Abstract

The Monte Gorzano area is underlain by a Tertiary pelagic succession, ranging from calcarenites of the Bisciaro Formation (Aquitanian p.p.-Burdigalian p.p.) to siliciclastic deposits of the Laga Formation (Messinian), and consists of a N-S trending anticline whose western limb is dissected by a WSW-dipping normal fault (Monte Gorzano Fault). This fault has a length of up to 30 km and vertical displacement in excess of 1300 m (Festa, 1999). The displacement decreases along strike to zero where the fold axis plunges away, with consequent disappearance of the marly-calcareous succession underneath the cover of the Laga Formation. The Monte Gorzano fault separates two tectonic units of the Laga Formation which have different structural settings. The western unit is characterized by a N-S striking east-verging fold-and-thrust system, while the eastern unit shows a gently east-dipping monocline. The geometry and kinematic evolution of the Monte Gorzano area are a result of a different mechanical response to shortening in the siliciclastic levels of the Laga Formation with respect to the underlying marly-calcareous Cerrogna Marls and Bisciaro Formation. The core of the anticline displays a complex ENE and WSW-verging imbrication system with tectonic repetitions involving tectonic slices of the Bisciaro Formation and Cerrogna Marls, with shear surfaces prevalently developed in Orbulina Marls, and slices of Laga Formation as well. This slice structure represents a "passive-roof duplex" (sensu Banks & Warburton, 1986) wedging underneath an east-dipping detachment, located on top of the Orbulina Marls, that caused the backthrust of the thick and poorly deformed roof succession of the Laga Formation. The Monte Gorzano normal fault, located in the back limb of the main detached fold and subparallel to its own axis, can be interpreted as kinematic accommodation consequent to a decrease of the normal stress components. This may have controlled the uplift of folded structures since the latest shortening stages (Middle-Late Pliocene) and not exclusively during Pleistocene time. This paper, based on 1:10.000 geological mapping and detailed structural analysis, describes the structural setting, kinematic evolution and shortening mechanisms of this area.

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