Pleistocene Change from Convergence to Extension in the Apennines as a Consequence of Adria Microplate Motion

Discussions about the Adria microplate offer differing views depending on the timescale and data considered. Neotectonic studies using earthquake mechanisms and GPS site velocities find Adria moving northeastward away from Italy, and bounded by an extensional boundary in the Apennines and convergent boundaries in the Dinarides and Venetian Alps. However, geologic data show that Adria was subducting southwestward beneath Italy during Mio-Pliocene time. We suggest that these views are consistent and reflect the recent spatio-temporal evolution of a multiplate system. We assume that during Mio-Pliocene time Adria was no longer part of Africa and had become an independent microplate. Convergence occurred as Adria moved northeastward with respect to Eurasia as at present, because the faster back arc spreading in the Tyrrhenian Sea caused Adria to move southwestward with respect to Italy. The transition from convergence to extension in the Apennines in the past 2 Ma resulted from the cessation of subduction in the Apennines accompanied by breakoff of the subducting Adria slab, and the associated cessation of back arc spreading in the Tyrrhenian Sea. As a result, western Italy became part of Eurasia, and Adria's northeastward motion produced a new extensional boundary along the Apennines.