Extensional veins and shear joint development in a thrust-fold zone: example from the Northern Apennines (Italy).

The Northern Apennines thrust system consists of clastic foredeep deposits that have been progressively involved in thrusting towards the northeast from the Oligocene to the Recent. The outcroping geological units are progressively younger towards the foreland implying that sedimentation and deformation developed together. The Umbrian Sequence is the most external unit and has been affected by Late Miocene to recent thrusting. The Marnoso-arenacea Formation, a marly arenaceous turbiditic sequence, belongs to this Unit. This formation provides good examples of pre-thrust and thrust-related structures. An extensional calcite vein system with two sub-vertical and orthogonal sets having constant orientation, developed before thrusting. A later conjugate shear joint system which cross-cuts the extensional veins, were developed in the early stages of thrusting. Folds related to the thrusting (i.e footwall synclines or hangingwall anticlines) spatially reoriented the previously formed extensional vein and shear-joint systems.