La geologia dell'alta Val Marecchia (Appennino tosco-marchigiano) - Note illustrative alla carta geologica 1:50.000.

A geological survey (1:10.000 scale) provided new data on the evolution of the Val Marecchia area. The deposits belong to Ligurian, Subligurian, epi-Ligurian and Umbro-Romagnan Domains outcrop in the mapped area; the lithostratigraphic units of the first three domains, included in Val Marecchia thrust-sheet, overthrust Umbro-Romagnan Domain (respectively Verghereto Marls, Ghioli di letto mudstones and Argille Azzurre) at the Lower-Upper Tortonian boundary, during Lower Messinian and at the end of Lower Pliocene (Globorotalia puncticulata Zone). The various units constituting Umbro-Romagnan Domain are also stacked by imbricated thrusts: every inner unit overrides the adjacent outer one and the age of thrusting is progressively younger towards NE, their formation being linked to the tectonic phases of the Northern Apennines. The structural pattern of Val Marecchia sheet is made up of imbricated slices, NE-verging and displaying an arcuate shape in a planar view. These imbrications are not only related to the emplacements of the thrust-sheet over the substratum tectonic Units but also to the Ligurian tectonic phase.New collected field data show the occurrence, inside the Ghioli di letto mudstones, of a wide belt characterized by resedimented deposits of Apenninic provenance (olistostromes and olistoliths of Ligurian and epi-Ligurian sources, turbiditic sandstones with Ligurian affinities), which were previously included in the Val Marecchia thrust-sheet. Furthermore piggyback basins filled by Ghioli di letto deposits have been recognized both in Umbro-Marchean-Romagnan Sequence and over Val Marecchia sheet. The presence of a fragile deformation style affecting the Val Marecchia thrust-sheet (arcuate thrusts, pop up structures, back thrusts, tear faults), led to envisage its emplacement as triggered by active tectonics, due to horizontal compressive pushes as in tapered sheets or in accretionary wedges. The emplacement of the Val Marecchia sheet occurs in a piggyback sequence of imbricated slices, filled not only by epi-Ligurian deposits, but also, exclusively at the thrust front and with a minor extension, by the terminal parts of the same deposits overriden by the thrust-sheet.Consequentely, a complex system composed of a duplex and a leading imbricate fan (in the substratum) arises. The role of transverse tectonics in Val Marecchia is strictly related to thrust-front activities: during every tectonic phase it acts in the outermost part of the thrust belt as a line dividing fronts with different degrees of advancement and it affects the sedimentation of those deposits that will be afterwards overthrust by Val Marecchia sheet.These elements have then been utilized to describe the structural evolution of the studied area during Miocene, according to the model of the migration of foredeep-thrust belt system, and to explain the tectonic relationships due to compressive horizontal forces. Collected data have also shown that gravitational episodes with an Apenninic provenance are of restricted importance and forerunners of Ligurian tectonic emplacements.