and our goal is provide new information about structure below the Moho discontinuity. One particularly long profile (CEL 05) extends from Russia to Hungary. The record section from this profile reveals a number of interesting phases from the upper mantle (P1, P2, and P3) that we presently interpret as reflectors. Although this is our best example, other record sections also contain mantle phases that will be investigated.

Acknowledgements

It is only through the participation of approximately 500 individuals that the preparation, execution, and data analysis for the CELEBRATION Experiment could be successfully completed. It is not possible to acknowledge everyone individually, but the following individuals played key roles: W. Czuba, E. Gaczynski, T. Janik, P. Sroda, M. Wilde-Piorko (Poland), T. Bodoky, E. Takacs (Hungary), J. Santavy (Slovakia), W. Chwatal (Austria), A. F. Morozov (Russia), K. Komminaho (Finland), G. Kaip, C. Snelson, S. Azevedo (USA) and many technical staff and students.

In Poland, this project was supported by the State Committee for Scientific Research (KBN, grant PCZ 06/21/2000), Ministry of the Environment, Polish Oil and Gas Company, and Institutes of Geophysics of the Polish Academy of Sciences and the University of Warsaw through the Association for Deep Geological Investigations in Poland (ADGIP). In the Slovak Republic, the Geological Survey and Academy of Sciences provided support. The Ministry of Environment of the Czech Republic also provided major support, as did the Eötvös Loránd Geophysical Institute in Hungary. Austrian participation was funded by the Austrian Academy of Sciences. IRIS/PASSCAL is supported by the U. S. National Science Foundation (NSF) and provided the majority (~700) of the instrumentation for this experiment, and most of these instruments were provided through grants to the University of Texas at El Paso (State of Texas Higher Education Coordinating Board, NSF/MRI, and DoD). The participation in the fieldwork by the University of Texas at El Paso was funded by NSF (EAR and Int) and State of Texas Higher Education Coordinating Board. Canadian participation included instrumentation from the Geological Survey of Canada and support of the Lithoprobe Secretariat. Danish participation was funded by the Carlsberg Foundation and the Danish Natural Science Research Council. In Turkey, the project was supported by the TUBITAK-MAM, Earth Science Research Institute, Gebze, Turkey. German participation was supported by the University of Jena and GeoForschungsZentrum. The Ministry of Natural Resources supported the activities in Russia, and the Central Geophysical Expedition in Minsk supported the activities in Belarus. Finnish participation was based on a long-standing exchange between the Finnish and Polish Academies of Sciences.