Northern Apennines and Southern Alps, two chains for the same foreland: recent kinematics and possible interactions

The kinematics of a fold-and-thrust belt or of parts of it can be influenced and modified by different factors, most of which have been thoroughly studied through years (syn-tectonic sedimentation, erosion, inherited structures, etc). Very few is known, however, on the possible influence that one thrust belt (or a single thrust) could exert on another one opposite verging. In the world, different cases of opposite verging thrust belts and shared foreland basins can be found (Rioja Through, Northern Spain; Middle Magdalena Valley, Colombia; Sevier fold-thrust-belt and Laramide foreland, North America, and others). An interesting case, characterized by a close distance between the outermost fronts of the two opposite verging orogenic fronts is represented by the Central Po Plain, in Italy. We investigated the propagation of the Northern Apennines on its foreland (Central Po Plain), in a sector where its outermost thrusts, buried under the Po Plain siliciclastic deposits, are close to those of an opposite verging chain, i.e., the Southern Alps. A regional cross-section, representative of this particular tectonic setting, has been restored in order to reconstruct the recent (post-Messinian) kinematics of the Apennines thrust fronts. An out-of-sequence propagation of the thrust faults was identified and its possible causes have been studied through analogue models. We reproduced in a sand-box the structural setting of the Central Po Plain, modeling the two opposite verging chains (Northern Apennines and Southern Alps). The experiment was set up to analyze the effects of an existing chain (Southern Alps) on an opposite verging one, which is developing toward the same foreland (Northern Apennines) while syntectonic deposits were sedimenting. The comparison between what observed through seismic lines and well logs and what shown by the models highlights that the presence of the Southern Alps fronts inhibits the propagation of the Northern Apennines fronts, leading to an out-of-sequence reactivation of the inner thrust fronts of the latter thrust belt. Comparison among the study area and its surroundings, along with a detailed analysis of the analogue models, revealed that syntectonic sedimentation cannot be the only factor responsible for the out-of-sequence reactivation of Northern Apennines inner fronts. Moreover, the presence of the Southern Alps buried fronts affects the Northern Apennines kinematics also when the two fronts are still far from colliding. The implications of this complex kinematics allow outlining some insights on the current seismotectonic setting of the area and on the possible evolution of this type of structures.