Episodic slab rollback in the Apennines, Italy, explained by subduction-to-collision-to-delamination transition

The transition from oceanic subduction to continental collision and, eventually, to delamination is thought to control dynamics, magmatism/metamorphism, and tectonic/sedimentation style in orogens. We propose for the first time that the alternation of slow and fast orogenic wedge advance and backarc opening in the retreating Apennines subduction zone (central Mediterranean area) was controlled by the transition from oceanic subduction to soft-collision (subduction of hyperextended continental lithosphere), evolving to hard-collision (subduction of the necking domain), and eventually to delamination. The coupling between slab dynamics and the evolution of the orogen is revealed by an unprecedented joint analysis of magmatism/metamorphism, of timing and rate of migration of forebulging, thrusting, and backarc extension and of seismic heterogeneities in the slab. Oceanic subduction and soft-continental collision, testified by high pressure-low temperature metamorphism, was characterized by fast orogenic and backarc extension migration, fast rotation of Corsica and Sardinia and vigorous magmatism. Low shear velocity anomalies observed in along-dip tomographic profiles across the Apennines are interpreted as signatures of slab damage events associated with the diachronous (at 21 Ma in the Northern and at 18 Ma in the Central/Southern Apennines) underthrusting of the necking domain of Adria in the Northern and Central-Southern Apennines. This hard-collision stage was characterized by slow orogenic wedge advance and backarc opening. The heating and weakening of the subducting continental crust produced by hard-collision promoted the transition from continental subduction to delamination of the Adria lithosphere. This process occurred at ca. 9 Ma and led to a relocalization of the subduction interface from the base of the sedimentary cover into the ductile middle crust and was associated with a renewal of fast orogenic wedge advance and backarc opening.