The Orobic Thrust: A Long-Lived Regional Fault Zone in the European Alps – Architecture, Evolution, and Geochronological Insights

The Orobic Thrust is a prominent regional-scale fault zone extending over 80 km, recognized as one of the largest structures in the retro-belt of the European Alps. It represents a significant tectonic boundary where the Variscan basement is thrust southward over the Upper Carboniferous to Lower Triassic volcano-sedimentary cover of the Southalpine Domain. Several well-exposed cross sections of the entire fault zone, approximately 250-300 m thick, allow a comprehensive reconstruction of its architecture and evolution.

A distinctive 20-25 m protomylonitic band at the top of the fault zone, coupled with thermal maturity analyses of clay minerals in the footwall, indicates temperatures of at least 300 °C during the early stages of activity. Field and microstructural analysis identified four distinct Brittle Structural Facies (BSF) within the fault zone: cataclasites, foliated cataclasites, pseudotachylyte-bearing cataclastic bands, and incoherent fault gouges. With the exception of fault gouges along undeformed planes, these facies exhibit mutual crosscutting relationships, evidencing a history of alternating seismic slip and aseismic creep.

SEM imaging, minerochemical analyses and quantitative microstructural analyses were performed in order to better characterize the BSF. Our results show that multiple BSFs can be observed at the microscale, with up to five seismic slip events recorded within a single thin section. Analyzing selective clast survival from melting, the clast-to-matrix ratio, grain size distribution, and mineralogical content can help discriminate between different rheological behavior during coseismic slip. Geochronological data provide absolute age constraints on fault activity. Pseudotachylytes yield ⁴⁰Ar-³⁹Ar ages ranging from 83 to 64 Ma, while illite from gouge material along a reverse fault plane at the core of the zone gives a K-Ar age of 53 Ma. Notably, pseudotachylyte ages show older values (79–83 Ma) at both the top and bottom of the fault zone, with younger ages (76–64 Ma) displaying a bottom-forward trend. These findings illustrate the fault's prolonged activity, with discrete illite gouge-decorated planes extending the activity to the Early Eocene.

The Orobic Thrust, active from the Late Cretaceous to the Early Eocene, functioned as a pre-collisional fold-and-thrust belt within the upper plate of the Alpine Tethys subduction system. Its extended 30-million-year history highlights the capacity of regional-scale fault systems to undergo multiple reactivations under changing thermal and stress conditions.