The structure of the window of Bobbio : (northern Apennines, Italy)

Abstract

Field work in the tectonic window of Bobbio has demonstrated the following tectonic units: Tuscan unit, the lowermost unit in the window. It is composed of a turbiditic sequence of Oligocene or Early Miocene age, folded into a large recumbent syncline with a gently NE-dipping NE limb and a steeply overturned SW limb. The Tuscan unit has been overridden from the SW by the Coli-Sanguineto Complex (CSC). This unit is composed of rocks in a sandy-marly facies and a chaotic shaly facies. The latter is interpreted as olistostromes, derived from the Monte Penice unit. The complex occurs in two areas between which the Tuscan unit crops out. We correlate the rocks of the areas upon their similar lithology and age, which is Late Oligocene or Early Miocene. Deformation along the CSC-thrust is dependent on the angle between the overthrust direction (from SW to NE) and the attitude of the beds in the relatively competent footwall. Where this angle is positive, a footwall syncline has been formed. Where it is negative, the footwall has been deformed by shear parallel to the beds and minor reverse faulting. We consider the so-called "Argilliti di Peli" " which often crop out along the CSC-thrust plane, as a severely sheared level and not as a stratigraphic entity. - The Ruffinati-Aveto unit. It includes two formations: Ruffinati Siltstone, which lithologically reminds of the CSC; its olistostromes are identical to those of the CSC. The Ruffinati Siltstone is stratigraphically overlain by the Aveto Sandstone. It includes andesitic tuffites, exotic conglomerates and turbiditic sandstones. The age of the formations could not be settled directly. There are indications for an Oligocene or Miocene age. 1 The Ruffinati-Aveto unit is only present in the SW part of the window. The contact with the CSC is tectonic; it is truncated by the overlying unit: - The Monte Penice unit, which includes a sequence of dark shales (Santa Maria Shale) and a calcareous flysch (Monte Penice Limestone). The age of the formations is Paleocene-Eocene. The thickness of the unit varies strongly. It is locally absent. - The Ligurid Complex is the highest Apenninic element. In the investigated area, the total slip of these overthrusts exceeds 70 km. The movement have taken place in Miocene times. The unit of the Antola Limestone- a Helminthoid flysch- has overridden the Ligurid Complex from the W. It crops out W of the window of Bobbio. Since Late Eocene times it is welded to the Ligurian Alps, as shown the unconformable, continuous cover of the Tertiary Piemont Basin series. This entire block has been emplaced in its present position after -or in a late stage of- the completion of the Apenninic pile of thrust sheets. It is limited in the N by the E-W striking VarziVillalvernia lineament, a sinistral strike-slip fault. Its eastern limit is a N-S trending thrust front at the base of the Antola Limestone.