Summit low-angle faults in the Central Apennines of Italy: Younger-on-older thrusts or rotated normal faults? Constraints for defining the tectonic style of thrust belts

Low-angle faults that juxtapose younger rocks over older ones are widely documented in fold-thrust belts, and reconstruction of tectonic style is strictly dependent on their interpretation. Various modes exist for generating hinterland-dipping low-angle faults with younger-on-older relationships. Indeed, in the Central Apennines of Italy, the hinterland-dipping younger-on-older low-angle faults, which rest on the summits of the major anticlines (i.e., summit low-angle faults), have been interpreted variously as younger-on-older thrusts within out-of-sequence thrust systems, postorogenic normal faults, gravity-driven slides, or as rotated prethrusting normal faults. In this study, we provide a new and robust structural-geological data set, corroborated with stratigraphic timing constraints and balanced geological cross sections, bringing an essential contribution to the interpretation of the hinterland-dipping younger-on-older low-angle faults as preexisting normal faults rotated within the shortcut anticlines during the Neogene thrust-related fold emplacement. A detailed geological and structural characterization carried out on four remarkable examples from the Central Apennines allowed reconstructing an inversion tectonics model. The lack or reverse reactivation of the rotated prethrusting normal faults here analyzed is consistent with fault bend and fault propagation folding models associated with break forward in-sequence thrust propagation implying more conservative estimates of shortening for the Central Apennines thrust system, compared to the previous out-of-sequence models. Taking into account the various possible causes for the development of hinterland-dipping younger-on-older low-angle faults, the structural-geological characterization presented in this study and the results achieved could be critically applied when examining similar structures in other thrust belts.