Upper plate deformation as marker for the Northern STEP fault of the Ionian slab (Tyrrhenian Sea, central Mediterranean)

The Eastern Tyrrhenian margin (ETM), the active boundary of the Tyrrhenian Sea backarc basin, is the key for understandingthe geodynamics of the central Mediterranean. Numerous seismic tomography studies have beencarried out in this region, proposing different reconstructions of the lower subducting plate and cause of theslab-break-off existing beneath the Southern Apennines. However, the area and mode of the recent deformationof the Tyrrhenian Sea are still not fully defined and understood. In this study,we combine the analysis of a recentseismic tomographymodel and geological data, in order to understand the relationship between the subductinglower plate and the tectonic evolution of the sedimentary basins formed on the upper plate.With this aim,we interpreted a large data set of seismic reflection profiles and several well logs. The results consistin 2D and 3D geologicalmodels of the basins, sedimentary infill, and fault networks. Taking into account thegeological data of the ETM and those of the adjacent inner flank of the Apennines, we observe: (i) a system oflinked sedimentary basins developed on a narrowdeformation belt bounded by transformfault zones; (ii) a polyphaseriftingwithin the upper plate; (iii) an abrupt change of the direction of extension (~90°), fromNE-orientedin the Lower Pleistocene to SE-oriented in the Middle Pleistocene. Since these ETM features are not the typicalexpressions of the current backarc extensional models, we propose a link between the evolution of upper plateand the onset and development of a STEP (Subduction-Transform-Edge-Propagator) fault along the northernmargin of the Ionian slab.