Polyphase extensional basins: interplay between tectonics and sedimentation in the Neogene Siena-Radicofani Basin (Northern Apennines, Italy)

Rift-basins are the shallow effects of lithosphere-scale extensional processes often producing polyphase faulting. Their sedimentary evolution depends on the mutual interplay between tectonics, climate, and eustasy. Estimating the role of each factor is generally a challenging issue. This paper is focused on the tectono-sedimentary evolution of the Neogene Siena-Radicofani Basin, a polyphase structural depression located in the inner Northern Apennines. Since Miocene, this basin developed after prolonged extensional tectonics, first as a bowl-shaped structural depression, later reorganized into a half-graben structure due to the activation of high-angle normal faults in the Zanclean. At that time the basin contained coeval continental and marine settings controlled by the normal faulting that caused the development of local coarse-grained depositional systems. These were investigated to: (i) discriminate between the influences of tectonics and climate on sedimentation patterns, and (ii) provide detailed time constraints on fault activity. The analysed successions were deposited in an interval between 5.08 and 4.52 Ma, when a climate-induced highstand phase occurred throughout the Mediterranean. However, evidence of local relative sea-level drops is registered in the sedimentary record, often associated with increased accommodation space and sediment supply. Such base-level fluctuations are not connected to climate changes, suggesting that the faults generally control sedimentation along the basin margins.