Messinian-Early Pliocene crustal shortening along the Tyrrhenian margin of Tuscany, Italy

This paper illustrates the results of structural studies carried out in the western margin of Tuscany along a major crustal structure. Surface deformation of sediments filling different basins aligned on top of this major structure (from north to south: the Fine Basin, the Sassa-Guardistallo basin, the Rio Guardigiano area in the Lustignano basin) allow us to date its tectonic activity to the Messinian-Early Pliocene. In these areas, structures such as reverse and strike-slip faults and mesoscopic folds are widely developed. Structural analysis determined a compressive stress field with the σ1 oriented from E-W to NE-SW active from Messinian to Early Pliocene. At the southern end of this crustal structure, the Gavorrano antiform and the granitic pluton (radiometric age of granite ~4.4 Ma) coring this fold correlate with a thrust ramp anticline at depth, and thus constrain thrust activity to the Early Pliocene. These data document a Messinian-Early Pliocene compressive activity that contrasts with models invoking continuous extensional tectonics affecting the hinterland since the Late Oligocene-Middle Miocene in the frame of a back-arc-slab retreating process. The results presented therefore raise the question of which geodynamical model could account for such a complex structural evolution of Northern Apennines hinterland.