Tectonic, sedimentary, and diapiric Formation of the Messinian mélange: Tertiary Piedmont Basin (northwestern Italy)

The Messinian mélange of the Tertiary Piedmont Basin is the product of different but interrelated processes (tectonic, gravitational, and diapiric) that operated sequentially over a short time span (intra-Messinian time) and in a geodynamic environment (episutural basin) for which mélanges have so far been poorly described. It is composed of different mappable bodies of (non-metamorphic) mixed rocks characterized by a strong facies convergence. Their geometric and stratigraphic position, the internal organization, and the nature of the bounding surfaces allow the defining of some criteria to distinguish different units of mixed rocks (tectonically disrupted unit, gravity-driven sedimentary unit, and diapiric disrupted unit), in each of which the role of a different prevailing mélange-forming process can be inferred. None of these processes operated in isolation. They were linked by complex and intimate mutual interactions and triggered by intra-Messinian tectonics. The latter produced self- generating processes of mélange formation in which gravitational and diapiric processes triggered and affected each other. Different pulses of overpressured fluids (often rich in methane) strongly governed sediment deformation and also played a crucial role in infl uencing the time relationships and causative links between the different mélange-forming processes. Faulting may have triggered gas hydrate dissociation, promoting the upward rise of overpressured fl uids. These fluids reduced the shear strength of the overlying sediments, promoting large-scale gravity-driven phenomena. Loading provided by rapid emplacement of the gravity-driven sedimentary bodies could have, in turn, developed new overpressured conditions necessary to promote the upward rise of poorly consolidated sediments and shale diapirism.