Rifting dismantling an orogenic belt: the interplay between polyphase extensional structures and magmatism in the northern Tyrrhenian Sea (Elba Island, inner Northern Apennines, Italy)

The rapid transition from thick to over-thinned continental crust under an orogenic belt is a fascinating process that also characterizes the Neogene evolution of the Northern Apennines, an Alpine belt formed during the Cretaceous-Early Miocene by the convergence and collision between Africa (Adria microplate) and Europe (Sardinian-Corsican Massif). Since the Miocene, the inner Northern Apennines have been affected by extensional tectonics, resulting in a current crustal and lithospheric thickness of about 25 and 45 km, respectively. The clearest evidence of this process is the opening of the Northern Tyrrhenian Basin over the former suture zone. Therefore, in this paper we focus on the interplay between the evolution of extensional structures and magmatism on the Elba Island, which is a key area in this context due to its exposures and geographic location. We have integrated existing field data into a new view that offers a different interpretation of the reflection seismic lines acquired in the surroundings of the Elba Island. Five extensional events have been defined as having developed during the Aquitanian-Zanclean period, albeit within a continuous process in which normal and transfer faults were simultaneously active. We describe the occurrence of Burdigalian-Tortonian shear zones developed during the formation of the Corsica Basin, which were exhumed and rotated to the west. Their exhumation was triggered by Tortonian magmatism, which determined the doming of the shear zones. The eastward ongoing extensional tectonics generated new shear zones, which in turn were deformed into a dome-shaped geometry during the emplacement of the new Monte Capanne and Porto Azzurro plutons, in the late Miocene. The progressive deformation created normal faults that formed grabens or semigrabens in the Pliocene, which are now well exposed in Tuscany and characterize the Tyrrhenian Basin. The coeval NE-trending transfer fault zones played the role of channelling magma volumes and overcoming inhomogeneities in the continental crust, subject to stretching. The doming of extensional shear zones simulate illusory reverse kinematics, reason why the tectonic context of the Northern Tyrrhenian Basin and the inner Northern Apennines is debated.