Internal structure of an ancient analogue of an erosive subduction channel: the Subligurian Units of the Northern Apennines, Italy

The Subligurian Units –SLU- were involved in the Apennine subduction system starting from the middle Eocene. At the time the SLU were part of the Ligurian prism forming the frontal portion of the European margin. The present geometric position of the SLU, sandwiched between the External Ligurian Units and the Tuscan Units, implies that later on these units have been underthrust all together moving from the toe of the prism to below the prismWith the subduction of the continental crust, the SLU show evidence to have accommodated the relative displacement between the two plates. In particular is possible to reconstruct a flow velocity gradient within the sub-Ligurian units that allows them to be considered a fossil analogue of a subduction channel as defined by Cloos and Shreeve (1988).The deformation of the SLU varies within the different stratigraphic components: the basal formation (i.e. Canetolo Shale and Limestone) shows the same deformation style of the External Ligurian Units, which has been interpreted as the product of frontal accretion during the subduction. Accordingly, the basal part of the SLU formed the frontal part of the middle Eocene accretionary prism. The unconformable overlying late Eocene-late Oligocene slope deposits (i.e. Passo del Ticchiano Shale and Limestone, Petrignacola Sandstone, Ponte Bratica Sandstone), do not maintain the same deformation style, but they rather seal this frontal part of the accretionary prism. Hence the SLU younger than late Eocene could be considered as epi-Subligurian units.We infer that from end of the epi-SLU deposition, the entire Subligurian package was incorporated in a subduction channel and, since the main portion of the material forming the subduction channel was previously part of the upper plate, the Northern Apennines entered a phase of subduction erosion. The input in the subduction channel was through frontal erosion, with incorporation of the prism toe, and basal erosion, represented by sparse blocks of External Ligurian Units found in the upper part of the subduction channel.The subduction channel then is structured in minor units that we can envision as sub-horizontal bodies separated by tectonic surfaces parallel to the boundaries of the subduction channel itself. These thrust-bounded packets are organized in a tectonic vertical stratigraphy with those including younger sediments laying closer to the basal décollement and those including older sediments laying closer to the roof décollement. This tectonic stratigraphy implies that more than one décollement was active within the subduction channel. In particular, it suggests that the different décollements subdivide portions of the subduction channel that moved at different velocities such as, considering the overlaying plate as fixed, the lower décollements moved faster. Décollements deactivation occurs at depth, where involvement of the SLU in the fold and thrust system of the Tuscan Units indicates complete locking.The subduction channel deformation is preserved within the epi-Subligurian formations and the Ponte Bratica Sandstone has been the focus of detailed structural analysis. As a main character the Ponte Bratica Sandstone has been affected by a regional boudinage both in NE-SW direction and in SE-NW direction. Within each package of unfolded strata, scattered mesoscopic, mostly disharmonic, tight to isoclinal folds are present together with evidence of soft sediment deformation. The attribution of this deformation features to processes active at the prism toe seems quite certain, but the driving force is still unclear. Subduction channel deformation features are, instead, the conjugate sets of extensional faults filled by calcite veins. They cut all the components of the subduction channel and often also the deactivated décollements suggesting a strong partitioning of the deformation among them. They also suggest that the shear stress is mainly accommodated in extension.