Recent extensional faulting in the Gulf of Taranto area: implications for nuclear waste storage in the vicinity of Scanzano Ionico (Basilicata)

The Gulf of Taranto in southern Italy provides an excellent case where it is possible to document the importance of normal faults in displacing terraced deposits. The study area is located at the front of the southern Apennines, that is a fold-and-thrust belt developed following the closure of the Mesozoic Tethys Ocean, and the deformation of the Adriatic passive margin during Tertiary and Quaternary times. The outer, eastern parts of the belt were structured in recent, i.e. up to Middle Pleistocene times. The front of the chain is partially sealed by Pliocene-Pleistocene foredeep deposits, which represent the infill of the Bradanic trough. The upper portion of the middle Pleistocene succession consists of marine sands and conglomerates that in the previous literature have been arranged in several orders of terraces. Analysis of aerial photographs and geomorphological mapping has shown the occurrence of prominent geomorphic lineaments, which appear to control the local drainage pattern. Some of these structures coincide with the map trace of normal faults that produce vertical offsets of the marine terrace surfaces in the order of ca. 10 m each. Most of the fault escarpments can be traced for up to 3-4 kilometres, and laterally reduce their elevation and terminate. Scarp height is comprised between 2 and 10 meters, although some faults are locally observed to produce displacements up to 50 m. Their mean trend ranges from ENE-WSW to N-S and defines an arcuate pattern that mimics the present coastline. An accurate geomorphological, sedimentological and stratigraphic analysis has been carried out in a selected area of the Bradanic trough (Pisticci transect) to investigate in detail the relationships between normal faults and the development of the terraces. This analysis allowed 5 facies associations to be recognised, related to the upper and lower beachface and to the neritic clays which represent the substratum of the terraces. More importantly, we observed that all the terraced deposits in the Pisticci transect could be referred to a single sedimentary body displaced by faults. The terraced deposits are related to an event of beach progradation, of Middle Pleistocene age, which has been documented in other areas of the Italian peninsula. These results outline an intimate relationship between the arcuate trend of the recognised fault set and the present coastline pattern. The development of the normal faults can be related to large-scale gravitational processes developed after the general tilting of the Bradanic trough towards the SE. The development of normal faults could be related to gravity tectonics connected to a shallow-dipping détachment horizon. These large-scale movements involve the southern part of the Bradanic trough. They appear related to a gravitational collapse towards the SE of the continental escarpment generated by the general tilting of the Bradanic trough. In addition, and more important, the recognition of recent extensional faulting and tectonic instability in the Taranto Gulf has a relevant implication for a matter of social security. In November 2003, the area of Scanzano Jonico was proposed as a site for nuclear waste deposit. The discovery of recent extensional faults undermine the generalised assumption of a long-lasting tectonic quiescence in the Taranto Gulf, and suggests that the entire investigated area may not be suitable for nuclear waste storage.