Coexistence of contractional and extensional tectonics during the northern Apennines orogeny: The late Miocene out-of-sequence thrust in the Elba Island nappe stack

In the northern Tyrrhenian Sea, the Elba Island is one of the westernmost portions of the northern Apennine inner belt. One of its noteworthy features is the anomalous tectonic repetition of continental-derived (Tuscan Unit) and oceanic-derived (Ligurian units) thrust sheets, lately intruded by late Miocene granitoids. Moreover, in detail, a slice of strongly deformed Ligurian peridotites results tectonically sandwiched between two thrust sheets of Tuscan units. This tectonic setting results from a middle Miocene folding and thrusting of the Apenninic nappe stack with development of large-scale antiform and out-of-sequence thrust. In central-eastern Elba Island, the folding of an imbricate stack is bracketed between Langhian (middle Miocene) and Messinian (late Miocene). Consequently, the anomalous repetition of Tuscan and Ligurian units thrust sheets gives evidence of middle-late Miocene shortening deformation post-dating nappe stack and pre-dating late Miocene-Pliocene granite emplacement. We suggest that the architecture of the Elba Island nappe stack documents the coexistence of early-middle Miocene contractional and extensional tectonics in an overall convergent tectonic setting in the westernmost zone of northern Apennines. Extensional tectonics in the upper portion of the wedge, balancing transient gravitational instabilities due to over-thickened conditions, were followed by a renewal of contractional deformation leading to development of large-scale out-of-sequence thrust responsible for inversion of the stack order.