Basement-controlled Neogene polyphase cover thrusting and basin development along the Chianti Mountains ridge (Northern Apennines, Italy)

The Chianti Mountains is an important sector of an E-verging regional thrus t-related fold (the so-called Tuscan Nappe) extending along the whole lengt h of the Northern Apennines. This thrust system involves the Tuscan Sequenc e superposing the Macigno sandstones onto Cervarola-Falterona sandstones, b oth of which are sedimented in adjacent foredeep basins. Detailed field map ping and analysis of superposition relations among tectonic structures, as well as correlation between structures and syntectonic deposition, has allo wed Chianti Mountain evolution to be interpreted in terms of three main sta ges of deformation. The D-1 stage resulted in the NE-directed synsedimentary thrusting of the M acigno onto the Cervarola-Falterona sandstones, while large NE to ENE-verge nt thrust-related folds developed during the two successive deformation sta ges (D-2 and D-3). Fault-propagation folds developed during the D-2 stage, and were affected by the Main Chianti Mountains Thrust (MCMT) during the su ccessive D-3 stage. In particular, the D-3 stage has been correlated to the development, during the Pliocene period, of the hinterland Upper Valdarno Basin, which was previously considered to be an extensional basin. In fact, this continental basin formed along the eastern margin of the Chianti Moun tains, ahead of the MCMT that also produced a shortening of the basin fill. With the beginning of the Quaternary period, the tectonic regime switched to extensional, as manifested by the development of a normal fault system o n the opposite basin margin. The data presented here allow us to infer that the Chianti Mountains thrust system (D-2 and D-3) developed during a time interval spanning from the La te Miocene (similar to 12 Ma) until the Late Pliocene (similar to 2 Ma) per iods. In the Northern Apennines, polyphase thrusting recorded by cover rock s has been related to the activity of basement thrusts, which have been rec ently evidenced by geophysical data. In this context, the two latest stages of deformation recognised in the Chianti Mountains have been attributed to the activity of the Abetone-Cetona crustal thrust, the deformational effec ts of which propagated forward in the sedimentary cover.