SUMMARY New seismic reflection profiles of the Italian deep crust project CROP provide new insights on the structure of the Ionian sea. In spite of the Apennines and Hellenides Neogene subduction zones, two conjugate passive continental margins are preserved at the margins of the Ionian sea, along the Malta escarpment to the southwest and the Apulian escarpment to the northeast. The Ionian sea is likely to be a remnant of the Mesozoic Tethys Ocean, confined by these two conjugate passive continental margins. The transition from continental to oceanic crust appears sharper to the northeast than to the southwest. The basin between southeast Sicily and southwest Puglia was about 330 km wide and suggests a low spreading rate. The inferred oceanic ridge should have been flattened by thermal cooling and buried by later sediments. Based on stratigraphic and structural constraints to the north in the Apennines belt, the ocean continued to the northwest. This palaeogeography is supported by the seismicity of the Apennines slab underneath the southern Tyrrhenian sea, which implies downgoing oceanic lithosphere. The adjacent absence or paucity of deep seismicity does not imply absence of subduction, but rather it can be interpreted as due to the more ductile behaviour of the subducted continental lithosphere. Surprisingly, we note that where the oceanic inherited basin is subducting underneath the Apennines, in the hangingwall of the subduction hinge there are outcropping slices of continental crystalline basement previously deformed by the Alpine orogen.

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Sicily is a thick orogenic wedge formed by (1) the foreland (African) and its Sicilian orogen and (2) the thick-skinned, Calabrian–Peloritani wedge. The crust under central Sicily, from the Tyrrhenian margin to\ud the coastline of the Sicily Channel, has been investigated by the multidisciplinary (SI.RI.PRO.) research project.\ud The project dealt with the nature and thickness of the crust and depth and geometry of the Moho, which is essential in formulating subduction models and improving the knowledge of African and Tyrrhenian–\ud European lithospheres. The results resolve features such as (1) the main orogenic wedge, (2) the very steep, NW–SE-trending regional monocline suggesting inflection of the foreland crust, (3) the deep Caltanissetta synform imaged, for the first time, to about 25 km, and (4) the top of the crystalline basement and the inferred\ud crust–mantle boundary. The SI.RI.PRO. transect confirmed that the NNW-dipping, autochthonous Iblean platform of SE Sicily and its basement extends all the way into central Sicily. Further NW, towards the NNW\ud end of the transect, a large uplift involves the Iblean platform and its underlying basement. The associated gravity anomaly is interpreted as the southern wedge edge of the Tyrrhenian mantle that splits the subducting Iblean–Pelagian (African) continental slab from an overlying synformal stack of allochthonous thrust sheets

In the western Sicilian fold and thrust belt, large clockwise rotations of allochthons occurred during late Cenozoic contraction of part of the southern Tethyan margin. The magnitude of rotation decreases stepwise from over 120° in the upper sheets, lying on the north coast of Sicily, to no appreciable rotation in the frontal portion of the belt flanking the southern coast of the island. The allochthons are composed of imbricate thrust sheets derived primarily from individual basin and platform assemblages of the old Tethyan margin. Paleomagnetic and structural data indicate that the rotation of the allochthons was accommodated by coherent torsional displacements on relatively low‐angle detachment surfaces. Timing relations for the imbrication history of the Sicilian fold and thrust belt are derived from stratigraphic overlap and local involvement of sediments deposited in a series of foreland and piggyback basins. The locus of deposition within successive foreland basins first migrated easterly then southerly during progressive deformation in the orogen. Imbrication began in the early Miocene (Burdigalian‐Langhian) and continued at least through the early Pleistocene and appears to be continuing today. Rotation is related to thrusting and accompanies a 70° change in the tectonic transport direction from easterly to southerly. Easterly striking, right‐oblique transpressional faults and associated northeasterly trending folds postdate thrust sheet rotation in the interior of the thrust belt and were active contemporaneously with south‐directed thrusting in the foreland region. Pleistocene and possibly older (late Pliocene?) extension strongly modified the older thrust morphology along the Tyrrhenian coast of northwestern Sicily, with the development of down‐to‐the‐north listric normal faults. The extensional structures apparently are related to the opening and subsequent deformation of the Tyrrhenian Sea to the north.

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