Tectono-thermal evolution of the Northern Apennines-Alpine knot: a case study from the Bobbio Tectonic Window

The structural and tectonic interactions between the S-verging Southern Alps and the NE-verging Northern Apennines fold-and-thrust belt, and their shared Po Plain foreland basin, represents a classic and long-debated issue in Alpine–Apennine geodynamics. We here investigate a cross section from the Bobbio Tectonic Window (BTW) in the Emilian Northern Apennines, to the central Po Plain subsurface, which records these important relationships. Previous studies focused on fault slip-rate measurements of the buried Northern Apennine thrust fronts, but a comprehensive tectono-thermal study of the Lower Miocene turbiditic sequence outcropping in the BTW is still lacking. In this work, we investigate the relationship between BTW development and the interaction of the Northern Apennines and Southern Alps thrust fronts buried below Pliocene-Pleistocene sediments in the central Po Plain. We analysed the cooling/exhumation history of rocks exposed at the core of the BTW by means of low-T thermochronology (apatite fission-track and U-Th/He) on samples from the Lower Miocene (Burdigalian) Bobbio Fm. and compared them with the slip-rate history of the Northern Apennines buried thrust front along the Emilian Arc. Our thermochronological results from the BTW show a maximum temperature of ca. 85-90°C (apatite fission-tracks partial annealing zone) reached soon after depositional age, followed first by a relatively slow cooling in the Early Miocene- Early Pliocene time window (17-6 Ma), and then by a fast cooling starting between ca. 6 and 4 Ma.  By comparing these results with the slip-rate trend of the buried Apennines thrust front, we interpret them as the signal of an out-of-sequence thrusting reactivation within the inner Northern Apennine fold-and-thrust belt due to the interaction between the Northern Apennines outermost fronts and the Southern Alps.  This study shows how far-field geological structures can influence the general kinematics of the thrust-fold belt, promoting out-of-sequence reactivation of internal tectonic structures and the exposure of deep tectonic units within the BTW.