Pre-thrusting mesoscopic extension in a syn-orogenic foredeep basin of the Umbria-Marche Apennines, Italy

Syn-orogenic deposits within foredeep basins undergo exten- sion related to the initiation of the basin, followed by contraction related to the migration of fold-and-thrust systems toward the fore- land. Extension and basin development can result from flexuring of a lithospheric plate induced by the load of a stacking thrust pile, so extension followed by contraction should be common in such set- tings. Contraction overprints the earlier extension, however, which is therefore difficult to observe in syn-orogenic deposits. We describe the complex mesoscopic structures in the Contessa section, in the Umbria-Marche Apennines, Italy. Several normal faults occur that are conjugate about vertical when the beds are restored to the hori- zontal. These normal faults have been tilted in the folds and are dis- placed by two generations of thrusts that are related to the Miocene- Early Pleistocene orogenic event responsible for the development of the Apennines. These results have important implications for Apen- nine tectonics, where similar, pre-thrusting extension is recognised in other, younger foredeep basins. A comparison of the modes of transition from extension to contraction at different scales may provide useful constraints for a better understanding of evolving belt-foredeep-foreland systems.