Hyblean foreland

The foreland consists of exposures of the Hyblean Plateau and the offshore areas of Gela. The subduction forms a northward-dipping monocline in the foreland with a dip of 16–18°, which is a high angle compared to other regional monoclines. It is suggested that this steep angle is caused by a higher subductibility of the African slab.^[6] Deformation in the foreland is dominated by normal faulting; most of these faults have been formed in the Neogene by plate flexure. Some of the faults are recognized to be much older, formed in the Mesozoic. The faults are suggested to have separated high-standing platforms from deeper basins.^[8]

The simplified stratigraphy includes the following units:^[6][8]

The post-Paleogene geological events of the foreland can be divided into four main stages:

• At early Miocene, thrusting and volcanic activity triggered a regional uplift at the foreland, which caused the emergence of a broad carbonate platform in the eastern Hyblean Plateau.^[9]
• During the Messinian of late Miocene, the Messinian salinity crisis occurred as a result of sea-level fall and major uplift, causing deposition of evaporites over the Mediterranean basin. The dry climate resulted in extensive erosion in high-standing regions of the plateau.^[9]
• At Early Pliocene, rapid subsidence occurred at the foreland. This subsidence event occurred simultaneously with a major rise in Mediterranean sea level, during which Plio-Pleistocene sediments were accumulated in the foreland basin.^[9]
• During the Late Miocene-Early Pleistocene, a massive basaltic volcanic activity occurred. After the emission of alkali basalts during the Late Miocene, major volcanic activity occurred during the Late Pliocene, in which large volumes of tholeiitic basalts were emitted and the foredeep north of the Hyblean foreland collapsed.^[9]

Gela foredeep

The Gela foredeep extends from the northern margin of the Hyblean plateau on land to the offshore Gela basin of southwestern Sicily. Based on fossil analysis, the basin has been developing since Late Pliocene; its formation is related to the bending of the carbonate platform due to loading at the front of the Gela nappe. The basin fill consists of Late Miocene-Pleistocene sediments, including marly limestones, Messinian evaporites and sandy clays.^[3]

The Apenninic-Maghrebian orogen

The central to western part of Sicily is made of the Maghrebian-Apenninic orogen and represents the fold and thrust belt of the subduction system. A combination of frontal accretion and low-angle detachments formed the present geological structure.^[11] The orogen consists of a stack of detached nappes that involves mostly the Mesozoic carbonate, which has subsequently been folded into a syncline, underlying a broad anticline of the Caltanissetta area.^[12]

The stratigraphy of the accretionary wedge is composed of the following units:^[5][6][13]

Calabrian crystalline basement

The northeastern part of Sicily consists of the Calabrian Arc unit, the tectonic unit that connects the Apennines with the North African Maghrebide. Paleozoic igneous and metamorphic basement rocks are exposed at this part of the island, suggested to have been exposed by exhumation processes between mid-Oligocene and Middle Miocene.^[14] The rapid migration of the Calabrian Arc towards the southeast was driven by the retreat of the Ionian slab east of Sicily.^[1] In early Miocene, Calabria was still attached to Sardinia. In Middle Miocene, the trench rolled back to the east and normal faulting occurred between Calabria and Sardinia, causing Calabria to split eastward from Sardinia. By Early Pliocene, the retreat of Calabria consumed the oceanic slab of the Ionian sea while new oceanic crust was created in the Tyrrhenian Sea by back-arc magmatism. Since Pleistocene, the eastern portion of the arc formed the Apennine mountain of Italy, while the Calabrian block slid to Sicily through right lateral strike-slip motion, forming the northeastern portion of the island.^[1] GPS measurements show that the Calabrian Arc still advances to the southeast at 2-3mm per year at present, due to the ongoing rollback of the Ionian slab.^[15]

The simplified stratigraphy consists of the following units:^[3]

Late Triassic–Early Jurassic period

During Late Triassic, the ancient Neotethys ocean was formed by a major continental rifting event and rapid tectonic subsidence. Traces of the tectonic event can be found at both the Hyblean plateau and western Sicily.^[9]

Early Jurassic–Late Cretaceous period

After the rapid subsidence during Triassic to Early Jurassic, there was a slow thermal subsidence at the Mediterranean basin from Jurassic to Late Cretaceous that formed the Neotethyan carbonate platform.^[9]

Late Cretaceous–Palaeogene period

During Late Cretaceous and Palaeogene, the collision of African and Eurasian plates generated compressive stress in the region. The Neotethyan ocean was closed by the collision and the continental margin became active.^[9]

Neogene–Quaternary period

Sicily has been undergoing a phase of uplifting and subduction since Neogene.^[9] The African slab retreats to the east as the collision of Eurasian and African plate continued.^[1] Since late Oligocene, the orogenic process started when sediments accreted onto the Calabrian block while flysch was deposited in the foreland basins. The Sardinia block then underwent counter-clockwise rotation and the Calabrian unit subsequently underwent clockwise rotation. During the Early–Middle Miocene, shallow seated thrusting occurred which formed the wedge-top basins. Since Late Miocene, thrusting occurred in the Sicilian orogen and Calabrian accretionary wedge.^[2] Extension and back-arc magmatism occurred in the Tyrrhenian slab due to the fast roll back of the Ionian slab, while Calabria migrated to the current position at northeast Sicily.^[1]