Deformation features within an active normal fault zone in carbonate rocks: The Gubbio fault (Central Apennines, Italy)
Résumé
The Gubbio fault is an active normal fault defined by an important morphological scarp and normal fault focal mechanism solutions. This fault truncates the inherited Miocene Gubbio anticline and juxtaposes Mesozoic limestones in the footwall against Quaternary lacustrine deposits in the hanging wall. The offset is more than 2000 m of geological throw accumulated during a poly-phased history, as suggested by previous works, and has generated a complex zone of carbonate-rich fault-related structures. We report the results of a multidisciplinary study that integrates detailed outcrop and petrographic analysis of two well-exposed areas along the Gubbio fault zone, geochemical analysis (fluid inclusions, stable isotopes, and trace elements) of calcite-sealed fault-related structures and fault rocks, and biostratigraphic controls. Our aims are: (i) the characterization of the deformation features and their spatial-temporal relationships, and (ii) the determination of the P/ T conditions and the fluid behaviour during deformation to achieve a better understanding of fluid-rock interaction in fault zones. We show that few of the observed structures can be attributed to an inherited shortening phase while the most abundant structures and fault rocks are related to extensional tectonics. The outcropping extensional patterns formed at depths less than 2.5-3 km, in a confined fluid system isolated from meteoric water, and the fault structures are the response to a small amount of cumulated displacement, 12-19% of the total geological throw.