GEOPHYSICAL CONSTRAINTS ON THE STRUCTURE OF THE EASTERN MARGIN OF THE EAST EUROPEAN CRATON

The SW margin of the East European Craton (EEC) is the most prominent lithospheric boundary in Europe to the north of the Alps. This is delineated by the Teisseyre-Tornquist Zone (TTZ) where a step in the Moho depth defines a boundary between the thick crust of the EEC and the thinner crust of the Palaeozoic platform to the west. However, the location and character of the TTZ have remained a matter of controversy because this is buried below a 8-10 km thick sedimentary pile of Cambrian to Cainozoic age. Being inaccessible by wells, the TTZ can only be studied using a large spectrum of geophysical data from seismic profiles to potential fields anomaly maps.

Two seismic reflection lines from the PolandSPAN™ experiment were used to image the deep structure at the SW margin of the East European Craton. Lines PL-5300 and PL-5400 run NE-SW in central and northern Poland, respectively. The lines cross the Teisseyre-Tornquist Zone, that is traditionally considered to delimit the edge of the EEC. The seismic interpretation down to top basement was integrated with the 2D gravity and magnetic modelling along the lines that highlight the structure of deep crust. The 2D models were constrained by the Moho and top lower crust horizons that were extracted from grids based on the joint results of earlier refraction seismic experiments.

Both the gravity and magnetic models show a suture welding two crustal blocks at the level of lower crust and Moho. The position of the suture coincides with the previously documented location of the TTZ. However, basement above the suture is uniformly sloping and overlain by undisturbed lower Palaeozoic and younger sediments. The lines studied show no evidence of compresional deformation of sedimentary cover until their SW terminations c. 30 km to the west of the modelled suture. By implication, the suture must have formed in the Precambrian and the crustal blocks on both its sides belong to the EEC. If the TTZ does not define the edge of the craton the tectonic suture formed owing to the closure of the Tornquist Ocean and the early Palaeozoic collision of Avalonia and Baltica must be localised farther to the west.

Acknowledgements

ION Geophysical is thanked for consent to use PolandSPAN™ seismic data for 2D modelling. MM and SM acknowledge financial support from the Polish National Science Centre grant UMO-2011/01/B/ST10/04713.