L. Csontos

csontos@ludens.elte.hu

Remarks to the basement map of the Danube basin

The attached map (Fig. 1) was basically prepared by compilation of former basement maps of Fuchs (1984), Fusán et al. (1987) and Fülöp and Dank (1987). In the southern part the map of Flügel (1988) was taken into consideration.

This compilation was modified or completed by own interpretations. A main modification was made in the basement of the Little Hungarian Plain. This map is the result of confidential seismic section interpretation. A striking difference with former maps is the presence of a Neogene-covered Penninic window north of the formerly known Kőszeg-Rechnitz window. This is based on a seismic section, where the strongly reflective lower boundary of Lower Austroalpine rises to the basement of the Neogene basin. Only the southern margin of the window was drilled and still Lower Austroalpine was found. The northern closure of the window was not seen, but inferred from Slovakian borehole data. Another small Penninic-Borinka window was interpreted in the Malé Karpaty Mts based on works of Plasienka (1987).

The formerely more uniformly shown Paleozoic units were subdivided based on their age of metamorphism. This work, made by Árkai and Balogh (1989) differentiated between two units. The first, closely attached to (transgressed by) the Permo-Mesozoic sequence of the Transdanubian Central Range (Tét unit) suffered Variscan metamorphism and structuring (Dudko and Lelkes-Felvári 1992). The second (Mihályi unit) is composed of weakly metamorphosed Paleozoic transformed in the Alpine phase (120 Ma). The former is closely affiliated to the Grauwacken zone, the second to the Graz Paleozoic (Ebner et al. 1998). The first unit appears also on the southern margin of the TCR unit, which forms a large NE-SW directed synclinorium.

There is a peculiar spot in the basement of the Little Hungarian Plain: the Ikervár area. Here boreholes revealed shales, radiolarites and mafic rocks of presumed Jurassic age (Bodzai 1966). Based on their similarity to rocks exposed in the Kőszeg area, this spot was interpreted as part of the Penninic unit. Seismic sections showed that this interpretation cannot be valid, since thick masses of Lower Austroalpine and Graz Paleozoic are in between the “Penninic” occurrences and no structure explaining a new, south-easetrn occurrence could be seen. In case the original borehole material interpretation was correct (i.e. part of the basin substratum) the only interpretation in my view is to consider this occurrence as part of the Meliata unit, which should be more closely attached to the TCR anyway. Another possible interpretation is that the drilled material was misinterpreted as basement and in fact a Middle Miocene cobble conglomerate was penetrated. Figure 1 reflects the former case.

Two spots coloured as TCR or Szilice-related units occur south of the TCR exposures. These rocks were found by boreholes. Several works (Bérczi-Makk et al. 1993, Haas et al. 1988, Haas et al. 2000, Balla et al. 1987) dealt with the area. The map reflects an own reinterpretation of the original descriptions and these works.

The colouring of the units reflects a suggested affininty of different nappes. The Juvavic is suggested to be closely related to the Szilice units (Kovács 1984) and thick carbonates in the Somogy basin. The TCR unit is suggested to be the counterpart of either the Juvavic or alternatively the Tirolic-Choc nappes. The second solution is more close to my present views.

Some tectonic lines are marked on the map. The most important are the Salzach-Vienna basin master left lateral fault (Neubauer and Genser 1990); the several branches of the Rába fault (including the circular detachments around the Penninic windows), the several branches of transpressive/thrust faults south of the TCR and a set of onion-shell structures which is present mainly in the West Carpathians. This latter is marked as a green line in Slovakian territory.

References:

Árkai, P. and Balogh, K. 1989. The age of metamorphism of the East Alpine type basement, Little Hungarian Plain, W Hungary. K-Ar dating of K white micas from very low and low garde metamorphic rocks. Acta Geol. Hungarica, v 32/1: 131-147.

Balla, Z., R. Tátrai M., Dudko, A. 1987. A Közép-Dunántúl fiatal tektonikája földtani és geofizikai adatok alapján. Annual Rep. of MáELGI Geophysical Institute from 1986: 74-94.

Bérczi-Makk, A., Haas, J., Rálisch-Felgenhauer, E., Oravecz-Schaffer, A. 1993. Upper Paleozoic-Mesozoic formations of the Mid-Transdanubian unit and their relationships. Acta Geol. Hungarica. V 36/3: 263-296.

Dudko. A. and Lelkes-Felvári, Gy. 1992. Variscan tectonics of the Transdanubian Range. Ann Rep of the Hung. Geol, Survey from 1990: 39-67.

Ebner, F., Kovács, S., Schönlaub, H.P. 1998. Stratigraphic and facial correlation of the Szendrő-Uppony Paleozoic (NE Hungary) with the Carnic Alps-South Karawanken Mts. And Graz Paleozoic (Southern Alps and Central Eastern Alps); some paleogeographic implications. Acta Geol. Hungarica v 41/4: 355-388.

Flügel, H. W. 1988. Steirisches Becken- Südburgenländische Schwelle; Geologische Karte des prätertiären Untergrundes 1:200,000. Geol. B.-A. (Wien).

Fuchs, W., 1984. Grosstektonische Neuorientierung in der Ostalpen und Westkarpaten unter Einbeziehung plattentektonischer Gesichtspunkte. Jahrb. der Geol. Bundesanstalt, 127, 4: 571-631.

Fusán, O., Biely, A., Ibrmayer, J., Plancar, J. and Rozloznik, L. 1987. Basement of the Tertiary of the Inner West Carpathians. G.U.D.S. Bratislava; 123 p.

Fülöp, J. and Dank, V., (Editors) 1987. Magyarország földtani térképe a kainozoikum elhagyásával. Map 1: 500 000, MAFI, Budapest.

Haas, J., Mioć, P., Pamić, J., Tomljenović, B., Árkai, P., Bérczi-Makk, A., Koroknai, B., Kovács, B., Rálisch-Felgenhauer, E. 2000. Complex structural pattern of the Alpine-Dinaride-Pannonian triple junction. Int, Journ Earth Sciences, Abstr. Vol. 89/2: 377-389.

Haas, J., Rálisch-Felgenhauer, E., Oravecz-Schaffer, A., Nagy, E., Bérczi-Nakk, A. 1988. Triassic key sections in the Mid-Transdanubian (Igal) structural zone. Acta Geol. Hungarica. V 31/1-2: 3-17.Kovács, S. 1984. North Hungarian facies types. A review. Acta Geologica Hungarica, 27, 3-4: 251-264.

Lelkes-Felvári, Gy., Árkai, P., Sassi, F.P. 1996. Main featuresof the regional metamorphic events in Hungary: a review. Geol. Carpathica 47/4: 257-270.

Neubauer, F., and Genser, J. 1990. Architektur und Kinematik der östlichen Zentralalpen: eine Übersicht. Mitt. Naturwiss. Steiermark, Graz, 120: 203-219.

Plašienka, D. 1987. Lithological, sedimentological and paleotectonic pattern of the Borinka unit in the Little Carpathians. Miner. Slov., 19, 3: 217-230. [In Slovakian with Engl. abstr.]

Schweigel, J. and Neubauer, F. 1997a. Structural evolution of the central Northern Calcareous Alps: significance for the Jurassic to Tertiary geodynamics in the Alps. Eclogae Geol. Helvetica 90: 303-323.

Schweigel, J. and Neubauer, F. 1997b. New structural, sedimentological and geochemical data on the Cretaceous geodynamics of the central Northern Calcareous Alps (Eastern Alps). Zbl. Geol. Paleont. Teil1 H3/4: 329-343.