On the evolution of orogens: Pressure cycles and deformation mode switches

Evidence of two burial-exhumation cycles that took place during a single orogeny has been found in rocks belonging to the Piemonte unit of the Western Alps. An early high pressure event, which resulted from tectonic burial down to pressures of 1.5 GPa, was followed by exhumation to ca. 0.20-0.35 GPa as a result of extensional deformation. Renewed shortening culminated in a second burial episode down to pressures of 0.65-0.80 GPa, before the final exhumation took place. Existing geochronological data allow only ca. 13-19 Ma for the completion of both burial-exhumation cycles. Therefore, we suggest that the evolution of orogens is characterized by multiple short-lived burial-exhumation cycles related to orogen-scale alternance between shortening and extensional deformation.