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

Abstract

Passive-roof duplexes accommodate shortening at the mountain front of many fold-and-thrust belts worldwide. These structures typically manifest at the surface by hinterland-verging backthrusts that decouple thin-skinned thrust sheets from underlying foreland-verging duplexes. Although the main factors controlling the development of passive-roof duplexes have mostly been identified, some of their intrinsic characteristics are still poorly defined. These relate to their spatio-temporal relationships to thrust faults located further inland in orogens, and their ability to transport younger rocks over older ones. 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-of-sequence 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-of-sequence thrusts active, and eventually promoted the development of the D₂ passive-roof duplexes.