Early orogenic normal faults and their reactivation during thrust belt evolution : the Gubbio Fault case study, Umbria-Marche Apennines (Italy).

Foreland (early-orogenic) and hinterland (late-orogenic) extensional domains have been widely documented in the Northern Apennines, where they are synchronous with contraction in the active part of the fold and thrust belt. The progressive eastward migration of the contractional front and the associated hinterland extensional field implies that early-orogenic extensional structures, developed in the foreland domains, may experience reactivation/inversion. We present new field data, integrated with seismic evidence, from the Gubbio normal fault, a 22 km long presently active fault, showing: a) evidence for early-orogenic extension since the Lower Miocene time; b) successive positive inversion during the Upper Miocene contraction; and c) renewed, late-orogenic, extension during the Quaternary age. Field data allowed two systems of mesoscale normal faults, respectively interpreted as early- and late-orogenic structures, to be recognised. Stress fields associated with contractional and early- late-orogenic extensional tectonic regimes, are characterised by an overall coaxiality, with directions of compression and tension consistently aligned NE-SW. The symmetry between the successive deformation stages is interpreted as a likely cause for the repeated reactivation of the Gubbio fault since the Miocene. Our analysis in the Gubbio area shows that normal faults play a key role during the evolution of a fold and thrust belt from the early-orogenic stages, when they influence the geometry and evolution of foredeep basins, to the late-orogenic stages when Quaternary activity, due to extension reactivation, controls the evolution of the intramountain basins