Crustal extension in the northern Apennines: The transition from compression to extension in the Alpi Apuane Core Complex

An interpretation of Northern Apennine geology is presented which relates the temporal and spatial occurrence of both compressional and extensional deformation features in terms of the changing dynamic evolution within an accretionary wedge, after a model proposed by Platt [1986], followed by the initiation and development of continental rifting. During Cretaceous to Eocene time an accretionary wedge formed as remnant Tethyan oceanic crust subducted beneath the rotating Corsica-Sardinia microplate. Microplate collision during the Oligocene was characterized by the rapid imbrication of buoyant continental crust of the Italian continental margin, the record of which is preserved within the duplex structure geometry of the Alpi Apuane region. The overthickened wedge geometry returned to a more stable configuration by developing extensional features during the Miocene: both listric normal faults at upper-crustal levels and shear zones indicating evidence of distributed ductile extensional strain at mid-crustal levels are recorded. It is proposed that large-scale regional extension with associated volcanism beginning in the Messinian was represented by the intrusion of asthenospheric material from the subducted plate into the already attenuated accretionary complex. Further rifting, perhaps aided by subduction and back arc processes in the Southern Apennines, led to the formation of the Tyrrhenian Sea as an oceanic basin. Both the Apennines and North American core complexes record evidence of crustal thickening followed by crustal thinning, and finally of continental rifting. This suggests that the similar histories of these regions with vastly different plate tectonic settings may both be explained by processes linked to the changing internal dynamics of accretionary wedges.