From Carboniferous convergence to Permian continental rifting – the interaction of Baltica with the Variscan belt of Europe at the time of the Pangaea assembly

In Western Europe, the Variscan belt contacts Avalonia along the Rhenohercynian Suture, a result of Early Carboniferous continental collision. Moving east of the Harz Mts., the Rhenohercynian suture disappears beneath a thick sedimentary sequence of the Permian-Mesozoic basin. Its extension is either truncated by major NW-SE strike-slip faults like the Elbe, Odra, or Dolsk faults or bends under the cover of a thick sedimentary succession. The extension of Avalonia into Poland is challenging to determine, with the thinned margin of Baltica considered the substratum of the Permian-Mesozoic basin. Deep seismic soundings show that the thinned margin of Baltica reaches the NW-SE oriented Dolsk or Odra fault, potentially bringing the crust of Baltica into direct contact with the crust of the Variscan internides of the Bohemian Massif. Along the Dolsk fault, there is the two-layered, low-velocity Variscan crust in the SW that contacts the three-layered Baltica crust. The geometry of this contact remains unknown, but the lower, high-velocity crust of Baltica may extend southwest to the Odra fault. In the basement of the sedimentary sequence between the Dolsk and Odra faults, low-grade metamorphosed phyllites with a metamorphic age of approximately 360 Ma are found. They apparently represent a fragment of Variscan metamorphic nappes.

The Variscan front is oriented NE-SW in Western Europe, but in Poland, it bends by 90° to the NW-SE direction, continuing to the border of Ukraine. In southeastern Poland, the front enters the slope of the East European Platform, constituting an undisputed example of a direct contact between the Variscan belt and Baltica. If the geometry of the Variscan front reflects the structure of the orogen, the edge of Baltica must have initially played the role of a transform margin with a right-lateral displacement. NW-SE strike-slip faults, parallel to this margin, truncated the Rhenohercynian and other Variscan sutures from the NE. The following accretion event resulted in NE-SW shortening, either thin-skinned, leading to folding of the external fold-and-thrust belt, or thick-skinned, resulting in the emplacement of the Variscan nappe stack on the Baltica margin.

The last folding of external Variscides in Poland occurred around 305 Ma and was immediately followed by the emplacement of a large igneous province at the Carboniferous to Permian transition. The centre of magmatism was in NE Germany, the area of greatest crustal thinning. The origin of the igneous province was linked to plate boundary forces leading to extension and continental rifting. The latter produced the Mid-Polish trough, an elongated continental rift running NW-SE parallel to the Teisseyre-Tornquist zone. Permian rifting further attenuated the Baltica margin and, jointly with coeval magmatism, reshaped the margin of Baltica masking its contact with the Variscan belt. Toward the east, the continuity of the Variscan internides was disrupted by early Mesozoic rifting in the area of the present-day Carpathians.