Oroclinal bendings in allochthonous units: influence on Epiligurian sedimentation and implications for a palaeogeographic reconstruction of the Northern Apennines during the Neogene.

Most fold-and-thrust belts show pronounced plan-view geometrical curves defined by lateral variations in structural trends. According to the transport direction, they can be subdivided in antitaxial curves (convex in the direction of the vergence) and syntaxial curves (concave in the direction of the vergence). Two types of bends are recognized: the first, non rotational arc, is a bend where the segments do not change strike during its development. The second, rotational arc or orocline, is a bend where the segments change strike during its development. In this way, the recognition of arcuate faults belonging to allochthonous thrust sheets, as oroclines, permits their formation to be linked to the thrusting phase of the allochthonous unit. At the same time the trasport direction of a bowed fault can be determined by: 1) the bisetrix of the chord connecting its terminal ends, 2) the strike of the tear faults bounding it, 3) the internal geometry of syntectonic deposits located atop the fault (piggy-back basins). In this context, the recognition of Neogene antitaxial oroclinal bendings in the Ligurian thrust sheet of the Northern Apennines, is of outstanding importance in understanding the relationships among thrusting direction, tectonic control on sequence boundaries and rotation of the Italian peninsula.Starting from late Oligocene the migration of the Northern Apennine thrust belt was coupled with the counterclockwise rotation of the Corsica-Sardinia block and of the northern Italian peninsula around a pivot north of Genoa. Following this opinion, various paleogeographic reconstructions indicated a NE-SW direction of the deformational front during late Oligocene, a N-S direction during early-middle Miocene and a NW-SE direction, similar to the recent position, during late Miocene-early Pliocene. Palinspastic restorations are essentially based on paleomagnetic measurements, absolute dating of the rotation of the Corsica-Sardinia block and distribution of sedimentary facies. A further support to the paleogeographic reconstructions comes from the tectonic transport direction registered by oroclinal bendings occurring in the Val Marecchia thrust sheet, cropping out between Tuscan and Romagna-Marche Apennines. In this allochthonous sheet some antitaxial arcs are characterized by different transport directions, reflecting a change of about 40 degrees in the direction of the Apennine deformational front during early Serravallian-early Tortonian. The timing of this change can be desumed both from the age of deposits underlying the allochthonous sheet and from arcuate Epiligurian deposits located on top of arcuate thrust faults.In a section view arcuate Epiligurian basins describe a wedge-shape geometry, with the maximum thickness localised in the back portion of the basin, which is confined by tear faults and couples of reverse faults or couples of frontal reverse faults and rear normal or vertical faults. These basins are filled by thick late Oligocene-early Pliocene deposits, that can be grouped in several depositional sequences. Each depositional sequence crops out always parallel to the arc curvature, with convex shape boundaries. Every younger sequence tends to occupy a more internal position within the arc, with a decreasing inclination of bedding planes up to a sub-horizontal attitude. The unconformities separating depositional sequences registered different Neogene tectonic events, related to step-wise advancements of the Ligurian thrust sheet. In conclusion, these peculiar syntectonic deposits are an excellent natural laboratory for gaining insight into arc-development processes and tectonic control on sequence boundaries in orogenic wedges. Following these data an accurate paleogeographic reconstruction of the timing of the various stages of Neogene rotations of the Northern Apennine deformational front is proposed