Internal structure and tectonic evolution of an underthrust tectonic mélange: the Sestola-Vidiciatico tectonic unit of the Northern Apennines, Italy.

The Sestola-Vidiciatico Tectonic Unit (SVTU) in the Northern Apennines is an underthrust tectonic melange presently sandwiched between the Tuscan-Umbrian foredeep units and the overlying Ligurian/Subligurian thrust-nappe. The SVTU has been generated during the collision between the European and the Adria plates and now it separates the former oceanic accretionary wedge -Ligurian/Subligurian thrust nappe-from the underlying fold-and-thrust belt formed by Adria sedimentary units. The collision caused an eastward migrating foredeep basin and the overthrusting of the frontal part of the Ligurian/Subligurian thrust-nappe on the subducting Adria margin. Part of the inner lower-slope sediments of the migrating foredeep basin have been unconformably deposited on a frontal prism formed by material already accreted in the Ligurian/Subligurian prism gravitationally and tectonically reworked. The frontal prism and its sedimentary cover have been progressively dragged down along the plate boundary zone generating the SVTU. The lower-slope sediments have been incorporated in the melange as they were not completely lithified, and they show a long deformation history ranging from continuous and pervasive soft-sediment deformation to discontinuous brittle deformation concentrated along faults and mainly controlled by cycles of fluid pressure as testified by the presence of crack-and-seal texture and implosion breccia in the veins.