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SYMPOSIUM III: Geodynamic of the Alpine Orogen

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SYMPOSIUM III: Geodynamic of the Alpine Orogen 142 SYMPOSIUM III: GEODYNAMIC O THE ALPINE OROGEN INTEGRATED MODELLING AND RHEOLOGICAL STUDY O THE WESTERN CARPATHIANS M. BIELIK1, H. ZEYEN2 and A. LANKREIJER3 1Geophysical Institute of the Slovak Academy of Sciences, Dúbravská cesta 9, 842 28 Bratislava, Slovak Republic; [email protected] 2Department des Sciences de la Terre, Universite de Paris-Sud, Bat. 504, 9-91405 Orsay Cedex, 9rance 3Institute of Earth Sciences, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands Key words: Integrated modelling, rheology, finite element algorithm, Western Carpathians. Introduction 9or the purpose of solving geological and geophysical problems in framework of the PANCARDI project we concentrated on the intergrated modelling of the geophysical fields. Alhough the West- ern Carpathians belong to the regions which are geophysically well surveyed, the study of its lithospheric structure has usually been performed by independent interpretation of the single geo- physical fields. Lateral and temporal changes in the lithosphere rheology of the Carpathian-Pannonian region have been shown to have pronounced effects on lithosphere dynamics (Lankreijer et al. 1996). Therefore rheological constraints on geodynamic models in the studied area are very important. Integrated modelling and rheological study of the Western Carpathian lithosphere was car- ried out along deep seismic reflection transect 2T. Methods ig. 1. Results of the integrated modelling along NNW-SSE Slovak Geotransect 2T. Explanations: (a) Surface heat flow. Commas represent The applied integrated modelling is capable of determining two- measured values with corresponding error bars. The solid line represents dimensional thermal lithospheric structure. On the basis of a finite calculated data. (b) Bouguer anomaly and (c) Elevation. Commas corre- element algorithm (Zeyen & 9ernandez 1994), the temperature spond to measured data and solid lines to calculated values. (d) Model ge- distribution in steady state regime given an assumed lithospheric ometry. Bodies: (1) Szolnok-Maramures flysch, (2) crystalline complex of thickness (here defined as the 1300 oC isotherm) and heat produc- the Tatricum, (3) Neogene volcanics, (4) Pannonian Basin 9ill, (5) Mol- tion profile is calculated. Densities are then calculated, based on lasse and 9lysch sediments of the external Carpathians, (6a) Lithosphere- asthenosphere boundary after Babuka et al. 1988, (6b) Lithosphere-as- this temperature distribution. The lithospheric thickness is adjust- thenosphere boundary after Horváth (1993). ed by trial and error until the model fits all the data. The input data are as follows: topography, gravity, heat flow density, lithology, thermal conductivity, heat production and density. Results Due to the importance of mantle density variations, it does not make much sense to do isostatic modelling using the crustal density The lithosphere thickness underneath the Polish foreland is about variations alone. 9or example the typical modelling that utilizes 100 km in our model (9ig. 1). It is much thinner than previously crustal seismics, gravity and topography does not make sense, if we modelled (Babuka et al. 1988; Horváth 1993). One could decrease do not also take the lithosphere-asthenosphere boundary into con- the heat flow there in order to thicken the European lithosphere. sideration, especially, in areas like the Carpathians and the Pannon- However, if we do this, the topography underneath the Polish fore- ian Basin with important variations in lithospheric thickness. land would become much too low. We also imply a decrease in up- In the uppermost level of the mechanically strong part of the lithos- per-crustal heat production in order to be compactible with the mea- phere, rheology is generally goverened by brittle failure (Bayerlees sured heat flow data. Taking into account that the calculated mean Law). At the temperatures exceeding roughly half the melting temper- value of the lithosphere thickness in Europe is about 100 km (Panza ature of rock, ductile creep processes become the dominant deforma- 1985), then the calculated thickness of the lithosphere by the inte- tion mechanism. The rheology of the lithosphere is predicted on the grated modelling in this area is similar (Zeyen & Bielik 1999). A basis of extrapolation of failure criteria, lithology and temperature presence of the lithospheric root can be observed underneath the models. The approach also allows us to predict effective elastic thick- highest topography of the Western Carpathians with a maximum be- ness (EET) variations in the lithosphere (Lankreijer et al. 1999). neath the northern slope of the Low Tatras Mts. A strong thickening 143 is needed in order to avoid an excessively high topography of