Active tectonics along the Nebrodi–Peloritani boundary in northeastern Sicily (Southern Italy)

• We report new evidence for active tectonics at the southern edge of the Calabrian arc.
• Rejuvenated fault scarps due to extensional reactivation of old strike–slip faults
• The analysed faults accommodate motion along the southern margin of a mobile block.
• The deformation of the margin included fault growth and widespread fracturing.
• The tectonic style fits well with the occurrence of seismic swarm in the region.

In the epicentral area of the seismic swarm of the June–September 2011, at southern edge of the Calabrian arc in NE Sicily, very recent extensional motions remobilised two main NW–SE dextral faults. The extensional reactivation of strike–slip faults responded to a new regional dynamic, also evidenced by GPS and seismological data. The inverted structures are aligned at the margin of a wide crustal block that is moving apart from the rest of the island and is uplifting faster than the adjacent regions. The active faults terminate to the northwest at the intersection with a prominent NNE trending fault that represents the western boundary of the mobile block. The vertical displacement along this border exactly matches the difference in elevation of the marine terraces resting inside and outside the block, respectively. On the contrary, only part of differential displacement of the marine terraces was actually accommodated as cumulative motion along the two NW oriented inverted faults, across the southwestern boundary of the block. Amounts of the vertical displacement were distributed on distinct fault planes of the previous dextral shear belts. The widespread fracturing is also the best explanation for the seismic swarm of the 2011, whose epicenters spread on a discrete rock volume rather than concentrated along a single fault plane. The diffuse fracturing seems to represent a peculiar style of deformation, connected to the tectonic inversion of previous strike–slip shear zones. Seismic swarm also affects the northern termination of the Calabrian arc where active extensional deformation reactivated previous strike–slip faults. The similarity of the two regions suggests that seismic swarm can be peculiar of extensional belts developed on previous strike–slip shear zones, along which the pre-existing geometry favours the dispersion of the tectonic motion on a network of small linked fault planes.