Formation of late-stage passive-roof duplexes in fold-and-thrust belts: 1 Thrusting sequence and thermochronologic constraints from the Northern Apennines (Italy)

This study explores these issues in the Casentino-Romagna axial sector of the Northern Apennines, which expose regional forethrusts and backthrusts. Detailed field mapping and analysis of superposed tectonic structures were integrated with apatite fission-track dating for constraining the timing of rock exhumation and correlated tectonic events. Collectively, the results have allowed us to interpret the evolution of the study area in terms of two main deformation stages. Specifically, a first, long phase (D1) progressed from NE-directed, in-sequence thrusting (∼18 to ∼10–9 Ma) to late out-ofsequence thrusting (∼8–5 Ma). A successive deformation phase, that we refer to as D2 (∼4–2 Ma), consisted of backthrusts and associated folds that were ubiquitous and systematically overprinted onto the foreland-verging D1 structures. Such retrovergent structures identify a late deformation phase dominated by the development of passive-roof duplexes that propagated hinterlandward into the orogen up to beyond the primary watershed ridge. Orogen-scale processes controlled the evolution of forelandward D1-phase thrusts, although late erosion could have played a major role by bringing the Apennine thrust wedge toward an undercritical state. The latter conditions could have contributed to keeping the out-ofsequence thrusts active, and eventually promoted the development of the D2 passive-roof duplexes