Tracing Alpine Tethys Closure: Insights from Detrital Rutile Geochronology in the Outer Western Carpathians and Eastern Alps

The collision between Europe and Alcapa (a segment of Adria) led to the formation and subsequent erosion of high-pressure rocks in the Carpathian and Alpine arcs. Metamorphic rutile, which forms under relatively high pressures, is a reliable indicator of subduction environment during orogeny. To enhance our understanding of the Alpine Tethys Ocean's closure in the Western Carpathians, U-Pb geochronology was performed on detrital rutile from medium-grained sandstones within the Magura and Silesian Nappes.

Twelve samples were collected along a transect in the Magura Nappe, with an additional three samples from the Silesian Nappe serving as a reference. An additional profile of three samples was collected from the Altengbach-Formation of the Rhenodanubian Flysch in the Greifensteiner-Decke for comparison. From each sandstone, approximately 200 rutile grains were extracted, and about half were selected for detailed analysis. The dated rutile grains exhibit significant variation in age and physical characteristics, indicating multiple source origins.

In the Magura transect the most prominent age peaks align with the Variscan (c. 400–280 Ma) and Alpine (c. 160–90 Ma) tectonic events, both of which are well-represented except oldest dated sample. Notably in the Magura transect, four distinct Alpine maxima were identified in the rutile dataset. Among these, the two dominant peaks at 137–126 Ma and 115–105 Ma appear in most samples. Two additional samples, deposited during the Eocene–Oligocene and the Late Cretaceous–Paleocene, reveal the youngest age peak at 94–90 Ma. A peak at 193–184 Ma is observed in these two samples and in another sandstone dated between the Paleocene and Eocene.

The Silesian samples consistently exhibit a prominent Variscan peak. Only the sample deposited in the Oligocene reflects Alpine tectonic events, with one dominant peak at 95 Ma and two minor peaks at 26 Ma and 180 Ma.

In the Altlengbach-Formation the Alpine peaks appear in the two youngest samples, whereas the Variscan peaks are prominent in all samples. The oldest sample is Lower Cretaceous whhereas the other two are Upper Cretaceous–Paleocene.

For the Carpathian samples, we tentatively propose that key tectonic events include the Jurassic subduction of the Meliata Ocean (~180–155 Ma) and the Cretaceous nappes stacking and exhumation of the Veporic and Gemeric megaunits (140–90 Ma). The widespread presence of Alpine-age rutile in all but the oldest sandstone indicates an open sedimentary pathway from the southern and central Alcapa to a basin located north of the alleged Oravic (Czorsztyn) continental sliver within the Alpine Tethys Ocean. The absence of Alpine ages in the oldest sandstone may reflect either a physical barrier separating the basin from the orogen or the unavailability of rutile-bearing rocks at the surface during that time.

More broadly, we suggest that the synorogenic deposits of the Outer Western Carpathians contain detritus derived from previously subducted, exhumed, and imbricated oceanic and continental crustal domains. Age peaks in the ~180–105 Ma range are probably related to the closure of the Neotethys Ocean (Meliata branch), while the youngest peak at 94–90 Ma possibly corresponds to the subduction of the Alpine Tethys beneath Alcapa.

Research is funded by the NSC, Poland, project no. 2021/43/B/ST10/02312.