Evolution in space and time of a regional-scale fault: example from the Orobic Thrust (European Alps, Southalpine Domain)

Despite the small volume they occupy in the crust, fault zones are of striking importance as they localize both seismic slip and aseismic deformation, as well as fluid migration at middle to shallow crustal levels. Regional-scale fault systems may benefit of long-lived activity as, due to their rheological weakness, they can be also reactivated by weak far-field stresses in a variety of tectonic settings. The architecture of fault zones that experienced multiple re-activation is complex and accurate structural analyses, including also the identification of (micro)structural facies (Brittle Structural Facies, BSF) and their crosscutting relationships, is mandatory to solve the spatial evolution and relative chronology of the fault zone.

The Orobic Thrust is a regional-scale fault zone, more than 80 km in length, representing one of the largest structures in the European Alps retro-belt. Along the thrust plane the Variscan basement is thrusted southward over the Upper Carboniferous to Lower Triassic volcano-sedimentary cover of the Southalpine Domain. In several areas the fault zone, ca. 250-300 m thick, is continuously exposed, allowing the detailed reconstruction of the fault architecture. A narrow (20-25 m) protomylonitic band marks the top of fault zone suggesting temperature of at least 300°C in the early stage of fault activity. Temperature in excess of 200°C are also supported by analysis of the thermal maturity of clay mineral assemblages (<2 µm size fraction) in terrigenous rocks in the footwall of the thrust plane. Four distinct BSF have been recognized: cataclasites, foliated cataclasites, pseudotachylyte bearing cataclastic bands and incoherent fault gouges. Apart from fault gouges occurring along discrete plane that appear to be undeformed, all the other 3 BSF display mutual crosscutting relationships, testifying for multiple switching between seismic slip and aseismic creep during fault history. The occurrence of pseudotachyltes and fault gouge allow to obtain absolute age constraints with ⁴⁰Ar-³⁹Ar and K-Ar illite dating, respectively. The ages obtained from pseudotachylytes span from 83 to 64 Ma whereas illite (<0.1 µm size fraction), separated from the gouge along a fault plane with a reverse kinematic at the core of the Orobic Thrust fault zone, provided and age of 53 Ma. Pseudotachylyte age distribution shows older ages 79-83 Ma occurring both at the top and the bottom of the fault zone, with a superposed pattern that display instead a bottom forward younging direction of ages between 76 and 64 Ma. Discrete fault planes decorated with gouges mark the end of the activity of the Orobic Thrust in the early Eocene.

Detailed meso-and microstructural analyses combined by absolute age constraints of the BSF allowed the reconstruction in space and time of the Orobic Thrust fault zone through its 30 Myrs long fault activity.