the bourg, 9rance, 4751. northern part of the central Western Carpathians whereas a density Zeyen H. & 9ernandez M., 1994: Integrated lithospheric modelling com- decrease along the upper part of the thicknened area helps to keep bining thermal, gravity and local isostasy analysis: Application to the the southern part of the central Western Carpathians at the observed NE Spanish Geotransect. J. Geophys. Res., 95, 27012711. Zeyen H. & Bielik M., 1999: Implication of the integrated lithospheric high topography. Beneath the Pannonian Basin the calculated mean modelling for the orogenic evolution of the Western Carpathians (in thickness of the lithosphere is 80 km. preparing). Rheological study clearly indicates a general decrease in strength Zoetemeijer R., Tomek È. & Cloetingh S., 1999: 9lexureal expression of from the Polish foreland, via the Western Carpathians to the Pan- European continental lithosphere under the Western Outer Car- nonian Basin. In the Polish foreland area a horizontal rheological pathians. Tectonics (in press). stratification of the lithosphere can be observed. Mechanically strong behaviour is predicted for the upper part of the crust, the up- permost part of the lower crust and the uppermost part of the man- tle. On the other hand mechanically weak behaviour was calculated OLISTOLITHS A PROO OR THE MIDDLE for the lower part of the lower crust and the lower part of the upper EOCENE MOBILITY O THE MAGURA BASIN crust. These zones could act as a detachment levels. On the basis of strength predictions EET of 12 km was predicted. In the Western Carpathians, lower crustal strength completely disappears. The J. BROMOWICZ lithospheric strength gradually decreases towards the Pannonian Ba- sin. This results from the increasing temperatures in this direction and the corresponding decrease of the thermally defined lithospher- 9aculty of Geology, Geophysics and Environment Protection, ic thickness. The EET values of 1523 km were calculated for the Chair of Raw Rock Material Deposits University of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Kraków, Poland; bromow@@uci.agh.edu.pl Western Carpathians. The Pannonian Basin is characterized by the extreme weakness of the lithosphere. Only one thin strong layer in Key words: flysch, olistoliths, lithofacies, synsedimentary tectonics, Eocene, the uppermost ten km of the crust can be observed. The EET is pre- Polish Carpathians. dicted at 510 km (Lankreijer et al. 1999). Olistoliths known from the Beloveza 9ormation (fm) are ex- Discussion posed in the Kamienica River valley, in the vicinity of £abowa, by the road Nowy S¹czKrynica (9ig. 1A, B). The rocks are located The overall agreements between the model that was calculated in the northern, marginal part of the Bystrica Subunit which is a by integrated modelling and the data are quite good. But some dif- fragment of the Magura Unit (Bromowicz 1998). To the north the ferences in topography still exist. The calculated elevation is too Bystrica Subunit is thrusted over the Raèa Subunit whereas to the low by about 200 m and in the Pannonian Basin it is too high by a south it plunges under the Krynica Subunit. The facial diversity of similar amount. Based on analysis of the results obtained we sug- the structural units is illustrated in 9ig. 1C, D and E. gest that the unexplained topography is due to the following rea- Both mono- and polylithic olistoliths were distinguished. The sons. In the Polish foreland it is, probably, due to the flexural former are represented by coarse- and fine-grained sandstones, bulge of the lithosphere. We have enough evidence to suggest and marls whereas the latter are bedded sets of sandstones, marls (Bielik 1985; Zoetemeijer et al. 1997; Krzywiec & Jochym 1997; and shales in variable proportions. Lankreijer et al. 1999) that the foreland lithosphere behaves as an Most of the olistoliths are composed of coarse-grained sand- elastic plate that is governed by the flexural bulge behaviour. In stones. The polylithic olistoliths consist of coarse-grained sand- the Pannonian Basin the problem could be associated with denser stones and marls interlayered with thin shale beds. These are ac- lithospheric mantle underneath this region. The lithospheric man- companied by olistoliths showing distinctly higher percentages of tle has to be 510 kgm-3 denser than usual subcrustal mantle which shales, thin-bedded sandstones and marls and/or olistoliths com- may be explained by enrichment due to a plume. posed exclusively of thick-bedded sandstones. The olistoliths are enclosed within dark-bluish-grey marly shales strongly disturbed by submarine slides. The marly shales References contain rare, mostly thin-bedded, fine-grained sandstones accom- Babuka V., Plomerová J. & ílený J., 1988: Spatial variations of P-residu- panied by irregular
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  • Tethyan Oceans
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