Interpreting the eastern termination of the Variscan Belt: Insights from gravity, magnetics, and structural evolution

The termination of the eastern Variscan belt has long been a topic of intense scientific debate due to its burial beneath extensive younger sedimentary cover. Competing hypotheses have sought to explain its geometry: the Variscan orocline model and the right-lateral strike-slip tectonics concept. To address this ambiguity, we compiled high-resolution gravity and magnetic anomaly maps spanning Czechia, Poland, and eastern Germany. These maps, coupled with geological and geophysical evidence, provide a robust framework to reassess the subsurface architecture and tectonic evolution of the region.

Our findings reveal a pronounced eastward deflection of the Rheno-Hercynian Suture. This structural trend takes a nearly 90° turn east of the Harz Mountains and extends south-eastward into Poland. This configuration supports the hypothesis of a semi-orocline that terminates abruptly against the Brunovistulian Block. The observed anomaly patterns, when integrated with geological evidence, point to a two-stage accretionary history in the eastern Variscan belt. The first stage involved W-E convergence during the early phases of Variscan orogeny. This process led to the development of NNE-SSW-trending structures, prominently preserved in the southern Bohemian Massif. These early tectonic fabrics were later overprinted during a subsequent, critical N-S shortening phase. This second stage reoriented the deformation patterns, producing WNW-ESE-trending structures that parallel the Baltica margin and dominate the region northeast of the Elbe Fault. Seismic imaging corroborates this structural interpretation, highlighting significant underthrusting of Baltica's crust beneath the Variscan belt at a distance exceeding 100 km.

The Variscan belt of Europe terminates in western Poland and Moravia, reaching the SW margin of Baltica and the western edge of the Brunovistulian Block. Although elements of the Variscan basement occur much farther east within the Carpathian belt, they cannot currently be correlated with the Variscan zones stretching between the Iberian Peninsula and western Poland. The presence of Variscides farther SE on the eastern side of the Brunovistulian Block is indicated by the direction of the Variscan deformation front running WNW-ESE up to the Ukrainian border. Particularly in SE Poland, Variscan shortening resulted in thin-skinned deformation of the EEC sedimentary cover.