Late Pliocene–Quaternary evolution of outermost hinterland basins of the Northern Apennines (Italy), and their relevance to active tectonics

Abstract We examine the tectonic evolution and structural characteristics of the Quaternary intermontane Mugello, Casentino, and Sansepolcro basins, in the Northern Apennines fold-and-thrust belt. These basins have been classically interpreted to have developed under an extensional regime, and to mark the extension-compression transition. The results of our study have instead allowed framing the formation of these basins into a compressive setting tied to the activity of backthrust faults at their northeastern margin. Syndepositional activity of these structures is manifested by consistent architecture of sediments and outcrop-scale deformation. After this phase, the Mugello and Sansepolcro basins experienced a phase of normal faulting extending from the middle Pleistocene until Present. Basin evolution can be thus basically framed into a two-phase history, with extensional tectonics superposed onto compressional structures. Analysis of morphologic features has revealed the occurrence of fresh fault scarps and interaction of faulting with drainage systems, which have been interpreted as evidence for potential ongoing activity of normal faults. Extensional tectonics is also manifested by recent seismicity, and likely caused the strong historical earthquakes affecting the Mugello and Sansepolcro basins. Qualitative comparison of surface information with depth-converted seismic data suggests the basins to represent discrete subsiding areas within the seismic belt extending along the axial zone of the Apennines. The inferred chronology of deformation and the timing of activity of normal faults have an obvious impact on the elaboration of seismic hazard models.