Long-lasting fault control on the Tiber River channel in Rome: did an ancestor of the Tiber Island exist in Pleistocene times?

Abstract

In the present paper we have reconstructed the geologic substrate in the area of Rome comprised between the Capitoline Hill and the Colosseum. The analysis of the stratigraphic logs of a large number of boreholes allowed us to highlight the occurrence of a buried fluvial channel of the Paleo- Tiber River, the geometry of which mimics, almost exactly, the fluvial bend hosting the present-day Tiber Island. Several 40Ar/39Ar dates allowed at discriminating two aggradational successions filling this paleo-channel, deposited during two consecutive glacio-eustatic cycles corresponding to Glacial Termination VIII (621 ka) and VII (534 ka). The buried paleo-channel corresponds with a partially obliterated NW-SE morpho-structural lineament affecting the present morphology, parallel to another, more marked lineament, hosting a tributary valley of the Tiber River, the Murcia Valley, 1 km to the southwest. Such lineaments match with the direction of the main Pleistocene extensional faults on the Tyrrhenian Sea margin of central Italy, which are re-activated under the present-day tectonic regime, exerting a close structural control on the drainage network of the Tiber River catchment in the area of Rome. The activity of one of these faults, running along the Murcia Valley, has been recognized to be responsible for the diversion of the Tiber course and the birth of the Tiber Island during the 6th century BCE. We conclude that a long-lasting structural control existed on this portion of the Tiber valley, which caused the repeated diversion of the river channel, around 650 ka and 550 ka, and ultimately in the 6th century BCE, creating the conditions for the origin of a fluvial island in correspondence with the fault-controlled river bend.