Geological oulaine of Sicily and lithospheric tectono-dinamics of its Tyrrhenian Margin from new CROP seismic data

Based on new CROP seismic data, integrated with other geological-geophysical information,an evolutive lithospheric tectono-dynamic reconstruction of the Sicilian region and its Tyrrhenian margin has been done with innovative results. The data and conclusions presented here are supported not only by the here annexed and described sections, but also by relevant results from the organic ensemble of the whole interpreted CROP dataset. The preorogenic (Lower Cretaceous) paleogeographic framework of the central Mediterranean, now much better constrained by direct CROP data, consisted of two inter-communicating ocean basins: the Alpine and Ionian Neotethys. These basins were interposed between Europe and Africa–Adria, and between the Adria and the Africa the Ionian one. Downward extension of the geophysical imaging to the upper mantle allowed a better understanding of the complex evolution of this wide Mediterranean area. One of the most significant and helpful new results obtained is represented by clear seismic imaging of the upper Ionian slab buried beneath a stack of allocthonous crustal units colliding with the paleo-Sicily(African) slope. Such a situation, previously postulated in uncontrolled, extrapolated models, is now well defined. The stratigraphic and structural data collected onland, and integrated with subsurface gophysical and borehole data allow us to reconstruct the geological setting of Sicily. The Foreland Domain is represented by the undeformed portion of the Pelagian block, exposed in the Hyblean plateau and the Sciacca platform. From the Late Miocene, the detachment of inner edge units gave rise to Triassic-through-Miocene carbonate platform imbricates: the Pelagian–Sicilian Thrust Belt. This external thrust system is exposed only in western Sicily. The lowermost carbonate platform imbrication reaches the surface in the area north of Sciacca as a large ramp anticline involving Early Pliocene deposits. Geoseismic data show that the wedge of Meso-Cenozoic carbonate platforms is underlying a tectonic melange, mainly composed of Oligo-Miocene Numidian nappes. The thrust belt outcrops in the structural highs of Rocca Busambra and M. Kumeta (“M. Alpi-like thrust”). The deep-seated thrust belt is tectonically overlain by “roof thrust systems”, composed of the Apenninic fold-and-thrust belt, originating from the deformation of both basinal and carbonate platform Mesozoic-Tertiary deposits overlain by crystalline units (Calabrides). The lowermost tectonic units of the chain are the Ionides (the Sicanian, Imerese, M. Judica basinal carbonate and siliceous thrust sheets). They deposited in the Ionian Tethys basin and are overridden by the Panormides. A large nappe, composed of Permo-Triassic rocks grading up into a Mesozoic-Paleogene carbonate sequence, the Lercara unit, lies widely over the Middle-Late Miocene top levels of the Sicanian and Imerese units. Therefore it is thought to pertain to a palaeogeographical area originally located along the margin of the Ionian Tethys, and provides useful information about the opening of this Neotethyan Sea. The Ionides are composed of complex basinal thrust sheets, overriding the Late Miocene–Pliocene top levels of the Sicilian–Pelagian Thrust System (Tyrrhenian stage). Sicily is characterized by a huge volume of Late Oligocene–Early Miocene Numidian Flysch, deriving from the substratum of thePanormides. The Numidian Flysch is overlain by the Alpine Tethydes, widely displaced as far as the frontal wedge of the chain. All reconstructed seismogeological sections show coherent results and thus allow us to invert in a regionally well-constrained manner the tectonic processes which occurred. In the Balearic stage (Oligocene–Lower Miocene) a NW-dipping subduction of the Alpine Tethys (remnant of a wider ocean closed in the Alps sector by the Adria–Europe collision)gave rise to the formation of a wide orogenic arc which included stack components of the Northern Apennines, the Southern Apennines, the Calabrian arc and the Sicilian Apennines–Maghrebian chain. The rotation of the Corso-Sardinian block stopped with the collision of Corsica against the NW Adria margin, and SE Sardinia against the Panormid promontory. In the late Balearic–early Tyrrhenian stage (Burdigalian) the Panormid promontory dissected the Sicilian Apennines from the Maghrebian chain to the south, and the Etrurian promontory (NW Adria) dissected the Northern Apennines from the Southern Apennines to the north, giving origin to the impressive tectono-dynamics of the SE-directed Ionian slab retreat and connected Tyrrhenain mantle dynamics (Middle Miocene to present). The Ionian slab subduction was accompanied by detachment and thrusting of Panormid units over the Ionian basin, detachment of the condensed sedimentary cover of the Ionian ocean and of its marginal zone sequence, with oblique thrusting over the Sicilian platform. Prominent amounts of Ionian Tethydes (Ionides = Lagonegrese, Imerese, Sicanian) made up the Sicilian Apennine wedge filling the Caltanissetta basin. In the Late Tyrrhenian stage, the collision of the migrating crustal block, and mantle flux pressure against the steep Sicilian slope generated a transcrustal thrust that duplexed the Sicilian platform. New ophiolite-bearing units obducted in the Eo-Alpine stage over the southern European margin are now discovered on the Calabrian units of the SE-stretched Sardinian margin. The innermost allochthonous units facing the back-arc Tyrrhenian basin are all formed by delaminated fragments of Eastern Sardinia within which the new Tyrrhenian ocean started to open.