On the Mesozoic Ionian Basin

New seismic reflection profiles of the Italian deep crust project CROP prov ide new insights on the structure of the Ionian sea. In spite of the Apenni nes and Hellenides Neogene subduction zones, two conjugate passive continen tal margins are preserved at the margins of the Ionian sea, along the Malta escarpment to the southwest and the Apulian escarpment to the northeast. T he Ionian sea is likely to be a remnant of the Mesozoic Tethys Ocean, confi ned 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 abou t 330 km wide and suggests a low spreading rate. The inferred oceanic ridge should have been flattened by thermal cooling and buried by later sediment s. Based on stratigraphic and structural constraints to the north in the Apenn ines belt, the ocean continued to the northwest. This palaeogeography is su pported by the seismicity of the Apennines slab underneath the southern Tyr rhenian sea, which implies downgoing oceanic lithosphere. The adjacent abse nce 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 s ubducted continental lithosphere. Surprisingly, we note that where the ocea nic inherited basin is subducting underneath the Apennines, in the hangingw all of the subduction hinge there are outcropping slices of continental cry stalline basement previously deformed by the Alpine orogen.