Paleomagnetic, Tectonic and Stratigraphic Correlation of Tertiary Formations in Slovenia and Hungary along the Periadriatic and Mid-Hingarian Tectonic Zone (Preliminary Communication)

Bogomir Jelen', Emo Márton\ László Fodor\ Maria Báldi*, Jože Čar\ Helena Rifelj', Dragomir Skaberne' & Marko Vrabec^ ^Institute for Geology, Geotechnics and Geophysics, Dimičeva 14, 1109 Ljubljana, Slovenia ^Eötvös Loránd Geophysical Institute of Hungary, Columbus u. 17-23, 1145 Budapest, Hungary 'Department of Applied and Environmental Geology, Eötvös Loránd University, Múzeum krt. 4/A, 1088 Budapest, Hungary ''Rákóczi u. 42, 2096 üröm, Hungary ^Geological Department, Faculty of Natural Sciences and Engineering, University of Ljubljana Aškerčeva 12, 1000 Ljubljana, Slovenia


Introduction
A project with the above title vjas conducted from 1995-1996 as a part of the Slovenian-Hungarian intergovernmental science and technology cooperation program under the No. 21/95. Although the thorough reporting of results u^ill be published soon, we provide here a preliminary account of our activities and preliminary results.

Preliminary Communication
The understanding of the stratigraphy of the Tertiary in Eastern Slovenia was improved with the application of a tectonostratigraphic model (Jelen et al., 1992). Stratigraphie research showed that the Donat tectonic zone is a contact between two tectonostratigraphic units, and that the role of the zone is equivalent to the role of the Buda line in Hungary (Jelen et al., 1992). This new aspects of the correlation of the Tertiary formations within the Periadriatic zone, the mid-Hungarian tectonic zone, and the adjacent areas, are important from the viewpoint of the continental escape tectonics along the Periadriatic zone, and mid-Hungarian zone (Jelen, 1994) because of the collision of the European and African tectonic plates (K á z m é r, 1984;Kázmér & Kovács, 1985). In the escape process the units of the Carpatho-Pannonian area must have moved eastward. Thus, the formerly uniform Paleogene basin may be found today in two pieces, the northern part in Hungary and southern Slovakia, and the southern part in Slovenia (C s o n t o s et al., 1992). The dextral separation of these pieces was estimated by Kázmér (1984) to be 450-500 km and by Tari (1994) to be 350-550 km.
Since the first application of the escape theory to the Alpine-Carpathian-Pannonian region in the last decade, many studies have dealt with different aspects of it. However, the problem is very complex, and several questions have remained unanswered. Our project concentrated on the following aspects of this problem: 1. Direct stratigraphie comparison (not using information from the literature only) of the Slovenian and the Hungarian Paleogene basins.
2. The study of the manner and timing of the tectonic movements in the "source" area of the displaced continental slivers in N Slovenia.
For these purposes, paleomagnetic method was applied for the first time in the Tertiary of Eastern Slovenia, along with microtectonic measurements and analysis which revealed the orientation of paleostress axes, enabled the separation of different stress regimes and their timing, and constrained the kinematics of movements along faults.
For direct stratigraphical comparison, the Slovenian team with the guidance of the Hungarian party made two field trips to Hungary and examined the following Tertiary formations (middle Eocene through Karpatian): Darvastó Formation, Szöc Formation, Padrag Formation, Dorog Formation, Szépvôlgy Formation, Piszke Formation, Buda Formation, Tard Formation, lower part of Kiscell Formation, Iharkut Formation, Mäny Formation, Csatka Formation, Eger Formation, Szécsény Formation, Pétervására and Budafok Formation, Gyulakeszi rhyolite tuff horizons, Garáb Formation and Egyházasgerge Formation. Micropaleontologic samples were taken for the biostratigraphic correlation. Nannoplankton biostratigraphy and biochronology of the Slovenian sections was established by M. Báldi. Lithostratigraphy, foraminiferal biostratigraphy and biochronology and stratigraphie correlation were performed at the Institute for Geology, Geotechnics and Geophysics, Ljubljana.
The stratigraphie correlation gave important results. Stratigraphie equivalents of the Szépvôlgy Limestone, Buda Marl and Tard Clay and sedimentary transition from Eocene beds into Oligocene beds have been found for the first time in Slovenia. Biostratigraphic analysis revealed nannoplankton biochronozone NP 21 and planktonic foraminifera biochronozone 7P17/P18 (uppermost Priabonian) for the Slovenian equivalent of the Szépvôlgy Limestone, nannoplankton biochronozone NP 21 and planktonic foraminifera biochronozone PI8 (uppermost Priabonian and lowermost Rupelian) for the Slovenian equivalent of the Buda Marl, and nannoplankton biochronozones from NP 21 through the lower part of NP 23 and planktonic foraminifera biochronozones from P18 through aproximately PI9 (lower Rupelian) for the Slovenian equivalent of the Tard Clay. These stratigraphie units which were found from Luče area (upper Savinja valley) in the west to Rogaška Slatina area in the east characterize a tectonostratigraphic unit bounded by Donat tectonic zone (in sensu Jelen et al., 1992) and Celje fault (in sensu В u s e r, 1979).
