Late-orogenic heating during exhumation: Alpine PTt trajectories and thermomechanical models

During the Alpine orogeny, the Penninic zone of the Alps was affected by Eoalpine high-pressure metamorphism. In the central and western Alps, this was followed by Lepontine medium-pressure, high-temperature metamorphism during exhumation. We compare the pressure–temperature–time (PTt) trajectories established in two key areas in the central and western Alps with 2-D numerical models of two possible causes of Lepontine metamorphism: (1) detachment or breakoff of a subducting slab, and (2) the presence of a wedge of accreted radiogenic material. Numerical models show that both mechanisms are capable of producing significant heating during orogeny. Heating by slab detachment is fast and transient (more than 100°C in up to 10 million years, depending on the location), whereas radiogenic heating requires time spans of the order of tens of millions of years and cessation of the subduction process. The combination of PTt trajectories and synthetic PT paths deduced from our thermomechanical modelling results suggests that the metamorphism observed in the central Alps has not been caused by radiogenic heating alone. Slab breakoff, on the other hand, seems a viable mechanism to account for the documented rise in metamorphic temperatures during exhumation. In view of the time constraints posed by the geological data, and acknowledging the effects of large-scale block rotations and out-of-section transport, slab detachment is also a more likely mechanism to have provided sufficient heat to cause re-heating in the western Alps.