The Paglia-Tevere Graben, Italy: Plio-Pleistocene sedimentation and structural architecture

Extensional basins in retro-arc and hinterland areas allow sediment accommodation and preservation of thick coverage in well organised depositional sequences. The Peri-Tyrrhenian area of the northern-central Apennines (Italy) is characterised by a widespread occurrence of syn-tectonic basins framed within a horst-and-graben structural setting produced in response to the SW-NE trending continental extension since the Miocene. Among these syn-tectonic basins, the Paglia-Tevere Graben represents a 60 km long, NNW-SSE-striking morpho-structural depression hosting marine-to-continental Plio-Pleistocene sediments up to 1 km-thick lying above substrate made of turbiditic sequences. The Plio-Pleistocene sedimentary succession is organised in two main syn-rift and high rank depositional sequences , from the open marine to coastal-transitional environments: the first one is Zanclean-early Gelasian in age, while the second spans from the late Gelasian to the early Calabrian. The continental succession ends with post-rift and syn-uplift fluvial terraces, travertines and volcanic deposits of the Vulsini Volcanic District, since the late Calabrian (1.3 Ma). Detailed surveys and geological mapping, combined with an intense bibliographic review of biostratigraphic, tephrostratigraphic and strontium isotopes data, were performed to improve the knowledge of the geological evolution of the graben in its northern part. Stratigraphic and sedimentological analyses have been devoted to detail the stratigraphic organisation of the Plio-Pleistocene sequences. We identified: i) minor transgressive-regressive cycles associated with recurrent stages of west-directed progradation and retrogradation of coarse-grained fan-deltas sourced from the western mountainsides of the Apennines; ii) transgressive marker beds of bioclastic calcarenite levels with Amphistegina (zone MPL5b), as indicators of climate warm phase. The structural analyses revealed the occurrence of multiple fault sets, variably dissecting both the Plio-Pleistocene sequences and the Miocene turbiditic substrate. A polygonal setting (in map view) of normal faults occurs in the eastern part of the graben, represented by decametre-to-hectometre-long fault segments striking NW-SE, N-S, and NE-SW, and accommodating a cumulative stratigraphic offset of tens of metres. Strike-slip faults , mostly striking NNE-SSW, locally cut the normal ones, contributing to the disarticulation of the morphostructural basin. Our preliminary data can be used to constrain the geometry, the opening rate and the sedimentary evolution of the Paglia-Tevere Graben during the Quaternary.