Maresca, Augusto The Tyrrhenian-central Apennines crustal transect: trench retreat tectonics during continental collision. [Tesi di dottorato]

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Abstract

I present a crustal-scale balanced cross-section across the Tyrrhenian back-arc basin and the central Apennines belt, alongside a kinematic reconstruction of the subduction-to-collision system for the last 20 Myr. Subduction of the oceanic lithosphere of Alpine Tethys and the accretion of its sedimentary cover, with the coeval slab rollback and opening of the Liguro Provencal back-arc basin, terminated around 18-20 Ma, with the arrival of the Adria rifted margin at the subduction zone. Docking of the Adria margin at the subduction zone, due to the inherent buoyancy of the continental crust, stopped the rollback of the Adria slab and decelerated the convergence. At about 12-10 Ma, the relocalization of the subduction interface from the base of the sedimentary cover into the ductile middle crust promoted the decoupling of contractional deformation into thin- and thick-skinned tectonics and the onset of crustal delamination, renewing the slab rollback. Since then, thrusting and folding have occurred in a dominant piggy-back sequence, while extension was re-established at the rear of the belt and progressively migrated toward the foreland, keeping a constant distance of ~100 km with the compressional front. To preserve consistency between the shortening in the basement and in the cover and to explain the observed delay between compression and extension in a crustal delamination environment, here is proposed a zip-like tectonic model for the Apennines. Delamination of the lower crust spreads from a forelandward migrating singularity point located in the ductile middle crust. Two bands propagate from this point: (i) one toward the foreland, featuring compressive structures, and (ii) the other toward the hinterland, delimiting extensional structures. The seismicity of the Apennines, the active and fossil forelandward-dipping low-angle normal faults, the Moho depth variation across the upper and lower plates, the unroofing pattern, and the structural elevation of the compressive mountain front can be explained in terms of the forelandward migration of the singularity point. As such, the central Apennines provide a comprehensive example of trench retreat in collisional systems.

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