Extension controls Quaternary tectonics, geomorphology and sedimentation of the N-Apennines foothills and adjacent Po Plain (Italy)

In the middle Pleistocene, thrusting in the N-Apennine fold-and-thrust belt came to an end and foredeep sedimentation in the adjacent Po Plain basin ceased. A new tectono-sedimentary regime was installed. Extensional faults were activated in the foothills and are limited to the northeast by a major, SW-dipping normal fault, the Bologna fault for which a vertical displacement of > 1000 m is estimated. The region northeast of the fault, i.e. its footwall, was progressively tilted towards the Po Plain. The SW sectors of the tilting region were progressively uplifted above sea level, subjected to erosion thereby feeding a deltaic system which prograded towards the northeast in the subsiding regions. Compressional structures without major deformation are recorded in the zone of maximum curvature. All these features are explained by the onset of extensional deformation along the Bologna fault and consequent flexural unloading of its footwall. The evidence, therefore, shows that the transition region between the N Apennine foothills and Po Plain has been under a tensional stress regime since the middle Pleistocene.