Structural measurment were carried out by a joint Hungarian-Slovenian team at 90 localities in Slovenia along the eastern terminantion of the Periadriatic zone and in the surrounding areas. The measured data were analysed and different deformation phases were separated. Paleostress calculations were carried out for sites where permitted by the data sets (the presence of slickensided fault planes). Tectonic and structural analyses and interpretations were made at the Department of Applied and Environmental Geology of Eötvös Loránd University and at the Department of Geology, University of Ljubljana.
Our analysis demonstrates repeated, progressive dextral transpression of the study area within and in-between the large shear zones of Periadriatic-Šoštanj and Sava-Celje faults. The brittle deformation is characterized by NW-SE to N-S compression and perpendicular tension. The penetrative strike-slip faulting was accompanied with folding and verticalization of beds (as in the Donat tectonic zone). Between the shear zones, in a rather rigid Savinja block, sinistral transpression occured in the west (Smrekovec area), and dextral transtension in the east due to the accompanying mechanisms of deformation. These observations indicate a modified tectonic model of relatively rigid domino blocks rotating clockwise between the two dextral strikeslip zones.
Dextral transpresion started in Early Miocene and occured in several more intensive periods until the recent time.
Detailed study of the young transtensional activity and the relative timing of tectonic events was made in the Velenje basin area with use of the subsurface data.
The paleomagnetic studies involved drilling and orienting of paleomagnetic samples in the field by a Hungarian-Slovenian team. So far 69 localities (577 samples) were measured, demagnetized in several steps and finally evaluated from paleomagnetic point of view in the Paleomagnetic Laboratory of Eötvös Loránd Geophysical Institute of Hungary. About half of the samples were collected and processed before the start of the present project, but the conclusions we make here are based on all observations.
The main results of the paleomagnetic studies are the following: 1. A relatively large area (with Eastern Alpine basement) north of the Periadriatic lineament and the Ljutomer fault is characterised by 30-40° uniform counterclockwise rotation. The age of the rotation is post-Badenian, not yet constrained more precisely. This area must have moved as a single microplate.
2. In the area with Dinaric basement, south of the Sava-Celje fault system, data from the Tertiary show very moderate (up to 20°) clockwise rotation.
3. The zone between the above described rotated blocks is a complex shear zone, characterised mostly by large (60-140°) clockwise rotations. The distribution of the paleomagnetic declinations in time indicates that the rotations occured in at least two tectonic phases, the first taking place in late Karpatian (about 16.5 Ma) and the second in post-Pontian (younger than 6 Ma) time.
4. In certain places, counterclockwise rotated or apparently non-rotated localities are found. This might be explained by the complex internal structure of the shear zones themselves which are right-lateral, yet allow counterclockwise rotation between a pair of secondary sinistral shear zones.
The orientation of stress axes, the presence and amount of tectonic rotation of the blocks, the chronology of deformation, and the resulting fault patterns were compared to similar data from central, north-eastern and southern Hungary. Our research confirmed the earlier suggestions that the Slovenian and the north-eastern Hungarian Paleogene basin segments must have formed a single uniform basin. Contrary to the current opinion, our results imply that the Slovenian part of the Paleogene basin is not a homogeneous unit, but rather an assemblage of tectonic slices that originated in different Paleogene basins. The Early Miocene dextral slip along the Periadriatic fault system and the shear zone of Slovenia continued into the mid-Hungarian zone between the Lake Balaton and the Mecsek area.
The results of this project are important for the future geodynamic interpretations and reconstructions of the Alpine-Dinaric-Carpathian-Pannonian region. The stress field evolution of the study area, the young rotations indicated by paleomagnetic measurements, the chronology of intesive tectonic periods and related faulting, especially the indications for the Quaternary to Recent activity, are meaningful for the regional seismic risk evaluation. The structural correlation with the Hungarian side of the study area, where hydrocarbon resources are present, gives new insights for the hydrocarbon exploration in Slovenia.
Our studies answered several questions concerning the stratigraphie and tectonic evolution of the Alpine-Carpatho-Pannonian area. At the same time, they suggest new fields of investigation especially in constraining the timing of the young movements, exact reconstruction of the formerly Paleogene basin and diagenesis.