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 40Ar - 39Ar 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 similar to 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.