An example of inversion in a brittle shear zone

A useful tool to document structural inversion inside a shear zone is the identification of overprinted structures, which can increase the complexity of the internal fabric of shear zones. In the case of thrust faults, shear zones can also be problematic due to the occurrence of structures connected to flexural slip along bedding in the recumbent forelimb. Therefore identification of superimposition among different structures can help to distinguish among these configurations inside a shear zone. In Central Apennines (Italy), favourable conditions along the outcrop of Mt. Tanciaʹs thrust allowed the discovery of two sets of superimposed fabric elements referring to two opposite senses of shear. The first is related to thrusting (top-to-E–NE), whereas the second is likely associated to reactivation in normal sense (top-to-W–SW). The fabric elements have been mapped and their distribution inside a semi-brittle shear zone has been defined. High angle extensional shear planes occurring only in the uppermost part of the shear zone (closest to the main thrust plane) have been interpreted as a new generation of C-planes, connected to negative reactivation, instead of Riedel shear planes. The overprinting relationship defined inside the shear zone helps in discriminating whether the observed structures developed in the footwall of the main thrust or on the recumbent limb of the hanging wallʹs anticline. The deformation inside the shear zone can be described using a simple shear model at the outcrop scale, where reoccurrence of fibrous calcite on shear C-planes should control rotation of S-planes; these are dissolution planes parallel to the XY-plane of the local strain ellipsoid. At thin section scale, instead, veins maintaining the same orientation suggest a coaxial deformation due to repeated crystallization events controlled by fluid pressure and differential stress variation.