Ancient accretionary complexes are a natural laboratory to investigate the mode and nature of mass-transport processes and their mutual interactions with tectonic and diapiric processes, and the formation of different types of mélange. The External Ligurian accretionary wedge and the related wedge-top basins (Epiligurian Units) in the NW- Apennines (Italy) provide different examples of ancient MTDs emplaced during the late Cretaceous Miocene evolution of this accretionary wedge. These MTDs consist of sedimentary mélanges or olistostromes and display heterogeneous deformation controlled by the degree of sediment consolidation and the velocity of gravitational processes. Crosscutting relationships between MTDs and coherent successions, tectonic mélanges - broken formation and shaly-dykes and diapirs allow us to document their time-progressive development, the correlation with tectonic and diapiric processes, and the material redistribution forming polygenetic mélanges in the frontal part of the External Ligurian accretionary wedge.
Out-of-sequence “megathrust” and strike-slip faulting, fluid overpressure and presence of low-permeable layers in the sedimentary column were the main factors that controlled the emplacement of various MTDs. In all the examples described, mass-transport was closely associated and had mutual interactions with tectonic and diapiric processes. Tectonics played the most prominent role (directly and indirectly), whereas fluid flow and overpressure strongly controlled the mechanical behavior of sediments and facilitated the emplacement of kilometer-to-hundreds of square kilometers of MTDs. All the examples described from the External Ligurian accretionary wedge here are comparable in size and geodynamic selling types to those documented from the frontal part of several modern accretionary and/or subduction complexes (e.g., Nankai accretionary wedge), providing important information on their evolution, slope instability and hazard mitigation.
