Late oligocene-neogene evolution of Europe-Adria collision: New structural and geochronological evidence from the Giudicarie fault system (Italian Eastern Alps)

Abstract

The Giudicarie fault system (Giudicarie sensu stricto, Meran-Mauls, Passeier, Thurnstein, and Jaufen faults) represents a sharp break in the generally E-W strike direction of the orogen-scale Periadriatic fault system and is a key element for understanding the late Oligocene-Neogene evolution of the Alpine chain. The kinematics, timing and magnitude of movements on the Giudicarie fault system are presented here, on the basis of a detailed structural study of the component fault segments combined with zircon and apatite fission track analysis from closely spaced samples. Two main tectonic phases are established: (1) back thrusting of the Austroalpine units over the Southern Alps around 32 Ma, recorded by basement and limestone mylonites along the Giudicarie and Meran-Mauls faults with transport directions toward 100°-110°, and (2) later sinistral transpressive displacement, characterized by structures at the ductile-brittle transition, which overprinted the top to E/ESE thrust-related mylonites but also partitioned into a major system of transcurrent faults in the Southalpine domain. It was during this later event that the Periadriatic fault attained its present-day geometry. However, the amount of sinistral displacement along the Giudicarie system was only ∼15-20 km. The magnitude is established here from the ∼15 km sinistral offset of the Jaufen mylonites across the Passeier brittle fault and the ∼20 km long gap in the otherwise continuous distribution of Oligocene tonalitic lamellae along the Giudicarie fault. A direct structural connection is also established between the Brenner and the Jaufen faults. This constrains the timing of the second phase, since it must postdate the main exhumation phase of the Tauern Window at 20-18 Ma. The results of this study argue against an originally straight Periadriatic fault. The Giudicarie fault formed a restraining bend in this part of the Periadriatic fault system since at least the late Miocene and probably since the late Oligocene.