Tectonic escape of Sicily micro plate in the framework of the Tyrrhenian-Apennine system evolution

The opening of the Tyrrhenian basin and the contemporary formation of the Apennine chain are described through the application of techniques of deformable plate kinematics and modeled with several dedicated software (Chapter 1). These methods have proven to be suitable to describe complex tectonic processes, such as those observed along the Africa–Europe collision belt, characterized by passive subduction processes. These results from the subduction of the residual Alpine Tethys and the Ionian lithosphere, and from the fragmentation of the Adriatic plate. As a first step, described in Chapter 2, the area of the Apennine Chain and of the Tyrrhenian basin have been divided into deformable polygons through a careful observation of the regional structures. The polygons are distinguished on the basis of sets of extensional structures that are coherent with unique Euler pole grids. The boundaries between these polygons coincide with large tectonic lineaments that characterize the Tyrrhenian–Apennine area. The tectonic style along these structures reflects the variability of relative velocity vectors between two adjacent blocks. The deformation of tectonic elements is accomplished, allowing different rotation velocities of lines that compose these blocks about the same stable stage poles. The angular velocities of extension are determined on the basis of the stratigraphic records of syn-rift sequences, while the finite rotation angles are obtained by crustal balancing. From north to south, the polygons are: (1) Northern Apennine; (2) Umbria–Marche Apennine Arc; (3) Southern Apennine; (4) Calabrian Arc; (5) Sicilian Chain. Then, to understand the geodynamic processes that guided the opening of Tyrrhenian back-arc basin, the geometry of the Adriatic-Ionian slab has been reconstructed and presented in Chapter 3. The slab is constituted by Liguride and Ionian oceans and by the Apulian-Adriatic continental margin. It is segmented by STEP faults, propagated along the COB of the Liguride and Ionian oceans and along the fracture zones of the Apulian-Adriatic margin, formed during the Triassic- Jurassic rifting. The continental lithosphere begins to subduct starting from the North and proceeds to the South with time intervals determined by the articulation of the Adriatic COB line. To the South, where the continental lithosphere is absent or scarce, the slab segments keep sinking into the upper mantle; on the contrary, the northernmost segments progressively decrease their sinking with the incoming of continental lithosphere into the subduction zone, until the buoyancy equilibrium is reached. A strong relationship between upper plate evolution and asynchronous subduction has been considered in the proposed kinematic model. Chapter 4 is dedicated to the specific aim of this PhD thesis: reconstruct the kinematic evolution of Sicily, considered as an independent plate starting from 4.5 Ma ago, and its role in the framework of the Tyrrhenian-Apennine System. Sicily is involved in a process of escape towards east-southeast induced by the African plate acting as an intender pushing toward north, during its convergence with the European plate, and by the Malta escarpment STEP fault, due to the retreat of the African-Ionian slab that created space toward east. The plates and microplates that I consider involved during the evolution of the last 4.5 Ma are Europe, Africa, Calabria and Tunisia. This last microplate is strictly related to the evolution of Sicily. The boundaries of the Sicily and Tunisia microplates are lithospheric structures known from the literature and identifiable from geological and geophysical datasets: high resolution bathymetric maps, seismic sections, geodetic data, focal mechanism of recent earthquakes, gravimetric maps, Moho depth maps and so on. The margin between Sicily and Europe is along the Drepano-Elimi chain (Ustica ridge), a E-W trending morpho-structure with a general transpressive kinematics; the margin with the Calabrian microplate is along the right-lateral Taormina line first and then along the ‘‘Aeolian-Tindari- Letojanni” line; the margin with Africa is expressed along part of the Malta Escarpment and along the Sicily Channel, where a series of troughs (Pantelleria, Linosa and Malta) were interpreted in literature as rift basins or as pull-apart basins related to a dextral shear zone. The margins of Tunisia instead follow the N-S and NE-SW structures that characterize the Central Atlas. Several attempts have been tried to obtain the Euler pole of rotation between Sicily and Africa, starting from the structures in the Sicily Channel and using the GPlates software. Then, also Sicily- Europe, Sicily-Calabria and Sicily-Tunisia poles and the respective velocity vectors have been derived and compared with the geological data. Finally, the proposed kinematic model of the Tyrrhenian-Apennine system, including Sicily and Tunisia, is shown in Appendix 1 and Movie 1, and described in 5 phases in Chapter 5. Briefly, during the first phase (19–12 Ma), an extension strip divided the proto-Apennine chain from Sardinia while a northern Apennine arc and a central Apennine arc were separated from an extensional area, in transverse direction, covered by top-wedge sediments. The Calabrian arc was divided in north Calabria and south Calabria–Peloritani mounts, separated by the Catanzaro Trough. The second phase (12–7 Ma) is marked by the separation of the Western Sicily chain from the Calabrian arc and the beginning of extension in the Caltanissetta basin; this, is still active in phase 3, for the whole Messinian age. Phase 3 (7–4.5 Ma) was characterized by the formation of the Vavilov basin, which records major changes in the Ionian slab-retreat process. A new nascent sector, the Southern Apennine Sector, including the Lazio–Abruzzi platform, was separated from the Apennine Arc by the Ancona–Anzio Line and rotated around a very close pole, located at the northern tip of the Vavilov Basin. During phase 4 (4.5-2.5 Ma) Sicily and Tunisia start to rotate around their poles: Tunisia is pushed upward while Sicily starts a clockwise rotation that produces extension in the northernmost part of the Sicily Channel. At this time the Taormina line is active as the boundary between Sicily and Calabria. At the beginning of the last phase (2.5-0 Ma) extension jumped eastwards of the Vavilov basin and the southern branch of the triple junction of the Southern Tyrrhenian rifts started to form the Marsili basin. A dextral shear zone is active in the Sicily channel and the movement between Sicily and Calabria jumps from the Taormina line to the ‘‘Aeolian-Tindari-Letojanni” line. At the end of this phase the Ionian STEP fault intersected the Malta Escarpment and the tear fault began to propagate along this structure.