Post-Tortonian mountain building in the Apennines. The role of the passive sinking of a relic lithospheric slab

At least five compressional phases have been recognized, which are responsible for post Tortonian mountain building in the Apennines; the single phases of orogenic transport (usually consisting of several deformation events evidenced by the structural analysis) occurred within short time intervals separated by longer periods in which the subsidence played an important role for the sedimentation of clastic deposits in the foredeep basinal areas. The deformation did not act cilindrically, that is to say the amount of transport and the tectonic style considerably changes across different segments of the mountain chain. The trend of surface and subsurface structures allows the recognition in the Apennines of two major arcs which have developed in post-Tortonian times. The amount of shortening is only some tens of kilometres in the north-western Apennines, while it reaches about three hundred kilometres in the Calabrian Arc; the average velocity of shortening, therefore, varies from less than 1 cm/y in the north to around 5 cm/y in the south, in agreement with the extension pattern of the Tyrrhenian Sea. The deep-focus earthquakes beneath the Southern Tyrrhenian Sea are related to the presence of a lithospheric slab which may represent, in its deepest part, the remnant of the Po-Adriatic-Ionian lithosphere which was subducting the Corsica-Sardinia block before the opening of the Tyrrhenian basin. Passive subduction of the Po-Adriatic-Ionian lithosphere by gravitational sinking with progressive retreat of the flexure zone appears as a reasonable mechanism to explain first-order contemporaneous phenomena such as mountain building in the Apennines and extension in the Tyrrhenian area, as well as the time-space migration of the thrust belt-foredeep- foreland system in post-Tortonian times. The Pleistocene and, probably, the present-day compression along the outer margin of the Apennines is confined at the apex of the northern Apenninic arc and in the External Calabrian Arc, suggesting the existence of two major zones of differential lithospheric sinking