The Alpine Tethys suture in the Western Carpathians revisited: new constraints from lithospheric-scale geophysical imaging

The Western Carpathians are commonly regarded as the eastern continuation of the Eastern Alps, where closure of the Alpine Tethys culminated in the collision between the European passive margin and the Adria-derived Alpine–Carpathian–Pannonian (ALCAPA) tectonic unit. In contrast to the Eastern Alps, however, the geometry and nature of the Alpine Tethys suture remains poorly constrained in the Western Carpathians. The suture is generally associated with the Pieniny Klippen Belt (PKB), a remarkably narrow yet laterally extensive unit separating the Outer and Central Western Carpathians. Current tectonic models invoke two oceanic domains within the Alpine Tethys—the Magura Ocean to the north and the Vahic Ocean to the south—separated by the continental Czorsztyn Ridge acting as basement for the PKB sediments. Nevertheless, geological and geophysical evidence for this configuration remains equivocal. To provide independent constraints on the tectonic architecture of the Alpine Tethys suture, we investigated the lithospheric structure of the Western Carpathians using passive seismic and potential-field methods.

The passive seismic experiment was conducted along a dense ~N-S transect to acquire data for Receiver Function (RF) analysis. Joint inversion of RFs, P-wave polarization and Rayleigh wave dispersion curves yielded lithospheric-scale seismic velocity models, which were subsequently used for Common Conversion Point (CCP) migration. The resulting images reveal a Moho depth of 30-35 km, interrupted by two major offsets: one beneath the PKB and another beneath the eastern continuation of the Hrubanovo–Diósjenő Fault. Integration of the seismic results into a structural cross-section indicates that the Europe–ALCAPA boundary is best represented by a Pyrenean-style crustal wedge, with the lower crust of ALCAPA acting as an indenter into the European margin.

A prominent southward-dipping to sub-horizontal interface identified within the upper mantle beneath ALCAPA is interpreted as the underthrusted attenuated European passive margin. Importantly, this lithospheric architecture does not provide categorical evidence for the existence of a subducted oceanic slab preceding continental underthrusting. Furthermore, seismic velocities suggest that the underthrusted European crust may terminate around the second Moho offset as mantle-like velocities dominate south of this Moho offset, potentially constraining the extent of continental underthrusting.

Forward gravity modelling based on the seismic cross-section required the incorporation of a low-density body within the upper crust of the European plate to achieve an acceptable fit to the observed gravity field. This feature is consistent with the pronounced Bouguer gravity low associated with the PKB and may reflect the presence of low-density Magura Basin sediments within the underthrusted European margin. The density model was also used to assess the isostatic contribution of the underthrusted crust by comparing scenarios with and without its presence in the upper mantle. The results show that the High Tatras are essentially compensated by the normal crust, whilst the underthrusted lower crust provides significant additional isostatic support leading to higher modelled topography than observed, especially in the Low Tatras.

Acknowledgments: This research was funded by the National Science Centre (Poland), grant no. 2021/43/B/ST10/02312. We also acknowledge the AdriaArray initiative, within which the passive seismic experiment was conducted.