Middle-Late Pleistocene transverse faulting in wedge-top units of the Northern Apennines (Italy): Implications for orogen-parallel extension during coeval foreland-ward migration of the compression and extension fronts

We examine the post-orogenic setting of the Northern Apennines (Italy) along the diffuse boundary between the belt's frontal domain, characterised by active crustal shortening through folding and thrusting, and its hinterland, where crustal stretching and extension by normal faulting prevail. This hinterland-foreland transition zone hosts middle Eocene-Upper Miocene wedge-top basins (Epiligurian units) that form the uppermost structural level of the belt. We analysed the structural architecture of the Epiligurian units to clarify the deformation style of this transitional domain. Integrated lineament remote-sensing and structural fieldwork reveal a complex array of extensional features, including NE-SW faults that cut transversely across the belt NW-SE axis. Syn-kinematic calcite mineralisations associated with these transverse extensional structures were dated by the U-Th method, yielding Middle-Late Pleistocene ages (280-65 ka). These data indicate that the studied wedge-top units record the onset of the belt's post-orogenic evolution though activation of extensional transverse structures. Combining our results with published data, we propose a model involving early orogen-parallel extension deforming the shallow part of the Northern Apennines since the Middle Pleistocene. This extensional phase occurred between the foreland-directed migration of the compressional front and the initiation of the present orogen-perpendicular extension emanating from the hinterland.