Lifting the orogenic lid – Assessing the European basement beneath the Alpine wedge using U-Pb geochronology

Overridden continental margins preserve critical records of crustal growth, magmatic activity, and pre-orogenic topography, yet they are rarely accessible in situ. In the European Alps, the distal southern European margin is only exposed within the Tauern Window. An additional, unique archive of this margin basement is, however, provided by exotic granitoid blocks and boulders preserved in the Ultrahelvetic nappe system in the Alpine fold-and-thrust-belt. In the Ultrahelvetic slope setting at the passive margin, coarse-grained rock fall and debris flow material was deposited in pelagic sediments during opening and closure of the Penninic Ocean. These sedimentary successions were later accreted into the Alpine wedge and transported northwards, allowing the exotic basement clasts to escape subduction and pervasive Alpine metamorphism. As a result, Ultrahelvetic granitoid boulders preserve a unique record of the southernmost European basement.

This archive forms the first sample suite and is complemented by sandstone samples hosting the boulders as well as drillcore samples from beneath the Alpine wedge. Whereas the drillcores sample the autochthonous basement adjacent to the Bohemian Massif, the samples from the Ultrahelvetic nappe system represent the most distal part of the European margin. To explore this largely hidden margin, we carried out U–Pb zircon dating on samples from all three archives.

Drillcores from five basement samples beneath the northern Alpine wedge and the Molasse Basin range from Early Proterozoic orthogneisses (Mank drillcore) to Permian granodiorites (Moosbierbaum and St. Corona drillcores). Ordovician protolith ages (Großgraben and Oberndorf drillcores) correlate with known Bohemian Massif units, while late Variscan granites document post-collisional magmatism beneath the Molasse Zone.

In contrast, the Ultrahelvetic exotic granitoid boulders provide direct information on the distal European margin. Previous geochronological data suggested exclusively Late Devonian ages and, in conjunction with geochemical analyses led to interpretation of these rocks as products of a marginal high (Frasl & Finger, 1988). Our new data reveal a much more differentiated record, with four magmatic pulses: Ordovician (~466–480 Ma), Late Devonian–earliest Carboniferous (~360–380 Ma), Carboniferous (~320–340 Ma), and Permian (~290–280 Ma). Following an Ordovician magmatic event, Late Devonian and Carboniferous ages record Variscan magmatism. Permian ages reflect post-Variscan extension preceding Jurassic rifting. We infer derivation of the exotic boulders from a topographically elevated marginal high, a characteristic feature of rifted passive margins. Notably, the exotic boulders are petrographically and geochronologically similar to the Zentralgneise of the Tauern Window, suggesting that this window exposes an equivalent distal margin basement.

In sum, our Ultrahelvetic samples revealed that the crustal rocks at the southern European margin were formed by multi-stage magmatism between the Ordovician and the Permian. Reworked boulders of these rocks occur in Paleogene slope deposits and can be used as a proxy for crustal domains of the distal European margin, allowing us to reconstruct Penninic rifting, Variscan tectonics, and passive-margin architecture by effectively “lifting the orogenic lid.”

Frasl, G., & Finger, F. (1988). The "Cetic Massif" below the Eastern Alps - characterised by its granitoids. Schweiz. Mineral. Petrogr. Mitt., 68, 433 - 439.