Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids

Collision tectonics and the associated transformation of continental crust to high-pressure rocks (eclogites) are generally well-understood processes, but important contradictions remain between tectonothermal models and petrological–isotopic data obtained from such rocks. Here we use 40 Ar– 39 Ar data coupled with a thermal model to constrain the time-integrated duration of an orogenic cycle (the burial and exhumation of a particular segment of the crust) to be less than 13 Myr. We also determine the total duration of associated metamorphic events to be ∼20 kyr, and of individual heat pulses experienced by the rocks to be as short as 10 years. Such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events (earthquakes). Such events triggered transient heat advection by hot fluid along deformation (shear) zones, which cut relatively cool and dry subducted crust. In contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust, our non-steady-state cold-crust model satisfactorily explains several otherwise contradictory geological observations. Mountain difficulty The duration of the geological processes that shape the Earth generates vigorous debate. Mountain-building, where crust is consumed continuously by burial and recycled as buried remnants rise to the surface, is one process where new dating methods can resolve the issue. High-precision dating of rock remnants left by continental collision in the Caledonides of southern Norway shows that the entire cycle can happen quickly, in about 13 million years. In addition, the rapid transport of hot fluid zones through the cold crust may explain many puzzling geological observations.