L'Unità di Colli/Tavarone in alta Val di Vara (Appennino ligure): caratteristiche litostratigrafiche ed assetto strutturale.

Abstract

The Colli/Tavarone Unit cropping out in the high Vara Valley belongs to the Internal Ligurid Units of the Northern Apennines, as well as the Bracco/Val Graveglia and Mt.Gottero Units. These Units, consisting of Jurassic ophiolite sequences and related sedimentary covers, are regarded as remnants of the Western Tethys oceanic litosphere deformed and thrust during Late Cretaceous/Eocene intraoceanic subduction and later continental collision. The Colli/Tavarone Unit is characterized by a sequence consisting of Jurassic ophiolites, only ultramaphites and Mg-gabbros, and related sedimentary cover represented by Palombini Shales (Berriasian - Santonian) and Val Lavagna Shales (Campanian - early Maastrichtian). This sequence is unconformably topped by the Colli/Tavarone Formation (early Paleocene) consisting of debris flow and slide deposits in shaly matrix. The deformation history includes a first phase D1 characterized by isoclinal folds with similar geometry. The penetrative axial plane foliation can be recognized in the shales as slaty cleavage showing the following metamorphic mineral assemblage: quartz + albite + chlorite + white mica (illite) + Fe oxides + calcite. This assemblage can be referred to P/T conditions of 3/4 kb and 250°/350°. Cataclastic shear zones bounding the main tectonic units have been developed during D1 phase. The second phase D2 is characterized by folds with parallel geometry and variable style associated to an axial plane foliation recognized as crenulation cleavage in the shales. These pre-Oligocene deformation is responsible of the complex structural setting consisting of large-scale fold structures dissected by thrusts. The following deformation phases D3 and D4 are characterized by gentle folds with steep axial planes related to the Miocene thrusting of the Ligurian Units onto the Tuscan Domain. The pre-Oligocene deformation history of the Colli/Tavarone Unit suggests a deep involvement of the oceanic litosphere of the Western Thetys in the subduction zone processes. The related D1 and D2 phases are interpreted as result of underplating and later uplift in the accretionary wedge