Anatomy of late orogenic extension: The Northern Apennines case

Abstract

Late orogenic extensional structures overprint the compressional structures of the Northern Apennines fold and thrust belt. The Tyrrhenian Sea is the result of major continental extension and rifting which started in the thickened zone of the Apulian/ Corso-Sardinian collision. It is believed to have commenced as early as in the Late Oligocene or Early Miocene in the Corsica basin. Extension post-dates the collision by approximately 20 Ma. The extensional stress field observed in the internal part of the Northern Apennines co-exists with compression in the foreland part of the belt. Field, borehole and refraction and reflection data were used to investigate the geometry of extension and how it affected the compressional abric in the Northern Apennines. Analysis of the collected data has revealed a complex geometry for the extension across the belt. A geological cross-section from the island of Elba, to the west of the Italian Peninsula, to Ancona, in the east, as well as a geodynamic model for the evolution of the Northern Apennines is presented and discussed. From the Early Miocene stretching of the collision-thickened crust created a N-S rift which evolved into the present Northern Tyrrhenian Sea. Typically, two sets of extensional faults, a low-angle E-dipping and a W-dipping one are observed across the chain. Usually the W-dipping faults have similar attitudes to those of the older reverse faults, reactivating some of the thrusts as normal faults. The E-dipping normal faults, however, cut down-section through the pre-existing thrusts and, in the internal part of the belt, accommodate most of the displacement associated with extension. A change in fault polarity occurs in the Tuscan region and major faults dip towards the west. A-type subduction of the Apulian crust under the Corso-Sardinian block occurred after collision leading to the delamination of the Apulian lithospheric mantle away from the crust. Collision-induced delamination of the mantle lithosphere away from the zone of collision resulted in mechanical thinning of the lithosphere beneath the orogenic belt. This process induced late orogenic crustal stretching in the internal zone of the Northern Apennines. As the convergence between the Apulian and Corso-Sardinian blocks continued the extensional and compressional fronts, accompanied by the delamination, migrated from the hinterland towards the foreland of the orogen.