Timing, thrusting mode, and negative inversion along the Circeo thrust, Apennines, Italy. How the accretion-to-extension transition operated during slab rollback

The evolution of the Apennine wedge has seen the time-space migration of the forebulge, foredeep, thrust wedge, and back-arc extension phases in the wake of the Eastward rollback of the subducting Adria slab. In this framework, thrusting and post-orogenic extensional faulting have occurred in two parallel forelandward-migrating ribbons, with extensional deformation overprinting or partly exploiting anisotropies of the inherited thrust system. Here, we explore the tectonic framework and the timing of thrusting and subsequent negative inversion of the Circeo thrust, one of the major thrusts in the inner portion of the central Apennines, with the main aim to constrain the timing and mode of the compression to extension switch. Structural analysis, carbonate C and O and clumped isotopes analysis, X-ray diffraction of clay minerals, and U-Pb dating of calcite slickenfibers have been integrated with seismic interpretation, cross-section balancing, and 1D burial and thermal modeling. We show that the Circeo thrust developed during Langhian-Serravallian time. Its extensional reactivation is dated at the Serravallian, during the stacking of an underlying thrust slice, before the onset of Pliocene back-arc extension in the area. Combination of our data with the age of thrusts, extensional basins, and base of the foredeep infill of the central Apennines, demonstrates that forelandward migration of the foredeep-thrust system occurred at variable velocities. Accelerations and decelerations are synchronous, respectively, with the opening of the Liguro-Provençal and Tyrrhenian back-arc basins and with the interluding quiescent period.