Pinboard Article to the
P.A. #001 (15 Aug. 2002)
csontos@ludens.elte.hu
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
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