Incipient Crustal Stretching across AN Active Collision Belt: the Case of the Siculo-Calabrian Rift Zone (central Mediterranean)

In the Central Mediterranean, the differential roll-back of the subducting Nubia Plate caused the Neogene-Quaternary extrusion of the Calabrian arc onto the oceanic Ionian slab, and the opening of the oceanic Tyrrhenian Basin, in the overriding Eurasia Plate. The differential motion at the edges of the arc was largely accommodated along transform faults that propagated across the orogenic belt. Since the Late Quaternary, the southern edge of the arc has been replaced by the roughly N-S oriented Siculo-Calabrian Rift Zone (SCRZ) that formed as the NNW-directed normal faults of NE Sicily, crossing the orogenic belt, have linked the NNE-oriented Tyrrhenian margin of southern Calabria with the NNW-trending Africa-Ionian boundary of southeastern Sicily. Our study focused on the Sicily shoulder of the SCRZ, where the transition zone between the extensional belt and the still active Nubia-Eurasia convergent margin is characterized by two distinct mobile crustal wedges, both lying on an upwarped Mantle, where a re-orientations of the σ1 is combined with volcanism (e.g. Etna, Aeolian islands) and a huge tectonic uplift. In southeastern Sicily, the Hyblean-Etnean region evolved, since about 0.85 Ma, as an indipendent crustal wedge, moving towards the NNW and pointing to the active Mt. Etna volcano. A local ENE crustal stretching accompanied the traslation of the block and pre-dated the ESE-oriented extension governing the propagation of the southernmost branch of the SCR, which started at about 330 ka B.P.. Similarly, the Peloritani-Aeolian region, flanked by the 125 ka-old NE-Sicily branch of the rift zone, represents a mostly submerged crustal wedge that migrates towards the NE, diverging from the rest of the Sicily collision zone and pointing to the Stromboli volcano. The Peloritani-Aeolian block is characterized by the occurrence of a wide central NE-oriented collapsed basin contoured by an actively uplifting region, whose tectonic boundaries are evidenced by a sharp increase in the elevation of the marine terraces of the last 600 ka. In the two analysed crustal blocks, the crustal doming with the related basin collapses and the concurrent tectonic motion without any back-stop can be respectively related to the thermal anomalies and to the drag forces exerted by a hot Mantle flow, triggered by the opening of a vertical slab windows at the southern edge of the Calabrian arc. This process has favored the migration of hot Mantle material towards the sectors of larger rollback of the subduction hinge, producing the spectacular tectonic uplift and active magmatism of the region. The increase of the volume of the Mantle wedged in the subduction zone of the Calabrian arc has been also effective to accommodate the differential roll-back between the adjacent segments of Nubia Plate, thus replacing the motion along the pre-existing transform faults dissecting the orogenic belt. In this frame, the deformation of the Hyblean-Etnean and the Peloritani-Aeolian blocks heralded the development of the two Sicily branches of the SCRZ. The delayed crustal stretching responsible for the propagation of the rift zone across the orogenic belt can be framed in the dynamic of a still active convergent margin if interpreted as the result of the sliding of the crust along the Ionian flank of the Mantle flow.