Pomeranian Caledonides, NW Poland - A collisional suture or thin-skinned fold-and-thrust belt?

- Pomeranian Caledonides represent a thin-skinned fold-and-thrust.
- They involve Ordovician and Silurian sediments of the Caledonian foreland basin.
- The deformation front was developed due to the buttressing effect of a basement ramp.
- Teisseyre-Tornquist Zone (TTZ) corresponds to a Precambrian suture zone.
- The suture is defined by a remnant crustal keel still preserved underneath the TTZ.

A main goal of this study was to understand the character of the Caledonian Deformation Front in Pomerania, NW Poland and its relationship to the adjoining Teisseyre-Tornquist Zone (TTZ), in the context of the Avalonia-Baltica early Palaeozoic collision. Since the Pomeranian Caledonides are concealed beneath 1-4 km thick platform cover of upper Palaeozoic and Mesozoic sediments, we used a combination of potential field, seismic and well data to investigate the basement architecture and structure of the lower Palaeozoic rocks. Starting from a qualitative review of gravity and magnetic data, we built a 2D gravity and magnetic model upon the PolandSPAN™ PL1-5600 seismic reflection line and applied 3D gravity inversion for a depth-to-basement study. Using well tops, a top lower Palaeozoic horizon and lower Palaeozoic isopach map were created. We found out that the Pomeranian Caledonides represent a thin-skinned fold-and-thrust belt involving Ordovician and Silurian sediments of the Caledonian foreland basin. The deformation front was developed due to the buttressing effect of a basement ramp occurring directly above the TTZ. The latter corresponds to a Precambrian suture zone overprinted by successive extensional tectonics and buried beneath the foreland basin. In northern Poland, the suture is defined by a remnant crustal keel still preserved underneath the TTZ, probably resulting from Precambrian collision during formation of the Rodinia paleocontinent. We propose that the extension of the Thor suture separating Baltica and Avalonia must exist west from the TTZ, probably in the Rügen area. In our interpretation, the TTZ represents an intra-cratonic crustal discontinuity comparable to its NW prolongation, the Sorgenfrei-Tornquist Zone that runs across the Baltic Sea, Sweden and Denmark. Consequently, the basement underlying the Palaeozoic Platform in NW Poland may be considered to have been part of Baltica in early Palaeozoic times.