Upper crust “boudinage” during post-collisional Miocene extension in Tuscany: Insights from the southern part of the Larderello geothermal area (Northern Apennines, Italy)

A geological study carried out in the southern part of the Larderello geothermal area (Northern Apennines) provides new information on the development mechanism and timing of the earlier extensional structures that formed during the Miocene post-collisional tectonics which affected the orogen. Staircase low-angle normal faults (LANFs) affected a multilayered thickened upper crust after the collisional stage, producing the lateral segmentation of the Tuscan Nappe, the deeper non-metamorphic tectonic unit of the Northern Apennines in the Tuscan area. The tectonic history recorded in two Tuscan Nappe discontinuous bodies revealed that the LANFs took place during the Middle–Late Miocene, displacing collisional structures developed from the Late Oligocene. These Tuscan Nappe bodies are delimited by detachment faults located at the base, within the Tuscan evaporites, and at the top within the Ligurian Units. Their western and eastern margins coincide with east-dipping ramps. These structures and the Tuscan Nappe bodies were later dissected by Pliocene–Quaternary high-angle normal faults. The reconstructed deformation history implies that the Tuscan Nappe bodies are extensional horses developed through an earlier asymmetrical east-dipping extensional duplex system, involved in block faulting during the later, Pliocene-Quaternary, stage of extension.