Geochronological constraints on the evolution of the Periadriatic Fault System (Alps)

Abstract

Fault rocks from various segments of the Periadriatic fault system (PAF; Alps) have been directly dated using texturally controlled Rb–Sr microsampling dating applied to mylonites, and both stepwise-heating and laser-ablation ⁴⁰Ar/³⁹Ar dating applied to pseudotachylytes. The new fault ages place better constraints on tectonic models proposed for the PAF, particularly in its central sector. Along the North Giudicarie fault, Oligocene (E)SE-directed thrusting (29–32 Ma) is currently best explained as accommodation across a cogenetic restraining bend within the Oligocene dextral Tonale-Pustertal fault system. In this case, the limited jump in metamorphic grade observed across the North Giudicarie fault restricts the dextral displacement along the kinematically linked Tonale fault to ~30 km. Dextral displacement between the Tonale and Pustertal faults cannot be transferred via the Peio fault because of both Late Cretaceous fault ages (74–67 Ma) and sinistral transtensive fault kinematics. In combination with other pseudotachylyte ages (62–58 Ma), widespread Late Cretaceous–Paleocene extension is established within the Austroalpine unit, coeval with sedimentation of Gosau Group sediments. Early Miocene pseudotachylyte ages (22–16 Ma) from the Tonale, Pustertal, Jaufen and Passeier faults argue for a period of enhanced fault activity contemporaneous with lateral extrusion of the Eastern Alps. This event coincides with exhumation of the Penninic units and contemporaneous sedimentation within fault-bound basins.