Abstract
Geophysical and geological data from the eastern sector of the Central European Variscan belt are presented and reviewed in the regional tectonic context. Matched filtering of isostatic gravity, guided by results of spectral analysis, along with other derivatives of gravity and magnetic fields reveal a dominant WNW–ESE-trending pre-Permian structural grain in the external zones of the Variscan belt in Poland. This trend is confirmed by regional distribution of dips in Carboniferous and Devonian strata that were penetrated by boreholes beneath Permian-Mesozoic sediments. Based on these data, two alternative concepts explaining the connection of the Variscan belt and its NE foreland, those of strike-slip tectonics versus oroclinal bending, are discussed. The WNW–ESE structural trend in the Variscan foreland is parallel to a set of major strike-slip fault zones in the area, including those of Upper Elbe, Intra-Sudetic, Odra, Dolsk and Kraków-Lubliniec. These faults are considered to convey a significant dextral displacement between Laurussia and Gondwana. The revised position of the Variscan deformation front shows a similar, uninterrupted, generally WNW–ESE trend, up to the SE border of Poland, which indicates an initial continuation of the Variscan belt into the area of the present-day Western Carpathians. The geometry of the Variscan deformation front along with the pattern of the Variscan structural grain are inconsistent with the idea of an oroclinal loop affecting the external, non-metamorphic Variscan belt. However, the data presented do not entirely rule out an oroclinal loop within the Variscan internides. The still possible options are (1) a semi-oroclinal model postulating ~ 90° bending of the Variscan tectonostratigraphic zones into parallelism with the WNW–ESE strike-slip faults or (2) an orocline limited only to the belt linking the Wolsztyn High and Moravo-Silesian non- to weakly-metamorphic fold-and-thrust belt. Regardless of the kinematic model preferred, our data indicate that structural evolution of the Polish Variscides was concluded with the end-Carboniferous NNE–SSW shortening that resulted in the present-day extent of the Variscan deformation front.
Highlights
The European Variscan belt sharply changes its trend in easternmost Germany and western Poland, with respect to that prevailing in Germany and eastern to central France (Fig. 1)
At the north-eastern end of the latter, the Variscan belt makes nearly a U-turn, encircling the Bohemian Massif from the north. No surprise that these radical changes of Variscan structural trends have been explained for almost a century by assuming a 180° oroclinal loop, in which the Rhenohercynian and Saxothuringian tectonostratigraphic zones inarm the Moldanubian/Tepla-Barrandian core (e.g. Kossmat 1927; Lorenz 1976; Ellenberger and Tamain 1980; Lorenz and Nicholls 1984; Engel et al 1983; Franke 1989a, b; Pożaryski and Karnkowski 1992; Pożaryski et al 1992; Tait et al 1997; Narkiewicz 2007)
A semi-oroclinal geometry i.e., a c. 90° bend of the North Phyllite Zone, masked by younger sediments in NE Germany (Fig. 12b; e.g. Winchester et al 2002; Franke and Żelaźniewicz 2000, 2002), is still possible. Such a pattern would imply a continuity of the Rhenohercynian Suture as a cryptic feature along the Odra Fault, with its ESE ending remaining enigmatic
Summary
The European Variscan belt sharply changes its trend in easternmost Germany and western Poland, with respect to that prevailing in Germany and eastern to central France (Fig. 1). At the north-eastern end of the latter, the Variscan belt makes nearly a U-turn, encircling the Bohemian Massif from the north It is, no surprise that these radical changes of Variscan structural trends have been explained for almost a century by assuming a 180° oroclinal loop, in which the Rhenohercynian and Saxothuringian tectonostratigraphic zones inarm the Moldanubian/Tepla-Barrandian core Kossmat 1927; Lorenz 1976; Ellenberger and Tamain 1980; Lorenz and Nicholls 1984; Engel et al 1983; Franke 1989a, b; Pożaryski and Karnkowski 1992; Pożaryski et al 1992; Tait et al 1997; Narkiewicz 2007) According to this classical view, the outermost tectonostratigraphic zone of the Variscan belt, the Rhenohercynian Zone of Germany, continues eastward in the deep substratum of the PermianMesozoic German-Polish Basin and reappears at the surface along the eastern rim of the Bohemian Massif as the Moravo-Silesian Zone in eastern Czechia and southern Poland (including the eastern part of the East Sudetes). In the somewhat modified version of the oroclinal concept, Franke and Żelaźniewicz (2000, 2002) correlated the area
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